Exam 3 Answers - Chemistry 227

Fall 1997 - Harding

1. (4 pts) Draw structures corresponding to each of the following names. Either condensed or line structures may be used. Use wedge-dash projections to indicate stereochemistry.

a) (S)-6-chloro-5-methyl-2-hexyne

b) 3-iodo-2,2,4,4-tetramethylpentane

2. (9 pts)

a) Place asterisks (*) at all the stereogenic centers in calciferol (Vitamin D₂) shown below.

b) In addition to stereogenic centers, there are other groups that result in stereoisomerism in calciferol. Circle those groups.

There are three C-C double bonds which can have E/Z stereoisomers.

3. (8 pts)

Consider the structure of testosterone shown below.

a) (2 pts) How many stereoisomers are possible for testosterone? (Shown your calculations)

6 Stereogenic centers (See * on structure)

2⁶ = 2 x 2 x 2 x 2 x 2 x 2 = 64

b) (6 pts) Assign R or S configuration to each indicated stereogenic center in testosterone. Place your answer in the appropriate box.

4. (13 pts) Consider the reaction shown below.

a) (2 pts) The reactive intermediates in this reaction are: (Correct answer is underlined)

A. carbocations

B. radicals

C. carbanions

D. free spirits

b) (2 pts) How many structural isomers are possible in this reaction?

4 isomers (The 1-bromo, 2-bromo, 3-bromo, and 4-bromo isomers). See Problem 10.4 in text; this is a simpler example than Problem 10.21

c) (3 pts) Draw the structural isomer expected to be the major product from this reaction.

Substitution at the 3^o hydrogen

d) (4 pts) Draw the structural isomer(s) of the product that is(are) chiral. (You do not have to show the stereoisomers.)




e) (2 pts) Will the total product mixture be optically active or inactive? (Explain your answer)

Optically inactive. Achiral starting materials with achiral reagents will lead to optically inactive products (all chiral products are formed as racemic mixtures). See Problem 10.21

5. (30 pts) Predict the major organic product(s) in each reaction below. Be sure to show stereochemistry when it is relevant.

a)

See Problem 8.22c

b)

See Problem 8.20e

c)

See Problem 8.21a

d)

See Problem 10.11

e)

See Problem 10.20f and g

f)

See Problem 10.6

g)

See pp. 356 and 366

h)

See Problem 8.21c

i)

See Problem 8.6a

6. (8 pts) For the following pairs of compounds indicate whether they are:

identical, enantiomers, diastereomers, or constitutional isomers.

a) enantiomers

b) diastereomers

c) identical

d) identical

7. (10 pts) An Aggie chemistry graduate student has just run the following addition reaction and separated the product mixture into two compounds with the same molecular formula.

a) (2 pts) The starting material A is: (Correct answer is underlined)

A. racemic

B. achiral

C. optically active (One stereogenic center with only one configuration present)

D. optically inactive

b) (4 pts) Draw the structures of the two expected products in the boxes above.

c) (4 pts) What simple experiment should the Aggie conduct with his two samples in order to be able to place the correct structure (B or C) on each of samples? Be sure to indicate what result would be expected from this experiment for each of your structures B and C.

Measure optical rotation. Sample corresponding to structure B will be optically inactive (meso isomer) while the sample corresponding to structure C will be optically active.

Compare with Problem 10.23

8. (6 pts) Write the complete reaction mechanism for the reaction below. Show all intermediate structures and show all electron flow with curved arrows.

See pp. 277 and 279

9. (2 pts) Draw the meso isomer of 2,4-dibromopentane.

See Problem 10.23

or any other representation of this structure.

10. (10 pts) Carbon-carbon bond forming reactions are important in organic synthesis. Starting with any organic starting materials of 4 or fewer carbons, show two different syntheses (using two different C-C bond forming reactions) of:

(You need to show all reagents and stable intermediate structures.)

See Problems 10.13, 10.19b, 10.20g, and 10.32