Question

Imines can exist as stereoisomers. The isomers are named using the E, Z system of nomenclature (Section 4.2). The lone pair has the lowest priority.

Draw the structure of each of the following compounds:

a. the (E)-hydrazone of benzaldehyde

b. the (Z)-oxime of propiophenone

Short answer

a.

(E)-hydrazone of benzaldehyde

b.

(Z)-oxime of propiophenone

Step-by-step solution

Stereoisomers

Some compounds will have the same atoms and connectivity, but the functional groups' orientation around the atom will be different. These compounds are called stereoisomers, and they have the same IUPAC names.

The stereoisomers formed due to the restricted rotation of double bonds are called configurational isomers.

E and Z configuration

The configurational isomers have the same IUPAC name. Therefore, to differentiate these compounds, configurations are used based on the priority of functional groups on a carbon atom.

The groups are prioritized based on the rules by Cahn, Ingold, and Prelog. The atom directly attached to the carbon atom is considered, and the priority increases with a higher atomic mass.

If the groups with higher priority are on the same side of the double bond, the isomer has Z-configuration. If the high-priority groups are on the opposite side, the isomer is an E-isomer.

Structures of given stereoisomers

a. The hydrazone of benzaldehyde has the higher priority group on the carbon atom is the phenyl group, and the priority of the amine group on nitrogen is higher than the lone pair.

In the E-isomer of the compound, the higher priority groups should be on the opposite side of the double bond. Therefore, the structure of (E)-hydrazone of benzaldehyde is:

(E)-hydrazone of benzaldehyde

b. The oxime of propiophenone has the chemical formula

The carbon atom has a phenyl group and an ethyl group. Therefore, the phenyl group has a higher priority.

The nitrogen atom has a lone pair and hydroxyl group. Therefore, the hydroxyl group has a higher priority.

In a Z-isomer, the higher priority groups are on the same side of the double bond. Therefore, the structure of (Z)-oxime of propiophenone is:

(Z)-oxime of propiophenone