Primary carbocations, instability

The first leads to formation of a primary carbocation. The instability of the primary carbocation is not so great that it is grounds for eliminating this step from consideration however, the intermediate ion also contains a four-membered ring, a structural feature not found in the product. The second alkyl shift leads to the carbocation encountered in the mechanism shown previously. 

Energetics This path avoids forming an unstable cation and therefore is a useful alternative to path Dn followed by path 1,2R. Expect to use this path if a Dn path fails its crosschecks because of cation instability, for instance, if it would have formed a primary carbocation, or formed an electronegative heteroatom with an incomplete octet. 

There are many reactions in which tertiary and secondary carbocations are intermediates but no reactions for which a simple primary or methyl cation is required. These intermediates are simply too unstable. In practice, this instability means that when you find yourself writing a mechanism using a primary carbocation, think again, it s almost certainly wrong ... 

The final stage on the route to the aziridine product involves an internal Sn2 ring closure in which the primary amine group attacks the a-carbon atom holding the remaining bromine from the backside to close an aziridine ring and displace bromide with inversion of configuration at the acarbon. This displacement has been shown to go exclusively by this mechanism in the case of the p-amino-a-bromoketones an S l reaction is less likely because of the instability of an a-keto carbocation. For a detailed discussion of the mechanism of the classic Sn2 substitution reaction, refer to Experiment [22],... 

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