Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following represent the correct order of nucleophillic addition for (I) HCHO, (II) \(\mathrm{CH}_{3} \mathrm{COCH}_{3^{+}}\) (III) \(\mathrm{CH}_{3} \mathrm{CHO}\), (IV) \(\mathrm{CH}_{3} \mathrm{COC}_{2} \mathrm{H}_{5}\) (a) \(\mid>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) \(\mathrm{I}>\mathrm{III}>\mathrm{II}>\mathrm{IV}\) (c) \(\mathrm{IV}>\mathrm{II}>\mathrm{III}>\mathrm{I}\) (d) \(\mathrm{I}>\mathrm{IV}>\mathrm{III}>\mathrm{II}\)

Step-by-Step Solution

Answer

(d) \\(\mathrm{I} > \mathrm{IV} > \mathrm{III} > \mathrm{II}\\) is correct.

Step 1: Understand the Concept of Nucleophilic Addition

Nucleophilic addition reactions involve the addition of a nucleophile to a carbonyl group. The reactivity in nucleophilic addition often increases with a decrease in the steric hindrance and the electron-donating effect of groups attached to the carbonyl carbon.

Step 2: Rank Based on Steric Hindrance

Compare the molecules based on steric hindrance near the carbonyl carbon. HCHO (I) has the least steric hindrance (only H atoms), hence it will be more reactive. Acetone, (II) \((\mathrm{CH}_{3} \mathrm{COCH}_{3})\), has bulky methyl groups causing more steric hindrance, making it less reactive.

Step 3: Rank Based on Electron Donating Groups

Evaluate the electron-donating effect of the groups. More electron-donating groups make the carbonyl less reactive as an electrophile. HCHO (I) is the most electron-deficient and reactive, followed by \((\mathrm{CH}_{3} \mathrm{CHO})\) in III, due to less electron donation compared to IV and II.

Step 4: Determine Correct Order

Combine the steric and electronic effects: \((\mathrm{I} > \mathrm{III} > \mathrm{IV} > \mathrm{II})\) fits the criteria. HCHO is most reactive due to least steric hindrance and no electron-donating groups, followed by \((\mathrm{CH}_{3}\mathrm{CHO})\), then \((\mathrm{CH}_{3}\mathrm{COC}_{2}\mathrm{H}_{5})\), and least reactive is II.

Key Concepts

Reactivity Order

Understanding the reactivity order in nucleophilic addition is crucial for predicting how different carbonyl compounds will behave in reactions. A nucleophilic addition involves the attack of a nucleophile on the electrophilic carbonyl carbon...
The reactivity of a carbonyl compound depends on various factors, including electronic and steric influences. In a nutshell, the following factors contribute to the reactivity order:

• The degree of electron deficiency at the carbonyl carbon: More electron-deficient the carbonyl carbon, more reactive it will be as an electrophile.
• Steric effects: Less sterically hindered the compound, more accessible the carbonyl carbon will be for a nucleophilic attack.

For example, among the compounds HCHO, extit{Acetone} is least reactive due to both higher steric hindrance and more electron-donating groups compared to formaldehyde (HCHO), which is most reactive due to minimal hindrance and electron deficiency.

Steric Hindrance

Steric hindrance refers to the physical blocking of the reactive site by surrounding groups or atoms. In organic chemistry, it's an important factor when considering how different molecules react.
In the case of nucleophilic addition reactions, the more bulky groups present around the carbonyl carbon, the more difficult it is for a nucleophile to attack. Consider the formaldehyde (HCHO), with no alkyl groups, it represents minimal steric hindrance, allowing nucleophiles to easily target the carbonyl carbon.
On the other hand, acetone, which has two methyl groups (\(\text{CH}_3\)), introduces significant steric hindrance, making nucleophilic attack more challenging. This accessibility directly correlates to their reactivity order, showcasing why smaller or less bulky substituents lead to more reactive carbonyl groups.

Electron Donating Effect

The electron donating effect directly influences the reactivity of the carbonyl group in nucleophilic addition reactions. Electron donating groups can reduce the positive character of the carbonyl carbon.
This decreased electrophilicity makes the carbonyl carbon less attractive to nucleophiles. Here's why:

• Groups like methyl (\(\text{CH}_3\)) can donate electron density via hyperconjugation or inductive effect to the carbonyl carbon, reducing its electrophilicity.
• In formaldehyde (HCHO), lacking electron-donating groups, the carbonyl carbon is significantly electron-deficient, enhancing reactivity.

Contrastingly, in molecules like acetone, the presence of these electron-donating alkyl groups diminishes its carbonyl reactivity compared to a molecule like formaldehyde.