Problem 148 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following substances: 1\. HCHO 2\. \(\mathrm{CH}_{3} \mathrm{CHO}\) 3\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{3}\) 4\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{2} \mathrm{CH}_{3}\) Correct order of reactivity towards nucleophillic addition reaction is: (a) \(1>4>2>3\) (b) \(1>2>4>3\) (c) \(1>3>2>4\) (d) \(1>2>3>4\)

Step-by-Step Solution

Answer

The correct order is (d) 1>2>3>4.

Step 1: Identifying Functional Groups

First, identify the functional group in each substance. The functional group in all these compounds is the carbonyl group (C=O), which is present in aldehydes and ketones.

Step 2: Understanding Reactivity

Understand that reactivity towards nucleophilic addition reactions is influenced by the steric hindrance and electronic effects surrounding the carbonyl group. Aldehydes are generally more reactive than ketones due to having fewer bulky groups attached to the carbonyl carbon.

Step 3: Ranking the Compounds

Rank the given compounds based on the number of alkyl groups attached to the carbonyl carbon, as they increase steric hindrance and decrease reactivity: 1. HCHO (formaldehyde) - No alkyl groups provide the least steric hindrance. 2. CH3CHO (acetaldehyde) - One alkyl group. 3. CH3CH2COCH3 (butanone) - Two alkyl groups. 4. CH3CH2COCH2CH3 (pentan-2-one) - Two alkyl groups, and more steric hindrance due to longer chains.

Step 4: Determining the Order

Based on the steric factors, the reactivity order is deduced by stating the least hindered to the most hindered with respect to nucleophilic attack on the carbonyl carbon: 1. HCHO > 2. CH3CHO > 3. CH3CH2COCH3 > 4. CH3CH2COCH2CH3.

Key Concepts

Carbonyl Group Reactivity

The carbonyl group \( \mathrm{C=O} \) is central to aldehydes and ketones, determining their chemical behavior. This group consists of a carbon atom double-bonded to an oxygen atom. The electronegative oxygen pulls electron density away from the carbon, making it partially positive. This electrophilic nature makes the carbon susceptible to attacks by nucleophiles. Nucleophilic addition involves a nucleophile donating an electron pair to form a new bond with the carbonyl carbon. The reactivity of a carbonyl compound in these reactions greatly depends on its structure. Specifically, the factors that influence reactivity include:

• Electronics: The positive charge distribution on the carbon varies with different substituents, affecting how easily it can attract nucleophiles.
• Substituents: Compounds with fewer bulky groups around the carbonyl carbon have less hindrance, making them more reactive in nucleophilic additions.

Understanding these details helps explain the reactivity sequence observed in different carbonyl-containing compounds.

Steric Hindrance

Steric hindrance is a crucial factor influencing the reactivity of carbonyl compounds. It results from the physical presence of groups around the reactive site, which can block or slow down reactions. This is particularly relevant in nucleophilic addition reactions involving carbonyl groups. When large groups surround the carbonyl carbon, the reactivity decreases because the nucleophile has more difficulty reaching the reactive center. In the exercise, formaldehyde (HCHO) serves as a prime example. With no alkyl groups attached to the carbonyl carbon, it offers minimal steric hindrance and thus exhibits high reactivity. In contrast, compounds like butanone (CH₃CH₂COCH₃) and pentan-2-one (CH₃CH₂COCH₂CH₃) have bulky groups that surround the carbonyl. These bulky groups limit access to the carbonyl carbon, significantly reducing reactivity. Therefore, understanding steric hindrance is key to predicting the order of reactivity in nucleophilic addition reactions.

Aldehydes and Ketones

Aldehydes and ketones are both categories of carbonyl-containing compounds but exhibit distinct behaviors due to their structural differences. Both have a carbonyl group, but aldehydes have at least one hydrogen atom attached to the carbonyl carbon, whereas ketones have two alkyl groups. This difference is pivotal when discussing reactivity and steric effects. Characteristics of Aldehydes:

• Aldehydes usually have higher reactivity due to less steric hindrance. An example is acetaldehyde (CH₃CHO), which has only one alkyl group, providing less blockage for nucleophiles approaching the carbonyl.
• The presence of hydrogen makes the carbonyl carbon more accessible for nucleophiles, facilitating the nucleophilic addition reactions.

Characteristics of Ketones:

• Ketones typically have reduced reactivity due to increased steric hindrance from two alkyl groups. This creates a significant barrier for nucleophiles.
• These alkyl groups can also donate electron density to the carbonyl carbon, diminishing its electrophilic character slightly, which again reduces reactivity.

In review, aldehydes tend to be more reactive than ketones toward nucleophilic addition, primarily due to structural factors contributing to steric and electronic effects.