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

1 Answer
Feb 7, 2018

Answer:

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.