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
The reaction is an electrophilic addition to an alkene. It is closely related to simple hydration but instead of water being the nucleophile, here it is an alcohol, so instead of the product being an alcohol, it's an ether.

The first step is protonation of the alkene with the H+ to give the more stable carbocation, in this case a 3o carbocation. Then the lone pairs on the O act as the nucleophile attacking the cation. Loss of the proton gives the product.   It is important to remember that in an acidic solution (we have H+ defined) that the amount of alkoxide ion (i.e. CH3O-) is minimal.... think of the pKa which for this dissociation would be about 16. So the nucleophile will be the alcohol itself (i.e. CH3OH).


B
The reaction is an electrophilic addition to an alkyne with excess HCl, so we will go from alkyne to vinyl halide to a geminal dihalide.  The regiochemistry follows Markovniknov's rule which says that the H will add to the end with the most H already. However, this is really based on carbocation stability, in the first step the secondary is favoured over the primary cation (since alkyl groups are electron donating due to hyperconjugation and their polarisability).  When HCl adds to the vinyl chloride, the secondary cation is further stabilised by a resonance contribution from the Cl atom, this leads to the geminal dichloride.

Note : the mechanism now accepted as being current and depicted in current textbooks is a termolecular process.


C
Not as complex as it looks, but care is needed to keep track.  Again it is an electrophilic addition. The first step is protonation of the alkene with the H+ to give the more stable carbocation, in this case a 3o carbocation. Then the alkene acts as the nucleophile, forming a new C-C bond and another 3o carbocation..... if this reacts with another alkene, the polymer continues to grow. If it looses a proton, then we get an alkene forming.

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