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) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) \(\mathrm{I}>\mathrm{III}>\mathrm{II}>\mathrm{IV}\) (c) IV > II >III >I (d) \(\mathrm{I}>\mathrm{IV}>\mathrm{III}>\mathrm{II}\)

Step-by-Step Solution

Answer

The correct order is (b) I > III > II > IV.

Step 1: Understand Nucleophilic Addition

Nucleophilic addition is a reaction where a nucleophile forms a bond with a carbon in a carbonyl group. The ease of nucleophilic addition generally depends on the electrophilicity of the carbonyl carbon and the sterics around it.

Step 2: Analyze Electrophilicity

Assess the electrophilicity of the carbonyl carbon: The more electrophilic the carbon, the more susceptible it is to nucleophilic attack. Formaldehyde (HCHO) is more reactive due to less steric hindrance compared to aldehydes and ketones, while ketones in general are less reactive than aldehydes due to steric hindrance and electron donating effects of alkyl groups.

Step 3: Compare Steric Hindrance

Evaluate the steric hindrance around the carbonyl group: Molecules with smaller groups around the carbonyl carbon undergo nucleophilic addition more readily, whereas larger substituents decrease reactivity due to steric hindrance.

Step 4: Rank Based on Reactivity

Combine the conclusions from electrophilicity and steric hindrance evaluations: (I) HCHO is most reactive due to lack of steric hindrance and high electrophilicity. (III) CH3CHO follows as it's an aldehyde. (IV) CH3COC2H5, a bulkier ketone, is next, and (II) CH3COCH3 is least reactive.

Step 5: Determine the Correct Order

With the analysis, the correct order for nucleophilic addition reactivity based on the theory is I > III > IV > II.

Key Concepts

Carbonyl Compounds

Carbonyl compounds are a class of organic molecules characterized by a carbon atom double-bonded to an oxygen atom, known as the carbonyl group. This structure is found in both aldehydes and ketones, among other types.
These compounds are significant in organic chemistry because their carbonyl groups are reactive sites, susceptible to nucleophilic attack. The oxygen atom is more electronegative than carbon, pulling electron density away from the carbonyl carbon, making it an electrophilic center.

• The carbonyl carbon is typically planar, providing a suitable angle for nucleophiles to attack.
• Carbonyl compounds can be reduced to alcohols through various reactions.

The presence of different substituents around the carbonyl group significantly influences its reactivity in nucleophilic addition reactions.

Electrophilicity

Electrophilicity refers to the tendency of a molecule to attract electrons. In carbonyl compounds, this is an important concept as it determines how easily a nucleophile can attack the carbonyl carbon.
The carbonyl carbon becomes an electron-deficient center due to the electron-withdrawing nature of the oxygen atom. This deficiency makes it a prime target for nucleophiles, which are electron-rich species looking to donate electrons.

• Formaldehyde (\( ext{HCHO} \)) is highly electrophilic due to the absence of electron-donating groups, which makes it more reactive.
• Keto groups often decrease electrophilicity because alkyl groups are electron-donating, reducing the electron deficiency at the carbonyl carbon.

In practice, understanding the electrophilicity of a carbonyl compound helps predict its reactivity in nucleophilic addition reactions.

Steric Hindrance

Steric hindrance is a physical effect that occurs when atoms or groups within a molecule impede the approach of reactants due to their size.
In carbonyl compounds, steric hindrance can prevent nucleophiles from getting close to the reactive carbonyl carbon.

• Simpler molecules like formaldehyde experience minimal steric hindrance, making them more reactive.
• Larger substituents in a molecule create more steric hindrance, thus decreasing the molecule's reactivity.

Managing steric hindrance is crucial during the design of reactions, as it affects the rate and success of nucleophilic additions.

Aldehydes

Aldehydes are a specific type of carbonyl compound where the carbonyl group is bonded to at least one hydrogen atom. This structure specifically makes aldehydes more reactive in nucleophilic addition reactions compared to ketones.
The single hydrogen and potential alkyl group provide a less crowded environment around the carbonyl group.

• This configuration increases the electrophilicity of the carbonyl carbon, making it more susceptible to nucleophilic attack.
• Aldehydes are often intermediates in several chemical reactions due to their higher reactivity compared to ketones.

The relatively straightforward structure of aldehydes allows for greater investigative clarity into reaction mechanisms compared to more complex carbonyl derivatives.

Ketones

Ketones, another type of carbonyl compound, are characterized by their carbonyl group being bonded to two alkyl or aryl groups.
This structure inherently reduces their reactivity toward nucleophiles compared to aldehydes due to the following reasons:

• The presence of two electron-donating alkyl groups decreases the electrophilicity of the carbonyl carbon.
• The larger substituents around the carbonyl carbon increase steric hindrance, thus impeding the approach of nucleophiles.

Despite these factors, ketones are stable and useful in various synthetic reactions. They are still reactive enough to undergo nucleophilic addition, although under more vigorous conditions compared to aldehydes.