Question

Why does the +I effect increases here?

Answer 1

The length of the carbon chain affects the acid strength of a carboxylic acid.

Explanation:

In acids, the electron-releasing `color(red)"inductive effect"` of the alkyl group increases the electron density on oxygen and thus hinders the breaking of the O-H bond, which consequently reduces the ionization. Greater ionization in formic acid when compared to acetic acid makes formic acid (pKa=3.74) stronger than acetic acid (pKa=4.76).

Furthermore, the strength of an acid depends on the tendency to release proton when the acid is dissolved in water.

`RCOOH⇒RCOO^−+H^+`

More negative is the charge on carboxylate ion, the more is its reactivity and more is its basic nature i.e. lesser is acidic nature.

Alkyl groups are electron donating species. Thus, they increase the negative charge on the carboxylate ion (`RCOO^−`) due to +I inductive effect. And hence, acidic strength decreases. Thus, acidic nature is:
`HCOOH>CH_3COOH>C_2H_5COOH>....`