Steric crowding, effects nucleophilic substitution

Tertiary haloalkanes react by an S l mechanism because 3° carbocation intermediates are relatively stable and tertiary haloalkanes are protected against backside attack. In fact, 3° haloalkanes are never observed to react by an mechanism. In contrast, halomethanes and primary haloalkanes are never observed to react by an mechanism. They have little crowding around the reaction site and react ty an Sj 2 mechanism because methyl and primary carbocations are unstable. Secondary haloalkanes may react by either 8, 1 or 8, 2 mechanisms, depending on the nucleophile and solvent. The competition between electronic and steric factors and their effects on relative rates of nucleophilic substitution reactions of haloalkanes are summarized in Figure 9.3. 

When the double bond is substituted with alkyl groups or other electron donating groups, the effect of the substituents on the regiochemistry leads to the trend known as Markovnikov addition. Markovnikov addition predicts the dominant product to arise from nucleophilic addition to the more substituted carbon. Note that nucleophilic addition to the more substituted carbon forms the more sterically crowded and thus less stable product (Eq. 10.7). Therefore, Markovnikov addition predicts the kinetic product, not the thermodynamic product. 

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