Question

Which chloro denvative of hexane on reaction with alcoholic KOH gives products which are only diastereomeric pair of each other? (A) 1-Chlorohexane (B) 2-Chlorohexane (C) 3-Chlorohexane (D) No such compound exist

Short answer

The chloro-derivative of hexane that reacts with alcoholic KOH to give products that are only a diastereomeric pair of each other is (B) 2-Chlorohexane. The reaction involves an E2 elimination, and the two resulting alkenes are 1-hexene and 2-hexene, which form a diastereomeric pair.

Step-by-step solution

Identify the structure of the given compounds

Let's first draw the structure of given chloro-derivatives of hexane. (A) 1-Chlorohexane: CH3-CH2-CH2-CH2-CH2-CH2-Cl (B) 2-Chlorohexane: CH3-CH(Cl)-CH2-CH2-CH2-CH3 (C) 3-Chlorohexane: CH3-CH2-CH(Cl)-CH2-CH2-CH3 (D) No such compound exist

Reaction with alcoholic KOH

Now, let's study the reaction of these compounds with alcoholic KOH, which is an E2 elimination reaction. The elimination reaction involves the removal of a halogen and an adjacent hydrogen atom, forming an alkene. (A) 1-Chlorohexane: No possible diastereomeric pairs (B) 2-Chlorohexane: Formation of diastereomeric pair of alkenes (C) 3-Chlorohexane: No possible diastereomeric pairs

Analyzing possible diastereomeric pairs

Diastereomers are stereoisomers that are not mirror images of each other. In this case, we must analyze the products formed during the E2 elimination reaction and compare them to see if they are diastereomeric pairs. (A) 1-Chlorohexane: No possible diastereomeric pairs, as there is only one alkene product possible: 1-hexene (B) 2-Chlorohexane: Formation of diastereomeric pair of alkenes - 1-hexene, and 2-hexene, which is a pair of diastereomers. (C) 3-Chlorohexane: No possible diastereomeric pairs, as there is only one alkene product possible: 2-hexene

Conclusion

Based on our step-by-step analysis, the correct answer is (B) 2-Chlorohexane, which on reaction with alcoholic KOH gives products that are only diastereomeric pair of each other.

Key concepts

1-Chlorohexane

1-Chlorohexane is a linear alkyl halide with the chemical formula: CH\(_3\)-CH\(_2\)-CH\(_2\)-CH\(_2\)-CH\(_2\)-CH\(_2\)Cl. This compound has the chlorine atom attached to the first carbon of the hexane chain. When subjected to a reaction with alcoholic potassium hydroxide (KOH), an E2 elimination reaction occurs.
E2 reactions result in the removal of the chlorine atom and an adjacent hydrogen atom, leading to the formation of an alkene. In the case of 1-Chlorohexane, the elimination can only produce 1-hexene. This is because both possible hydrogens for elimination are at the terminal positions, resulting in the same alkene.
As a result, no stereochemical isomers, like diastereomers, are formed from 1-Chlorohexane in this reaction. This is because there are no structural configurations to yield such isomeric diversity from a simple linear chain with terminal reaction sites.

2-Chlorohexane

2-Chlorohexane is a secondary alkyl halide represented by the formula: CH\(_3\)-CH(Cl)-CH\(_2\)-CH\(_2\)-CH\(_2\)-CH\(_3\). Here, the chlorine atom is bound to the second carbon, creating a potential for more complex reaction outcomes. When it undergoes an E2 elimination reaction with alcoholic KOH, different alkenes can be produced.
The elimination can remove the chlorine and a hydrogen from either the first or third carbon, leading to the formation of different alkenes: 1-hexene and 2-hexene. These products illustrate stereoisomerism, as they have different configurations and are not mirror images, creating diastereomers.

• 1-Hexene: A linear alkene, resulting from hydrogen removal from the first carbon.
• 2-Hexene: Comes in two configurations, cis and trans, leading to a diastereomeric pair.

Thus, 2-Chlorohexane uniquely forms products that are diastereomeric pairs during an E2 reaction, unlike its chloro-hexane counterparts.

3-Chlorohexane

3-Chlorohexane is characterized by its structure: CH\(_3\)-CH\(_2\)-CH(Cl)-CH\(_2\)-CH\(_2\)-CH\(_3\), with the chlorine atom attached to the third carbon. In an E2 elimination reaction with alcoholic KOH, similar to other chloroalkanes, the chlorine and an adjacent hydrogen are eliminated to form alkenes.
The potential hydrogens that can be removed are on the second and fourth carbons, leading primarily to the formation of 2-hexene. Despite this, there are no alternative configurations that result in diastereomers here.
This is because the reaction sites still allow the formation of linear or non-diastereomeric configurations, producing just one significant product, 2-hexene. Therefore, unlike 2-Chlorohexane, 3-Chlorohexane doesn't yield diastereomeric pairs when it reacts under similar conditions.