Write structures of various carbocations that can be obtained from $2-$methylbutane. Arrange these carbocations in order of increasing stability.

(N/A) $2-$methylbutane $(CH_3-CH(CH_3)-CH_2-CH_3)$ has four different sets of equivalent $H$ atoms.
Removal of one hydrogen from each set gives four different carbocations:
$(a) (CH_3)_2CH-CH_2-CH_2^+ \text{ (primary } 1^{\circ} \text{ carbocation)}$
$(b) (CH_3)_2C^+-CH_2-CH_3 \text{ (tertiary } 3^{\circ} \text{ carbocation)}$
$(c) CH_3-CH^+-CH(CH_3)_2 \text{ (secondary } 2^{\circ} \text{ carbocation)}$
$(d) ^+CH_2-CH(CH_3)-CH_2-CH_3 \text{ (primary } 1^{\circ} \text{ carbocation)}$
Stability order of carbocations is $3^{\circ} > 2^{\circ} > 1^{\circ}$.
Comparing the two $1^{\circ}$ carbocations $(a)$ and $(d)$,$(a)$ is more stable than $(d)$ because the alkyl group attached to the positively charged carbon in $(a)$ is $(CH_3)_2CH-CH_2-$,which provides a stronger inductive effect than the group in $(d)$.
Thus,the increasing order of stability is: $(d) < (a) < (c) < (b)$.