AIIMS 2003 Chemistry Conformational Analysis Assertion Reason Question

To analyze the question regarding the conformation of cis-1,3-dihydroxycyclohexane, we need to evaluate the assertion and reason provided.

Assertion:

Cis-1,3-dihydroxycyclohexane exists in boat conformation.

Reason:

In the chair form, there will not be hydrogen bonding between the two hydroxyl groups.

Explanation:

1. Understanding the Assertion:

   • Cis-1,3-dihydroxycyclohexane has two hydroxyl (-OH) groups located at the 1 and 3 positions on the cyclohexane ring and is in the cis configuration. This means that both hydroxyl groups are on the same side of the cyclohexane ring.
   • Cyclohexane can adopt two primary conformations: chair and boat. The chair conformation is more stable due to minimized steric strain and torsional strain, while the boat conformation is less stable because of steric interactions and torsional strain.

2. Evaluating the Reason:

   • In the chair conformation of cyclohexane, the hydroxyl groups at the 1 and 3 positions would both occupy axial positions if both are on the same side (cis). This arrangement can lead to 1,3-diaxial interactions, which introduce steric hindrance and destabilize the molecule.
   • However, when these hydroxyl groups are positioned in the boat conformation, they can be oriented such that intramolecular hydrogen bonding can occur, which is favorable. In the chair form, there is no opportunity for hydrogen bonding between the two hydroxyl groups due to their axial positioning.

3. Conclusion:

   • The assertion is correct because cis-1,3-dihydroxycyclohexane can exist in the boat conformation to allow for favorable interactions like hydrogen bonding.
   • The reason is also correct because, in the chair form, the hydroxyl groups cannot interact favorably due to their axial orientations, leading to no hydrogen bonding.

Correct Answer:

D) Both Assertion and Reason are correct, and Reason is the correct explanation for Assertion.

Why Other Options are Incorrect:

• Option A (Assertion is true, Reason is false): This option is incorrect because both the assertion and reason are true.
• Option B (Assertion is false, Reason is true): This option is incorrect because the assertion is true; cis-1,3-dihydroxycyclohexane does exist in a boat conformation.
• Option C (Assertion is false, Reason is false): This option is incorrect as both statements are true.

Key Concepts:

• Conformational Analysis: The study of the different shapes (conformations) that a molecule can take and how they can influence the properties of the molecule.
• Hydrogen Bonding: A type of attractive interaction between a hydrogen atom which is covalently bonded to an electronegative atom and another electronegative atom.
• Steric Hindrance: The prevention of reactions due to the size of groups within a molecule.

Summary:

In summary, cis-1,3-dihydroxycyclohexane prefers the boat conformation over the chair conformation due to the potential for hydrogen bonding between hydroxyl groups in the boat form, while the chair form does not allow for such favorable interactions. Thus, both the assertion and reason provided are correct, leading us to the conclusion that option D is the correct answer.