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Acidity, Effect of Substituents on Acidity and Important Reactions of Benzoic Acid

In an electron-donating group, benzoic acid's conjugate base is destabilized. It becomes less acidic as a result.

Acidity

Electron-withdrawing groups

In an electron-donating group, benzoic acid's conjugate base is destabilized. It becomes less acidic as a result.



Electrophilic attack on the benzene ring is stopped by electron-withdrawing groups, making benzoic acids more acidic.


Electron-donating groups

The electron-withdrawing groups in the conjugate base of benzoic acid stabilize it. It therefore keeps its acidity.



Benzene rings are activated to electrophilic attack by electron-withdrawing groups, leaving benzoic acids less acidic.



Effects of substituents on acidity

Inductive effect

Acidities of carboxylic acids can vary widely. Butanoic acid (Ka = 1.51 X 10-5) has a pH of less than 60,000 times higher than trifluoroacetic acid (Ka = 0.59). Inductive effects of substituents attached to carboxylic acids probably account for most of these vast differences in acidity. The transmission of charge by atoms within a molecule results in a permanent dipole in a bond. This effect has been experimentally observed. When halogens (such as fluorine) are present on adjacent carbons of a carboxylic acid group, the carboxylate conjugate base is stabilized and the acidity of the carboxylic acid group is increased.

Withdrawing inductive effect

Fluorine atoms are more electronegative than hydrogen atoms, and therefore they have the ability to attract the electron density of covalent bonds towards themselves. As the electrons of the F-C covalent bond are drawn toward the fluorine in the fluoroacetate anion, the carbon is partially positively charged. In turn, the positively charged carbon creates a stabilizing effect by drawing electron density away from the carboxylate anion, which is then disperse to create a stable state. In addition to stabilizing carboxylate anion, carboxylic acid is made more acidic. An electron-withdrawing group for the fluorine substituent is thus being formed.



Fluorine withdraws electrons from fluoroacetate anion to stabilize it
Similar effects are observed when -CH2CO2H is present with other electron-withdrawing groups. Carboxylic acids drop in pKa as their electron-withdrawing power becomes stronger.



Multiplier effects compound the inductive effect, which increases the acidity of carboxylic acids. Dichloroacetic acid is a weaker acid than chloroacetic acid and trichloroacetic acid.



An electrostatic potential map of acetic and trichloroacetic acids (Left) demonstrates the inductive effect of chlorine. The dark blue color of trichloroacetic acid is caused by its strong polarization of O-H bonds, thereby making it stronger than acetic acid.



The amplifying effect of carboxylic acid on electron-withdrawing groups rapidly fades as the number of sigma bonds between them increases. Due to the nature of covalent bonds, ionic effects are not effectively transmitted. The chlorine's inductive effect in 4-chlorobutanoic acid is almost completely eliminated when three sigma bonds are formed, giving it a similar pKa value as unsubstituted butanoic acid.



Donating inductive effect

Inductively electron-donating groups (hydrocarbons) are alkyl groups. The inductive effects in this case lead to an increase in electron density on the carboxylate anion, resulting in a destabilizing effect and a decrease in acidity.



Inductive effects are more readily enhanced by lengthening alkyl chains of carboxylic acids, but the effect ceases to be profound once the chain is about three carbons long.



Important reactions of benzoic acid

Bromination of benzoic acid

Since benzoic acid has a carboxyl group that withdraws electrons, it is a meta-directing group. Bromination of benzoic acid leads primarily to methyl bromo benzoic acid or m-bromo acid, which are similar to brominated products.
Reaction



Reaction mechanism



Nitration of benzoic acid

M-nitrobenzoic acid or meta-nitrobenzoic acid is one of the major products derived from nitrating benzoic acid. If concentrated sulfuric acid and fuming nitric acid are mixed, 3,5-dinitrobenzoic acid may form as a byproduct after nitrating concentrated sulfuric acid. An important deactivating group on benzene is its carboxylic acid group. Deactivating groups are usually meta-directing groups. By the electron donating effects of activating groups, electron density is pushed onto ortho- and para-positions, and by the electron removing effects of deactivating groups, electron density is pulled away from ortho- and para-positions. Both positions are almost unaffected by electron density shifts, though the meta-position becomes more reactive when electron density is removed from the ortho- and para-positions.

Reaction



Reaction mechanism

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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