Problem 226 - A Complete Resource Book in Chemistry for JEE Main

Question

The increasing order of the rate of HCN addition to compounds \(\mathrm{A}-\mathrm{D}\) is (a) HCHO (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{PhCOCH}_{3}\) (d) PhCOPh (a) \(a

Step-by-Step Solution

Answer

Option (c): d < c < b < a.

Step 1: Identify the Compounds

We are given four compounds: HCHO (formaldehyde), CH\(_3\)COCH\(_3\) (acetone), PhCOCH\(_3\) (acetophenone), and PhCOPh (benzophenone). These display different reactivities towards HCN addition.

Step 2: Understand Mechanism of HCN Addition

HCN addition is a nucleophilic addition reaction where the cyanide ion (CN\(^-\)) attacks the carbonyl carbon. The ease of this reaction depends on the electrophilicity of the carbonyl carbon, which in turn depends on the nature of the substituents.

Step 3: Evaluate Electrophilicity of Carbonyl Carbon

HCHO has no substituents that donate electron density to the carbonyl carbon, making it the most electrophilic. In contrast, PhCOPh has two phenyl groups that donate electron density through resonance, decreasing electrophilicity. Acetone and acetophenone fall between these extremes.

Step 4: Increase in Order of Rate

Based on electrophilicity, the order of the increasing rate of HCN addition is determined by decreasing electrophilicity of the carbonyl carbons: d (PhCOPh) < c (PhCOCH\(_3\)) < b (CH\(_3\)COCH\(_3\)) < a (HCHO).

Step 5: Match with Given Options

Comparing our order with the options, we match it to option (c): d < c < b < a.

Key Concepts

Nucleophilic Addition

In chemistry, nucleophilic addition is a type of reaction where a nucleophile, which is a molecule or ion with a pair of electrons, seeks out a positive or partially positive center to bond with. In the case of the HCN addition, the nucleophile is the cyanide ion (\(\text{CN}^{-}\)), which is attracted to the carbon of the carbonyl group. This reaction is common among carbonyl compounds.

• Typically involves a nucleophile approaching a carbonyl carbon.
• Results in a conversion of a double bond in a carbonyl group into a single bond.

Understanding the basic mechanism helps simplify larger reactions involving carbonyl compounds. As the nucleophile attaches to the positive site, such reactions are foundational in organic synthesis. Breaking down this mechanism can demystify many organic processes.

Electrophilicity

Electrophilicity refers to the tendency of a chemical species to attract electrons. In the context of nucleophilic addition reactions like the one with HCN, the electrophilicity of the carbonyl carbon is a key factor in determining the rate of reaction. A carbonyl carbon is more electrophilic when it lacks surrounding electron-rich groups.

• Alkyl groups can donate electron density, lowering electrophilicity.
• Phenyl groups contribute electron density through resonance, further lowering electrophilicity.

This concept is crucial because the carbonyl carbon's electrophilicity directly impacts how easily a nucleophile, like the cyanide ion, can react with it. More electronegative substituents or fewer electron-donating substituents increase the electrophilic nature of carbonyl carbons.

Carbonyl Compounds

Carbonyl compounds contain a carbon-oxygen double bond, known as a carbonyl group. This group is highly reactive due to the partial positive charge on the carbon, making it susceptible to nucleophilic attack.

• Examples include aldehydes, ketones, carboxylic acids, and esters.
• Aldehydes generally possess greater electrophilicity than ketones due to lesser electron-donating substituents.

In HCN addition, this reactivity is harnessed as the nucleophile attacks the electrophilic carbonyl carbon, leading to significant changes in the molecule's structure. Understanding the nature of the carbonyl group aids in predicting and explaining the behavior of many organic compounds.

Reactivity Order

Reactivity order in nucleophilic addition reactions is determined by the ease of nucleophilic attack, which primarily depends on the electrophilicity of the carbonyl carbon. In simpler terms, compounds with more electrophilic carbonyl carbons are generally more reactive.

• HCHO (formaldehyde) is highly reactive due to no substituents neutralizing its positive charge.
• Acetone and acetophenone fall in the middle, with differing levels of reactivity due to their substituents.
• Benzophenone (PhCOPh) is less reactive because phenyl rings decrease electrophilicity with resonance donation.

Arranging compounds by their electrophilicity helps determine their reactivity in nucleophilic addition processes. In the exercise, correctly ordering these compounds is key to understanding their respective reactivities.