Organic Chemistry

712 CHAPTER 17 Carbonyl Compounds I

A carboxylic acid can be converted into an acyl chloride by heating it either with

thionyl chloride or with phosphorus trichloride Acyl bromides can be

synthesized by using phosphorus tribromide

All these reagents convert the OH group of a carboxylic acid into a better leaving group

than the halide ion.

Therefore, when the halide ion subsequently attacks the carbonyl carbon and forms a

tetrahedral intermediate, the halide ion is notthe group that is eliminated.

Notice that the reagents that cause the OH group of a carboxylic acid to be replaced by

a halogen are the same reagents that cause the OH group of an alcohol to be replaced

by a halogen (Section 12.3).

Once the acyl halide has been prepared, a wide variety of carboxylic acid deriva-

tives can be synthesized by adding the appropriate nucleophile (Section 17.8).

an anhydride

RROCl − R O R

+ + Cl−

an ester

ROH

ROR

+ + HCl

an amide

2 RNH 2

R NHR

+ + RNH 3 Cl−

+

C

O

C

O

C

O

C

O

C

O

C

O

RCl

C

O

RCl

C

O

O

C

OO

S

R O

++SO 2

Cl−

O−

Cl−

Cl

Cl

O

C

R Cl

S

RC O Cl

ROCl

C

O

S

O

RO

C

O

PCl 2

RO

C

O

PBr 2

good leaving groups

acetic acid

CH 3

thionyl

chloride

SOCl 2

OH

+

acetyl chloride

CH 3

SO 2

Cl

Cl

∆ + + HCl

benzoic acid phosphorus

tribromide

3

OH

+ PBr 3 3

benzoyl bromide phosphorous

acid

∆ + H^3 PO^3

butanoic acid phosphorus butanoyl chloride

trichloride

CH 3 CH 2 CH 2 CH 3 CH 2 CH 2

• PCl (^3) ∆ + H 3 PO 3
  C
  O
  OH
  C
  O
  C
  O
  Br
  C
  O
  C
  O
  C
  O
  33
  (PBr 3 ).
  (SOCl 2 ) (PCl 3 ).