Synthesis of alkyl halides:

Tertiary alcohols can go through the S_N1 reaction to generate tertiary alkyl halides. As the reaction needs the loss of the hydroxide ion (a poor leaving group), a little bit of 'trickery' is needed to convert the hydroxyl moiety into a better leaving group. This can be acquired under acidic conditions with the make use of HCl or HBr. The acid works to protonate the hydroxyl moiety since the ?rst step and then a normal S_N1 mechanism occurs where water is lost from the molecule to make an intermediate carbocation. A halide ion then makes a bond to the carbocation center in the third step.

The ?rst two steps of this mechanism are exactly similar as the elimination reaction explained above. Both reactions are performed under acidic conditions and one might ask why elimination does not take place. The variation here is that halide ions serve as good nucleophiles and are present in high concentration. The elimination reaction explained earlier is performed using concentrated sulfuric acid and just only weak nucleophiles are present (that is water) in low concentration. Having said that, some elimination can take place and even though the reaction of alcohols with HX generates mainly alkyl halide, some alkene by-product is generally present.

Because primary alcohols and some secondary alcohols do not go through the S_N1 reaction, nucleophilic substitution of these compounds has to be involves an S_N2 mechanism. One time again, protonation of the OH group is needed as a ?rst step after that the reaction includes simultaneous attack of the halide ion and loss of water. The reaction carries on with good nucleophiles like the iodide or bromide ion, but fails along with the weaker nucleophilic chloride ion. In this case, a Lewis acid requires to be added to the reaction mixture. The Lewis acid makes a complex with the oxygen of the alcohol group, resultant in a much better leaving group for the subsequent S_N2 reaction.