Reaction of Alcohols with Hydrogen Halides

Reaction type: Nucleophilic Substitution (S_N1 or S_N2)

Summary:

• When treated with HBr or HCl alcohols typically undergo a nucleophilic substitution reaction to generate an alkyl halide and water.
• Alcohol relative reactivity order : 3^o > 2^o > 1^o > methyl.
• Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity order).
• Reaction usually proceeds via an S_N1 mechanism which proceeds via a carbocation intermediate, that can also undergo rearrangement.
• Methanol and primary alcohols will proceed via an S_N2 mechanism since these have highly unfavorable carbocations.
• The reaction of alcohols with HCl in the presence of ZnCl₂ (catalyst) forms the basis of the Lucas test for alcohols.

SN1 MECHANISM FOR REACTION OF ALCOHOLS WITH HBr Step 1: An acid/base reaction. Protonation of the alcoholic oxygen to make a better leaving group. This step is very fast and reversible.  The lone pairs on the oxygen make it a Lewis base.       Step 2: Cleavage of the C-O bond allows the loss of the good leaving group, a neutral water molecule, to give a carbocation intermediate. This is the rate determining step (bond breaking is endothermic)         Step 3: Attack of the nucleophilic bromide ion on the electrophilic carbcation creates the alkyl bromide.

Reaction of Alcohols with other Halogenating agents (SOCl₂, PX₃)

Reaction type: Nucleophilic Substitution (S_N1 or S_N2)

Summary:

• Alcohols can also be converted to alkyl chlorides using thionyl chloride, SOCl₂, or phosphorous trichloride, PCl₃.
• Alkyl bromides can be prepared in a similar reaction using PBr₃.
• Used mostly for 1^o and 2^o ROH
• In each case a base is used to "mop-up" the acidic by-product.
• Common bases are triethylamine, Et₃N, or pyridine, C₆H₅N.
• In each case the -OH reacts first as a nucleophile, attacking the electrophilic center of the halogenating agent.
• A displaced halide ion then completes the substitution displacing the leaving group.
• Note that it is not -OH that leaves, but a much better leaving group.
• The advantage of these reagents is in that the reaction is not under the strongly acidic conditions like using HCl or HBr.