Chem 334 - Summer 1998 - Organic Chemistry I Dr. Carl C. Wamser

FINAL EXAM Answers - Chapters 1 - 9

1. (15 pts) Write complete IUPAC names for each of the following compounds, including designation of stereochemistry if it is specifically shown:

a)

(2S,3R)-2-chloro-1,3-butanediol

b)

5-(1-methylcyclopentyl)-1-pentanethiol

c)

(R)-5-ethoxy-2-ethyl-1-pentanol

2. (15 pts) Write accurate structures for each of the following:

a) sec-butyllithium

b) DMSO

c) 1,3-dioxacyclopentane

d) the best chair conformation of cyclohexanethiol

e) the transition state for E2 elimination from 2-bromopropane by methoxide

3. (15 pts) The structure shown below is a typical steroid.

Place the six substituents shown on the structure at left onto the structure at right in their appropriate places (we are considering the two identified H's to be substituents for purposes of this question).

On the structures, identify each of the six substituents as either axial or equatorial.

Explain why each of the ring junctures is called "trans".

The pairs of carbon bonds that extend from each ring to make the next ring are all trans to one another.

4. (15 pts) Arrange the compounds in each series in order with respect to the property indicated. Write "MOST" and "LEAST" under the appropriate compounds in each group.

a) acidity

MOST / / LEAST / / MIDDLE

b) radical stability

MOST / / MIDDLE / / LEAST

c) nucleophilicity

MIDDLE / / LEAST / / MOST

d) reactivity in an SN2 mechanism

LEAST / / MOST / / MIDDLE

e) amount of axial Cl at equilibrium

MIDDLE / / MOST / / LEAST

5. (15 pts) Complete the following reactions by adding the missing part: either the starting compound, the necessary reagents and conditions, or the final product(s). Indicate stereochemistry when it might be expected to be specific.

a)

b)

c)

d)

e)

6. (15 pts) Complete the following reactions by adding the missing part: either the starting compound, the necessary reagents and conditions, or the final product(s). Indicate stereochemistry when it might be expected to be specific.

a)

b)

c)

d)

e)

7. (15 pts) Devise a synthetic sequence for either one of the two molecules below. Allowable starting compounds include any alcohol having four or fewer carbons, ethylene oxide (oxacyclopropane), plus any needed solvents or inorganic reagents.

Note - in each of the schemes above, there is a primary alcohol that could also have been prepared by Grignard addition to oxacyclopropane, e.g.,

8. (15 pts) Use the table of bond dissociation energies to calculate delta H for each of the following reactions. If the exact data are not available, select the most appropriate analog.

a) hydrolysis of 1-bromopropane

b) hydrolysis of 2-bromopropane

c) From the delta H values calculated above, can you tell which isomer is more reactive? Explain.

No. Reactivity depends on the rates of reactions, which are determined by Ea values rather than delta H.

9. (16 pts) Write a complete mechanism for the bromination reaction shown below. Show all steps.

(2 pts) If the reaction above began with the (R) enantiomer, the product would be expected to be (circle one):

(R) (S) racemic unknown other

Racemic

(2 pts) If the reaction above began with the (+) enantiomer, the product would be expected to be (circle one):

(+) (-) (±) unknown other

(±)

10. (20 pts) Write complete mechanisms for the following reactions.

Show all steps.

a)

b)

11. (15 pts) Write structures for all the different isomers that would have the name 2,3-dimethylcyclopropanol.

Identify enantiomer pairs and meso compounds, but it is not necessary to assign (R) and (S) to the stereocenters (unless you find it helpful).

12. (15 pts) The reaction below has been used as evidence that the shift of a methyl group to a primary carbon is concerted and anti to the leaving group.

(D represents heavy hydrogen.)

Write out the complete mechanism, and explain why this is considered important information about the methyl shift.

Because the reaction occurs with inversion at the stereocenter, this suggests that the CH3 shift occurs at the same time as the leaving group is leaving, in other words, a concerted shift, like an internal SN2 displacement.

13. (10 pts) In order to determine how your body oxidizes ethanol, the two stereospecifically labeled forms of ethanol (shown below) were prepared and subjected to enzymatic oxidation. (D represents heavy hydrogen.)

NAD+ is a cofactor, specifically an oxidant, often used in conjunction with enzymes -- you do not need to know its structure to answer this question.

Explain the significance of the following experimental results.

The stereospecific removal of either H or D during the oxidation reaction indicates that the enzyme (or NAD+) can distinguish the two hydrogens on the alpha (CH2) position of ethanol based on the stereochemical orientation.