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E1 using KOt-Bu

Question asked by Emily Garrison on Feb 29, 2020
Latest reply on Mar 1, 2020 by Emily Garrison

On a recent exam I was asked to identify the mechanism when 1-bromo-1-methylcyclohexane was reacted with KOt-Bu in HOt-bu, no other conditions were given (temp, concentration, etc).  

 

I inferred the lack of conditions to imply equal amounts of base and solvent at room temperature; I said E2; this was wrong.  My professor explained that because the base was so bulky, the substrate was both tertiary and bulky, and the solvent was protic, a C+ would form before the base could "get" over to the substrate and this would proceed through an E1 mechanism.

 

I am having a difficult time understanding this explanation because it is my understanding that the conjugate acid of a strong base is often used as the solvent, that strong bulky bases will proceed through E2 (Hoffman) mechanism given a 1°, 2°, or 3° substrate, with 3° being the best (unless the substrate is a benzene or allylic which prefer E1).

 

I have found many examples using the same substrate and base, but without a solvent listed; they are all E2.  I have found several references stating that sterics do not effect basicity.   I have also found many examples of bulky 3° alkyl bromides, that are not 1-bromo-1-methylcyclohexane, reacting with KOt-Bu in HOt-Bu which proceed via E2.

 

Can anyone explain this and/or outline some guidelines for differentiating between E1 and E2 given similar scenarios?

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