[briteyellowness]

2,2,3-trimethyl butane reacts with chlorine gas (Cl2) in the presence of light and the question asks for the primary yield.  The answer given is 2-chloro-2,3,3-trimethylbutane.  I thought it was 1-chloro-2,2,3-trimethylbutane because although the tertiary spot is more selective, there are more primary spots available.  So if there was only one tertiary spot, then 5 (selectivity) * 1 (spot) < 1 (primary selectivity) * 9 (primary spots).  I don't understand how it can still "primarily yield" at the tertiary location.

[movies]

Well, even if you were to form the primary radical first, it could rapidly rearrange to the more stable tertiary radical via a hydrogen atom shift.

[briteyellowness]

but in the reaction of 2-methylpropane with chlorine, it forms 63% 1-chloro-2-methylpropane and 37% 2-chloro-2-methylpropane due to the greater number of sites available to react at the primary position, and even if it is able to move to the more stable tertiary position, the numbers are given that the halogenation at the primary position is the primary yield.  So how is this different from that of the example I gave above?

[laotree]

The tertiary radical is much more table than primary, and your condition is a thermodynamic control rather than kinetic control.

[movies]

The tert-butyl group is a much better electron releasing group than methyl so this tertiary radical would be yet more stable than the 2-methylpropane example.

[laotree]

well said!

[briteyellowness]

so would you say primary yield would be how reactive it is to form the product?  but the equation

selectivity = reactivity / # sites to react

is not used?

[movies]

I think that "equation" is way oversimplifying the problem.

[briteyellowness]

kay, thanks so much for your *delete me*