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Let us consider SN1 and SN2 reactions and concertedness. As my premise that there are no concerted reactions, that is, if time is infinitely divisible, then it is not possible for three objects to collide at exactly the same time. One collision will always precede the other.
For SN1 and SN2 reactions, then the notion of SN1 reactions is easy. Bond breaking precedes bond formation. That is the essence of SN1 reactions. Bond breaking leads to a carbocation intermediate.
However, that would force SN2 reactions to occur with bond making preceding bond cleavage. That fits the transition state model. Further, it implies that the transition state would behave as an intermediate. However, that is quite different than how I have written these problems and what you will find in any textbook. A transition state is the necessary construct one arrives at in a concerted reaction. That is, if bond making and breaking occurred at the same time, then the substituents attached to a carbon atom must invert in forming a new product.
On the other hand, if we consider that bond breaking must occur after bond formation (as the opposite of bond breaking occurring first as in an SN1 reaction), then that can be a convenient construct to test for an SN2 reaction. Are the electrons of the nucleophile sufficiently available to collide with the electrophilic carbon before the carbon-halogen bond can break? Therefore, even though the reactions might be written as concerted (wrongly according to the premise), we might think of SN2 reactions as bond formation then bond cleavage reactions.
SN1 and SN2 Reactions
Thursday, December 28, 2006
