Problem 32 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An optically active alcohol \(A\left(\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}\right)\) absorbs two moles of hydrogen per mole of \(A\) upon catalytic hydrogenation and gives a product \(B\). The compound \(B\) is resistant to oxidation by \(\mathrm{CrO}_{3}\) and does not show any optical activity. Deduce the structures of \(A\) and \(B\).

Step-by-Step Solution

Answer

\(A\) is 3-hexene-2-ol, and \(B\) is 2-hexanol, which loses optical activity.

Step 1: Analyze Absorption of Hydrogen

The information that alcohol \(A\) absorbs two moles of hydrogen means that it likely contains two units of unsaturation (like alkenes or alkynes) that can be reduced to form saturations. This suggests compound \(A\) initially has two double bonds that are converted to single bonds in compound \(B\).

Step 2: Consider Optical Activity

Compound \(A\) is optically active, meaning it has a chiral center. Chiral centers are carbon atoms bonded to four different groups. For \(A\) to lose optical activity when converted to \(B\), this possibly indicates either the destruction of the chiral center or a symmetry reached whereby the optical rotation cancels out.

Step 3: Understand Resistance to Oxidation

Compound \(B\) being resistant to oxidation by \(\text{CrO}_3\) indicates it is a saturated alcohol or possibly a non-reactive ether. Normally, primary and secondary alcohols are oxidizable by \(\text{CrO}_3\). Thus, \(B\) must be a tertiary alcohol or another structure altogether.

Step 4: Deduce Structure of Compound A

Considering the given molecular formula \(\text{C}_6\text{H}_{10}\text{O}\) and the need for two units of unsaturation, a structure that fits is 3-hexene-2-ol, an optically active alcohol with one chiral center at carbon-2.

Step 5: Deduce Structure of Compound B

Upon the hydrogenation of 3-hexene-2-ol, both double bonds are reduced, leading to the hydrogenated product, which is 2-hexanol. Since 2-hexanol can exist as a racemic mixture after reduction, it becomes a non-optically active compound.

Key Concepts

Chiral Center

In organic chemistry, a chiral center is a carbon atom that is connected to four different groups or atoms. This unique configuration makes the molecule non-superimposable on its mirror image, thus exhibiting chirality.
The presence of a chiral center is a key factor in generating optical activity within a compound.
For compound \(A\) in our exercise, it needs to have a chiral center to display optical activity. When compound \(B\) shows no such activity post-reaction, it indicates a change at the chiral center, potentially involving the creation of symmetry or the complete removal of this center. To provide a clearer picture:

• The chiral center plays a crucial role in the observed properties of the molecule.
• Losing optical activity can result from the loss or transformation of the chiral center.

Catalytic Hydrogenation

Catalytic hydrogenation is a common technique used to reduce unsaturation, such as double or triple bonds, in organic compounds. This process involves the addition of hydrogen in the presence of a catalyst, usually a metal like platinum, palladium, or nickel.
In the exercise, compound \(A\) undergoes catalytic hydrogenation, absorbing two moles of hydrogen which suggests the presence of two double bonds, hinting at unsaturation.
This reduction transforms compound \(A\) into \(B\) by converting double bonds into single bonds, making \(B\) saturated. The process results in a decrease in the molecule's reactivity and can also influence its optical properties, such as removing optical activity if it leads to the formation of a racemic mixture.

• During hydrogenation, double bonds are reduced to single bonds.
• Changes in structure can lead to changes in optical activity.

Optical Activity

Optical activity refers to a substance's ability to rotate the plane of polarized light. Chiral molecules often exhibit this property, and it is commonly measured using a polarimeter. Optically active substances typically have chiral centers that are not symmetrical.
For compound \(A\), its optical activity indicates the presence of a chiral center. Post-hydrogenation, the resulting compound \(B\) does not exhibit optical activity.
This difference is likely due to \(B\) either forming a racemic mixture, where the optical rotations cancel each other out, or by losing its chiral center. Therefore, hydrogenation changed the arrangement such that the molecule became symmetrical or balanced. When analyzing optical activity, consider:

• Optically active compounds have chiral centers or enantiomers.
• Symmetrical or racemic mixtures do not show optical activity.

Structural Determination

Structural determination involves identifying the molecular structure using various analytical techniques and intellectual deduction. A deep understanding of the chemical behavior, especially concerning reactivity and stereochemistry, is essential for this process.
In the problem, determining the structure of compound \(A\) involves considering its unsaturation (double bonds) and its chiral nature.
Concluding that \(A\) is 3-hexene-2-ol involved recognizing the presence of two double bonds and a chiral center. Once subjected to hydrogenation, converting \(A\) to \(B\) (2-hexanol), added hydrogen and removed all unsaturations.
Key points in structural determination include:

• Considering all forms of possible bonds and their transformations, such as double bonds becoming single bonds.
• Using knowledge of optical activity and reactivity to deduce chiral centers and possible molecular symmetries.
• Recognizing that changes in optical activity, like moving from an optically active to an inactive state, can hint at structural changes.