Alkyl halides - Mechanism of nucleophilic substitution. Level 2

Some more features of mechanism of nucleophilic substitution in alkyl halides

• Any factor that prevents the approach of the nucleophile to the carbon bearing the nucleofuge (leaving group) will not promote S_N2 pathway.
• Non polar solvent are good for S_N2 reactions. But the best is a polar aprotic solvent like DMSO. The solvent selectively solvates the cation making the anion a strong nucleophile thus promoting the reaction. Crown ethers have similar influence.
• If the alkyl halide can form a stable carbocation then the reaction is going to be facoured by S_N1 mechanism. Tertiary halides, allylic and bezylic halides substitute this way.
• If the reaction takes place at the stereo center then S_N2 will lead to inversion in configuration and S_N1 will result in a racemic mixture.
• S_N1 reactions may involve rearrangements, when there is a possibility of formation of a more stable carbocation exists.
• A good leaving group is the one that forms a more stable anion.
• Solvolysis is a reaction in which the solvent itself acts as a nucleophile.
• Nucleophilic substitution of halogen in vinyl and aryl halides is very difficult. The halogen is attached to a sp2 carbon. The lone pair on the halogen is involved in delocalisation, hence the C-Cl bond is strong.