Synthesis of Alkyl Halides

Alkyl halides are synthesized using either addition or by substitution reactions.

• Addition of a hydrogen halide $\text{H}\text{X}$ ($\text{H}\text{X}$ = $\text{H}\text{Cl}$, $\text{H}\text{Br}$ or $\text{H}\text{I}$) to an alkene yields the corresponding monohalogenated alkene (Markovnikov addition). The addition of bromine or chlorine to alkenes results in the corresponding vicinal alkyl dihalides.

• The radical substitution of alkane hydrogens with bromine or chlorine is yet another method of synthesizing alkyl halides. However, the practicality of this method is limited because mixtures of alkyl halides with varying degree of halogenation are obtained.

Nevertheless, a selective monohalogenation (bromination) in allylic position is possible by using N-bromosuccinimide (NBS) as the reagent. This method was first introduced by Karl Ziegler in 1942.

A standard method of synthesizing alkyl halides is the treatment of alcohols with either $\text{H}\text{Cl}$, $\text{H}\text{Br}$ or $\text{H}\text{I}$. The reaction is a nucleophilic substitution in which the hydroxy group of the alcohol is exchanged for the halide. Protonation of the hydroxy group, normally a poor leaving group, enhances the leaving group character. The protonated hydroxy group is a good leaving group and by leaving generates an intermediate carbenium ion. At moderate temperatures, the reaction proceeds at a high rate only with tertiary alcohols; the reaction time of secondary and primary alcohols is significantly slower and the reaction requires higher temperatures. Tertiary alcohols can be converted to the corresponding halides in just a few minutes by passing pure HCl or HBr through the alcohol. However, the hydrogen halides required for the syntheses of alkyl halides are more frequently generated in situ by treating halide ions with phosphoric or sulfuric acid.

HBr and HI are used usually for the synthesis of alkyl bromides and iodides, respectively. However, $\text{P}{\text{Br}}_3$ may also be applied. Aside from HCl, inorganic acid halides, such as thionyl chloride ($\text{S}\text{O}{\text{Cl}}_2$), phosphorus trichloride ($\text{P}{\text{Cl}}_3$), phosphorus pentachloride ($\text{P}{\text{Cl}}_5$), or phosphorus oxychloride ($\text{P}\text{O}{\text{Cl}}_3$), are common chlorinating agents used in the syntheses of alkyl halides. In general, the conversion of alcohols into alkyl halides using these chlorination agents proceeds under mild conditions and very few side-poducts generated by acid-catalyzed rearrangments are obtained .

Exercise: Synthesis of alkyl halides