Question

p-Nitrobenzaldehyde is more reactive toward nucleophilic additions than p-methoxybenzaldehyde. Explain.

Short answer

In nucleophilic addition reaction, the nucleophile$\left({\text{Nu}}^{\text{-}}\right)$adds to the substrate, which is electron deficient. More is the “electron deficiency” on the substrate more is the reactivity of the nucleophile towards the substrate.

Aldehyde is more reactive toward nucleophilic addition reaction than ketone due to less electron density in the carbonyl carbon of aldehyde.

Step-by-step solution

Step 1:Reactivity of Nucleophilic addition Reaction 

In nucleophilic addition reaction, the nucleophile$\left({\text{Nu}}^{\text{-}}\right)$adds to the substrate, which is electron deficient. More is the “electron deficiency” on the substrate more is the reactivity of the nucleophile towards the substrate.

Aldehyde is more reactive toward nucleophilic addition reaction than ketone due to less electron density in the carbonyl carbon of aldehyde.

More Reactivity of p-nitro benzaldehyde than p-methoxy benzaldehyde toward Nucleophilic addition reaction

In the nucleophilic addition reaction, the upcoming nucleophile is attracted more toward the compound, having more electron deficiency (more positive charge) due to its affinity toward positive charge species.

Compound p-Nitrobenzaldehyde has the electron-withdrawing group nitro $\left({\text{-NO}}_{\text{2}}\right)$attached to the para position. It will pull the electron density toward it, making the carbonyl group of the p-Nitrobenzaldehyde more positive or nucleophile, resulting in a fast attack of the upcoming nucleophile.

Compound p-methoxy benzaldehydehas the electron-donating group methoxyattached $\left({\text{-OCH}}_3\right)$to the para position. It will donate the electron density toward it, making the carbonyl group of p-methoxy benzaldehydethe more negative or electrophile, resulting in a slow attack of the upcoming nucleophile.

More reactivity of p-Nitrobenzaldehyde than p-methoxy benzaldehyde