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UNIVERSITY OF CENTRAL FLORIDA / OTHER / CHM 2210 / what's the definition of stereochemistry?

what's the definition of stereochemistry?

what's the definition of stereochemistry?

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School: University of Central Florida
Department: OTHER
Course: Organic Chemistry 1
Professor: Reuben kimwomi
Term: Fall 2018
Tags:
Cost: 50
Name: Orgo 1 Study Guide for Exam 2
Description: *Stereochemistry *Understanding organic reactions *Alkyl halides *Nucleophilic substitution
Uploaded: 09/28/2018
12 Pages 3 Views 4 Unlocks
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Ch. 5 Stereochemistry 


what's the definition of stereochemistry?



o Stereochemistry is the 3D structure of a molecule o Stereoisomers exist only when the 3D structures differ in  their arrangement, but their molecular formula remains the same (i.e. how the atoms are oriented in space

o Compare this to constitutional isomers that differ in their  connectivity of atoms

o A configuration is the specific 3D structure of a  

stereoisomer

o When looking at mirror images, some are identical and some are  different

o Molecules are chiral if their mirror images are  

nonsuperimposable (i.e. not identical)

o Molecules are achiral if their mirror images are  

superimposable (i.e. identical)

 These usually contain a plane of symmetry, which  cuts a molecule in half, evenly, having each half as a  

reflection of the other

o Enantiomers are nonsuperimposable mirror images o Stereogenic centers are carbon atoms bonded to four different groups (tetrahedral); these may or may not be part of a ring o Molecules can have 0 or more stereogenic centers-  If there are none, the molecule is achiral


What is chiral?



 If there is one, the molecule is always chiral and  

exists as a pair of enantiomers

 If there is two or more, the molecule can be chiral or  achiral

o To designate a stereogenic center as either R or S, assign  priorities (1-4) to the groups bonded to the central carbon atom  o The larger atom gets the higher priority Don't forget about the age old question of Sociology did not become widely accepted in the USA until when?

o If two atoms are the same, move down the chain until you  spot a difference, then assign the higher priority to the  larger atom

o If no difference is recognized, this is not a stereogenic  center

o Treat a multiple bond as if it were the number of single  bonds attached to the central carbon

o Groups with priorities moving clockwise are R and groups  with priorities moving counterclockwise are S

**remember this by drawing both a capital letter R and S and watching which direction your pencil moves over the top curve of each letter** 


Enantiomers defined as what?



Don't forget about the age old question of what is the meaning of Internal attribution?

o Compounds with two or more stereogenic centers are more  complex, due to the fact that they allow for more stereoisomers  o 2n, where n equals the number of stereogenic centers

o this indicates the maximum number of stereoisomers that  are possible, but there can be fewer

o at the point of having multiple stereoisomers,  

diastereomers are recognized

 these are stereoisomers that are not mirror images of each other

o Compounds that have stereogenic centers but are also achiral  are referred to as meso compounds

o these contain a plane of symmetry

o For compounds with two or more stereogenic centers, there will  be more than one R or S configuration

o The same R and S configurations are found throughout  identical molecules

o Opposite R and S configuration are found in enantiomers o A mix of one opposite and one configuration that is the  same is found in diastereomers

o Enantiomers have identical physical and chemical properties,  except when they react with chiral molecules

o Physical properties:

 Using a polarizer, light is only allowed to pass  

through in one plane, creating plane-polarized  

light

 A polarimeter is then used to pass this light through a tube containing an organic substance

 Achiral compounds do not change the light at all,  making them optically inactive

 Chiral compounds rotate the light, making them  optically active

∙   The angle the light is rotated through is α and  Don't forget about the age old question of who is George Whitefield?

rotation can be clockwise (dextrorotatory

(+)) or counterclockwise (levorotatory (-))

o Two enantiomers cancel out each others rotations because  they are rotating equally, but in opposite directions

 This is called a racemic mixture/racemate

 Optically inactive

o The number of chiral molecules interacting with the  polarized light determines the observed rotation

 Further depends on the sample tube and its contents  Specific rotation [α] is used with sample tube  length (l) (in dm), temperature, wavelength, and  

concentration (c)  

∙    α/l x c

o When there is a mixture of two enantiomers, where one is  in excess, enantiomeric excess (ee)/optical purity is

used to determine how much more of one enantiomer  there is

 ee = % of one enantiomer - % of the other  Don't forget about the age old question of What are the Basics of Social Psyc?

enantiomer

 if two quantities are known, a different equation can  be used to find the excess

∙    ee = [α] mixture / [α] pure enantiomer x 100%

o the optical rotation and physical properties of  

diastereomers are different

 this allows for them (along with constitutional  

isomers) to be easily separated, unlike enantiomers

o Chemical properties:

 Enantiomers react at different rates when they are  reacting with chiral molecules

Ch. 6 Understanding Organic Reaction 

o Organic reactions are drawn with a single arrow

o Reagent reacts with the organic compound- may be drawn on the left side of the reaction or above the reaction arrow o The solvent and temperature of the reaction can be drawn  above or below the reaction arrow (“hv” (light) or “Δ”  (heat))

o When reactions occur without an intermediate, the steps  can be numbered above or below the arrow (indicates the  sequence and timing of the reaction- i.e. when reagents  are added, etc.)

 Usually not present on product side

o Types of reactions:

o Substitution- atom/group replaces another

 Involve σ bonds on a carbon atom (one forms, one  breaks)

o Elimination- starting material elements are “lost”

 Π bond is formed, two σ are broken

o Addition- elements are added to a starting material  Two σ are formed, π is broken

o Reaction mechanisms provided details of what and how bonds  are broken/ formed as starting material becomes product o Provide order and rate of bond breaking/formation If you want to learn more check out Multidomestic Company is what?

o Explains which products are formed

o Concerted reaction occurs in one step

 Starting material becomes product, directly

o Stepwise reaction has more than one step and involves a reactive intermediate

o Bonds are broken via homolysis (equal dividing of  electrons) or heterolysis (unequal dividing of electrons)

 In heterolysis, the atoms have different  

electronegativities, therefore the electrons will move  towards the more electronegative atom

 Both processes require energy

 Hemolysis creates uncharged intermediates,  

heterolysis creates charged intermediates

o Curved arrows are used to show the movement of the  electron pairs, a half-headed curved arrow shows the  movement of a single electron

 Radical = reactive intermediate with single lone  electron (hemolysis)

 Carbocation= two electrons go to heteroatom, none to carbon (+)-> unstable, electrophile

 Carboanion= two electrons go to carbon, none to  heteroatom (-)-> unstable, nucleophile

 Carbocations and carboanions can be intermediates  in polar reactions

o One electron from two radicals can combine to form a  bond, or an atom with a negative charge can donate two  electrons to form a bond with an atom with a positive  charge We also discuss several other topics like what is monopolistic competition?

 Energy is always released by bond formation **Table 6.1 in textbook- types of arrows in chemical  reactions**

o Dissociation energy can be used to measure bond breaking, as it is the energy needed to break a covalent bond o Enthalpy (ΔH°) is the heat absorbed or released in a  reaction

 (+) when the reaction is endothermic and energy  is absorbed; more energy is needed to break  

these bonds than what is released

 (-) when the reaction is exothermic and energy is  released; more energy is released than what is  

needed to break these bonds

 Indicates relative strength of all bonds in a  

reaction

o Bond dissociation energy is always positive

o Homolysis is always endothermic

o Stronger/ shorter bond= higher dissociation energy  As atoms increase in size, dissociation energy  decreases

o Determining enthalpy:

 Balance the equation

 Add amount of energy needed to break bonds (+)  and the amount needed to form bonds (-)

 Find the sum of all of these numbers

o A reaction equilibrium should favor the products (Keq<1) and  the reaction rate should be decent to form the products in a  reasonable time

o Consider:

 Thermodynamics- energy and equilibrium,  

comparing that of the reactants and products

 Kinetics- reaction rates (i.e. how fast the reaction occurs)

o Equilibrium constant (k) compares the amount of  starting material and the amount of product at  

equilibrium (determines equilibrium position)

 Keq>1 = equilibrium favors products, = -ΔG°

 Keq<1 = equilibrium favors reactants/starting  materials, = +ΔG°

 Determined by the relative energy of both sides of the reaction  

 Gibbs free energy (G°) -> change in free energy (ΔG°) is the difference between the energy of the  products and reactants

o Compounds are more stable when they are lower in  energy (i.e. a reaction will favor the more stable  

products when they have less energy than the  

reactants)

o Even small changes can affect the overall difference  between the products and reactants at equilibrium o Reactions can be coupled, or paired, when they work  together in an unfavorable process

o The energy from one reaction powers the next reaction o Combined the two reactions form the net reaction  Depicted by adding the substances from both and  subtracting the ones that are present in both

 Add the energies from the individual steps to find  the overall energy change

o Free energy change can be affected by enthalpy changes and  entropy change (ΔS°)- measure of the change in  randomness/disorder of products and reactants

o Entropy measures the movement and randomness of a  reaction; more movement= more energy

o Increased entropy is favored

o Entropy increases when two products are made by a  single starting material

o Entropy decreases when a ring is formed by an acyclic  compound (becomes more structured and less able to  move)

 ΔG° = ΔH° -T ΔS° T= temperature in Kelvin

o Entropy is often completely omitted when it is relatively  small, the resulting equation is ΔG° ≈ΔH° 

 (-)ΔH° = product is favored (net gain in bond  

strength)

 (+)ΔH° = reactant is favored (net loss in bond  

strength)

o Energy diagrams are used to depict reactions as they occur over time

o Shows how many steps are involved, how quickly the  reaction proceeds, and compares the reactants,  

intermediates, and products

o Energy is on the y-axis, the reaction coordinate is on the x-axis

o The transition state will not ever be isolated, as it is  connected to other parts of the reaction at the “top of  the hill”

o The activation energy (EA) is the difference between  the starting materials and the transition state

 This is the minimum amount of energy in order to  break bonds, and ultimately determines how quickly  a reaction takes place

 The height of the energy barrier is also determined  by the activation energy

o Enthalpy and activation energy are independent of each other

**be familiar with identifying parts of an energy diagram and  how to analyze its data for both a one- and two-step reaction  mechanism** 

o In a two-step reaction mechanism, the rate-determining  step is the step with the highest transition state

o Kinetics studies reaction rate, or how quickly a reaction moves o The energy barrier is the amount of energy that must  be overcome for the reactants to form the products o A larger EA = a slower reaction

o The reaction will occur quicker as the concentration  increases (more collisions by more molecules)

o The reaction will occur quicker as the temperature  increases (kinetic energy increases with heat, causing  more collisions)

o Reaction rate is not impacted by reaction quantities o Free energy, enthalpy, and entropy have no control on  reaction rate

o Rate law shows the correlation between the rate and  concentration of a reaction

 Rate = k [reactants] 

o Fast reaction = large k; slow reaction = small k

o The rate equation for a one-step reaction has the  

concentration of all reactants; the rate equation for a  two-step reaction has only the concentration of reactants  from the rate-determining step

 A reaction with two reactants = bimolecular

o Rate equation order is the sum of the exponents on the  terms

o The rate-determining step indicates how quickly the  reaction occurs, this is why the concentrations of these  reactants only appear in the rate equation

o Catalysts are added to a reaction to speed it up; this remains  unchanged after the reaction and is not present in the products o They lower the activation energy needed in a reaction o Catalysts are added to a reaction in small amounts  (catalytic amount)

o Enzymes are proteins that are a type of biological  catalyst

 They contain an active site that binds to the  

reactant (substrate)= enzyme-substrate  

complex

 Enzymes may help a reaction by holding the  

reactant for a smoother reaction; may provide sites  

with a different pH (basic/acidic) for a certain  

reaction to take place

 The enzyme releases the substrate after the  

reaction takes place and goes to find another

Ch. 7 Alkyl Halides and Nucleophilic Substitution o Alkyl halides contain a halogen (X) atom and a carbon atom  (sp3 hybridized)

o Depending on the degree of the carbon it is bonded to, the alkyl  halide can be primary, secondary, or tertiary (very important  for determining chemical reactions)

o Vinyl halides- carbon-carbon double bond attached to a  halogen

o Aryl halides-benzene ring attached to a halogen o Neither of these participate in reactions because they are  bonded to sp2 carbons

o Allylic halides- carbon- carbon double bond is adjacent to the  halogen and carbon bond

o Benzylic halides- benzene ring is adjacent to the halogen and  carbon bond

o Naming an alkyl halide:

o Identify parent carbon chain, name as an alkane o Name using other rules of nomenclature as usual o –ine changes to –o for a halogen substituent 

o –ine changes to –ide for a halogen bonded to the alkyl  group 

o Physical properties: (Table 7.1)

o Alkyl halides are weakly polar

o Have dipole-dipole interactions because of the polar bond  (C-X), even though the rest of the molecule is nonpolar o The polar C-X bone determines the overall chemistry and  reactivity of the alkyl halide

o Manly participate in substitution and elimination reactions o React with electron-rich reagents (nucleophiles) because  the carbon is electrophilic

o Nucleophilic substitution:

o Must have a halogen bonded to an sp3 carbon (alkyl group) o The halogen (X) is referred to as the leaving group o Nucleophiles (:Nu-) contain a π bond or lone pair, which  they donate to the alkyl halide

o These reactions are Lewis acid-base reactions

o A curved arrow shows the movement of electrons o Nucleophiles replace all the leaving groups

o A good leaving group is a weaker base, with a strong  conjugate acid

o It can accept electron pairs more easily when it is more  stable (X:-)

o As basicity decreases, the function of a leaving group  increases

o (examples of good leaving groups in Table 7.2 & 7.3) o Prediction of the equilibrium direction can be simply found  by comparing the basicity of the leaving group and the  nucleophile

 When the leaving group is a weaker base than the  nucleophile, the reaction favors the products

o The structure of bases and nucleophiles is very similar (i.e. they  both have a π bond or a lone pair)

o The difference is in their actions-> nucleophiles attack  carbons (i.e. electron-deficient atoms); bases attack  protons

o Parallels:

 The stronger base is the stronger nucleophile, when  two nucleophiles have the same nucleophilic atom

 A nucleophiles with a negative charge is always  

stronger than its conjugate acid

 As basicity increases, nucleophilicity increases

o Steric effects are detected when two atoms are forced  close together in a small space

 Steric hindrance is the decrease in reactivity due to bulky groups; impacts the similarities between  

nucleophiles and bases

 Steric hindrance does not affect basicity, but is  

decreases nucleophilicity  

∙ Nonnucleophilic bases are bases that are  

poor nucleophiles

o The solvent used in a substitution reaction impacts an  atoms nucleophilicity

 Polar protic solvents- are able to hydrogen bond  (along with dipole-dipole interactions)

∙ Can solvate both anions (by hydrogen bonding)

and cations (by ion-dipole interactions)

∙ As the size of an atom increases, the  

nucleophilicity increases (opposite of basicity) 

 Polar aprotic solvents- only use dipole-dipole  

interactions  

∙ Only solvate cations (by ion-dipole interactions)

∙ Stronger base = stronger nucleophile 

o Understanding a nucleophilic substitution mechanism: o Identify the order of the bonds breaking and forming  Both occur at the same time (one step)

 Breaking occurs before forming (two steps)

 Forming occurs before breaking (two steps)- not  normally considered in these reactions

o Types of mechanisms:

o SN2- bimolecular, 2nd order kinetics

 Both the nucleophile and the alkyl halide appear in  the rate equation, and changing the concentration of  either one would impact the rate

 One-step: concerted reaction

∙ Bond breaking and forming occur at the same  

time

 Frontside attack-> nucleophile attacks from the  same side as the leaving group

∙ the configuration around the stereogenic  

center stays the same

 Backside attack-> nucleophile attacks from the  opposite side of the leaving group

∙ The configuration around the stereogenic  

center is inverted

∙ All SN2 reactions utilize backside attack  

by the nucleophile 

 Different compounds are formed as a result of both-> enantiomers

 The rate of an SN2 reaction decreases as the amount  of R groups on the carbon with the leaving group  

increases (steric hindrance)

∙ Methyl/ 1°= easiest to react (unhindered  

halides)

∙ 2°= reacts more slowly

∙ 3°= do not react

o SN1- unimolecular, 1st order kinetics

 the reaction rate is not affected by the concentration  or identity of the nucleophile

 Two-step: bond breaking occurs before bond forming  The reactive intermediates are carbocations

∙ It is trigonal planar, and sp3 hybridized with an  

empty p orbital (above and below)

 A racemic mixture is formed when one starting  material forms two products that are enantiomers of  each other

 The rate of an SN1 reactions increases as the amount  of R groups on the carbon with the leaving group  

increases (carbocation stability)

∙ 3°= easiest to react

∙ 2°= reacts more slowly

∙ Methyl/1°= do not react

o Carbocation stability:

o Can be primary, secondary, or tertiary according to the  number of R groups on the charged carbon atom

 As this number of groups increases, the stability of  the carbocation increases

o Inductive effects-electronic effects within the σ bonds  Electron-donating groups (alkyl groups) are  used to stabilize a positive charge

∙ These groups are more polarizable and more  

able to donate electrons

∙ As the R groups replace the more unreactive  

groups, the carbocation becomes more stable,  

because the positive charge is more spread out

o Hyperconjugation- overlapping an empty p orbital with  an adjacent σ bond, thus spreading the charge  

 A 3° carbocation is therefore more stable, because  the charge is more spread out (delocalized)

 Larger R groups cause a carbocation to be more  stable

o Hammond Postulate

o As a carbocation increases in stability, the rate of an SN1  reaction increases 

o The Hammond postulate estimates the transition state  energy, therefore determining the rates of two reactions   The transition state resembles the species  

(reactants/products) that is the most similar in  

energy

∙ Endothermic= closer to products

∙ Exothermic= closer to reactants

o By lowering transition state energy-> the reaction rate  increases

->the activation energy decreases

o A reaction will occur faster when the transition state forms  a more stable product (i.e. it is lower in energy)

 Lowering product energy does not affect the  

transition state energy, because the activation  

energy is similar no matter what

o SN1 reactions:

 The rate that the carbocation is formed is affected by the stability of it

o Factors to determine when a reaction is SN1 or SN2: (Table  7.7)

o Nucleophile- strong or weak

 Strong (high concentration) favors SN2- usually have  a negative charge

 Weak favors SN1

o Leaving group- good or bad

 A good leaving group favors the rate of both reaction  types

o Solvent- protic or aprotic

 Protic favors SN1

 Aprotic favors SN2

o Alkyl halide- CH3X, RCH2X, R2CHX, R3CX (most important)  Increasing favors SN1

 Decreasing favors SN2

 Methyl/1°- only SN2

 2°- both

 3°- only SN1

Additional Study Guide Tips/Review: 

o UTILIZE PRACTICE PROBLEMS THROUGHOUT THE  CHAPTERS AND AT THE END OF THE CHAPTERS

o Familiarize yourself with isomers, enantiomers, diastereomers,  and meso compounds

o Be able to locate stereogenic centers

o Properly identify S and R configurations

o What is a racemic mixture?

o What are the main types of organic reactions?

o Bond formation and bond breaking

o What is bond dissociation energy?

o Be familiar with the concepts of thermodynamics, kinetics, and  equilibrium

o What are coupled reactions?

o Be able to identify and read an energy diagram

o What are catalysts and how are they used? What are enzymes? o Understand how to name alkyl halides

o What is nucleophilic substitution?

o What is a leaving group?

o Differentiate between the two types of solvents

o Different between the two types of reaction mechanisms o What does the Hammond postulate state?

o What are the factors that determine a reactions mechanism? o Review biological nucleophilic substitution and other real world  examples of all substances throughout the three chapters

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