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by: Jayda Abrams


Marketplace > Virginia Commonwealth University > Chemistry > Chem301 > ENTIRE SEMESTER OF ORGANIC CHEMISTRY ONE
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These notes cover the entire semester of organic chemistry one! Covers topics like resonance, mechanisms for reactions, Newman projections, chair drawing, nomenclature and much much much more! Thes...
Organic Chemistry 1
Mr. Jon Baker
Organic Chemistry, MCAT Organic Chemistry, Organic Chem, Newman Projections, resonance, mechanisms, Wade8thEdition
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This 53 page Bundle was uploaded by Jayda Abrams on Friday September 2, 2016. The Bundle belongs to Chem301 at Virginia Commonwealth University taught by Mr. Jon Baker in Fall 2015. Since its upload, it has received 28 views. For similar materials see Organic Chemistry 1 in Chemistry at Virginia Commonwealth University.

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Date Created: 09/02/16
Resonance Compounds can have more than one Lewis structure that represents them correctly. The biggest difference between structures are where the electrons are placed in each one. These are called resonance structures or resonance forms. They are not different compounds but they are drawn in different ways. Resonance molecules will usually show characteristics of the different resonance structures even though it is something completely different. The actual molecule will be a resonance hybrid of the resonance forms. Since the actual molecule is a hybrid, this means the individual resonance structures are not completely accurate, and are not real. This can be compared to a rhino. The hybridization (or coming together) of a dragon and a unicorn will create a rhino. Even though dragons are not real, and unicorns are not real, rhinos are. This is the same in resonance. One structure that is not real combines with another structure that is not real and it creates a real structure. When comparing resonance forms, the more stable form is the major contributor and the less stable form is the minor contributor. Using Arrows in Resonance When drawing resonance structures arrows are very important. When showing the flow of electrons curved arrows are used. When showing the flow of a single electron a curved fishhook arrow is used. When showing that two structures have resonance a double sided arrow is used. Do not use a single sided arrow because this means that the two structures are in equilibrium! Flow of Electrons Flow of Electron Resonance Remember With Resonance… … ALL structures must be VALID Lewis structures. …you want to have as many octets as possible. …only electrons are moving between structures. Nuclei cannot be moved and bond angles stay the same! Also remember that the number of unpaired electrons as well as the formal charge will stay the same. …the most stable compounds have no unpaired electrons. …the major contributor is the one with the lowest energy. Good contributors usually have octets filled, have as many bonds as possible and have little charge separation. …negative charges are more stable on more electronegative atoms (like O, N and S). …resonance stabilization is important when it is used to delocalize a charge over two or more atoms. …curved arrows are used to show the flow of electrons. This diagram shows six different resonance structures of the same molecule. Notice how each individual structure has the same components and has the same total formal charge. Also notice the double headed arrow between structures and that the overall structure of the molecule has not changed. Chapter 1 The Basic Formulas Formula Description Example Shows which atoms are bonded to which. There Structural Formula are two types of Structural Formulas. There are Lewis Structures and there are Condensed Structural Formulas. Written without bonds and has the central atoms Condensed Structural and the atoms attached to Formula it listed out beside it. Nonbonding electrons are rarely shown. Show individual bonds Lewis Structures and often also shows the physical electrons. Also referred to as stick figure or skeletal structures. Bonds are represented by lines and carbons are assumed at Line Angle Formula the ends, and bends. Hydrogen atoms are usually not shown and it is assumed that carbon atoms have enough hydrogen atoms to give it a total of four bonds. Gives the number of Molecular Formula atoms of each element in a C 6 O12 6 molecule of a compound. Empirical Formula Compares the ratios of elements present. C 2 5 Chapter 2 Intermolecular Forces A molecular dipole moment is the dipole moment of the molecule taken as a whole. It is a good measure of a molecules overall polarity. Bond polarities can range from nonpolar to ionic. The polarity of an individual bond is measured as its bond dipole moment. When two molecules come close to each other they either attract or repel each other. The 3 major kinds of attractive forces that cause molecules to associate into solids and liquids are: 1. Dipole Dipole forces, 2. London Dispersion Forces and 3. Hydrogen bonds. Dipole Dipole Forces: An intermolecular force resulting from the attraction of one positive end and one negative ends of dipole moments on polar molecules. Dipole Dipole interaction only occur with two polar molecules and can occur with molecules that have hydrogen in them, but do not have hydrogen bonds (ex: HCl). London Dispersion Forces: An intermolecular force that attracts nonpolar molecules. London Dispersion Forces are a type of Van Der Wal Force and the attraction of two molecules occurs when two molecules have temporary dipole moments. Hydrogen Bonding: Hydrogen bonding is not a true bond, and is essentially a strong dipole dipole interaction. A bond is considered to be a hydrogen bond if there is a hydrogen bonded to a Nitrogen, Fluorine or Oxygen. These elements are extremely electronegative and will give hydrogen a positive charge. Even though hydrogen bonding is strong it is not as strong as covalent bonding. Summary of Hybridization and Molecular Geometry: Hybrid Orbitals Hybridization Geometry Approximate Bond Angle 2 s+p=sp Linear 180˚ ???? 3 s+p+p=s???? Trigonal 120˚ 4 s+p+p+p=s???? ???? Tetrahedral 109.5˚ 3 Bicyclic Molecules How Bicyclic Molecules and The Human Body are Similar! Two or more rings can be joined together into bicyclic or polycyclic systems. There are three ways two rings can be joined. They can be fused, bridged or spiro. The combination of two things coming together creating a system is like the human body because the human body has several parts that are put together to make a system. The two rings will touch at a carbon and the number of carbons touching defines the bicyclic as fused, bridged or spiro. The most common form is fused. Bridges is the second most common and spiro is very very rare. Definitions: Spirocyclic Compounds- When two rings share only ONE carbon atom. Bridged Rings- When two rings share TWO carbon atoms that are not adjacent to each other. Fused Rings- When two rings share TWO or more carbon atoms that are adjacent to each other. How many carbons shared between two rings can be remembered with the human body and specific body parts. 1. Skull starts with S and so does Spiro. The human body only has one skull and spirocyclic compounds only share one carbon. 2. Fused starts with F and does fingers. The human body has at least two fingers and fingers are side by side. Fused has at least two carbons with the carbons side by side. It is important to remember that to be fused the carbons must be adjacent. 3. Bridged starts with B and so does Bones. The human body has 2+ bones and they are not all side by side. Like bones in the body bridge rings have two carbons being shared but they are not side by side. All about Alkanes! Melting Point and Boiling Point Alkanes are nonpolar and are hydrophobic. This characteristic makes them good for preserving metals. The boiling point of alkanes depends on the number of carbons present. The more carbons the higher the boiling point. In alkanes branching lowers boiling point. If there are two alkanes with the same number of carbons the straight chain will have a higher boiling point than the branched chain. This is because there is less surface area for London Dispersion Interactions. Melting points with alkanes are a bit different. Alkanes with an odd number of carbons melt at lower temperatures than alkanes with an even number of carbons. However like the boiling point the more carbons present the higher the melting point will be. Uses of Alkanes: The job an alkane can do depends on the number of carbon it has. 1-2 Carbons: Used for Natural Gas 3 Carbons: Used for Propane 4 Carbons: Used for Butane 5-10 Carbons: Used for Gasoline 11-16 Carbons: Used for Diesel 16-20 Carbons: Used for Lubricant Oil 20-30 Carbons: Used for Petrolatum 30+ Carbons: Used for Asphalt Reactions of Alkanes Alkanes’ reaction include combustion, cracking, hydrocracking, and halogenation. Combustion is the rapid oxidation that takes place at high temperatures. Combustion produces water. Cracking of large hydrocarbon at high temperatures produces smaller hydrocarbons. In hydrocracking hydrogen is added to give saturated hydrocarbons. Cracking without hydrogen gives mixtures of alkanes and alkenes. Halogenation is initiated with heat or light. Chairs The chair conformation is the most stable. There are six carbons and two types of bonds: Axial and Equatorial. Like on a globe the axial is like the axis and runs up and down while the equatorial is like the equator and runs from side to side. In chair conformations the odd numbered carbons have the axial bonds up and equatorial bonds down, and the even numbered carbons have the equatorial bonds up and the axial bonds down. A chair is the most stable when the biggest groups are equatorial. If you are changing a ring into a chair conformation it is important to keep cis and trans in mind if specified. If it is cis make sure both hydrogens are pointing in the same direction and if it is trans make sure both hydrogens are pointing in opposite directions. When converting a chair to a cyclohexane ring the groups going down are dashes, and the groups going up are wedges. Key Things to Remember during Ring Flips: Remembering to shift the numbered carbons and to change all equatorial substituents to axial and changing all axial substitutes to equatorial. In this case the picture reads from left to right. Notice how in the first image of the chair the yellow dot (which highlights the tip of the chair) is facing up to the right. After the flip has occurred the yellow dot is facing down to the right. Also notice that numbered carbons have shifted one to the right. In this case the picture reads from right to left. Notice how in the first image of the chair the yellow dot (which highlights the tip of the chair) is facing up to the left. After the flip has occurred the yellow dot is facing down to the left. Also notice that the numbered carbons have shifted one to the left. Newman Projections It is important to know that Newman Projections have several different STAGES! STAGES STaggered Anti Gauche Eclipsed Syn The five main stages of a Newman projection are: Staggered, Anti, Eclipsed, Gauche and Syn. The difference between each stage depends on the angle between two large groups. 1. The eclipsed position has the two large groups 0˚, 120 ˚ and 240 ˚ away from each other. 2. The staggered position has the two large groups 60˚, 180 ˚, and 240˚ away from each other. 3. Syn is a type of eclipsed position and is located at 0 degrees. 4. Gauche and Anti are types of staggered positions; gauche has large groups 60 ˚ and 300 ˚ away from each other and anti has the two large groups 180 ˚ away from each other. Know that the energy between the different conformations of each Newman projection is different. Anti has the lowest energy and eclipsed has the highest energy. Chapter 3 All about Nomenclature Nomenclature is a fancy word for naming. There are two types of names a structure can go by. There is a systematic name and there is an IUPAC name. Both names are commonly known and are globally understood. To give a structure an IUPAC name the IUPAC rules must be followed. The IUPAC rules have been accepted as the standard method for naming an organic molecule. The rules can also be followed as steps to naming a molecule. Step one: Find the longest carbon chain. This chain does not need to be a straight chain and can curve or come off of a substituent. This will act as the “last name” of the compound. These include methyl, ethyl, propyl, butyl, etc. Step two: Number the main chain. This will help locate where the substituents have fallen and will make labeling easier. Start numbering at the end of the chain with the closest substituent. Step three: Name the substituents as alkyl groups. Give the location of the groups by the number of the main chain carbon it is attached to. Step four: Give prefix if needed. The prefixes are di-, tri-, tetra-, penta-, hexa-, hepta-, etc. This only occurs when naming compounds with more than one of the same substituent. When there are two or more different substituents, list them in alphabetical order and use prefixes (if needed). Make sure each substituent has a position number, and know that position numbers can repeat! The complete name should be in alphabetical order and there should be dashes separating numbers and letters, and commas separating two numbers side by side. To double check your work make sure the molecule is named like a person. Like a person, the complete name will have: a prefix, first name, last name, and suffix. Using these rules/steps, any structure’s name can be found, and using the name any structure can be drawn! Chapter 4 The Study of Chemical Reactions The most important part of studying chemical reactions involves understanding the MTK of each reaction. MTK stands for Mechanism, Thermodynamics, and Kinetics. Mechanism: The complete step by step description of which bonds break, which form and in what order to give the observed products. Thermodynamics: The study of the energy and the changes that accompany chemical and physical transformations. The stability of reactants and products can be compared and the compounds favored by equilibrium can be predicted. Kinetics: The study of reaction rates, which products are formed fastest and the prediction of how a rate would change if the reaction conditions changed. How People And Chain Reaction Mechanism Are Alike! The steps of a chain reaction mechanism are Initiation Propagation, Termination. The process of a person having a bad day and the process of a reaction are pretty similar. For example you are driving to work and another car hits you while texting and driving. No one is hurt but you are very angry because your car was wrecked and now you are late to work. This is the event that initiates your bad mood for the day. In molecules the initiation step is the formation of a reactive intermediate. You being angry means you will be very reactive and very irritable towards other people. Once you get to work a coworker accidently spills coffee on you and you lash out and yell at them, now putting them in a bad mood. This represents the propagation step. In molecules propagation is when a reactive intermediate reacts with a stable molecule to form a product and another reactive intermediate. You were in a bad mood and your coworker who spilled the coffee was in a good mood until you yelled at them. This now makes your coworker just as reactive as you are (creating another reactive intermediate). In molecules this process continues until the supply of reactant is used or the reactive intermediate is destroyed. During your lunch break a friend who is also having a bad day calls and asks if you want to go to the gym with them. You agree because working out calms you down and you know it will help get you out of your bad mood. This is the termination step during a reaction. Termination is a side reaction that destroys reactive intermediates and tend to slow or stop the reaction. The meeting/combination of two free radicals is always termination because it decreases the number of free radicals. You seeing your friend and going to the gym causes your bad mood to slow down and by the end of your workout it has completely stopped. 4 Intermediates- Carbanions, Carbocations, Free Radicals and Carbenes Carbon with three bonds and one lone pair A high electron density Resonance is important Basic (Lewis Base) Amine resemblance Nucleophilic Intermediate in organic reactions Opposite stability of carbocations (stability is methyl > 1 > 2 > 3) Negative charge S???? 3 hybridized Carbanion: A carbanion is a trivalent carbon atom that has a negative charge. There are eight electrons around the carbon (3 bonds and 1 lone pair) and it is NOT electron deficient. It is a strong 3 nucleophile (Lewis base), and is ???????? hybridized. The stability of a carbanion reflects their high electron density and the order of stability is opposite of carbocations meaning that tertiary is the least stable and methyl is the most stable (methyl > 1 > 2 > 3). Cyclopropane rings formed when added to double bonds A carbon with two bonds and one lone pair Reactive intermediate with no charge Basic carbene formula is :CH 2 Empty p orbital AND lone pairs Nucleophile or Electrophile S???? hybridized Carbene: A carbene is an uncharged reactive intermediate containing a divalent carbon atom. The simplest carbene has the formula of:CH . A carbene can be generated by forming a 2 carbocation that can expel a halide ion. Carbenes only have six electrons in a valence shell and is ???????? hybridized with a trigonal planer molecular geometry. A carbene has both a lone pair and an empty p orbital, so it can react as a nucleophile or an electrophile. The most common synthetic reaction of carbenes is their addition to double bonds to form cyclopropane rings. Charge is positive Alkyl substitutes help with stabilization Resonance delocalization Bonded to three other atoms Opposite stability of carbanion (stability is 3 > 2 >1 > methyl) Can be called carbonium ions or carbenium ions Acidic (Lewis acid/electrophile) Trigonal planar Inductive effect Only six electrons in valence shell Nothing in the p orbital 2 S???? hybridized Carbocation: A carbocation is a species that contains a positively charged carbon. It has a carbon with 3 bonds and 0 nonbonding electrons and has 6 electrons in its outer valence shell (???????? ). The hybridized p orbital is empty. Carbocations are electrophiles and are Lewis acids. They will react with any nucleophile they meet and they are unlikely to be found in basic solutions. Carbocations are electron deficient and are stabilized by alkyl substitutes by 1) The Inductive effect or 2) Through the partial overlap of filled orbitals with empty ones. Resonance delocalization is effective in stability. Unsaturated carbocations are stabilized by resonance stabilization. Free radicals have one more electron that carbocations Resonance Electron deficient Energy needed to break a C-H bond to form a more highly substituted radical is lower. Ranked stability is 3 > 2 > 1 > methyl Alkyl groups help with stability Delocalization over two carbon atoms helps with stability Intermediate for organic reactions Carbon with three bonds and one electron A single electron is not effective enough to have a charge cloud Lacks octet 2 S???? hybridized Free Radicals: Free radicals are carbons with three bonds and one electron. They are ????????2 hybridized and have a trigonal planer molecular geometry. They are not tetrahedral because the single electron does not have enough of an impact to have its own charge cloud. Like carbocations they use alkyl substitutes to help with stabilization. Stability is ranked 3 > 2 > 1 > methyl and the p orbital holds the lone electron. Chapter 5 All about Chirality!: Objects that have a left handed and right handed version of each other are chiral. This includes hands, feet, shoes and gloves. It can be said that the left and right handed versions of an object can be described as mirror images. Objects that are not chiral (or do not have a left and right handed version of each other/or are not mirror images of each other) are achiral. Achiral objects include a chair, a spoon or a cup of water. The difference between chiral and achiral can be remembered like this: RAL= Right and left. This is because chiral objects have a CHIRAL RIGHT AND LEFT version of each other! A…RAL= Anti Right and Left. This is because achiral objects ACHIRAL do NOT have a RIGHT AND LEFT version of each other. Two objects are said to be superimposable if one of the objects can lay on top of the mirror image of the other object perfectly and have all the pieces line up. To draw the mirror image of a molecule draw the same structure with the left and the right reversed! Notice that the structure of the molecule is unchanged! However in picture one the CH3 group is on the left and in picture two it is on the right. In picture one the hydrogen is on the right and in picture two it is on the left. Also notice how in both picture the carbon is in the center and the COOH group is still going up. The OH group is on a wedge in BOTH pictures and the H group is on a dash in both pictures! Nothing else has changed except the side the groups are on! How do you Know If a Molecule is Chiral?: The most common distinguishing factor of a chiral molecule is a carbon atom bonded to four different groups. A carbon with four different groups is also called an asymmetrical carbon atom and can be designated with an * beside it. An asymmetric carbon is the most common example of a chiral center (or a chirality center). Chirality centers belong to a group called stereocenters and stereocenter belong to stereoisomers. The best test for chirality is to determine if the molecule’s mirror image is the same or if it is different. If a compound does not have any asymmetric carbons it is achiral. If a compound has one asymmetric carbon it is chiral. If a compound has more than one asymmetric carbon it can be chiral or achiral. Any molecule that has an internal mirror plane of symmetry cannot be chiral (even though it may have asymmetric carbon atoms). Enantiomers (stereoisomers that ARE mirror images of each other) are told apart by the way they rotate polarized light. All the other properties (boiling point, melting point and density) are the same! The difference between two enantiomers can be specified by classifying the molecule as R or S. This is done by the Cahn-Ingold-Prelog Convention. Here is the procedure: 1. Assign priority to the groups- Atoms with a higher atomic number receive a higher priority. The highest priority should be numbered 1 and the lowest should be numbered 4. In most cases (but not always) hydrogen will be the lowest priority group. Make sure you are only looking at the FIRST atom of the group! *If there is a tie continue until the tie is broken! This is done by finding the higher priority group by comparing two groups. Make sure you are only looking at ONE atom at a time! *Treat double and triple bonds as if they are separate atoms. 2. Put the fourth priority group in the back- Use a 3D drawing or a 3D structure. 3. Assign R or S- Connect groups 1-3 in numerical order. If the arrow points clockwise it is R if it points counter clockwise it is S. If the fourth priority group is in the plane you can switch the number of the fourth priority and the one going back. After that you reverse the letter. If it is R it changes to S if it is S it changes to R. Fischer Projections Everytime I think of a Fischer projection I think of this part of the Patty Hype Spongebob episode. This helps me because the fry helps me remember the set up of a Fischer projection. When placing particals on a Fischer projection the wedges are placed horizontally (like the fry’s bowtie) and the dashes are placed vertically (like the crinkle cut of the fry.) This picture helps with the process of converting a drawing into a Fischer projection or placing parts on a Fischer projection however it does not represent the final drawing. Unlike a line angle drawing Fischer projections do not show dashes and wedges and the final projection has all lines “in the plane” with the chiral carbon in the center. It is important to know that Fischer projections are used for compounds with two or more asymmetric carbon atoms. They can be rotated by 180 degrees because the vertical lines will still be vertical and the horizontal lines will still be horizontal. However they cannot be rotated 90 degrees. Rotating a Fischer by 90 degrees would produce an enantiomer (Type of stereoisomer that has mirror images at the stereo-center(s)). When naming a Fischer projection with the IUPAC system the longest carbon chain will be the vertical line (line running from top to bottom). Process of converting a line angle to Fischer projection: C C The picture shows the first diagram that we are familiar with carbon in the middle with four groups coming off of it. The second diagram shows those same four groups on the bowtie french-fry. The bow tie acts as the wedges and the crinkle cut acts as the dashes. The groups are placed where they are because the angle observed to make this conversion. The observer would be looking up from the bottom between the red and yellow group. Meaning the red and yellow group would be coming forward which is why they are on the bowtie, and the green and purple group would be going back which is why they are on the crinkle part. The last diagrams shows the exact same thing without the dashes and wedges in the diagram and this is the final projection. Drawing Mirror Images of Fischer Projections The biggest part of drawing any type of perspective drawing is remembering to reverse the right and left and keeping the up and down (or front and back) in the same position. Swapping out the groups on the horizontal part of the cross is the key part of creating a mirror image of a Fischer projection because this completes half of the work. The other half is complete because the vertical groups are not changing and have not moved. It is important to know that interchanging the groups on the horizontal part of the diagram will reverse the R and S configuration. If something is R it turns to S and if something is S it turns to R. Testing for enantiomers is important and simple with Fischer projections. If the mirror image cannot be made to look the same as the original with a 180˚ degree rotation then the two mirror images are enantiomers. If the mirror images are the same the structure is achiral. Original Mirror Image 180 Rotation If the mirror images are different then the structure is chiral. In Fischer projection the planes of symmetry can also run horizontal. If they do the structure is chiral. Original Mirror Image 180 Rotation Isomers Isomer Definition Example Conformational Made up of the same things. Have the same configuration. Have a different shape. Constitutional Made up of the same things. Put together in a different way. Stereoisomers Made up of the same things. Different configuration. Diastereomers Type of stereoisomers. The stereocenters are not mirror images of each other. Enantiomers Type of stereoisomers. The stereocenters are mirror images of each other. Chapter 6 Substitution and Elimination Reactions of Alkyl Halides Vocab & Break Down: Prefix Meaning Hal- Halogen is involved Nucleo- Nucleus Philic- Loving Phobic- Hating Electro- Electron/negative A halide is a compound that is part radical and part halogen. An alkyl halide is a group of chemical compounds derived from alkanes containing one or more halogen. A leaving group is the atom or groups of atoms that departs during a substitution or elimination. The leaving group can be charged or uncharged but it leaves with the pair of electrons that originally bonded the group to the remainder of the molecule. Substitution is when another atom replaces the halide ion. A halide ion is a halogen atom with a negative charge. Substitution is when another atom replaces a halogen atom and its negative charge. When the halide ion (a halogen atom with a negative charge) leaves with another atom or H+ and forms a new pi bond (double or triple bond) the reaction is elimination. Elimination is when a halogen atom with a negative charge (halide ion) leaves with another atom or H+ and forms a new pi bond (double or triple bond). Elimination where a molecule of H-X is lost from the alkyl halide (a group of chemical compounds derived from alkanes containing one or more halogen) to give an alkene is called dehydrohalogenation. In a nucleophilic substitution a nucleophile (nucleus lover and donor of electron pairs) replaces a leaving group (the atom or groups of atoms that departs during a substitution or elimination can be charged or uncharged but it leaves with the pair of electrons that originally bonded the group to the remainder of the molecule) from a carbon atom using its lone pair of electrons to form a new bond to the carbon atom. Substrates are the compounds that are attacked by the reagent. Concerted reaction takes place in a single step with bonds breaking and forming at the same time. The transition state is a point of maximum energy, rather than an intermediate. An intermediate is a point of minimum energy. On the diagram below the green dots show the transition states and the purple dots show the intermediates. Polar aprotic solvents will dissolve salts. They lack acidic hydrogen and are not hydrogen bond donors, but they are acceptors. They can be described as having both high dielectric constants and dipole moments. Solvents include DMF, Acetone, Acetonitrile, DMSO and HMPA. Polar protic solvents have O-H or N-H bonds and can participate in hydrogen bonding. These solvents can serve as sources of protons (or acids). Solvents include water and alcohol. An Alkoxide is a CONJUGATE BASE OF AN ALCOHOL! It consists of an organic group bonded to a negatively charged oxygen. They can be written as RO and they are strong bases and good nucleophiles when the R group is not bulky. They are usually most stable in polar protic solvents like water and alcohol. In polar protic solvents: Nucleophilicity increases as the attacking atom is changed − − − − down a group of the periodic table. Ex: ???? > ???????? > ???????? > ???? . Also Nucleophilicity decreases left to right across a row of the periodic table. Ex: ???? < ???? . In polar aprotic solvents: Nucleophilicity decreases as the attacking carbon atom is changed down a group of the periodic table. Nucleophilicity also decreases from left to right across a row like basicity. Arrow Key: Yellow: Increase of nucleophilicity in polar protic solvents Green: Decrease of nucleophilicity in polar protic solvents Red: Decrease of nucleophilicity in polar aprotic solvents Purple: Decrease of nucleophilicity in polar aprotic solvents Orange: Increase of the strength of acids Blue: Increase of the strength of acids SN2 The best way to think of an SN2 reaction is trade. The halide ion is being traded for the nucleophile. The process for an SN2 reaction is done in a single step and the strong nucleophile attacks the electrophilic carbon forcing the leaving group to leave. The mechanism of the process can be remembered with the scenario of buying new clothes. You are out shopping and you ripped the pair of jeans you were wearing and you go to Nordstrom to buy new ones. Once you find a comfortable pair you place them in your cart. This represents the first part of the mechanism. In the mechanism the halide ion is still attached to the carbon and the nucleophile is looking to come in. In the scenario you are the carbon, the nucleophile is represented with the NEW jeans and the halide ion represents the RIPPED jeans. You still have your jeans on (they are still attached to you) but you have the other ones in the cart (who will come in) because you have not purchased them. You purchase the new jeans and you find the restroom This represents the transition state of the mechanism because in the mechanism the carbon has both the nucleophile and the halide ion just like how you have both the new jeans and ripped jeans with you in the bathroom. Finally you put on the new jeans, throw out the old ones and continue shopping. This represents the last part of the mechanism. Your ripped jeans are the leaving group and the product is you, in (or attached to) your new jeans or the nucleophile attached to the electrophilic carbon. SN2 stands for Substitution, Nucleophilic, Bimolecular; and it is important to keep in mind that for the SN2 reaction all of these steps are occurring in ONE SINGLE STEP. The scenario is also one step because you only went to one store to find the new jeans and throw out the old ones. This scenario is good for remembering the process/ mechanism of the reaction. For SN2 reactions: You MUST show the breaking off all bonds and the flow of electrons with curved arrows. If a bond is breaking it must be shown with an arrow starting at the bond and curved away from the substituents. For the rate limiting step, doubling the concentration of either component will double the rate. Details of SN2: The Basics, Inversion and What Makes a Good Leaving Group? The Basics: A SN2 reaction occurs when a nucleophile attacks the substrate to give a transition state in which a bond to the nucleophile is forming at the same time as the bond to the leaving group is breaking. The reaction occurs quickly in polar aprotic solvents. Polar aprotic solvents have higher dielectric constants than nonpolar solvents. They lack O-H and N-H bonds and they have dipoles but cannot hydrogen bond. In SN2s the electrophile is the antibonding orbital. As the antibonding orbital is populated with electrons, the bond to the leaving group is weakened. Collision at a primary or secondary spots are acceptable and primary is faster than secondary. However at a tertiary spot is so slow that it does not happen. (Order of Reactivity is:3CH ????>1>2>>3). The Rate Limiting Step for SN2 reactions is as follows: Rate = k[Substrate][Nucleophile] Inversion: The mechanism proceeds through backside attack and the stereochemistry proceeds with inversion. Inversion occurs after the nucleophile has attacked a carbon from the back, and the groups that were previously there flip to the other side. This can be imaged with an umbrella being hit with a gust of wind. The umbrella is the substrate and the wind is the nucleophile. After the wind hits the umbrella (nucleophile hits the substrate) the umbrella flips inside out (the substrate’s substituents flip to the other side). Notice on the left side of the picture the 3H , the H and the CH 3H g2oups are on the left hand side and on the right side of the picture the CH3, the H and the CH3CH 2roups are on the right hand side. This diagram shows inversion because during the process of the nucleophile hitting the substrate the groups inverted (or flipped) sides. What Makes a Good Leaving Group?: The best leaving groups are WEAK BASES! Good leaving groups have low (close to or less than zero) pKas of conjugate acids. Bad leaving groups have high pKas of conjugate acids. Bad leaving groups are strong bases and have high pKas of conjugate acids or have strong bonds (Ex: F). Something can be made into a good leaving group by adding a STRONG ACID! For example HO- is not a good leaving group, but H2O is. − − − − − Good LG I Br Cl TsO H2O AcO pKa of Conj -9 -8 -7 -3 -1.7 4 Acid Bad LG F− HO − RO − H2N − H − Alkyl,Alkenyl pKa of conj. 1 16 16-18 38 42 >45 Acid Details of SN2: The Strength of the Nucleophile A strong nucleophile is a molecule that reacts faster than a weak nucleophile during an SN2 reaction. A general trend is that species with a negative charge are stronger nucleophiles than a similar species that has a neutral charge. Another general trend is that a base is always a stronger nucleophile than its conjugate acid. It is important to know that basicity and nucleophilicity are different properties. Basicity is the equilibrium constant for abstracting a proton. Nucleophilicity is the rate of an attack on an electrophilic carbon atom. It is important to note that in both cases a nucleophile (or a base) forms a new bond. If new bond is reaching a proton it is a base and if the new bond is touching a carbon it has reacted as a nucleophile. Details of SN2: Reactivity of the Substrate Steric hindrance is when bulky groups interfere with a reaction because of their size. It has little effect on basicity; and bulky groups cannot approach a carbon atom easily. Most bases are nucleophiles capable of attacking a proton or an electrophilic carbon. *If we want a species to act as a base, we want a bulky reagent. If we want it to react as a nucleophile we use a less hindered reagent. Polar Aprotic Solvents SN2s favor polar aprotic solvents. Polar aprotic solvents are solvents that will dissolve salts. They lack acidic hydrogen and are not hydrogen bond donors, but they are acceptors. They can be described as having both high dielectric constants and dipole moments. Solvents include DMF, Acetone, Acetonitrile, DMSO and HMPA. The SN2 displacement is a good example of a stereospecific reaction. In a stereospecific reaction different stereoisomers react to give different stereoisomers of the product. This can be seen in SN2 mechanisms if the reaction kinetics are second order or if there is a situation where it is not known if an inversion has occurred, like with an asymmetric carbon or a ring. Notice how the in the first step the H is up and in the last step H is down. Also notice the inversion taking place. The inversion only happened to the carbon the reaction impacted and NOT to the entire molecule. Parts of the molecule not involved in the reaction should be LEFT ALONE! Also notice how the molecule started cis and ended trans. SN1 The best way to think of an SN1 reaction is with divorce and marriage. A bond is broken leaving one group positive (and looking for a new bond) and the other group has left and is negative. The process for an SN1 reaction is done in at least two steps. The mechanism of the process can be remembered with a scenario about a relationships. Richard and Elizabeth have been married for 3 years, and on their anniversary Elizabeth finds out her husband has been cheating. She wants a divorce and she understands it is a slow process that takes at least a year. After a year the divorce is finalized and she is happy and is surround with her friends. Richard however is not happy and is alone. This represents the rate limiting step in an SN1 reaction. In this step the initial molecule breaks apart and the leaving group is alone and negative (like Richard), and the carbocation is positive and is still attached to its other groups (like the Elizabeth). Elizabeth starts dating again, and after 21 days she is married to Zack. Zack is happy to be married to Elizabeth but Elizabeth wished they had dated longer before getting married. Elizabeth is not sad about being married to Zack, but she also isn’t happy about it. This represents the second part of the SN1 reaction. The carbocation (Elizabeth) meets a nucleophile (Zack) and they bond together quickly. The nucleophile becomes positive and the carbocation becomes neutral. SN1 stands for Substitution Nucleophilic Unimolecular; and it is important to keep in mind that for the SN1 reaction there is more than one step. This scenario is good for remembering the process/ mechanism of the reaction because it shows the breaking up and new reattachment (in more than one step) on the molecule, like in relationships. Like in SN2 reactions, in SN1 reactions: You MUST show the breaking off all bonds and the flow of electrons with curved arrows. If a bond is breaking it must be shown with an arrow starting at the bond and curved away from the substitute. Details of SN1: The Basics, The Leaving Group, Solvent Effects The Basics: SN1 is a reaction that has two steps. In SN1 reactions a nucleophilic attack on a carbocation occurs. SN1s can have additional steps if the nucleophile being added is water or an alcohol. If the nucleophile is a water or an alcohol the third step is to deprotonate to form the product. With SN1s the first step is slow (and endothermic) and the step/steps after that fast (and exothermic). The nucleophile is usually weak and if the reaction creates a chiral carbon, the chiral carbon becomes racemized. Carbocation stability is 3>2>1>CH X. Pr3mary and methyl additions are so unstable that they never happen. It is important to know that the rate limiting step for SN1 reaction is as follows: Rate = k[Substrate][Nucleophile/Base] Doubling the concentration of the substrate doubles the rate, but doubling the concentration of nucleophile/base has no effect. Rate Limiting Transition State Intermediate ???? + ???? − ????????????: The Leaving Group Should Be… …a weak nucleophile. …very stable after it leaves, with the pair of electrons that bonded it to carbon. …negatively charged. …similar to SN2 leaving groups. Solvent Effects: Carbocations are ions that are polar, so according to the “like dissolves like” rule, polar solvents (specifically polar protic solvents) are favored. Polar protic solvents have O-H or N-H bonds and can participate in hydrogen bonding. These solvents can serve as sources of protons (or acids). SN1s require highly polar solvents that strongly solvate ions. Although most alkyl halides are not soluble in water they often dissolve in highly polar mixtures of acetone or alcohol with water. 3 Factors That Stabilize Carbocations in SN1 Reactions Intro: Carbocations frequently go through changes to form more stable ions. Resonance and substitution at carbons helps stabilize carbocations. These two effects are seen in 3 major factors: rearrangement, creation of a double bond, and attacks from either side of a flat carbon. ALL of these occur with a carbocation after the rate limiting step is complete. Understanding these three factors is best remembered with scenarios or examples that connect the dots. The three factors can be remember by thinking of: Robin Hood, Baseball, and a Racecar. 1. Rearrangement/ The Robin Hood Effect Usually rearrangement is done to create a more stable ion and can be done before the leaving group has left. Rearrangement can be thought as the Robin Hood Effect. The concept of Robin Hood is that he stole from the rich and gave to the poor. During rearrangement a neighboring carbon pays the carbocation with electrons it steals from the hydrogens. The carbon beside the carbocation takes from the hydrogen (who in this case is rich with electrons because it is stable and not positive) and gives to the poor (the carbocation who has less electrons and carries a positive charge. Rearrangement occurs with either a hydride shift (~H) or a methyl shift (~CH ).3 Secondary carbocations can rearrange to tertiary carbocations through these shifts. Stability is ranked as follows: Tertiary carbocations are more stable than secondary carbocations. Secondary carbocations are more stable than primary carbocations. Primary carbocations are more stable than methyl carbocations. (3>2>>1>methyl). A hydride shift is the movement of a hydrogen atom with its bonding pair of electrons Methyl shift (also called an alkyl shift) is the movement of a methyl group with its electrons. Methyl shifts can only occur with a quaternary carbon. 2. E1/ Baseball A double bond is created when a neighboring atom donates a pair of electrons to the electron-poor carbon. This creates double bond and is often called a Pi Donation. This mechanism is two steps and can be thought of like a baseball game. In a baseball game the ball has to be hit before anyone can run. This is similar to the leaving group in this reaction. The leaving group has to leave before anything else happens. (After this a carbocation is created.) Once everyone starts running the person at first slides to second. This happens in the second part of the mechanism. A neighboring carbon’s hydrogen’s electrons slide into the spot to stabilize the carbocation and creates a double bond. This is also known as an E1 reaction. In an E1 reaction the hydrogen that donates its electrons is released and becomes a part of the product. 3. Racemization/Racecar During a racemization the carbocation is planar and achiral. This means that a nucleophile can attack from either the front or the back and giving both enantiomers of the product. If a nucleophile attacks the carbocation from the front it shows retention of configuration. If a nucleophile attacks the carbocation from the back it shows inversion of configuration. This can be remembered with the word Racecar because the word is the same from the front and the back and it doesn’t matter which way you read it from. This scenario is like racemization because it doesn’t matter which way you add to the carbocation. E1 A double bond is created when a neighboring atom donates a pair of electrons to the electron-poor carbon. This creation of a double bond is often called Pi Donation. This mechanism is two steps and can be thought of like a baseball game. In a baseball game the ball has to be hit before anyone can run. This is similar to the leaving group in this reaction. The leaving group has to leave before anything else happens. (After this a carbocation is created.) In a baseball game once everyone starts running the person at first slides to second. This happens in the second part of the mechanism. A neighboring carbon’s hydrogen slides into the spot to stabilize the carbocation and creates a double bond. It is important to remember that E1 mechanisms are TWO STEPS. Like in the scenario one group leaves and one group slides in. The E1 reaction is always in competition with the SN1 reaction because they have the same characteristics. They both use WEAK nucleophiles (nucleophiles that are uncharged. For example: H2O or ROH) and they both can occur on secondary or tertiary positions. E1 reactions include a carbocation meaning that the stability is ranked 3>2>>1>methyl. In E1 reactions the nucleophile is weak and the rate limiting step is: Rate Limiting Step = k[sub][nuc/base] Since E1 reactions are always in competition with SN1 reactions, one forms the major product and one forms the minor product. E2 During an E2 reaction, the electrons on hydrogens are moving and re-bonding after collision. A hydrogen is hit with a strong base and the electrons will be donated to the carbon it was attached to forming a double bond. This mechanism reminds me of a scenario at a football game. For example the Denver Broncos and the New England Patriots are playing against each other to determine which team gets to advance to the Super Bowl. The Broncos are on offense and the Patriots are on defense. When the ball is snapped the offensive Broncos player hits the defensive Patriots player and while falling he knocks over the quarterback. As the quarterback falls the ball is dropped and neither team has possession of it. In the scenario the offensive Denver player is the strong base, the defensive Patriot player is a hydrogen and the quarterback is the carbon. The strong base collides with a hydrogen and the hydrogen’s bond “falls” to the side. (The offense player hits defense player, defense player hit quarterback) Then the leaving group gets loose and leaves. (Like the ball after it was let go and in neither team’s possession.) This diagram shows an E2 reaction. (The OH is the defensive player, the H is the offensive player, Br is the ball.) It is important to remember that the E2 mechanism is ONE STEP. It is also important to remember that this scenario is summary of what is occurring in the reaction and what it looks like. Known that when the base and hydrogen collide they are a bond and the hydrogen leaves its electron behind. That electron is the one that allows the double bond to form, and the hydrogen bond DOES NOT physically fall. E2 reactions have some special characteristics that have to be kept in mind. The stability of E2 reactions is methyl<1<2<<3. E2 reactions must have a strong base! These can include (but are not limited to: KOH,LiOH,RbOH,Ba(OH) . T2ese reactions are favored by heat and form double bonds. Basicity increases going up the periodic table and F > Cl > − Br > I . The reaction can occur in a variety of solvents and basicity increases going left of the periodic table. Meaning C: > N: > O: > F. The overall rate limiting step is: − Rate = k[Substrate][Base] OR Rate = k[RX][B : ] Doubling the concentrations of either compound will double the rate. Rearrangement in the mechanism is IMPOSSIBLE and will NEVER occur, and a good leaving group is needed. A coplanar transition state is required for the stereochemistry of this reaction. Stereo specificity with E2 reactions It is important to know that E2 reactions are stereospecific. This means for E2 reactions to occur the alkyl halide and the .-----C-C·----- hydrogen must be anti-coplanar fromithey areer Sometimes it is necessary to rotate and flip bonds and! x/ -, molecules to make this happen! StepbyStep: Rotating Bonds in an E2 Mechanism with a Newman Projection: Step 1: Identify and label the alpha and beta carbons. > \< Step 2: Dr~~ Newman by looking either up or down the alpha carbon. On the Newman have the dashes on the left and the wedges on the right. H -&"~ Step 3: Rotate the pieces of the Newman so the halogen and the hydrogen from the beta carbon are anti-coplanar. crl~1-\ ~~CH3 Step 4: Draw the sawhorse projection. 6Y" (\1,rl Step 5: Snow mechanism. 13.... c\13 H ~ ""} Y t-I c13 .t:!_~ Steply" HightIghrtthe backbone. Step 7: Draw the double bond. Step 8: Add piecesipieces are cis or trans by using the Newman projection. Tips and Tricks for Chapter 6 include: Summary/ Important Characteristics of SN1, SN2, E1 and E2 Reactions!, Carbocation Stability, Polar Protic and Polar Aprotic SN1 or SN2?, Periodic Trends, How to tell what reaction to use, Cis/Trans, R/S, Diagnostic Cheat Sheet, and all about nucleophiles! SN1 E1 SN1 reactions occur in Polar Protic Solvents! Base is usually weak Possible rearrangements Alkene is formed Rate = k[Sub][Base/Nuc] Solvent should be good for ionizing Or Rate = k[B][R-X] Elimination reaction with Two times the substrate 2 steps means two times the rate BrH, HI, HCl O4,H 2O 4 BUT doubling the base/nucleophile has no Always rearranging effect Leaving group should be I is worst, F is best! good I<Br<Cl<F Limiting step = Carbocations k[Sub][Nuc/Base] (3>2>1>methyl) SN2 E2 Sn2 reactions occur in aprotic solvents! F<Cl<Br<I, F is worst I is best Aprotic OHK, LiOH, RbOH etc Positions don’t Overall Rate limiting step rearrange is Rate = k[sub][Base] Rate = k[R-X][Nuc: ] Tertiary and secondary Or Rate = k[sub][nuc] positions Twice as much Base has to be strong concentration of either Alkene is formed component means double the rate. Lots of solvents can be used I is best, F is worst! (I<Br<Cl<F.) Left along periodic table increases basicity. C: > N: CH X>1>2 3 > O: > F: The word carbocation tells the order of the CARBOCATION stability of carbocations! The word starts with the letter C and C is the 3 2 1 third letter of the alphabet, skip 2 letters and the next letter is B. B is the second letter in 3 > 2 > 1 the alphabet, skip 2 letters and the next letter is A and a is the first letter in the alphabet. The stability for carbocations is 3 > 2 > 1. Tertiary is greater than secondary. Secondary is greater than primary. Primary is greater than methyl. Methyl is technically apart of this ranking but a carbocation on a methyl is so reactive and unstable it does not happen! Carbocations occur in SN1 and E1 reactions and the stability ranking is the exact OPPOSITE for SN2 and E2 reactions. In SN2 and E2 reactions the stability is 1 > 2 >3 (primary is greater than secondary. Secondary is greater than tertiary.), and in this case bonding to a tertiary group does not happen! Polar Protic and Polar protic solvents are used in SN1 mechanisms and polar aprotic solvents are used in SN2 mechanisms. This can be remembered like this: “Polar aprotic starts with Polar Aprotic two different letters (P and A) so it is SN2. Polar protic starts with the same letter (P) so it is SN1”. It is also SN1 or SN2? important to remember polar protic solvents have hydrogen bonding whereas polar aprotic do not. Periodic Trends! Periodic trends can be remembered with the two letters A and P! Acidity starts with A and increases down and to the right of the periodic table. Aprotic starts with A and these characteristics go together. (Shown on letter A with green and purple arrows). Polar protic starts with P and increases nucleophilicity down the periodic table. The letter P looks like there is a capital D on the upper right hand side. This represents the decrease of nucleophilicity in polar protic solvents to the right of the periodic table. How to Tell If a Reaction is: SN1, SN2, E1 or E2! The most important rule is to classify the leaving group’s RULE 1: CLASSIFY position. By doing this you can (usually) eliminate two of THE POSITION OF THE LEAVING the four options. GROUP! If the leaving group is on a primary carbon, the reaction cannot be an SN1 or E1. This is because SN1 and E1 reactions have carbocations, and carbocations are not stable on primary carbons and do not occur. If the leaving group is on a tertiary carbon, this reaction cannot be SN2. This is because SN2 reactions occur with backside attacks and are affected with large bulky groups. A tertiary position would not occur with a SN2 reaction because of steric strain. DMSO DMF Acetone Acetonitrile If the leaving group is on a secondary carbon it can be either SN1, SN2, E1 or E2. This is when the other characteristics of the reaction come into play. If you see DMF, DMSO, Acetonitrile, or Acetone the reaction is SN2. If there is an 3gNO this is a sign that the reaction is SN1. If there is EVER a double bond the reaction is either E1 or E2. If there is a strong base (any salt/group 1 metal and an OH or OR) it is an E2 reaction. Front side Attack: In a front side attack, the nucleophile R and S attacks the electrophilic center on the same side as the leaving with group. When a front side attack occurs, the stereochemistry of the product remains the same; that is, there is retention of Inversion! configuration. S will stay S and R will stay R. Backside Attack: In a backside attack, the nucleophile attacks the electrophilic center on the side that is opposite to the leaving group. When a backside attack occurs, the stereochemistry of the product does not stay the same. There is inversion of configuration. S will be R, and R will be S. Cis and Trans For E2 reactions to occur the alkyl halide and the hydrogen must be anti-coplanar from each other. BUT in a ring they must be trans! Sometimes it is necessary to rotate and flip bonds and molecules to make this happen! Using a Newman projection and a sawhorse drawing is extremely helpful to show this rotation and the final product properly. Diagnostic Cheat Sheet Alkyl Halide Strong Nucleophile Strong Nucleophile Weak Nucleophile Weak Base Strong Base (H2O,ROH) (RCOO , CN, SH (OR, OH) SR, N3, X, N3 or amines) Methyl SN2 SN2 No reaction Primary SN2 SN2 No Reaction Secondary SN2 E2 SN1/E1 Tertiary No reaction E2 SN1/E1 Different things in the solution may instruct for different reactions to occur, however the strongest characteristic wins!


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