Frontside or Backside Attack Stereochemistry of the Sn2 Reaction

Consider the reaction of (8)-2-bromobutane with iodide ion. Frontside displacement should give rise to 2-iodobutane with the same (S) configuration as that of the substrate backside displacement should furnish a product with the opposite configuration. 

What is actually observed It is found that (5)-2-bromobutane gives (/ )-2-iodobutane on treatment with iodide This and all other Sf/2 reactions proceed with inversion of configuration. A process whose mechanism requires that each stereoisomer of the starting material transform into a specific stereoisomer of product is described as stereospecific. The Sn2 reaction is therefore stereospecific, proceeding by a backside displacement mechanism to give inversion of configuration at the site of the reaction. 

Stereochemistry of Spj2 Displacement at a Primary Carbon Atom 

The nucleophile, azide ion (N3 ), gives rise to stereospecific backside displacement of chloride, giving the azidoalkane product with the inverted configuration at the chiral carbon. 

Write the structures of the products of the Sn2 reactions of cyanide ion with (a) w 5 -2,4-dibromopentane (double 8 2 reaction) (b) ra 5-l-iodo-4-methylcyclohexane. 

---