Question

How do we come to know much aldehyde is stronger reducing agents-Methanol,ethanol,propanal,butanal

Answer 1

In metal hydrides reductions the subsequent alkoxide salts are insoluble and should be hydrolyzed (with care) before the alcohol product can be isolated. In the sodium borohydride reduction, the methanol solvent system accomplishes this hydrolysis naturally. In the lithium aluminum hydride decrease, water is generally included in a subsequent advance. The lithium, sodium, boron, and aluminum end up as solvent inorganic salts toward the finish of one or the other response.

Complete step by step answer:
The lone contrast between these $4$ is the length of their alkyl chain. Alkyl groups will in general be electron donating, this is the reason trimethylamine is a stronger base than ammonia or why the $pKa$ of acidic acid is higher than formic acid. Alkyl groups.
The fundamental reactions at carbonyls (like in the aldehyde groups) is some kind of nucleophilic substitution at the carbon group, and to do this, the carbon must be part of the way positive. Carbonyls are now truly energized, thus lots of these reactions are catalyzed by H+ since this will result in protonation of the Oxygen, and through resonance, the carbonyl carbon turns into a carbocation (${C^ + }$). In this expression, some weak nucleophile like water can assault the carbon...and you get your oxidation.
Thusly the main issue is brought to light - making the carbon receptive by making it more electrophilic. Furthermore, that is the place where the alkyl bunches become possibly the most important factor. My speculation is that the more electron-donating your alkyl bunches are (longer, for the most part), at that point, the more stabilized your carbocation will become, and less electrophilic and a less fortunate reducing agent.
So I'm going to figure Methanal is the best reducing agent, since it doesn't have the alkyl groups to "balance out" the carbocation by electron-donating effects.

Note: Strong reducing agents effectively lose (or give) electrons. An atom with a moderately enormous atomic tends will in general be a superior reducant. In such species, the separation from the nucleus to the valence electrons is long to the point that these electrons are not unequivocally attracted