'Stability of carbocations depends upon the electron releasing inductive effect of groups adjacent to positively charged atom involvement of neighbouring groups in hyperconjugation and resonace''.
Write structure of various carbocations that can be obtained from 2-methylbutane. Arrange these carbocations in order of increasing stability

Explanation

2-Methylbutane has four different sets of equivalent hydrogen atoms marked as a, b, c and d.
`{:(overset(a)(C)H_(3)-overset(b)(C)H-overset(c)(C)H_(2)-overset(d)(C)H_(3)),(" |"),(" "underset(a)(C)H_(3)):}`
Removal of one hydrogen form any of these equivalent sets of hydrogens gives four different carbocations I, II, III and IV as shown below :

Now stability of carbocations decreases in the order : `3^(@) gt 2^(@) gt 1^(@)`. Since III being the `3^(@)` carbocation, it is the most stable. This is followed by carbocation (II) which is `2^(@)`. Out of the other two `1^(@)` carbocations, carbocation (IV) has an electron-donating `CH_(3)` group at `alpha`-carbon while carbocation (I) has a `CH_(3)` group at `beta`-carbon. Since +I effect decreases with distance, therefore, carbocation (IV) is more stable than carbocation (I). Thus, the overall stability of these four carbocations increases in the order : `I lt IV lt II lt III`.