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 of reactivity is: (d) 1>2>3>4.

Step 1: Identifying Functional Groups

All of these compounds contain a carbonyl group (C=O) which can undergo nucleophilic addition reactions. The presence of two groups attached to the carbonyl carbon affects the reactivity.

Step 2: Understanding Reactivity Trends

In nucleophilic addition reactions, aldehydes are typically more reactive than ketones. This is because aldehydes have less steric hindrance and electron-donating groups compared to ketones, which makes the carbonyl carbon more electrophilic.

Step 3: Arranging the Compounds

Let's break down the compounds: 1. HCHO is an aldehyde (formaldehyde), 2. CH₃CHO is an aldehyde (acetaldehyde), 3. CH₃CH₂COCH₃ is a ketone (acetone), 4. CH₃CH₂COCH₂CH₃ is a ketone (pentan-3-one). Aldehydes 1 and 2 are generally more reactive than ketones 3 and 4.

Step 4: Comparing Within The Same Class

Among aldehydes: formaldehyde (1) is more reactive than acetaldehyde (2) due to less steric hindrance. Among ketones: acetone (3) might be slightly more reactive than pentan-3-one (4) due to the smaller size of its groups.

Step 5: Final Order of Reactivity

Based on sterics and electronic effects, the order of reactivity is: 1 (most reactive) > 2 > 3 > 4 (least reactive).

Key Concepts

Reactivity of Aldehydes vs Ketones

In nucleophilic addition reactions, aldehydes and ketones both feature a carbonyl group, which is the site where reactions typically occur. However, aldehydes are generally more reactive than ketones. This difference in reactivity is due to several key factors.
Aldehydes have only one substituent attached to the carbonyl carbon, whereas ketones have two. This molecular structure makes aldehydes less crowded, allowing nucleophiles to approach the carbonyl carbon more easily.
Additionally, aldehydes have a hydrogen atom instead of a carbon-based substituent, which means less electron donation to the carbonyl carbon. This increases the electrophilicity of the carbonyl carbon in aldehydes, making them more attractive to nucleophiles, thereby enhancing their reactivity compared to ketones.

Steric Hindrance in Carbonyl Compounds

Steric hindrance plays a critical role in determining the reactivity of carbonyl compounds in nucleophilic addition reactions.
In general, the greater steric bulk around the carbonyl carbon, the less reactive the compound will be. Aldehydes, with only one carbon-containing substituent, face less steric hindrance, making their carbonyl carbons more accessible to nucleophiles.
On the other hand, ketones have two carbon-containing groups that create a larger steric barrier, hindering the approach and attack of nucleophiles. In detail, acetone, with two methyl groups, faces less steric hindrance compared to larger ketones like pentan-3-one, which has bulkier ethyl groups. Therefore, steric factors contribute greatly to why an aldehyde like formaldehyde is often more reactive compared to ketones.

Carbonyl Group Reactivity

The reactivity of the carbonyl group itself is central to understanding nucleophilic addition reactions. The carbonyl carbon, a part of the C=O bond, is slightly positive due to the electronegative oxygen pulling electron density away, making it an attractive target for nucleophiles.
The factors affecting the reactivity of this group include electronic effects and steric hindrance. In aldehydes, the carbonyl carbon remains highly accessible due to less crowding and also remains highly electrophilic because of the presence of only one electron-donating group.
Furthermore, electron-withdrawing groups or the lack of additional electron-donating substituents can make the carbonyl carbon in aldehydes even more reactive as compared to ketones, which is why aldehydes tend to undergo nucleophilic addition reactions more readily.

Aldehyde and Ketone Comparison

When comparing aldehydes and ketones, it becomes evident why aldehydes are typically favored in reactivity. Aldehydes are simpler, often exhibiting higher reactivity toward nucleophilic agents due to less steric hindrance. For example, formaldehyde, with no additional carbon-based substituents, allows for the easiest nucleophilic attack.
In contrast, ketones have more complex substituent groups, creating a wider range of steric interference and a tendency to stabilize the carbonyl carbon, making them less reactive.
Therefore, when arranging compounds in terms of reactivity towards nucleophilic addition, aldehydes such as formaldehyde and acetaldehyde (HCHO and CH₃CHO) will generally come out on top, followed by simpler ketones like acetone before larger, more hindered ketones such as pentan-3-one.