Introduction
When a molecule has a carbonyl group, it is susceptible to nucleophilic attack. The carbonyl carbon atom in a molecule is a strong electrophile, and it attracts nucleophiles that are negatively charged. In this question, we are asked to identify the most susceptible compound to nucleophilic attack at the carbonyl group.

Explanation

The compounds given are:

1. MeCOCl
2. MeCHO
3. MeCOOMe
4. MeCOOCOMe

Among these, the most susceptible compound to nucleophilic attack at the carbonyl group is MeCHO.

The reason for this is that MeCHO has an aldehyde functional group, which is more reactive than other carbonyl functional groups. The carbonyl carbon in an aldehyde is less hindered than in other carbonyl compounds, which increases the electrophilicity of the carbon atom. This makes the carbonyl carbon more susceptible to nucleophilic attack.

In contrast, MeCOCl and MeCOOMe have an electron-withdrawing group attached to the carbonyl carbon, which decreases the electrophilicity of the carbon atom. This makes these compounds less susceptible to nucleophilic attack.

MeCOOCOMe has an ester functional group, which is less reactive than an aldehyde. The carbonyl carbon in an ester is more hindered than in an aldehyde, which decreases the electrophilicity of the carbon atom. This makes MeCOOCOMe less susceptible to nucleophilic attack than MeCHO.

Conclusion

In conclusion, among the given compounds, MeCHO is the most susceptible to nucleophilic attack at the carbonyl group due to the presence of an aldehyde functional group, which increases the electrophilicity of the carbonyl carbon.

Among aldehydes and acid derivatives, acid chlorides are most susceptible to nucleophilic attack because of strong -I effect and weak +R effect of Cl atom, due to this the carbonyl carbon has highest electron deficiency .