Animated Solution for Chemistry - Organic Chemistry: The increasing order of the reactivity of the following compounds in nucleophilic addition reaction is : Propanal, Benzaldehyde, Propanone, Butanone

Nucleophilic Addition

Welcome to a classic organic chemistry problem! Today, we are exploring the reactivity of carbonyl compounds towards nucleophilic addition. In these reactions, a nucleophile (an electron-rich species) attacks the electrophilic carbonyl carbon. The reactivity of the carbonyl compound depends entirely on how 'inviting' that carbon is to the nucleophile.

Two major factors govern this invitation: Steric Hindrance and Electronic Effects (Inductive and Resonance effects).

Ketones: The Golden Rule

The fundamental rule of thumb is that aldehydes are generally more reactive than ketones.

Why? Let's look at the structure. A ketone has two alkyl groups attached to the carbonyl carbon, while an aldehyde has only one alkyl group and one small hydrogen atom.

1. Steric Effect: Two bulky alkyl groups in a ketone physically block the incoming nucleophile, making the attack much harder. 2. Inductive Effect: Alkyl groups are electron-donating via the $+I$ effect. Two alkyl groups in a ketone pump more electron density into the carbonyl carbon, reducing its partial positive charge and making it less electrophilic.

Propanone vs. Butanone

Let's pit our two ketones against each other: Propanone ($C{H}_{3}COC{H}_3$) and Butanone ($C{H}_{3}COC{H}_{2}C{H}_3$).

Propanone has two methyl groups. Butanone has one methyl and one ethyl group. The ethyl group is significantly bulkier than a methyl group, increasing steric hindrance. Furthermore, the ethyl group has a stronger $+I$ effect, which further neutralizes the electrophilicity of the carbonyl carbon.

Therefore, Butanone is less reactive than Propanone.

Propanal vs. Benzaldehyde

Now, let's evaluate our aldehydes: Propanal ($C{H}_{3}C{H}_{2}CHO$) and Benzaldehyde ($PhCHO$).

Propanal is a standard aliphatic aldehyde. Benzaldehyde, however, is an aromatic aldehyde. In Benzaldehyde, the carbonyl group is directly attached to a benzene ring. The $\pi$-electrons of the benzene ring are in conjugation with the carbonyl group. Through the $+R$ (resonance) effect, the ring donates electron density to the carbonyl carbon.

This resonance stabilization significantly reduces the electrophilicity of the carbonyl carbon. Consequently, Benzaldehyde is less reactive than Propanal.

We know that ketones are less reactive than aldehydes. Among the ketones, Butanone is the least reactive. Among the aldehydes, Benzaldehyde is less reactive than Propanal.

It is crucial to note that despite the resonance stabilization in Benzaldehyde, it is still generally more reactive than aliphatic ketones because it only has one bulky group attached to the carbonyl carbon, whereas ketones have two.

Putting it all together, the increasing order of reactivity is:

This perfectly matches option (b).