R O+ O
H
Cl^ − + −ZnCl 2 Cl R + [Zn( H)Cl 2 ]−
[Zn(OH)Cl 2 ]– + H+ ZnCl 2 + H 2 O
11.14 ALKYL HALIDES FROM THE REACTION OF ALCOHOLS WITH
PBr 3 OR SOCl 2
- 1° and 2° alcohols react with phosphorous tribromide to yield alkyl bromides.
+PBr 3 + H 3 PO 3
(1oor 2o)
3 R OH 3 R Br
1) The reaction of an alcohol with PBr 3 does not involve the formation of a
carbocation and usually occurs without rearrangement ⇒ PBr 3 is often
preferred for the formation of an alcohol to the corresponding alkyl bromide.
- The mechanism of the reaction involves the initial formation of a protonated alkyl
dibromophosphite by a nucleophilic displacement on phosphorus:
RCH 2 OH O
H
Br P Br +
Br
+
Protonated
alkyl dibromophosphite
RCH 2 PBr 2 + Br^ −
1) Then a bromide ion acts as a nucleophile and displaces HOPBr 2.
RCH 2 Br
A good leaving group
Br^ − RCH 2 O
H
+ + PBr 2 + HOPBr 2
i) The HOPBr 2 can react with more alcohol ⇒ 3 mol of alcohol is converted to 3
mol of alkyl bromide by 1 mol of PBr 3.
