DAT - Dental Admission Organic Chemistry: Stereochemistry Questions and Answers

Question 1

A compound has a specific rotation of +32°. What would be the specific rotation of its enantiomer?

Accepted Answer

-32°

Answer

Enantiomers are non-superimposable mirror images that rotate plane-polarized light to an equal magnitude but in opposite directions. If one enantiomer is dextrorotatory (+), its mirror image will be levorotatory (-) by the same amount.

Question 2

Which of the following terms best describes the relationship between (2R,3S)-2,3-dibromobutane and (2S,4R)-2,4-dichloropentane?

Accepted Answer

Constitutional isomers

Answer

Constitutional isomers have the same molecular formula but different connectivity. The two compounds have different carbon skeletons (butane vs. pentane) and different substituents (bromo vs. chloro), meaning their connectivity is different, though they are not isomers at all as their molecular formulas are different. However, among the choices, constitutional isomers is the only one describing different connectivity, which is the fundamental difference here. Enantiomers and diastereomers are types of stereoisomers, which must have the same connectivity.

Question 3

What is the maximum number of stereoisomers possible for a molecule with the structure of 3,4-dimethylhexane?

Accepted Answer

4

Answer

The maximum number of stereoisomers is calculated by the formula 2^n, where 'n' is the number of chiral centers. 3,4-dimethylhexane has two chiral centers (at C3 and C4). Therefore, the maximum number of stereoisomers is 2^2 = 4. These stereoisomers exist as two pairs of enantiomers.

Question 4

Which of the following molecules is a meso compound?

Accepted Answer

(2R,3S)-Tartaric acid

Answer

A meso compound is an achiral molecule that contains chiral centers and an internal plane of symmetry. (2R,3S)-Tartaric acid has two chiral centers but possesses an internal plane of symmetry, making it achiral and thus a meso compound. The optical rotations from its two chiral centers cancel each other out.

Question 5

A 50:50 mixture of two enantiomers is known as what?

Accepted Answer

A racemic mixture

Answer

A racemic mixture, or racemate, is a 50:50 mixture of two enantiomers. Because the two enantiomers rotate plane-polarized light equally but in opposite directions, their effects cancel out, and the racemic mixture is optically inactive.

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DAT - Dental Admission Organic Chemistry: Stereochemistry Questions and Answers