The correct order of decreasing reactivity towards nucleophilic addition of the following compounds are:

\(HCHO, C_6H_5CHO, CH_3COCH_3, C_2H_5 COC_2H_5\)

\(HCHO > C_6H_5CHO > CH_3COCH_3 > C_2H_5 COC_2H_5\)

The correct answer is option 2. \(HCHO > C_6H_5CHO > CH_3COCH_3 > C_2H_5 COC_2H_5\).

In nucleophilic addition reactions, a nucleophile (electron-rich species) reacts with a molecule containing a carbonyl group \((C=O)\), which is electron-deficient. The key factor influencing the reactivity of the carbonyl group is the ease of access and positive character of the carbonyl carbon. Here's a breakdown of the factors affecting reactivity in the given compounds:

Steric Hindrance:

• Aldehydes \((RCHO)\) generally have a hydrogen atom \((H)\) bonded to the carbonyl carbon, creating minimal steric hindrance. This allows the nucleophile to approach the carbonyl carbon more easily and form a bond.

• Ketones \((RCOR’)\) have two bulky alkyl groups \((R)\) attached to the carbonyl carbon, causing steric hindrance. The bulkier the groups, the harder it is for the nucleophile to access the carbonyl carbon and react.

Electronic Effects (minor influence in this case):

• In this specific case, the effect is minimal. However, the phenyl group \((C_6H_5)\) in benzaldehyde \((C_6H_5CHO)\) can slightly withdraw electrons from the carbonyl carbon through resonance. This withdrawal makes the carbon slightly less positive, reducing its attraction towards the nucleophile.

Applying these concepts to the compounds:

1. \(HCHO\) (Formaldehyde): Being an aldehyde with only a hydrogen atom, it has minimal steric hindrance and the most reactive carbonyl carbon.

2. \(C_6H_5CHO\) (Benzaldehyde): While an aldehyde, the presence of the bulky phenyl group introduces some steric hindrance, making it slightly less reactive than \(HCHO\). The electron-withdrawing effect of the phenyl group is a minor contributor to its lower reactivity.

3. \(CH_3COCH_3\) (Acetone): This ketone has two methyl \((CH_3)\) groups causing steric hindrance around the carbonyl carbon, hindering nucleophilic attack compared to the aldehydes.

4. \(C_2H_5COC_2H_5\): Similar to acetone, this ketone has two ethyl \((C_2H_5)\) groups creating steric hindrance, making it the least reactive in the list.

Therefore, the order of decreasing reactivity towards nucleophilic addition is \(HCHO > C_6H_5CHO > CH_3COCH_3 > C_2H_5 COC_2H_5\).