Nucleophilic Substitution and Elimination at Saturated Carbon Atoms

2 Nucleophilic Substitution and Elimination at Saturated Carbon Atoms 

Nucleophilic Displacement of Halogens at Saturated Carbon Atoms 

With our first example of chemical reactions, we want to get acquainted with a very important type of reaction in organic chemistry, that is, with nucleophilic substitution at a saturated carbon atom. Since halogens are very common constituents of man-made organic chemicals, we consider their displacement by environmentally relevant nucleophiles. In these cases the halogen plays the role of the leaving group. 

Sn2 Mechanism. In this first case, the standard free energy of activation A (7°, and thus the rate of the reaction, depends strongly on both the capability of the nucleophile to initiate a substitution reaction and the willingness of the organic molecule to undergo that reaction. The former factor may be expressed by the relative nucleophi-licity of the nucleophile, an entity that can be quantified (see discussion below). The latter contribution to A 7°, however, is more difficult to quantify since it incorpo- 

If a reaction occurs by this first mechanism, it is commonly termed an SN2 reaction (i.e., substitution, nucleophilic, bimolecular). It represents an example of a simple elementary bimolecular reaction, as we discussed in Section 12.3, and it therefore follows a second-order kinetic rate law  

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