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Stereochemistry

Stereochemistry relates to the three-dimensional arrangement of atoms and molecules and the effect of this spatial arrangement on chemical reactions. Stereoselectivity is the preference for a reaction to occur on one molecular face over another, resulting in the preferential formation of one stereoisomer. If there's more than one stereoisomer possible from a reaction, and the configuration of at least one chemical bond is not known, that bond can be represented with a wavy bond.

A planar carbocation has 3 groups attached to it: a hydrogen, an ethyl group, and a methyl group. The ethyl group is coming towards us and is shown with a wedged bond. The hydrogen atom is going away from us and is shown with a dashed wedge bond. The bromide anion can attack from above or below the carbocation and donate electrons into the carbocations p orbital. The two products which form are stereoisomers. One product is S 2 bromo butane where the bromine points up, the ethyl points to the left and down, the hydrogen points to the back and down, and the methyl points to the front and down. The other product is R 2 bromo butane where the bromine points down, the ethyl points up and to the left, the hydrogen points to the back and up, and the methyl points to the front and up.

Figure 1. Stereoselectivity in bromide ion addition to a carbocation. Note the solid and hashed wedged bonds.

A reaction is said to be stereoselective if the reaction mechanism selects the formation of one stereoisomer over the other. In contrast, a stereospecific reaction is a reaction which only allows the formation of one stereoisomer (e.g. the stereochemistry is already set by the starting material).

Referred from:

Article
Selectivity in the electrophilic addition reaction
Article
The SN1 Reaction