Question

The number of stereoisomers of the compound 2-chloro-4-methylhex-2-ene is (Hen CEET 2000) (1) 1 (2) 2 (3) 4 (4) 16

Short answer

4

Step-by-step solution

Identify possible stereocenters

2-chloro-4-methylhex-2-ene contains both a double bond (which can have cis/trans or E/Z isomers) and a potential chiral center at the carbon with the chlorine atom attached.

Determine number of E/Z isomers

The double bond between carbon 2 and carbon 3 in 2-chloro-4-methylhex-2-ene can have E/Z isomerism. This gives 2 possible isomers: E and Z.

Identify chiral centers

The carbon at position 2 has four different groups attached: a chlorine atom, a hydrogen atom, an ethyl group (–CH2CH3), and a propyl group (–CH2CH2CH3). Therefore, it is a chiral center.

Calculate total stereoisomers

The total number of stereoisomers can be determined by multiplying the number of E/Z isomers by the number of chiral center configurations. Since there is 1 chiral center (2 configurations: R and S) and 2 E/Z configurations: Total stereoisomers = 2 (E/Z) * 2 (R/S) = 4.

Key concepts

double bond isomerism

Double bond isomerism occurs when there is a rigid double bond between two carbon atoms. This rigidity means the atoms or groups attached to these carbons cannot freely rotate around the bond. Because of this, two distinct arrangements, or isomers, can exist.
In 2-chloro-4-methylhex-2-ene, the double bond between carbon 2 and carbon 3 can result in different spatial arrangements. These are known as E (entgegen) and Z (zusammen) isomers:

• E Isomer: The higher priority groups (based on the Cahn-Ingold-Prelog priority rules) on each carbon of the double bond are on opposite sides.
• Z Isomer: The higher priority groups are on the same side.

Recognizing these two forms is crucial for determining all possible stereoisomers of the molecule.

chiral centers

A chiral center is a carbon atom bonded to four different groups. This arrangement leads to non-superimposable mirror images, known as enantiomers. In 2-chloro-4-methylhex-2-ene, the carbon at position 2 is a chiral center because it has four different groups attached: a chlorine atom, a hydrogen atom, an ethyl group (–CH2CH3), and a propyl group (–CH2CH2CH3).
To identify chiral centers in any molecule, look for carbons connected to four distinct groups. Each chiral center will have two possible configurations, R or S. This designation is based on the clockwise or counterclockwise arrangement of the priority groups around the chiral center.

E/Z isomerism

E/Z isomerism is a type of stereoisomerism that results from the restricted rotation around a double bond. According to the Cahn-Ingold-Prelog priority rules, the groups attached to the double-bonded carbons are assigned priorities. Then, based on their positions, the isomer is designated as E or Z:

• E (entgegen): The higher priority groups on each carbon are on opposite sides of the double bond.
• Z (zusammen): The higher priority groups are on the same side of the double bond.

Applying this to 2-chloro-4-methylhex-2-ene, we get two possible E/Z configurations, contributing to the overall number of stereoisomers.

R/S configuration

The R/S configuration system is used to describe the absolute configuration at a chiral center, based on the Cahn-Ingold-Prelog priority rules. Follow these steps to assign R or S:

• Identify the four different groups attached to the chiral center and rank them by priority (atomic number).
• Orient the molecule so the lowest priority group points away from you.
• If the sequence of the remaining three groups (1 to 3) is clockwise, the chiral center is R (rectus).
• If the sequence is counterclockwise, it is S (sinister).

In the case of 2-chloro-4-methylhex-2-ene, the carbon at position 2 having four different groups can be in one of two configurations: R or S. This contributes to the total number of stereoisomers when combined with E/Z isomerism: 2 (E/Z) * 2 (R/S) = 4 stereoisomers.