Part 6: MECHANISMS
Note that no other reagents are needed in order to complete any of these sequences, you should only be using what is there.

A.
This is an acid catalysed dehydration of an alcohol. with a C+ rearrangement. The first step is protonation of the -OH with the H+ to give the leaving group. Loss of the leaving group gives the 2o carbocation. This rearranges via a 1,2-methyl shift to give the more stable 3o carbocatio. Loss of the proton gives the product.


B.
This is a slight variation of the formation of a halohydrin, the difference here is that the nucleophile is an amine and the reaction is intramolecular. So the alkene reacts with the bromine to form the cyclic bromonium ion. This cationic intermediate is attacked by the lone pair on the N acting as the nucleophile in an SN1 like reaction as the nucleophile attacks the more substituted end of the bromonium ion due the greater +ve character. Loss of the proton gives the product.   The base could be the bromide ion is no other bases are added)


C.
The conjugated diene undergoes direct or 1,2-addition under kinetic control in accord with Markovnikov's rule to give the product shown. Reaction goes via protonation to give the more stable carbocation. Mono substitution is the product because the diene is more reactive than the product alkene since it is more electron rich (more nucleophilic).


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