Why does water attack the more substituted carbon in oxymercuration?
Using 1-methyl-1-cyclohexene as an example, the mercurinium ion is not symmetrical. If it were, the positive charge would all be on mercury. The ion is unsymmetrical with part of the positive charge on the more substituted carbon. Why this carbon? Follow Markovnikov's Rule: tertiary carbocations are more stable than secondary carbocations. This bias is manifested in the transition state permitting water to attack at the more substituted carbon. The addition is anti. I chose 1-methyl-1-cyclohexene as an example because adding a tertiary butyl group to the 4-position produces more subtle aspects of the reaction mechanism.



While structures 1 and 2 above are both tertiary alcohols, only alcohol 1 is formed. This selectivity is controlled not only by the anti-addition but also by the more favorable chair-like transition states (red arrows) over the boat-like transition states (blue arrows). These issues are provided in greater detail at
http://ursula.chem.yale.edu/~chem220/chem220js/StudyAids.html#Mechanisms as PowerPoint and pdf files.