Stereochemistry of SN2 Substitutions

The reason for the inversion of configuration is that SN2 reactions take place with a backside entry by the nucleophile on the bond between the C atom and the leaving group. In the transition state of the SN2 reaction, the reacting C atom has five bonds. The three substituents at the reacting C atom not participating in the SN reaction and this C atom itself are for a short time located in one plane  

3 A Refined Transition State Model for the SN2 Reaction Crossover Experiment and Endocyclic Restriction Test 

The purpose of every crossover experiment is to determine whether reactions take place intra- or intermolecularly. In a crossover experiment two substrates differing from each other by a double substituent variation are reacted as a mixture. This substrate mixture is subjected to the same reaction conditions in the crossover experiment that the two individual substrates had been exposed to in separate experiments. This double substituent variation allows one to determine the origin of the reaction products from their structures, i.e., from which parts of which starting materials they were formed (see below for details). 

The product mixture is then analyzed. There are two possible outcomes. It can contain nothing other than the two products that were already obtained in the individual experiments. In this case, each substrate would have reacted only with itself. This is possible only for an intramolecular reaction. The product mixture of a crossover experiment could alternatively consist of four compounds. Two of them would not have arisen from the individual experiments. They could have been produced only by crossover reactions between the two components of the mixture. A crossover reaction of this type can only be intermolecular. 

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