Conversion of Alcohols to Alkyl Halides with HX

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The reaction of alcohols with HX (X = Cl, Br, I) is a general method to prepare 1°, 2°, and 3° alkyl halides. 

More substituted alcohols usually react more rapidly with HX RCH2—OH R2CH-OH 

Whenever there is an oxygen-containing reactant and a strong acid, the first step in the mechanism is protonation of the oxygen atom. 

The reaction of CH3CH2OH with HBr illu.strates the Sfj2 mechanism of a I ° alcohol (Mechanism 9.5). Nucleophilic attack on the protonated alcohol occurs in one step the bond to the nucleophile X is formed as the bond to the leaving group (H2O) is broken. 

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Alcohols react with HX to form alkyl halides, but the reaction works well only for tertiary alcohols, R,COH. Primaiy and secondary alkyl halides are normally prepared from alcohols using either SOClj or PBr ). Alkyl halides react with magnesium in ether solution to form organomagnesium halides, or Grignard ret ents (RM O- Since Grignard reagents are both nucleophilic and basic, they react with acids to yield hydrocarbons. The overall result of Grignard formation and protonation is the conversion of an alkyl halide into an alkane (RX— RM RH). 

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