Substitution and Elimination Reaction Mechanisms
its ”caged” state, the second step, the attack of the nucleophile, is so much faster than the
first step, that the ”caging” can simply be ignored.
56.1.4 Summary
SN1 ,SN2 ,E1, andE2, are all reaction mechanisms, not reactions themselves. They are
mechanisms used by a number of different reactions. Usually in organic chemistry, the goal
is to synthesize a product. In cases where you have possibly competing mechanisms, and
this is particularly the case where anSN1 and anE1reaction are competing, the dominating
mechanism is going to decide what your product is, so knowing the mechanisms and which
conditions favor one over the other, will determine your product.
In other cases, knowing the mechanism allows you to set up an environment favorable to that
mechanism. This can mean the difference between having your product in a few minutes,
or sometime around the next ice age.
So when you’re designing a synthesis for a product, you need to consider, I want to get
product Y, so what are my options to get to Y? Once you know your options and you’ve
decided on a reaction, then you need to consider the mechanism of the reaction and ask
yourself, how do I create conditions that are going to make this happen correctly and happen
quickly?
56.2 Elimination Reactions
Nucleophilic substitution reactions and Elimination reactions share a lot of common char-
acteristics, on top of which, theE1andSN1 as well asE2andSN2 reactions can sometimes
compete and, since their products are different, it’s important to understand them both.
Without understanding both kinds of mechanisms, it would be difficult to get the product
you desire from a reaction.
In addition, theSN1 andSN2 reactions will be referenced quite a bit by way of comparison
and contrast, so it’s probably best to read that section first and then continue here.
Elimination reactions are the mechanisms for creating alkene products from haloalkane^10
reactants. E1andE2elimination, unlikeSN1 andSN2 substitution, mechanisms do not
occur with methyl halides because the reaction creates a double bond between two carbon
atoms and methylhalides have only one carbon.
NoteIn the notationE1andE2,
Estands forelimination
1 : stands forunimolecular(the concentration of only one kind of molecule determines the rate of
the reaction)
2 : stands forbimolecular(the concentration of two types of molecules determine the rate of the
reaction)10 Chapter 42 on page 165