Question

which is the most reactive acid among the carboxylic acids?? and which is the least reactive one?

Answer 1

The stability of any type of carboxylic acid derivative is generally determined by the ability of its functional group to donate electrons to the rest of the molecule. This means that the more electronegative the atom or group attached to carbonyl group, the less stable the molecule.

This readily explains the fact that the acyl halides are the most reactive, because halides are generally quite electronegative. It also explains why acid anhydrides are unstable; with two carbonyl groups so close together the oxygen in between them cannot stabilize both by resonance - it can't loan electrons to both carbonyls.
The following derivative types are ordered in decreasing reactivity (the first is the most reactive):
Acyl Halides (CO-X) > Acyl Anhydrides (-CO-O-OCR) > Acyl Thioester (-CO-SR) > Acyl Esters (-CO-OR) > Acyl Amides (-CO-NR2)

Any substance in the preceding list can be readily transformed into a substance to its right; that is, the more reactive derivative types (acyl halides) can be directly transformed into less reactive derivative types (esters and amides). Every type can be made directly from carboxylic acid (hence the name of this subsection) but carboxylic acid can also be made from any of these types.

Answer 2

Hope this suffices..........☺