Question

How do we come to know which aldehyde is stronger reducing agent? methanal,ethanal,propanal,butanal

Answer 1

Methanal
See Below

Explanation:

The only difference between these 4 is the length of their alkyl chain. Alkyl groups tend to be electron donating, this is why trimethylamine is a stronger base than ammonia...or why the pKa of acetic acid is higher (10x weaker) than formic acid. Alkyl groups.

The main reactions at carbonyls (like in the aldehyde group) is some sort of nucleophilic substitution at the carbon group, and in order to do this, the carbon has to be partially positive. Carbonyls are already really polarized, and so lots of these reactions are catalyzed by H+, since this will result in protonatioin of the Oxygen and through resonance, the carbonyl carbon becomes a carbocation (C+). In this state, some weak nucleophile like water can attack the carbon...and you get your oxidation.

Therein lies the rub - making the carbon reactive by making it more electrophillic. And that is where the alkyl groups come into play. My guess is that the more electron donating your alkyl groups are (longer, generally), then the more stablized your carbocation will become, and less electrophillic....and a poorer reducing agent.

So I'm gonna guess Methanal is the best reducing agent, since it does not have the alkyl groups to "stabilize" the carbocation by electron donating effects.