Acidity, effect of substituents on acidity and important reactions of benzoic acid.

 Acidity, effect of substituents on acidity and important reactions of
benzoic acid.

Benzoic acid

Conversion into functional derivatives:

a)     
à  acid chlorides

b)     
à  esters

“direct” esterification:                        H⁺

                RCOOH  
+   R´OH   D   RCO₂R´   +   H₂O

                -reversible
and often does not favor the ester

                -use an
excess of the alcohol or acid to shift equilibrium

                -or
remove the products to shift equilibrium to completion

“indirect” esterification:

                RCOOH   +  
PCl₃   à   RCOCl  
+   R´OH   à   RCO₂R´

                -convert
the acid into the acid chloride first; not reversible

c)      
à
amides

“indirect” only!

RCOOH   +   SOCl₂   à   RCOCl  
+   NH₃   à   RCONH₂

                                                                                  
amide

Directly reacting ammonia with a carboxylic acid results in
an ammonium salt:

                                RCOOH   +   NH₃   à   RCOO^–NH₄⁺

                              acid             base                 

Reduction:

                                RCO₂H   +  
LiAlH₄; then H⁺  
à   RCH₂OH

                                                                                                    1^o alcohol

Carboxylic acids resist catalytic reduction under
normal conditions.

                                           RCOOH   +   H₂,
Ni   à   NR

Alpha-halogenation: 
(Hell-Volhard-Zelinsky reaction)

                RCH₂COOH   +   X₂,
P   à   RCHCOOH  
+   HX

                                                        X

                                                     
α-haloacid

       X₂ =
Cl₂, Br₂

EAS:  (-COOH is
deactivating and meta– directing)

Acidity of Substituted Benzoic Acids

•       Electron-withdrawing
groups

•       The
conjugate base of benzoic acid is destabilized by electron-donating
groups.  This makes the acid less acidic

•       Electron-withdrawing
groups deactivate the benzene ring to electrophilic attack and make benzoic
acids more acidic.

Electron-donating
groups

•       The
conjugate base of benzoic acid is stabilized by electron-withdrawing
groups.  This makes the acid more acidicx

•       Electron-withdrawing
groups activate the benzene ring to electrophilic attack and make benzoic acids
less acidic.

Substituent Effects on Acidity

•       Water
is less acidic than hydrogen peroxide because hydrogen is less electronegative
than oxygen, and the covalent bond joining these atoms is polarized in the
manner shown. Alcohols are slightly less acidic than water, due to the poor electronegativity
of carbon, but chloral hydrate, Cl₃CCH(OH)₂, and
2,2,2,-trifluoroethanol are significantly more acidic than water, due to
inductive electron withdrawal by the electronegative halogens (and the second
oxygen in chloral hydrate). In the case of carboxylic acids, if the
electrophilic character of the carbonyl carbon is decreased the acidity of the
carboxylic acid will also decrease.