Posted by: Mark Foreman | February 13, 2010

Alkyl halides III, the E2 and E1 reactions

OK, listen up we have had our fun with SN2 and SN1 now it is time to think about an alternative.

Rather than replacing the halide group with something else we are now removing a molecule of HX (a hydrogen halide) from the molecule to make an alkene. This is an important reaction which will lead towards the way in which alcohols can be converted into alkenes (and the other way around).

First we need an example of why we need to know about these reactions, DDT is converted by the loss of HCl to form DDE. DDE is a fat soluble substance which builds up in animals. For bats all is well as long as they stay fat, but when they go for a long haul flight they burn up their fat and the DDE concentration in their bodies goes up. Then the bats drop dead. Sad !

DDT and DDE

But we need to know a little more about E1 and E2 than just that they convert DDT into the thing which kills bats.

The E2 reaction is a concerted process which has an associative mechanism. The base attacks the acidic hydrogen to form a new bond at the same time as the C-H and C-X bonds break. Also at the same time the pi bond between two carbons forms.

Everything happens in a single step just like it did with the SN2 reaction. The difference this time is that the steric hinderance at the carbon bearing the halogen is not so important. The main steric issue here is the approach of the base to the hydrogen.

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