Question

Which is good nucleophile HCOOH OR SO3H

Answer 1

Carboxylic acids can act as nucleophiles of course. Consider reactions with isobutene or thionyl chloride (although base certainly helps in this latter case). However, whether it's in SN2
S
N
2
or SN1
S
N
1
reaction mechanisms is another matter. If you beast something with enough heat it will usually react. I would not be surprised if a carboxylic acid reacted with triflouromethanesulfonic anhydride without base. It's still SN2
S
N
2
as mechanistic studies have shown the S=O bonds do not open up like acyl electrophiles. I suspect you would need to resort to a continuous as opposed to a batch approach to push a reaction between the acid and an alkyl halide without using any base. Expose the reaction to high temperature (I'm thinking 250C and higher) for 5 seconds or less under continuous liquid flow.