Re: When synth. t-butylCl&Br from t-bu-alcwhy do Br&Cl have same nucleophilicit

Hollie,
You are correct in the fact that bromine is a better nucleophile than 
chlorine.  However, I think that you are seeing a stereoscopic effect.  
That is, the size of the bromine is preventing it from being the better 
nucleophile.  Chlorine being smaller and the tert-butyl radical being 
bulky allows it to bond quicker than the bromine.  A bromine ion is a very 
big radical and has difficulty getting access to the carbon atom at the 
center.

You have not given enough facts about the reaction or the conditions of 
the chromatograph.  It has been a while since I ran any SN1 reactions, but 
temperature plays an important role in which product will dominate.  The 
cooler the temperature, say -20 centigrade, the rate of reaction for both 
the bromine and chlorine will slow down, with bromine slowing faster than 
chlorine.  This may be another factor.  Another factor is the temperature 
of the G.C.  at too high of a temperature, you may be actually rearranging 
the products in the inlet of the G.C. or the column itself.

But what you are observing is most likely a stereoscopic effect.

Dan Berger adds:
According to physical organic theory, the rate of an SN1 reaction is NOT 
AFFECTED by the identity of the nucleophile; thus, you would expect equal 
amounts of t-butyl chloride and bromide if you start with equal amounts of the 
two nucleophiles. The product ratio, to at least a first approximation, will 
depend on the concentration ratio of the nucleophiles but NOT on their relative 
strengths.