a) The cis stereochemistry of the two methyl groups in the 1,2-dimethylcyclohexane product indicates that the two H atoms that have added to 1,2-dimethylcyclohexene must have added to the same face of the double bond. This is not consistent with a carbocation intermediate. If a carbocation were formed, then the planar geometry of the carbocation would allow the second H to add to either side giving a mixture of cis- and trans- products. If we look at the cis- product we see that this requires one methyl to be in a less favourable axial position, whereas the trans- could have two equatorial methyl groups, which would be more stable. Therefore, if the carbocation were formed we should also expect to see the trans- product.

b) The hydrocarbon 18-annulene is aromatic since in contains a pi system that is conjugated, cyclic, planar, and would satisfy the Huckel rule 4n+2 rule (here n = 4, 9 pairs of e-). The H-NMR data indicates that 18-annulene is similar to benzene (d = 7-8 ppm) rather than an alkene system (d = 5- 6 ppm). The chemical shifts of the annulene are explained by the magnetic anisotropy of aromatic systems. The H on the outside of the ring are in the deshielding region but the H inside the ring are in the shielding region which causes the dramtic shift to a -ve value !