Part 8: MECHANISM
The remaining diagrams will be added as time allows.

 

I.
Alcohols react with thionyl chloride to produce an alkyl chloride, usually via an SN2 type of reaction (see below).  However phenols do not undergo nucleophilic substitution reactions for the following reasons (i) SN1 type would require the formation of an unfavourable carbocation, (ii) the geometry of the ring prevents the backside attack and hence an SN2 reaction, and (iii) the aromatic ring is already electron rich.


II.
These reactions are E2 reactions since we have a secondary bromide in the presence of a strong base. The E2 reaction is a concerted process that requires the H and LG to be at 180o (antiperiplanar).  In cyclohexanes, this requires that the Br be in an axial position.
In the trans-isomer, elimination to the less stable alkene occurs since the Me group is also in the axial position.
In the cis-isomer, the more stable alkene can be produced since the H is in the correct orientation.		 E2 elimination of subs. bromocyclohexanes


III.
Markovnikov and anti-Markovnikov hydration of alkenes.  Aq. H2SO4 reactions by protonation of the alkene to the more stable carbocation followed by attack of H2O as the nucleophile and finally loss of H+ to get the Markovnikov alcohol.  In contrast, borane (BH3) undergoes a concerted addition, so there is no carbocation intermediate. The reaction is controlled by electronic and steric factors.  The B atom is the d+ and H  d- in this process (check the electronegativities of B and H). Thus, the B adds at the least hindered end. During the oxidation step, an O atom is inserted into the C-B bond to give the C-O-B system that generates the alcohol on reaction with NaOH.

IV.
A classic nucleophilic substitution versus elimination problem. The presence of ethoxide, EtO-, a strong base, promotes  elimination via an E2 pathway leading to the more stable, more highly substituted alkene.

In contrast the much less basic EtOH favours substitution via an SN2 pathway, leading to an ether product.
 		 Substitution and Elimination of 2-bromopentane

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