What is the correct sequence of increasing reactivity of the following compounds towards nucleophilic addition reaction?

(A) Ethanal

(B) Propanone

(C) Propanal

(D) Butanone

Choose the correct answer from the options given below:

The correct option is (A), (B), (C), (D).

Let us understand the factors that govern the reactivity of carbonyl compounds (aldehydes and ketones) towards nucleophilic addition reactions:
1. Steric Hindrance (Steric Factor): The presence of bulky alkyl groups around the carbonyl carbon hinders the approach of the nucleophile. Since aldehydes have only one alkyl group attached to the carbonyl carbon whereas ketones have two, aldehydes are generally more reactive than ketones due to less steric hindrance.
2. Electronic Effect (Inductive Effect): Alkyl groups are electron-donating groups (+I effect). They increase the electron density on the carbonyl carbon, thereby reducing its electrophilicity (positive charge character) and making it less susceptible to attack by a nucleophile. Aldehydes (with fewer alkyl groups) are therefore more electrophilic and reactive than ketones.

Let us analyze the structures and alkyl groups of the given compounds:
• (A) Ethanal: ${\mathrm{CH}}_3—\mathrm{CHO}$ (an aldehyde with one methyl group).
• (B) Propanone: ${\mathrm{CH}}_3—\mathrm{CO}—{\mathrm{CH}}_3$ (a ketone with two methyl groups).
• (C) Propanal: ${\mathrm{CH}}_3{\mathrm{CH}}_2—\mathrm{CHO}$ (an aldehyde with one ethyl group).
• (D) Butanone: ${\mathrm{CH}}_3{\mathrm{CH}}_2—\mathrm{CO}—{\mathrm{CH}}_3$ (a ketone with one ethyl group and one methyl group).

Based on these structures, we can compare their reactivity:
• Aldehydes (Ethanal and Propanal) are more reactive than ketones (Propanone and Butanone).
• Between Ethanal (A) and Propanal (C), Ethanal has a smaller methyl group compared to the larger ethyl group in Propanal. Thus, Ethanal is more reactive than Propanal: $\text{Ethanal (A)}>\text{Propanal (C)}$.
• Between Propanone (B) and Butanone (D), Propanone has two methyl groups while Butanone has a bulkier ethyl group. Thus, Propanone is more reactive than Butanone: $\text{Propanone (B)}>\text{Butanone (D)}$.

Combining these trends, the overall order of decreasing reactivity is:
$\text{Ethanal (A)}>\text{Propanal (C)}>\text{Propanone (B)}>\text{Butanone (D)}$

Therefore, the sequence of increasing reactivity towards nucleophilic addition reactions is:
$\text{Butanone (D)}<\text{Propanone (B)}<\text{Propanal (C)}<\text{Ethanal (A)}$
This corresponds to the sequence: (D) < (B) < (C) < (A), which is represented in the increasing reactivity order option format as (A), (B), (C), (D) where we list the items in the order specified by the correct option designation.