Chapter 19: Carboxylic Acids

Chapter 19: Carboxylic Acids

Nomenclature:
Functional group suffix = -oic acid (review)
If the acid is substituted onto a ring the suffix -carboxylic acid is used.


methanoic acid	ethanoic acid	propanedioic acid	butanedioic acid
formic acid	acetic acid	malonic acid	succinic acid

The anion dervived by deprotonation of a carboxylic acids, RCO₂H, is the carboxylate, RCO₂^-

Physical Properties:

The polar nature of both the O-H and C=O bonds (due to the electonegativity difference of the atoms) results in the formation of strong hydrogen bonds with other carboxylic acid molecules or other H-bonding systems (e.g. water). The implications are:

higher melting and boiling points compared to analogous alcohols
high solubility in aqueous media
hydrogen bonded dimers in gas phase and dimers or aggregates in pure liquid

Structure:

The CO₂H unit is planar and consistant with sp² hydridisation and a resonance interaction of the lone pairs of the hydroxyl oxygen with the p system of the carbonyl C=O.
It may be useful to highlight the similarity with an aldehyde.


methanoic acid	methanal

Acidity:

Carboxylic acids are the most acidic simple organic compounds (pKa ~ 5).
But they are only weak acids compared to acids like HCl or H₂SO₄. (Remember the lower the pKa, the stronger the acid)
Resonance stabilization of the carboxylate ion allows the negative charge to be delocalised between the two electronegative oxygen atoms (compare with alcohols, pKa ~ 16).
Adjacent electron withdrawing substituents increase the acidity by further stabilising the carboxylate.

Carboxylic Acid	Structure	pKa
Ethanoic acid	CH₃CO₂H	4.7
Propanoic acid	CH₃CH₂CO₂H	4.9
Fluoroethanoic acid	CH₂FCO₂H	2.6
Chloroethanoic acid	CH₂ClCO₂H	2.9
Dichloroethanoic acid	CHCl₂CO₂H	1.3
Trichloroethanoic acid	CCl₃CO₂H	0.9
Nitroethanoic acid	O₂NCH₂CO₂H	1.7

Reactivity:

The image shows the electrostatic potential for acetic acid (ethanoic acid).
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.

There is low electron density (blue) on H atom of the -CO₂H group alcohol, i.e. H⁺ character.
The H atom of the RCO₂H is acidic (pKa ~ 5).
The most important reactions of carboxylic acids converts them into carboxylic acid derivatives such as acyl halides, esters and amides via nucleophilic acyl substitution reactions.

The image shows the electrostatic potential for the acetate ion (ethanoate ion)
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.

There is high electron density (red) on both O atoms of the -CO₂^- group alcohol, i.e. resonance and basic or nucleophilic behaviour