[Dawek]

Hello guys, i took this exercise from my Organic textbook, and i need a help.

a. Identify the substitution products that form when 2-bromo-2-methylpropane is dissolved in a mixture of 80% ethanol and 20% water.
b. Explain why the same products are obtained when 2-chloro-2-methylpropane is dissolved in a mixture of 80% ethanol and 20% water.

Letter b is pretty easy. Since the mechanism and the reactants of these reactions are the same, the product is the same.

I'm confused about the letter a. The substract is tert, so it goes under a S_N1 mechanism, in which the concentration of nucleophile doesnt matter.
Ethanol is a higher concentration and it dieletric constant is low, which minimizes the ionization of water, that could produces some OH^-, a good nucleophile.
So, i need to decide what is major product, if is ethanol acting as nucleophile or if is water. 
Thaanks! 

[Dawek]

It is about steric hindrance of the base?

[Babcock_Hall]

Do you know what the intermediate is in this reaction?

[clarkstill]

It's true that the concentration of base doesnt affect the overall rate of reaction, but that doesn't mean that it can't effect the distribution of products.

In an SN1, the RDS is loss of the halide, and the carbocation is so reactive that it will immediately form a bond with any nucleophile that approaches it (much more quickly than loss of the halide). I would suppose that you get 80% the ethanol addition product, and 20% the water product, since the distribution of products should just reflect the statistical likelihood of encountering either potential nucleophile.