CH 610A/618A

Second Exam

Fall, 2002

Prof.N.L.Bauld

 

 

 

 

                                                            __________________________________________                                                                              Name (Last, First)

 

 

                                                            __________________________________________                                                                            Social Security Number     

 

 

                                                                                               

 

 

                                                                                               

Question

Possible Points

Page/Pts/Points Earned

Score

I.

21

2/7/

3/9/

4/5/

 

II.

 

22

5/16/

6/6/

 

III.

 

15

7/12/

8/3/

 

IV.

19

9/11/

10/8/

 

V.

 

 

23

11/14/

12/9/

 

Totals

100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I. Acids and Bases in Organic Chemistry

 

 

A.  General Effects on Bronsted Acidity.

1.   [3 pts] Consider the acid dissociation of a general acid H-A in aqueous solution as written below. Cite the two general effects which are considered to influence the relative acidities of acids where only the nature of A is varied. Indicate in what sense (increase or decrease) they affect acidity and briefly explain why.

 

(1)  The bond strength or bond dissociation energy (D) of the H-A bond. The stronger the H-A bond, the less acidic is HA.

(2)  The stability of the anion A-. The more stable this anion is, the stronger the acid HA.

2. [2 pts each = 4 pts] For each of the series given below, indicate in which direction acidity increases, which one of the general effects you have mentioned is dominant, and and supply an explanation of why this particular effect is dominant.

 

q      The acidity increases from left to right, i.e., from HF to HBr. The bond strength effect dominates. This is because the bond strength decreases greatly in a series which proceeds down a column of the periodic table, but the anion stability varies only slightly.

 

 

 

 

q      Acidity increases from right to left, i.e., from methane to water. Anion stability dominates. This is because in a series which proceeds across arrow of the periodic table, bond strength varies relatively little, but electronegativity increases tremendously toward the right side, so that anion stability increases greatly as we go from methide to amide to hydroxide anion.

 

3. [3 pts each = 6 pts] For each of the pairs of Bronsted acids listed below, indicate the order of relative acidities , name the specific effect which is considered to cause this order of acidities, and explain the detailed basis for the effect, including a depiction of the effect..

 

.

q      Acetic acid is more acidic than ethanol, because the anion of acetic acid, i.e. the acetate anion, is resonance stabilized , but the anion of ethanol is not. In both anions, the negative charge is on oxygen, so the resonance effect is essentially the only differentiating effect.

 

 

q      Acidity increases from left to right, i.e., is greatest for ethyne and least for ethane.

q      This is because of a hybridization effect. Thek anion of ethyne has the unshared electron pair in an sp AO, which has 50% s  character, whereas the anion from ethene has the unshared pair in an sp2 AO, which only has 33% s character and that from ethene in an sp3 AO, which has 25% s character.

q      Since a 2s AO is substantially lower in energy than a 2p AO, the greater the percent s character the lower in energy is the orbital.

       

 

q      Chloroacetic acid is more acidic than acetic acid.

q      The general effect is anion stability and the specific effect is termed an inductive effect. We will also accept “field effect” or “dipolar effect”.

q      The C-Cl dipole of the chloroacetate anion is oriented so that the positive end of the dipole (the carbon end) is closer to the anionic oxygen centers than is the negative end of the C-Cl dipole. Consequently, the attractive interaction is dominate and results in stabilization of the anion by this inductive effect. .

 

 

 

B.  Acid/Base Equilibria

1.   [3 pts] Consider the following acid/base reaction and determine the position of the equilibrium using both a qualitative and a quantitative criterion. State both criteria which you are using and show precisely how they are applied.

 

 

a. Qualitative Criterion: [1 pt]

q      The stronger acid is converted to the weaker acid.

q      The lower the pKa the stronger the acid, so acetic acid is stronger than the methylammonium ion.

q      The equilibrium proceeds to the right.

 

b. Quantitative Criterion [2 pts]

q       

q      The reaction proceeds toward the right.

q       

C. Lewis Acids/Bases

1.    [3 pt] For the reaction shown below, supply the correct structure of the product and indicate which reactant species is the Lewis acid and which the Lewis base. Then explain what specific aspect of each reactant molecule enables it to function as the Lewis acid or base.

q      BF­3 has a vacant 2p AO, which enables it to function as the Lewis acid or electrophile.

q      NH3 has an unshared electron pair, which enables it to function as a Lewis base or nucleop

            2. [2 pts] Consider the structure of the acid/base reaction product of the reaction above. Could this molecule serve as a Lewis base? Why or why not? Could it plausibly serve as a Bronsted acid? Why or why not?

q      The product could not be an effective Lewis base, because it lacks an unshared electron pair or any other readily shared electron pair. Although the boron is negatively charged, it has a filled second main shell and no other electrons.

q      It could plausibly serve as a Bronsted acid, because there are acidic protons attached to the positively charged nitrogen.

 

II. Stereochemistry

 

 

A. Definitions

1. [2 pt each = 6 pts] Provide the appropriate stereochemical term corresponding to each of the following definitions:

a.     Stereoisomers which are not mirror images-- diastereoisomers

 

b.    A molecule which has chiral centers but is achiral—a meso compound

 

c.     The process of separating two mirror image isomers--resolution

 

 

B. Chiral vs. Achiral Molecules

            1. [2 pt each = 4 pts]  Indicate whether each of the following molecules is chiral or achiral and specify what criterion you are applying in order to arrive at this conclusion.

q      The structure on the left is achiral, since it has no stereocenters. (The central carbon atom, e.g., has two ethyl groups attached).

q      The structure on the right is chiral because it has a stereocenter, viz., the carbon which has OH, H, methyl, and ethyl groups attached. Also, it has no other stereocenters, so it could not be meso.

 

C. Molecules Having Two Stereocenters

            1. [4 pts] Draw the Newman projection structure of the mirror image of the molecule shown below and designate its configuration using the R,S nomenclature (you should specify the position locant of the R or S configuration as is done in the name shown below (e.g. 3S,2R). Now draw the Newman projection structures of the other two stereoisomers of 3-chloro-2-butanol and designate their configurations in the analogous way.

 

 

 

            2. [2 pts] Indicate the relationships between the various isomers, i.e., what type of isomers they are with respect to each other. Are all of these molecules chiral? Explain why or why not.

q      These are shown above.

q      Yes. Molecules with stereocenters are only achiral if the stereocenters are equivalent. These stereocenters are non-equivalent.

 

E. Separation of pure enantiomers from each other

            1. [3 pts] Consider a 50:50 mixture of R and S-lactic acid (shown below as Fischer structures). Indicate the Fischer structures of the products (salts) which are formed when this mixture reacts (a simple acid/base reaction) with a single enantiomer (the R enantiomer) of 1-phenylethanamine (also shown below. Also, provide the stereochemical designations of each of the products (e.g., R,R or S,S etc.).

 

 

            2. [2 pts] What is the stereochemical relationship between these two products, and how can they be separated? Explain why this separation method works.

q      They are diastereoisomers.

q      They can be separated by re-crystallization.

q      Since they are not mirror images, they have different physical properties, including solubility.

 

            3. [1 pt] Draw the Fischer structure of the mirror image of one of these products. Is it identical to either of the products? What is its stereochemical designation?

 

 

q      This is the mirror image of the (R,R) product.

q      It is different from either of the products of the original reaction.

III. Alkenes: Nomenclature and Stability

 

A.  IUPAC Nomenclature

            1. [3 pts] Provide the correct and complete IUPAC name for the following alkene, including any appropriate cis- or trans-designation:

q      Trans-2-methyl-3-hexene. Incidentally, numbering from the left side would give also give a 3-hexene, but the substituent methyl group would then be at the 5-position (not acceptable).

 

            2. [3 pts] Provide the correct and complete IUPAC name for the following alkene, including the designation as E or Z. In the latter connection, explain and show your priority rankings.

q      E-3-ethyl-2-methyl-3-hexene.

q      Note that there are three different numbering systems that give a hexene and all three would be 3-hexenes, but only this one gives the first substituent at the 2- position. 

q      The ethyl group is listed before the methyl because of alphabetics.

q      The high priority groups are trans, so this is the E isomer.

q      Ethyl has a higher priority than H, because C has a higher priority (atomic number) than H. Isopropyl has a higher priority than ethyl, because although the alpha atoms are carbon in both cases, the isopropyl group has 2 beta carbons, while the ethyl has just one.

 

B. {3 pts] Provide the correct structure of the alkene which has the IUPAC name Z-2-chloro-4-methyl-2-hexene.

C. Alkene Stability

            1. [3 pts] Classify the following alkenes with respect to their degree of substitution (mono-, di-, etc), and indicate their relative order of thermodynamic stabilities (1 is the most thermodynamically stable).

q      The first structure on the left is trisubstituted. It is the most stable.

q      The second structure is monosubstituted. It is the least stable.

q      The third structure (cyclohexene) is disubstituted. It is of intermediate stability.

 

D. Pi Bonding In Alkenes.

1. [3 pts] Illustrate by means of Newman projection structures, and also showing the relevant overlap of 2p orbitals in pi bonding, what happens when rotation around the C-C bond of cis-2-butene occurs to ultimately give trans-2-butene. Is this easy or difficult? How much of the pi bond remains at the 90 degree angle of rotation? Explain.

 

 

q      The overlap is weakened as rotation occurs around the bond and vanishes completely at the 90 degree angle of rotation. Therefore, this rotation is very hard to do, since it completely breaks the pi bond.

q      At the 90 degree angle of rotation, there is still some spatial overlap, but the amount of positive, bonding overlap just equals the amount of negative, antibonding overlap, and the net result is no bonding at all.

 

IV. Reaction Mechanisms: General; Carbocation Intermediates

 

A. Definitions:

1. [2 pt each = 4 pts] Provide appropriate definitions for the following terms:

                        a. transition state – the highest energy structure on the lowest energy pathway of a reaction.

                       

                        b. rate-determining step – a step of a multi-step reaction the rate of which is equal to the rate of the overall reaction.

 

B.  Reaction Path Diagrams

      1. Consider the following two step reaction:

 

a.     [3 pts] Sketch a reaction path diagram which realistically could represent such a reaction, indicating the position on the reaction coordinate of R, I, and P and any transition states (TS).

q      I can’t draw smooths path with my software, but the identical reaction scheme and the corresponding path can be found in your notes. It should be similar to the following scheme.

q      The scheme also contains the answer to the b. part of this question.

b.    [2 pts] Indicate on the same diagram the energy quantity which controls the rate of the reaction and also the energy quantity which controls the position of an equilibrium between R and P, in the event that it is established.

q      The quantity Eact or activation energy controls the rate of the reaction (the energy difference between R and TS1).

q      The energy difference between P and R determines the position of the equilibrium.

 

 

c.     [2 pts] Cite, and illustrate on the diagram, two specific conditions which must be satisfied in order for the first step to be rigorously rate-determining.

q      The intermediate must essentially always go on to product, never back to reactants. This means that the barrier between I and TS1 (the activation energy for the reverse of step 1)  must be greater than the barrier between I and TS2 (the activation energy for step 2).

q      The concentration of the intermediate must not build up to a significant level, i.e., it must go on rapidly to the products. This requires that the barrier to the second step be very low.

 

C. Carbocation Intermediates

      1. [2 pts] Which of the carbocations depicted below, the methyl carbocation or the ethyl carbocation is more thermodynamically stable and by how much (approximately, in solution)?

q      The ethyl carbocation is more stable than the methyl carbocation by about 15 kcal/mol in solution (much more in the gas phase).

 

2. [3 pts] Provide a resonance theoretical description of the resonance stabilization which exists in one of these carbocations. What specific sub-type of resonance effect is this?

q      This is called hyperconjugation.

 

3. [3 pts] Indicate the type of carbocation moiety (primary, secondary, or tertiary)  present in each of the species shown below, and  indicate the order of stabilities, with 1 representing the most thermodynamically stable.

 

 

q      The first, on the left, is a primary carbocation (it has only one substituent, the cyclohexyl group). It is the least stable.

q      The middle structure is secondary, and is intermediate in stability.

q      The last structure is tertiary, and is the most stable.

V. Mechanism of H-Cl Additions to Alkenes

 

A. The Reaction Mechanism.

            1. [4 pts] Write the detailed reaction mechanism for the addition of H-Cl to ethene, employing all of the conventions of this course.

 

 

 

 

            2. [2 pts] If one of the steps of this mechanism is rate-determining, explain in energetic terms why it is this step in particular which is rate-determining and not the other step.

q      The first step is rate determining. It involves breaking two bonds and forming just one, so it is highly endothermic (endergonic) and slow. The second step involves breaking no bonds and forming one, so it is highly exothermic and fast.

 

B. Transition State Models.

1. [5 pts] Provide a resonance theoretical treatment of the transition state for the rds of this reaction, summarize this as a DL/PC (dotted line/partial charge) structure, and characterize the TS as specifically as you can (without yet applying the Hammond Principle).

 

 

 

            2. [3 pts] State the Hammond Principle, and use it to further refine your TS model, explaining the steps in your logic.

q      The TS state of a reaction more strongly resembles the higher energy partner of the reactant or product.

q      Since the product of step 1 is a very high energy carbocation intermediate, the TS of this step strongly resembles the carbocation intermediate.

q      Consequently we can now say that the TS has extensive or highly developed carbocation character.

 

 

C. Selectivity.

1. [5 pts] Consider, now, the addition of H-Cl to propene. Using the TS model you have already generated, with appropriate slight modification for the case of propene, apply the Method of Competing Transition States to explain the preferred sense of addition of H-Cl to this unsymmetrical alkene (i.e., to which of the alkene carbons the proton adds).

 

q      Since secondary carbocation character is much more favorable than primary carbocation character, the first TS on the left is the favored one.

 

 

            2. [2 pts] Draw the structure of the predominant product of this reaction. What general term is used to describe this kind of selectivity or specificity (not looking for Markovnikov or anti-Markovnikov here)?

q      Regioselectivity or regiospecificity.

 

            3. [2 pts] What is the symbolic mechanistic designation of this reaction?

q      AdE2