# Reactivity of alkyl halides

I am unable to understand how to decide reactivity of alkyl halides based on inductive effect in a nucleophilic substitution reaction. I have 2 explanation for deciding. Consider 2 alkyl halides:

• (A) 2-chloro-2-methylbutane
• (B) 2-chloro-3-methylbutane

First explanation:

In (A) we have a $$3^\circ$$ carbon so if we break the $$\ce{C--Cl}$$ bond a $$3^\circ$$ carbocation will be formed, which is stabilized by +Inductive effect of surrounding groups. Therefore we can say that as breaking of $$\ce{C--X}$$ bond in (A) leads to the formation of a more stable carbocation than that formed in (B){$$2^\circ$$ carbocation}. And due to this the reactivity will be more of (A).

Second explanation:

As in (A) $$3^\circ$$ carbocation is formed, it will get more stabilized than $$2^\circ$$ carbocation formed in (B). So this means that $$2^\circ$$ carbocation in (B) becomes more positive than $$3^\circ$$ carbocation in (A), making it easier for a nucleophile to attack on (B)'s carbocation. This would make (B) more reactive.

Which one of them is correct?

Assuming your concern is on $$\mathrm{S_N1}$$ type nucleophilic substitution reactions, you are in the correct path with your first explanation. However, everything has gone south from that point on. As you found out, the stability of carbocations in $$\mathrm{S_N1}$$ reactions are: $$3^\circ \gt 2^\circ \gt 1^\circ$$. However, reaction rates of $$\mathrm{S_N1}$$ reactions do not depend on electron density (or lack of it) on carbocation, because rate determine step is rate of carbocation formation step:
As depicted in the diagram, $$3^\circ$$-carbocation from 2-chloro-2-methylbutane forms faster than $$2^\circ$$-carbocation from 2-chloro-3-methylbutane because the stability of $$3^\circ$$-carbocation is much larger than that of $$2^\circ$$-carbocation as you predicted in your first explanation. Also, you need to understand that in these reactions, 1,2-hydride (or methide) transfer to give more stable $$3^\circ$$-carbocation from $$2^\circ$$-carbocation is possible if there are available $$\alpha$$-hydrogen or $$\alpha$$-methyl group in the sought $$2^\circ$$-carbocation, alike in the case of 2-chloro-3-methylbutane here. Regardless of this rearrangement, the rate of carbocation formation of 2-chloro-2-methylbutane is still faster than that of 2-chloro-3-methylbutane, because it is strictly the stability of $$3^\circ$$- versus $$2^\circ$$-carbocations.