Question

compare a nucleophilic addition to a electrophilic atomatic substitution. how are the mechanisms alike and how are they different?

Answer 1

In a Nucleophilic addition

a nucleophile attacks the partially positively charged /positively charged sp2 carbon to

form a tetrahedral intermediate (sp2 C changes to sp3 C).

Then the tetrahedral intermidate takes up aproton to give the addition product.

In EAS

an elecrophle attacks an electron rich sp2 Carbon of aromatic ring to

form a tetrahedral carbocation(sp3 C ,arenium ion) as an intermediate

This intermediate is resonance stabilised .

The tetrahedralintermediate changes back to sp2 C( aromatic system) by loss of proton.

similarities

the slow and RDS involves attack of reagent on sp2 C to change it into sp3 C

diffeences

1) in Nu additon , the Nu is the attacks e deficient C while in EAS electrophile attacks on e- rich carbon

2) In addition a proton is added to the intermediate to give tetrahedral(sp3C) product. while in EAS the intermediate loses a proton to give back sp2 C that is planar product.