Question

Distinguish between $C{H}_{3}C{H}_{2}COOH$ (propionic acid) and HCOOH (methanoic acid).

Answer 1

Formic acid is unique amongst simple carboxylic acids because it contains a hydrogen atom instead of an alkyl group (H-COOH, all other carboxylic acid contains an alkyl group attached to C instead of hydrogen on left side) thus formic acid may be regarded both as an aldehyde as well as a carboxylic acid. Like aldehydes it can be easily oxidised to carbonic acid $\left(H_{2}C{O}_3\right)$ which decomposes to give $C{O}_{2}and{H}_{2}O$.
$HCOOH+\left[O\right]\to \left[H_{2}C{O}_3\right]\to C{O}_2+H_{2}O$
Thus like aldehydes, it reduces Fehling's solution to red ppt. of $C{u}_{2}O$ Tollen's reagent to a silver mirror.
Proponoic acid, on the other hand, has alkyl group attached to it and hence doesn't behave like aldehydes. They, therefore fail Tollens and Fehlings test.