Question

Why is tertiary carbon atom more reactive towards sn1 reaction even though it is the most stable one??

Answer 1

BECAUSE the formal cation that results from the tertiary carbon is intrinsically the MOST stable...

Explanation:

Now `S_N1` reactions are essentially bond-breaking, and these are conceived to occur when a carbon-halogen bond is BROKEN to give a carbocation intermediate....

Consider....

`H_3C-CHX-CH_3 stackrel("rate determining step")rarrH_3C-stackrel(+)CH-CH_3+X^(-)`

The fast step is the reaction of some nucleophile, `X'^-`,...which could be the solvent or it could be even the initial leaving group, to give a new species....

`H_3C-stackrel(+)CH-CH_3 ++X'^(-)stackrel("fast step")rarrH_3C-CHX'-CH_3`

Now because secondary carbocations are stabilized with respect to methyl, and primary carbocations, and tertiary carbocations are stabilized with respect to secondary carbocations, the order of reactivity in terms of `S_N1` mechanisms should be....

`underbrace("Methyl, primary, secondary, tertiary")_rarr`
`"RATE OF REACTIVITY IN BOND-BREAKING REACTION"`

And this order reflects the stability, the lifetime if you like, of the reactive carbocation intermediate. Sterically encumbered carbocations thus have faster rates of reactions in bond-breaking scenarios.