Q.Benzaldehyde gives a positive Tollens' test but a negative Fehling's test. Explain why an aromatic aldehyde behaves this way, while an aliphatic aldehyde such as acetaldehyde gives a positive result with both reagents.

The same ring-conjugation that lowers benzaldehyde's reactivity towards nucleophilic addition (§ reactivity of aldehydes/ketones) also lowers its reactivity towards oxidation: the aromatic $\pi$ system delocalises some of the carbonyl carbon's electrophilic character into the ring, making the carbonyl-carbon hydrogen less easily abstracted by a comparatively mild oxidant. Fehling's reagent (alkaline copper(II)-tartrate) is a genuinely mild oxidant, sufficient to oxidise an ordinary aliphatic aldehyde such as acetaldehyde (giving the brick-red $\text{Cu}_2\text{O}$ precipitate) but not strong enough to oxidise the deactivated carbonyl carbon of benzaldehyde. Tollens' reagent, though also considered a mild oxidant relative to something like $\text{KMnO}_4$, is nonetheless strong enough to still oxidise benzaldehyde, giving a positive silver-mirror result. Acetaldehyde, …

Recognise that 'mild oxidant' is not a single fixed bar -- Tollens' and Fehling's reagents differ in strength, and an aromatic aldehyde's …

Assuming all aldehydes give identical results with Tollens' and Fehling's reagents -- aromatic aldehydes are a genuine, testable exception (positive To …