Mechanisms, IUPAC leaving groups

Sn2 stands for substitution nucleophilic bimolecular. The IUPAC designation (p. 290) is AnDn. In this mechanism there is backside attack the nucleophile approaches the substrate from a position 180° away from the leaving group. The reaction is a one-step process with no intermediate (see, however, pp. 297-298 and 305). The C—Y bond is formed as the C—X bond is broken ... 

Both the Se2 (front) and Se2 (back) mechanisms are designated DFAE in the IUPAC system. With substrates in which we can distinguish the possibility, the former mechanism should result in retention of configuration and the latter in inversion. When the electrophile attacks from the front, there is a third possibility. A portion of the electrophile may assist in the removal of the leaving group, forming a bond with it at the same time that the new C—Y bond is formed ... 

This mechanism is the same as the simple electrophilic one shown on p. 734 except that the charges are reversed (IUPAC AN + AE or AN + AH). When the olefin contains a good leaving group (as defined for nucleophilic substitution), substitution is a side reaction (this is nucleophilic substitution at a vinylic substrate, see p. 335). 

In this mechanism the two groups leave at about the same time and bond to each other as they are doing so. The designation is Ei in the Ingold terminology and cyclo-DEDNA in the IUPAC system. The elimination must be syn and, for the four- and five-membered transition states, the four or five atoms making up the ring must be coplanar. Coplanarity... 

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