Question

Question: Answer the following questions about compounds A–D.

a. How are the compounds in each pair related? Choose from constitutional isomers, stereoisomers, or identical molecules: A and B; A and C; B and D.

b. Label each compound as a cis or trans isomer.

c. Draw B as a hexagon with wedges and dashed wedges to show the stereochemistry of substituents.

d. Draw a stereoisomer of A as a hexagon using wedges and dashed wedges to show the orientation of substituents.

Short answer

Answer

a.

A and B are constitutional isomers.

A and C are identical structures.

B and D are stereoisomers.

b.

Groups in A are cis to each other.

Groups in B are trans to each other.

Groups in C are cis to each other.

Groups in D are cis to each other.

c.

d.

Step-by-step solution

Check the molecular formula

Check the molecular formula of the given compounds. If their molecular formulas are the same, then check for isomerism. If their molecular formulas are not the same, then the molecules are different, i.e., they cannot be identical molecules or isomers.

Check for constitutional isomerism

If the molecular formulas of the given two compounds are the same, then check for the bonding of atoms. If the two compounds differ in the bonding of atoms or groups, then the two structures are constitutional isomers.

Check for stereoisomerism

If the molecular formulas and the sequence (or bonding) of atoms and groups are the same, then check for the arrangement of atoms and groups in the three-dimensional space.

• If the spatial arrangements of the atoms and groups in the two compounds are different then the compounds are stereoisomers.
• If the spatial arrangements of the atoms and groups in the two compounds are not different then the two compounds are identical.

Wedged and dashed-wedged representation

A solid wedge represents groups that are above the plane of the chemical structure. A dashed wedge represents groups that are below the plane of the chemical structure.

Isomerism in the given compounds

a. A and B differ in the bonding of atoms. Hence, A and B are constitutional isomers.

Structure A and B

The methyl and isopropyl groups in both A and C are on the upper side. Hence, compounds A and C are the same in structure, bonding, and spatial arrangement. The two structures are identical.

Identical structures A and C

In B, the methyl group is down, and the isopropyl group is up. While in D, both the methyl group and the isopropyl groups are up.

Hence, B and D differ in the spatial arrangement of groups, and are stereoisomers.

Stereoisomers B and D

b.

In A, both substituent groups are up. Hence, the groups are cis to each other.

In B, one group is up and the other group is down. So, the two groups in B are trans to each other.

In C, both groups are up. Hence, the groups are cis to each other.

In D, both groups are up. Hence, the groups are cis to each other.

c.

The up isopropyl group is shown with a solid wedge on the hexagonal ring. The down methyl group is shown with a dashed wedge on the hexagonal ring.

Hexagonal wedge and dashed-wedge structure of B

d.

In A, both substituent groups are up. To form a stereoisomer of A, flip any one of the two groups toward the downside of the compound.

Structure of A and stereoisomer of A