Exam 2 Study Guide
Exam 2 Study Guide CHMY 321-001
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This 15 page Study Guide was uploaded by Rebeka Jones on Sunday October 16, 2016. The Study Guide belongs to CHMY 321-001 at Montana State University taught by Holmgren, Steven in Fall 2016. Since its upload, it has received 4 views.
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Date Created: 10/16/16
Exam 2 The first step in naming an alkane is to identify the longest chain called the parent chain. If there is two chains of equal length, then choose the chain with the greater number of substituents. Once the parent has been identified list all substituents. When naming branched substituents number each carbon going away from the parent chain. Place the numbers on the longest chain in the substituent. Treat the complex substituent as if it is a mini-parent with its own substituents when naming it place it in parentheses. - An alkyl being 3 carbons can only be branched in one way and it is called an isopropyl group. - Alkyl groups with 4 carbons can be branched in 3 different ways. - Alkyl groups with 5 carbons can be branched in many different ways. In order to assemble the systematic name number, the carbons of the parent chain. These numbers are called locants. Rules - If one substituent is present it should be assigned the lowest possible number. - When multiple substituents are present, assign numbers so the first substituent receives the lowest number. - If there is a tie the second locant should be as low as possible. - If there is still a tie, then the lowest number should be assigned alphabetically. *when a substituent appears more than once in a compound a prefix is used to identify how many times *hyphen separate numbers from letters and commas separate numbers from numbers. *once all substituents have been identified and assigned arrange alphabetically (ignore all prefixes, sec, and tert) In summary - Identify parent chain - Identify name and substituents - Number parent chain and assign locants - Arrange the substituents alphabetically To number the parent, start at the bridge head that gives the substituent the lowest locant following numbering systems from the longest path to the shortest. Regardless of the position of substituent, the parent must be numbered beginning with the longest path first. But must start at the bridge head giving the substituent the lowest possible number. For an alkane, the number of possible constitutional isomers increases with increasing molecular size. - Avoid draw the same isomer twice o Use IUPAC names to know for sure In order to compare stabilities of alkanes we compare the heat released when combusted. Processes that are employed to increase gasoline yield. 2 - Cracking: process where C-C bonds of larger alkanes are broken producing smaller alkanes suitable for use as gasoline. o Done by high temperatures o Generally, leads to straight chain alkanes - Reforming: process of many reactions that lead to converting straight chain alkanes into branched hydrocarbons and aromatic compounds o More desirable Newman projections – drawing specifically designed to show the conformation of molecules - One carbon directly in front of the other - Point in the center of drawing = front carbon - Circle represents back carbon Drawing Newman projections - Identify the three groups connected to front carbon atom - Identify the three group connected to the back carbon atom - Draw Newman projection Torsion angle = separated by 60 degrees Staggered conformations – lowest energies Eclipsed conformations – highest energies Torsional strain – difference in energy between staggered and eclipsed Anti-conformation = 180 degrees Gauche interactions – type of steric interactions Angle strain – the increase in energy associated with a bond angle that has deviated from the preferred angle of 109.5 degrees 3 Cyclohexane can adopt many conformations some examples are the chair and the boat. - Bond angles close to 109.5 = little angle strain - Significant different is caused by torsional strain o chair has none o boat has two sources – many H’s are eclipsed and H’s on either side of the ring experience flag pole interactions. Can alleviate by twisting to form a twist boat. Each carbon atom in a cyclohexane ring can bear two substituents - Axial position: parallel to a vertical axis passing through the center of the ring - Equatorial position: approximately along the equator of the ring For a ring containing only one substituent two possible chair conformation can be drawn The substituent can be in an axial position or in a equatorial position. These two possibilities represent two different conformations that are in equilibrium to each other. The term “ring flip” is used to describe the conversion of one chair conformation into the other done by rotating all C-C bonds. When two chair conformations are in equilibrium, the lower energy conformation will be favored. When the substituent is in axial position, there are strain interactions with the other axial H’s on the same side of the 4 ring. The substituent’s electron cloud is trying to occupy the same region of space causing steric interactions. These are called 1,3-diaxial interactions. The presence of 1,3-diaxial interactions causes the chair conformations to be higher in energy when the substituent is in the axial position. When the substituent is in an equatorial position these 1,3-diaxial interactions are absent. The two chair conformations will generally favor the conformation with the equatorial substituent. When drawing chair conformations of compound with two or more substituents we must also consider the 3D orientation of each substituent. Stereoisomers – different compounds with different physical properties, and they cannot be interconverted via conformational change. Superimposable – mirror image is identical Non-superimposable – mirror image is different Objects that are non-superimposable are called chiral objects Object that are superimposable are called achiral objects Chirality center – tetrahedral carbon bearing four different groups 2 *sp cannot have a chirality center *also ignore CH 2nd CH gr3ups When a compound is chiral, it will have one non- superimposable mirror image called its enantiomer. 5 A chiral compound will have exactly one enantiomer. Easiest way to draw enantiomers is to replace dashes with wedges and vice versa. WARNING! This does not always work. Assign Priority The atom with the lowest atomic number is assigned the lowest priority Steps - Prioritize all four groups connected to the chirality center - Rotate the molecule so the fourth priority group is on a dash - Determine is the sequence clockwise or counterclockwise Counterclockwise = S Clockwise = R *when you come across the same atom move to the next level of atoms to determine priority. If the lists are identical move further away from the chirality center and repeat. *double bonds are considered as two separate single bonds same procedure for triple bonds. *switching any two groups in a chirality center will invert the configuration when naming a chiral compound, the configuration of the chirality center is indicated at the beginning of the name, 6 italicized and surrounded by parentheses. When there are multiple each configuration must be proceeded by a locant. Enantiomers exhibit identical physical properties Optically active – rotate the plane of polarized light Chiral compounds are chirality active while achiral compounds are not. specificrotat∝]==∝ cxl Enantiomers will rotate the plane in equal amount but opposite directions Dextrorotatory – exhibiting positive rotation Levortatory – exhibiting negative rotation Optically pure – solution containing only one enantiomer Race mixture – solution containing equal amounts of both enantiomers (optically inactive) Isomers 7 Constitutional Stereoisomers Isomers Same molecular formula and order of - Same molecular connectivity but different spatial formula but arrangement of atoms different order of connectivity (makes them different compounds) Enantiomers Diastereomer Mirror images s Not mirror images Diastereomers – stereoisomers that are not mirror images (different physical properties) n Maximum number of stereoisomers = 2 Axis of symmetry – if you rotate the molecule 180 degrees and it still looks the same (trans) Plane of symmetry – one side is reflected on the other *chirality is not dependent on an axis of symmetry any compound that possess a plane of symmetry in any conformation will be achiral. *the absence of a plane of symmetry does not mean its chiral inversion – reflection about a point 8 Summary - The presence or absence of rotational symmetry is irrelevant to chirality - A compound that has a plane of symmetry will be achiral - A compound that lacks a plane of symmetry will most likely be chiral Fischer projections – drawing style that is often used when dealing with compounds bearing multiple chirality centers For each chirality center, the horizontal lines are considered to be coming out of the page, and the vertical lines are considered to be going behind the page. Assign configuration - Draw one horizontal line as a wedge and draw one vertical line as a dash Resolution – separation of enantiomers For bond breaking reactions H is primarily determined by the amount of energy necessary to break the bond homolytically. Homollytic bond cleavage generates two uncharged species, called radicals each of which bear an unpaired electron. - Fishhook arrows used to break bond Heterolyic bond cleavage is illustrated with two headed arrows generating charged species called ions. 9 Bond dissociation energy – energy required to break a covalent bond. Heat of reaction (H) – total change in enthalpy for a reaction - Sign of H indicates the direction in which the energy is exchanged. +H indicates the system has increased in energy -H indicated the system has decreased in energy exothermic – system gives energy to the surroundings endothermic – system receives energy from the surroundings. G=−RTlnKeq R = 0.008314 G determines the maximum yield of products -G products favored (Keq > 1) +G reactants favored (Keq <1) in order for a reaction to be useful G must be negative Rate = K [A] [B] = experimentally Exponents x and y are determined by how the rates are affected with different concentrations of A and B First order Second Order Third Order Doubling A Doubling A double Doubling A doubles the rate rate as does quadruples the and B has no doubling B rate while double affect B has the effect of 10 doubling the rate (exponent of A is 2 and of B is 1) Sum of Exponents Sum of exponents Sum of exponents is 1 is 2 is 3 The rate constant depends on three factors Energy of Activation (Ea) – the energy between the reaction and products Temperature – raising temperature will make the rate increase. Rule of thumb raising the temperature by 10C causes the rate to double. Steric Considerations – the geometry of the reactants and the orientation of their collusions can have an impact on the frequency of collisions that lead to reactions. Catalyst is a component that can speed up the rate of a reaction without itself being consumed in the reaction - Lowers activation energy Kinetic refers to the rate of a reaction, while thermodynamics refers to the equilibrium concentration of reactant and products When look at an energy diagram the valleys are intermediates, while the peaks are transition states. A transition state is a state though which the reaction passes. Transition states cannot be isolated. Bonds are in the process of being broken, and or formed simultaneously. 11 Intermediates have a certain by short lifetime. An intermediate is not the process of forming for breaking. In an exothermic process the transition state is closer in energy to the reactants than to the products and vice versa. This principal is called the Hammond Postulate. Ionic or polar reactions involve participation of ions as reactant, intermediates, or products. Ionic reactions happen when one reactant has a site of high electron density and the other reactant has a site of low electron density. Nucleophilic – electron rich center - Characterized by its ability to react with a positive or partial positive charge. Electrophilic – electron deficient center - Characterized by its ability to react with a negative or partial negative charge A nucleophilic center is an electron rich atom that is capable of donation a pair of electrons (Lewis Base) - Any atom that possess a localized lone pair - Pi bonds Polarizability – described the ability of an atom to distribute its electron density evenly in response to external influences *effects strength of nucleophilic 12 An electrophilic center is an electron-deficient atom that is capable of accepting a pair of electrons (Lewis Acid) Carbocation – positively charged carbon - Has an empty p orbital o Functions as a site for electrons Nucleophiles Electrophiles Inductive effects Inductive effects Lone pair Empty P orbital Pi bond A mechanism shows how a reaction takes place. The tail of every curved arrow shows where the electrons are coming from and the head of every curved arrow shows where the electrons are going. Patterns of electron flow - Nucleophilic attack: characterized by a nucleophile attacking a electrophile - Loss of a leaving group: something leaves or breaks off - Proton Transfers: something is being protonated - Rearrangements – hydride or methyl shift All ionic mechanisms, regardless of how complex, are just different combinations of the four characteristic patterns seen. Concerted process – using two patterns at once. In both types of rearrangements, a secondary carbocation is converted into a more stable tertiary carbocation. In order to 13 determine when a carbocation rearrangement will occur, we must determine whether the carbocation will become more stable via rearrangement. *carbocation rearrangements generally do not occur when the carbocation is already tertiary unless a rearrangement will produce a resonance-stabilized carbocation <- called a allylic carbocation Nucleophilic attack - Reversible arrow generally used. If the nucleophile is capable of functioning as a good leaving group after the attack has occurred while an irreversible arrow is used if the nucleophile is a poor leaving group. Loss of leaving group - A reversible arrow is generally used if the leaving group is capable of functioning as a good nucleophile Proton Transfer - All proton transfers are technically reversible - Generally speaking, irreversible reactions arrows are used for reactions in which the acids differ in strength by more than 10 pKa units. - When the different in pKa values is between 5 and 10 pKa units, either reversible or irreversible reaction arrows may be used, depending on the context of the discussion Carbocation ion rearrangement - Technically an equilibrium will be establishing in which all the possible carbocations are present, with the most stable dominating the equilibrium 14 - The different in energy between the possible carbocations is often significant, so carbocation arrangements will generally be drawn as an irreversible process. 15
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