Download 20-2 Chemistry of Acyl Halides and Anhydrides(PPT)

Organometallic reagents convert alkanoyl chlorides into ketones.
Ketone formation is best achieved by using diorganocuprates rather than RLi or
RMgX. The latter are unselective and tend to attack more than once leading to
alcohol formation.
Reduction of alkanoyl chlorides results in aldehydes.
Alkanoyl chlorides are reduced to alcohols when sodium borohydride or lithium
aluminum hydride are used directly.
The reaction stops at the aldehyde if LiAlH4 is first reacted with three molecules of
2-methyl-2-propanol (tert-butyl alcohol), which reduces the nucleophilicity of the
remaining hydride ion.
20-3
Chemistry of Carboxylic Anhydrides
Carboxylic anhydrides are named by adding the term “anhydride” to the acid
name (or names, if a mixed anhydride). This method also applies to cyclic
derivatives.
The reactions of anhydrides with nucleophiles are the same as for alkanoyl halide,
only less vigorous.
The leaving group is a carboxylate instead of a halide.
Cyclic anhydrides undergo similar reactions which lead to ring opening.
Alkanoyl halides are difficult to store for extended periods without undergoing
hydrolysis from atmospheric moisture.
Anhydrides, although less reactive towards nucleophiles, are more stable and
many are commercially available.
For these reasons, anhydrides are often preferred for the preparation of many
carboxylic acid derivatives.
Strong bases catalyze the hydrolysis of esters through an addition-elimination
mechanism. The strong base converts the poor nucleophile H2O into the higher
nucleophilic ion OH-.
Unlike acid-catalyzed hydrolysis, base-catalyzed hydrolysis is driven to completion
by the last step, which converts the carboxylic acid into a carboxylate ion.
Ester hydrolysis is often carried out using the hydroxide ion itself, in at least
stoichiometric amounts.
Transesterfication takes place with alcohols.
The direct conversion of one ester into another without proceeding through the
free carboxylic acid can be carried out by reacting a second alcohol with an ester
in the presence of strong acid.
This process is called transesterification and is reversible. To shift the equilibrium,
a large excess of the second alcohol is used.
Lactones may be ring opened by transesterification.
Acid-catalyzed transesterifications proceed by protonation of the carbonyl oxygen
and subsequent attack by the alcohol.
Base-catalyzed transesterifications proceed by deprotonation of the alcohol and
subsequent attack at the carbonyl carbon.
Amines convert esters into amides.
Amines are more nucleophilic than alcohols.
Esters readily transform into amides by treatment with an amine and subsequent
heating. (A catalyst is not required.)
Grignard reagents transform esters into alcohols.
Two equivalents of a Grignard reagent will react with a normal ester to form a
tertiary alcohol. In the case of a formate ester, a secondary alcohol is formed.