General Considerations of Substitution Reactions

Thermochemical data for the solvation of ions as used in the preceding calculations are difficult to measure and even to estimate. Therefore this kind of calculation of AH° for ionic reactions involving organic molecules in solution usually cannot be made. As a result, we have considerably fewer possibilities to assess the thermodynamic feasibility of the individual steps of polar reactions in solution than we do of vapor-phase radical processes. Bond energies are not of much use in predicting or explaining reactivity in ionic reactions unless we have information that can be used to translate gas-phase AH°. values to solution AH° values. Exercise 8-3 will give you a chance to see how this is done. 

Exercise 8-3 Calculate AH° for the polar reaction of one mole of bromomethane 

We now wish to discuss displacements by nucleophilic reagents (Y ) on alkyl derivatives (RX). These are ionic or polar reactions involving attack by a nucleophile at carbon. A typical example is the reaction of hydroxide ion with bromomethane to displace bromide ion  

The electron pair of the C-O bond can be regarded as having been donated by the hydroxide ion, while the electron pair of the C-Br bond departs with the leaving bromide ion. The name for this type of reaction is abbreviated SN, S for substitution and N for nucleophilic. 

Reactions of this type are very useful. They can lead to compounds in which the new bond to carbon in the alkyl group, R, is to chlorine, bromine, 

---