1.10: Stereochemistry Exercises, Part 2 Answers

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PART A - Molecules (a) through (m) below have been drawn in a way that makes their symmetry apparent if they are in fact symmetric. All the molecules labeled chiral can exist as enantiomeric pairs. All molecules with two chiral carbons and a plane of symmetry represent meso compounds, namely (e), (k), and (m). Chiral carbons have been marked with an asterisk.

dichloromethane achiral

1-bromo-1-chloroethane one chiral carbon chiral

2-bromopropane achiral

2-chlorobutane one chiral carbon chiral

cis-1,2-dichlorocyclopropane 2 chiral carbons achiral

trans-1,2-dichlorocyclopropane 2 chiral carbons chiral

trans-1-bromo-3-chlorocyclobutane achiral

cis-1-bromo-2-chloroethene achiral (planar)

trans-1-bromo-2-chloroethene achiral (planar)

(2S,3R)-2,3-dibromobutane 2 chiral carbons achiral

(2R,3R)-2,3-dibromobutane 2 chiral carbons chiral

meso-1,3-dimethylcyclohexane 2 chiral carbons achiral

PART B - Before assigning configuration to carbons, make sure they are chiral!

a) Chiral molecule and its mirror image - enantiomers.

b) R-isomer on the left, S-isomer on the right - enantiomers.

c) -OH and -Br are in the same positions, but -H and -CH₃ have been exchanged - enantiomers.

d) R-isomer on the left, R-isomer on the right - same molecule.

e) Both molecules are chiral, but they do not have the same groups attached to the chiral carbon - unrelated.

f) Each molecule is chiral (no plane of symmetry) and they are mirror images - enantiomers.

g) The easiest way to approach this one is to assign configurations to the chiral carbons. The molecules have the same molecular formula and the same connectivities, but their 3D arrangement is different. They are stereoisomers. The configurations of their chiral centers mirror each other, which makes them enantiomers.

h) The molecule is not chiral (easiest to see in top view) and they’re both cis-isomers - same molecule.

cis-1-bromo-3-methylcyclobutane.

i) A pair of cis/trans isomers - diastereomers

j) Two trans-isomers (chiral) and mirror images - enantiomers.

k) Rotating the molecule on the left as shown leads to the molecule on the right - same molecule.

l) Cis-isomers, same substituents on the same carbons (1 and 3), but different conformations - same molecule.

m) Each molecule has two chiral carbons and a plane of symmetry. Although they mirror each other, they are the same molecule (a meso compound).

n) Both structures represent the S-isomer - same molecule.

o) S-isomer on the left, R-isomer on the right - enantiomers.

p) Both molecules represent 2,3-dibromobutane, but the molecule on the left has all the groups (-H, -Br, -CH₃)

anti to each other. One can rotate the front carbon until all the groups eclipse and match each other. That is not the case with the molecule on the right. Both molecules have two chiral carbons. The one on the left has a plane of symmetry, which makes it a meso compound. The molecule on the right has no symmetry and therefore it is chiral. They are diastereomers. For added clarity turn the molecules around to view them from the side.