Part 7: MECHANISMS

I
Alkyl bromides react with sodium iodide in acetone to produce an alkyl iodide via an SN2 type of reaction (strong nucleophile, polar aprotic solvent).  However aryl bromides do not undergo nucleophilic substitution reactions. The planar geometry of the ring prevents the backside attack (180o) and an SN2 reaction.  The SN1 type would require the formation of a phenyl cation which is an unfavourable carbocation, between primary and methyl in stability, making it unfavourable.  Plus the aromatic ring is already electron rich so it not usually attacked by nucleophiles.


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 both the Br and the H that is removed be in axial positions.

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 axial orientation.

 E2 elimination of subs. bromocyclohexanes

III
The overall process is the radical substitution of the ethane to give the ethyl bromide as the major product, with butane as a minor product. The steps in the radical chain process are shown below.  Most of the ethyl bromide is formed in the 3rd step, a propagation step, and the butane forms in a termination step where 2 ethyl radicals couple.
alkane radical substitution

IV
Here we have two acids reacting with an alcohol but one giving elimination to the alkene, the other giving substitution to an alkyl chloride. The difference arises because of a difference in the conjugate bases of the acids.  The chloride ion functions as a nucleophile, and attacks the carbocation intermediate giving substitution but the hydrogen sulphate ion is resonance stabilised (charge delocalised) and is a poor nucleophile, and a base removes a proton generating the alkene. The base could be the alcohol, hydrogen sulphate or a water molecule.
acid reactivity with ROH