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)       àamides

“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.

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