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Topic: Protic and Aprotic Solvents  (Read 6227 times)

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Offline lancenti

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Protic and Aprotic Solvents
« on: October 27, 2009, 03:59:41 AM »
Hello there, everyone. I was hoping I could get some clarification with my thought process regarding this problem:

I had to do a kinetics experiment today to compare the rates of reaction between two solvents, 50/50 water/acetone and 50/50 water/isopropanol (both by volume). What I found was that the water/acetone system had a faster reaction rate than my water/isopropanol system. The reaction was the hydrolysis of tert-butylchloride by water.

I think that this is correct, i.e. SN1 reaction is faster in my aprotic solvent, because...

1. Acetone will not crowd around the carbocation

Now, my problem is that Wikipedia suggests that it should be my protic solvent (i.e. water/isopropanol system) that should have a faster rate of reaction.

If it weren't tert-butylchloride, I could probably get around this problem by saying 'hey, it's going by an SN2 mechanism instead'. I did, however, find that polar aprotic solvents dissolve the nucleophile but do not solvate it by hydrogen-bonding. This then would suggest that it is at a higher energy, and would more likely attack my carbocation as opposed to the water having to overcome interactions with the isopropyl alcohol molecules as well.

I'm not too convinced with myself at the moment so I was wondering if any of you had any sources to help me out.

Offline jj74

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Re: Protic and Aprotic Solvents
« Reply #1 on: October 27, 2009, 05:54:41 AM »
in an SN1 reaction you have a reaction coordinate diagram in which the substrate are not charged, but in the transition state you have partial charges developed on C and Cl (the intermediate is the carbocation):

(CH3)3C-Cl  :rarrow: [(CH3)Cd+ - - Cld-:rarrow: (CH3)3C+ + Cl-

A protic solvent stabilizes more a charged chemical species than a less charged one (due to a better solvatation): so in this case the transition state is stabilized more than the substrate (that has no charge except the dipole moment of the C-Cl bond). If you use an aprotic solvent you get a substrate more stabilized than the TS which lead to an activation energy higher than the case with a protic solvent.
Hope that I've been clear (it's clear in my mind but difficult to say this in english ::))

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Offline lancenti

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Re: Protic and Aprotic Solvents
« Reply #2 on: October 28, 2009, 05:38:02 AM »
I think I get it. Thank you!

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