19.5 Definition and Structure

Carboxylic acids are organic compounds that contain the carboxyl functional group ($-COOH$) in their molecules. The name "carboxyl" is a combination of carbonyl ($>C=O$) and hydroxyl ($-OH$).

The general formula for a carboxylic acid is:

$R-COOH$

where:

• R can be a hydrogen atom (H), an alkyl group (e.g., $-C{H}_3$, $-C_2{H}_5$), or an aryl group (e.g., $-C_6{H}_5$).
• The carboxyl group consists of a carbonyl group ($C=O$) and a hydroxyl group ($-OH$) attached to the same carbon atom.

The carbon atom in the carboxyl group is $s{p}^2$ hybridized, resulting in a trigonal planar structure around the carbonyl carbon.

Classification of Carboxylic Acids

Carboxylic acids are broadly classified based on the nature of the R group:

1. Aliphatic Carboxylic Acids: The $-COOH$ group is attached to an open-chain alkyl group or hydrogen.
   • Examples: Formic acid ($HCOOH$), Acetic acid ($C{H}_{3}COOH$)
2. Aromatic Carboxylic Acids: The $-COOH$ group is attached directly to a phenyl or other aryl group.
   • Example: Benzoic acid ($C_6{H}_{5}COOH$)

Physical Properties

Carboxylic acids exhibit unique physical properties due to their ability to form extensive hydrogen bonds.

Boiling Points

Carboxylic acids have higher boiling points than alcohols of similar molecular mass because they form stable cyclic dimers through two intermolecular hydrogen bonds per pair of molecules. This strong association requires more energy to break.

Solubility

The first four aliphatic carboxylic acids ($C_1$–$C_4$) are very soluble in water due to hydrogen bonding with water molecules. Solubility decreases as the length of the non-polar hydrocarbon chain (R group) increases, because the hydrophobic alkyl portion dominates.

Acidity of Carboxylic Acids

Carboxylic acids are more acidic than alcohols and phenols. When a carboxylic acid loses a proton ($H^{+}$), it forms a carboxylate ion ($RCO{O}^{-}$):

$RCOOH\rightleftharpoons RCO{O}^{-}+H^{+}$

The carboxylate ion is stabilized by resonance — the negative charge is delocalized over both oxygen atoms equally:

$RCO{O}^{-}⟷R-\stackrel{-}{C}(=O)-OH$

This resonance stabilization of the conjugate base makes carboxylic acids significantly stronger acids than alcohols (where no such stabilization exists).

Effect of Substituents on Acidity

Electron-withdrawing groups (EWGs) such as halogens ($-Cl$, $-Br$, $-F$) increase acidity by further dispersing the negative charge of the carboxylate ion through the inductive effect, making the conjugate base more stable.

• More EWGs → greater acidity
• EWG closer to $-COOH$ → greater acidity
• Example: $C{l}_{3}CCOOH>C{l}_{2}CHCOOH>ClC{H}_{2}COOH>C{H}_{3}COOH$

Electron-donating groups (EDGs) such as alkyl groups decrease acidity by destabilizing the carboxylate ion.