Answer the question on basis of passage given below:

Organic compounds containing carbon -oxygen double bonds (C=O) are called carbonyl compounds. These are highly polar molecules. Hence, their boiling points are higher than hydrocarbons and ethers. Aldehydes can be prepared from dehydrogenation or oxidation of alcohols and by reduction of acylhalides. Ketones are prepared by oxidation of secondary alcohols or by hydration of alkynes etc. Aldehydes and ketones undergo nucleophilic addition reaction with the number of nucleophiles such as HCN, NaHSO, ammonia derivatives. Carboxylic acids are prepared by the oxidation of primary alcohols, aldehydes and alkenes, by hydrolysis of nitriles. Carboxylic acids are more acidic than alcohols and phenols.

Which of the following is most reactive towards nucleophilic addition reaction?

Ethanal

The correct answer is Option 3. Ethanal

Nucleophilic addition reactions involve a nucleophile attacking the electrophilic carbon atom of a carbonyl group \((C=O)\). The carbonyl carbon is electrophilic due to the partial positive charge it carries, created by the polar nature of the \(C=O\) bond.

Factors Affecting Reactivity

Steric hinderance: Larger groups around the carbonyl carbon can hinder the approach of the nucleophile, reducing reactivity.

Electrophilicity: The more electrophilic the carbonyl carbon, the more reactive the compound is toward nucleophiles. Aldehydes tend to be more reactive than ketones because they have a hydrogen atom attached to the carbonyl carbon, leading to less steric hindrance.

Resonance and Inductive Effects: The presence of electron-donating or withdrawing groups can also influence reactivity. Electron-withdrawing groups increase electrophilicity, while electron-donating groups decrease it.

Analysis of Each Compound

1. Butanone (4-Carboxybutan-2-one):

Type: Ketone with a four-carbon chain.

Reactivity: As a ketone, it is less reactive than aldehydes due to steric hindrance from the two alkyl groups. The carbonyl carbon is less electrophilic.

2. Propanone (Acetone):

Type: Ketone with a three-carbon chain.

Reactivity: Similar to butanone, propanone is a ketone and also experiences steric hindrance from the two methyl groups. Therefore, it is less reactive than aldehydes.

3. Ethanal (Acetaldehyde):

Type: Aldehyde with two carbons.

Reactivity: Ethanal has one hydrogen atom attached to the carbonyl carbon, resulting in less steric hindrance and greater electrophilicity. Aldehydes are generally more reactive than ketones because they are less hindered and more prone to nucleophilic attack.

4. Propanal:

Type: Aldehyde with three carbons.

Reactivity: Like ethanal, propanal is also an aldehyde and more reactive than the ketones mentioned. It has slightly more steric hindrance compared to ethanal but is still less hindered than ketones.

Comparing Aldehydes to Ketones

Aldehydes: More reactive than ketones due to having only one alkyl group, making the carbonyl carbon more electrophilic and less sterically hindered.

Ketones: Less reactive than aldehydes because they have two alkyl groups, increasing steric hindrance and decreasing the electrophilicity of the carbonyl carbon.

Conclusion

In this case, between the aldehydes (ethanal and propanal) and ketones (butanone and propanone):

Ethanal is the least hindered and the most reactive due to its smaller size and the fact that it has only one methyl group attached to the carbonyl carbon.

Propanal is slightly less reactive than ethanal but still more reactive than both butanone and propanone.

Thus, the most reactive compound towards nucleophilic addition is: Ethanal.

This reactivity pattern is crucial for understanding the behavior of these compounds in organic chemistry, particularly in synthetic applications where nucleophilic addition is a key mechanism.