Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Number of stereoisomers of the compound, 2 -chloro4-methylhex-2-ene is (a) 2 (b) 4 (c) 6 (d) 7

Step-by-Step Solution

Answer

There are 4 stereoisomers of 2-chloro-4-methylhex-2-ene.

Step 1: Identify the Stereocenters

First, we need to identify the stereocenters in the compound 2-chloro-4-methylhex-2-ene. A stereocenter is a carbon atom that is attached to four different groups. In this compound, the carbon at position 2 has a double bond, which can contribute to stereoisomerism due to potential E/Z (cis/trans) isomerism. Additionally, the carbon at position 4, which is connected to a methyl group, hydrogen, a chlorine attached chain, and the rest of the carbon chain, may also act as a stereocenter for optical isomerism.

Step 2: Determine Types of Isomerism

The compound can show both geometric (E/Z) isomerism at the C=C double bond and optical isomerism at a potential chiral center. The C2=C3 double bond can have two geometric forms: E or Z. Additionally, if the carbon at position 4 is a chiral center due to different attached groups, it can have two optical isomers (R/S). Hence, we can apply the formula for calculating stereoisomers: \[ 2^n \]where \( n \) is the number of stereocenters/geometric isomerisms.

Step 3: Calculate the Number of Geometric Isomers

For the double bond at position 2 (C2=C3), there are 2 possible geometric isomers (E and Z configuration).

Step 4: Calculate the Number of Optical Isomers

At the chiral center at position 4, given the presence of four different groups around it, there are 2 optical isomers (R and S configuration). This is calculated as \( 2^1 = 2 \)

Step 5: Combine the Types of Isomerism

The total number of stereoisomers is the product of geometric and optical isomers. Since there are 2 geometric isomers and 2 optical isomers, we calculate:\[2 imes 2 = 4\]

Step 6: Conclusion: Determine the Correct Answer

The compound 2-chloro-4-methylhex-2-ene can have a total of 4 stereoisomers, based on the combination of one geometric and one chiral center.

Key Concepts

Geometric Isomerism

In chemistry, geometric isomerism is a type of stereoisomerism associated primarily with compounds that have a carbon-carbon double bond (C=C). This kind of isomerism occurs because the restricted rotation around the double bond leads to different spatial arrangements of the atoms or groups attached to the carbon atoms. Because the double bond does not allow free rotation, groups can only be in specific positions relative to each other.

Two common geometric isomers are the E (entgegen, opposite) and Z (zusammen, together) configurations:

• The E isomer has the higher priority groups on opposite sides of the double bond.
• The Z isomer has the higher priority groups on the same side of the double bond.

The E/Z naming system helps distinguish the spatial arrangement, which can significantly impact the properties and reactivity of the isomers. Geometric isomerism is evident in the 2-chloro-4-methylhex-2-ene compound because the C2=C3 double bond can exist in either the E or Z configuration.

Optical Isomerism

Optical isomerism, also known as enantiomerism, occurs when a molecule has non-superimposable mirror images, much like left and right hands. This isomerism is all about how the molecule interacts with light. In particular, optical isomers or enantiomers rotate plane-polarized light in different directions.

To have optical isomerism, the compound needs to contain at least one chiral center. A chiral center is a carbon atom that is bonded to four different groups. If a molecule has such a center, it can exist in two forms that are mirror images of each other:

• The R (rectus, right) configuration.
• The S (sinister, left) configuration.

In 2-chloro-4-methylhex-2-ene, if the carbon at position 4 is attached to four different groups, it becomes a chiral center, allowing the existence of R and S isomers.

Chiral Center

A chiral center is a key concept in stereochemistry, as it leads to optical isomerism. Specifically, it refers to a carbon atom bonded to four distinct groups. This unique arrangement allows for two non-superimposable configurations, leading to the formation of enantiomers.

The presence of a chiral center in a molecule is what gives rise to its optical activity. If light is passed through a solution of these molecules, each enantiomer will rotate the light in opposite directions. This can be detected and measured using a polarimeter.

• A chiral molecule with one chiral center can have two possible isomers: R and S configurations.
• The study and manipulation of chiral centers are crucial in fields such as pharmaceuticals, where often only one enantiomer is biologically active.

In our compound, 2-chloro-4-methylhex-2-ene, the carbon at position 4 could potentially be a chiral center, resulting in optical isomerism. Understanding this allows chemists to predict and explain the physical and chemical characteristics of stereoisomers.