Organic Chemistry Multiple Choice Questions on “Enantiomerism”.
1. Which of the following is the definition for enatiomerism?
a) A pair of stereoisomers each of which has two chirality centres
b) A pair of stereoisomers that are not mirror images of one another
c) A pair of stereoisomers that are non-superimposable mirror images of one another
d) Any pair of stereoisomers
Answer: c
Clarification: The relationship between two stereoisomers having molecules that are mirror images of each other; enantiomers have identical chemical and physical properties in an achiral environment but form different products when reacted with other chiral molecules and exhibit optical activity. A pair of stereoisomers that are non-superimposable mirror images of one another are enantiomers.
2. Which of the following statements is not true regarding pairs of enantiomers?
a) They have identical boiling points
b) They rotate plane polarized light in opposite directions
c) They react at identical rates with chiral reagents
d) They have identical melting points
Answer: b
Clarification: This statement is true. All (+) enantiomers are levorotatory, corresponding to rotation of plane polarized light in a clockwise direction.
3. Which of the following statements regarding optical rotation is not true?
a) All R enantiomers are dextrorotatory
b) All (+) enantiomers are laevorotatory
c) All (-) enantiomers rotate plane polarized light in a counter clockwise direction
d) (+) and (-) enantiomers rotate plane polarized light in opposite directions
Answer: c
Clarification: This statement is false. Pairs of enantiomers react at different rates with chiral reagents.
4. How many enantiomers are there of the molecule shown below?
a) 6
b) 2
c) 0
d) 1
Answer: c
Clarification: Chiral centers are carbons that have four non-identical substituents on them and are designated as either of R stereochemistry or S stereochemistry. If a molecule has one stereocenter of R configuration, then in the mirror image of that molecule, the stereocenter would be of S configuration, and vice-versa. And above compound does not satisfy these conditions to show enantiomerism.
5. Which of the following notations is not used to distinguish between pairs of enantiomers?
a) R and S
b) E and Z
c) + and –
d) D and L
Answer: b
Clarification: All four of these notations are used to distinguish between isomers, but only three are used specifically in the context of enantiomers. The fourth – E and Z – is used to differentiate between geometric isomers, not enantiomers.
6. Which among the following is true about enantiomerism?
a) Assignments of R and S labels and (+) and (–) labels are not connected
b) The labels R and S refer to different conformers
c) The labels (+) and (–) are used to distinguish enantiomers
d) The specific rotation of enantiomers is equal and opposite
Answer: b
Clarification: R and S not refers to different conformers. Chiral centers are carbons that have four non-identical substituents on them and are designated as either of R stereochemistry or S stereochemistry. If a molecule has one stereocenter of R configuration, then in the mirror image of that molecule, the stereocenter would be of S configuration, and vice-versa.
7. Which of the following molecules does not possess enantiomers?
a) CH3CH2CH2CHBrCH3
b) CH3CH2CBr2CH3
c) CH3CHBrCH2CH3
d) CHBr2CH2CHBrCH3