Additional Problems

Which of the following structures are identical? (Green = Cl.)

(a) (b) (c)
(d)

Assign R or S configurations to the chirality centers in the following molecules (blue = N):

Which, if any, of the following structures represent meso compounds? (Blue = N, green = Cl.)

Assign R or S configuration to each chirality center in pseudoephedrine, an over-the-counter decongestant found in cold remedies (blue = N).

Orient each of the following drawings so that the lowest-ranked group is toward the rear, and then assign R or S configuration:

Which of the following objects are chiral?

(a) A basketball (b) A fork (c) A wine glass (d) A golf club (e) A spiral staircase 
(f) A snowflake

Which of the following compounds are chiral? Draw them, and label the chirality centers.

Draw chiral molecules that meet the following descriptions:

Eight alcohols have the formula C₅H₁₂O. Draw them. Which are chiral?

Draw compounds that fit the following descriptions:

Erythronolide B is the biological precursor of erythromycin, a broad-spectrum antibiotic. How many chirality centers does erythronolide B have? Identify them.

Which of the following pairs of structures represent the same enantiomer, and which represent different enantiomers?

What is the relationship between the specific rotations of (2R,3R)-dichloropentane and (2S,3S)-dichloropentane? Between (2R,3S)-dichloropentane and (2R,3R)-dichloropentane?

What is the stereochemical configuration of the enantiomer of (2S,4R)-2,4-octanediol?

What are the stereochemical configurations of the two diastereomers of (2S,4R)-2,4-octanediol? (A diol is a compound with two – OH groups.)

Orient each of the following drawings so that the lowest-ranked group is toward the rear, and then assign R or S configuration:

Assign Cahn–Ingold–Prelog rankings to the following sets of substituents:

Assign R or S configurations to each chirality center in the following molecules:

Assign R or S configuration to each chirality center in the following molecules:

Assign R or S configuration to each chirality center in the following biological molecules:

Draw tetrahedral representations of the following molecules:

Assign R or S configuration to each chirality center in the following molecules:

Assign R or S configurations to the chirality centers in ascorbic acid (vitamin C).

Assign R or S stereochemistry to the chirality centers in the following Newman projections:

Xylose is a common sugar found in many types of wood, including maple and cherry. Because it is much less prone to cause tooth decay than sucrose, xylose has been used in candy and chewing gum. Assign R or S configurations to the chirality centers in xylose.

Draw examples of the following:

Draw the meso form of each of the following molecules, and indicate the plane of symmetry in each:

Draw the structure of a meso compound that has five carbons and three chirality centers.

Ribose, an essential part of ribonucleic acid (RNA), has the following structure:

On reaction with hydrogen gas in the presence of a platinum catalyst, ribose (Problem 5-54) is converted into ribitol. Is ribitol optically active or inactive? Explain.

Identify the indicated hydrogens in the following molecules as pro-R or pro-S:

Identify the indicated faces in the following molecules as Re or Si:

One of the steps in fat metabolism is the hydration of crotonate to yield 3-hydroxybutyrate. The reaction occurs by addition of  – OH to the Si face at C3, followed by protonation at C2, also from the Si face. Draw the product of the reaction, showing the stereochemistry of each step.

The dehydration of citrate to yield cis-aconitate, a step in the citric acid cycle, involves the pro-R “arm” of citrate rather than the pro-S arm. Which of the following two products is formed?

The first step in the metabolism of glycerol, formed by digestion of fats, is phosphorylation of the pro-R – CH₂OH group by reaction with adenosine triphosphate (ATP) to give the corresponding glycerol phosphate plus adenosine diphosphate (ADP). Show the stereochemistry of the product.

One of the steps in fatty-acid biosynthesis is the dehydration of (R)-3-hydroxybutyryl ACP to give trans-crotonyl ACP. Does the reaction remove the pro-R or the pro-S hydrogen from C2?

Draw all possible stereoisomers of 1,2-cyclobutanedicarboxylic acid, and indicate the interrelationships. Which, if any, are optically active? Do the same for 1,3-cyclobutanedicarboxylic acid.

Draw tetrahedral representations of the two enantiomers of the amino acid cysteine, HSCH₂CH(NH₂)CO₂H, and identify each as R or S.

The naturally occurring form of the amino acid cysteine (Problem 5-63) has the R configuration at its chirality center. On treatment with a mild oxidizing agent, two cysteines join to give cystine, a disulfide. Assuming that the chirality center is not affected by the reaction, is cystine optically active? Explain.

Draw tetrahedral representations of the following molecules:

Assign R or S configurations to the chiral centers in cephalexin, trade-named Keflex, the most widely prescribed antibiotic in the United States.

Chloramphenicol, a powerful antibiotic isolated in 1947 from the Streptomyces venezuelae bacterium, is active against a broad spectrum of bacterial infections and is particularly valuable against typhoid fever. Assign R or S configurations to the chirality centers in chloramphenicol.

Allenes are compounds with adjacent carbon–carbon double bonds. Many allenes are chiral, even though they don’t contain chirality centers. Mycomycin, for example, a naturally occurring antibiotic isolated from the bacterium Nocardia acidophilus, is chiral and has [α]_D = −130. Explain why mycomycin is chiral.

Long before chiral allenes were known (Problem 5-68), the resolution of 4-methylcyclohexylideneacetic acid into two enantiomers had been carried out. Why is it chiral? What geometric similarity does it have to allenes?

(S)-1-Chloro-2-methylbutane undergoes light-induced reaction with Cl₂ to yield a mixture of products, among which are 1,4-dichloro-2-methylbutane and 1,2-dichloro-2-methylbutane.

How many stereoisomers of 2,4-dibromo-3-chloropentane are there? Draw them, and indicate which are optically active.

Draw both cis- and trans-1,4-dimethylcyclohexane in their more stable chair conformations.

Draw both cis- and trans-1,3-dimethylcyclohexane in their more stable chair conformations.

cis-1,2-Dimethylcyclohexane is optically inactive even though it has two chirality centers. Explain.

We’ll see in Chapter 11 that alkyl halides react with hydrosulfide ion (HS⁻) to give a product whose stereochemistry is inverted from that of the reactant.

Draw the reaction of (S)-2-bromobutane with HS⁻ ion to yield 2-butanethiol, CH₃CH₂CH(SH)CH₃. Is the stereochemistry of the product R or S?

Ketones react with sodium acetylide (the sodium salt of acetylene, Na^+– : C$\text{≡}$CH) to give alcohols. For example, the reaction of sodium acetylide with 2-butanone yields 3-methyl-1-pentyn-3-ol:

Imagine that a reaction similar to that in Problem 5-76 is carried out between sodium acetylide and (R)-2-phenylpropanal to yield 4-phenyl-1-pentyn-3-ol: