Which of the following compounds is most reactive towards nucleophilic addition reactions?

1. 
   
2. 
   
3. 
   
4. 
   

The correct option is A

$\text{Reactivity of Carbonyl Compounds in Nucleophilic addition reaction}$

$reactivityofcarbonylcompound$
$\propto electrophilicityofcarbonylcarbon$

$reactivityofcarbonylcompound$
$\propto \frac{1}{sterichinderanceatcarbonylcarbon}$

$\text{Analyzing the options}$

Option (A)

Acetadehyde is most reactive towards nucleophilic addition reaction among all given compounds because it has only one methyl group, although it shows $+H$ and $+I$ effect but do not decrease the partial positive charge on carbonyl carbon to a large extent and making it the most reactive.


Option (B)

Among aldehydes and ketones, aldehydes are more reactive towards nucleophilic addition reactions because the electron donating nature of alkyl groups decreases the positive charge on ketones to a large extent.

So, propanone is less reactive than acetaldehyde. Also, ketone is sterically more hindered which decreases the ease of nucleophilic attack.

Option (C)

Benzaldehyde contains benzene ring attach to carbonyl carbon. Also, benzene ring exhibits $+R$ effect which increases the electron density upon the carbonyl carbon and decreases the partial positive charge. Thus, decreases reactivity of benzaldehyde towards nucleophilic addition reaction.



Option (D)

Among aldehydes and ketones, aldehydes are more reactive towards nucleophilic addition reactions because the donating nature of alkyl/aryl group decreases the positive charge in ketones to a large extent. Also, benzene ring exhibits $+R$ effect and thus decreases reactivity of acetophenone towards nucleophilic addition reaction.


$\text{Reactivity Order}$

Overall reactivity order for the given compounds will be:


Hence, The correct answer is option $A.$