Chapter 16 part 1 Carboxylic Acids
Carboxylic acids (RCO2H) are a common and important functional group and provide the point
of access to the carboxylic acids derivatives (acyl chlorides, acid anhydrides, esters, amides, etc).
Carboxylic acids are the most acidic of the common organic functional groups.
Physical Properties:
The polar nature of both the O-H and C=O bonds, results in the formation of strong
hydrogen bonds with other carboxylic acid molecules or other H-bonding systems, the
results are:
Higher melting and boiling points compared to analogous alcohols
High solubility In aqueous media
Hydrogen bonded dimmers in gas phase or pure liquid
Structure:
The CO2 H unit is planar, with sp2 hydridisation and a resonance interaction of the one
pairs of the hydroxyl oxygen with the ∏ system of the carbonyl.
Nomenclature:
Ethanoic acid CH3CO2H
Propanoic acid CH3CH2CO2H
Acidity:
Carboxylic acids are the most acidic simple organic compounds (pKa~ 5).
But they are only weak acids compared to acids like HCI or H2SO4.
Resonance stabilization of the carboxylate ion allows the negative charge to be
delocalized between the two electronegative oxygen atoms.
Adjacent electron withdrawing substituent’s increase the acidity by further stabiling the
carboxylate.
Ethanoic Acid CH3CO2H PKa
4.7
Trichloroethanoic Acid CCl3CO2H 0.9
Chapter 16 part 1 Carboxylic Acids
Preparations of Carboxylic Acids
(overview)
Ozonolysis of
Alkynes
Ozonolysis of
Alkenes
Carbonation of
Grignards
Hydrolysis of
Nitriles
Oxidation of 1o
Alcohols
Oxidation of
Aldehydes
Oxidation of
Alkyl Benzenes
Chapter 16 part 1 Carboxylic Acids
Reaction of Carboxylic acids:
acyl chlorides
acid anhydrides
esters
amides
carboxylates
Chapter 16 part 1 Carboxylic Acids
1- Preparation of Acyl Chlorides
Reaction type: Nucleophilic Acyl Substiution
Acyl chlorides are prepared by treating the carboxylic acid with thionylchloride, SOCl 2, in the
presence of a base.
Acyl chlorides are by far the most commonly encountered of the acyl halides.
2. Preparation of Acid Anhydrides
Reaction type: Nucleophilic Acyl Substiution
Symmetrical anhydrides can be are prepared by heating the carboxylic acid
Symmetrical anhydrides are by far the most commonly encountered, e.g.acetic anhydride.
3. Preparation of Esters
Reaction type: Nucleophilic Acyl Substiution
This reaction is also known as the Fischer esterification.
Esters are obtained by refluxing the parent carboxylic acid with the appropraite alcohol with an
acid catalyst.
The equilibrium can be driven to completion by using an excess of either the alcohol or the
carboxylic acid, or by removing the water as it forms.
Alcohol reactivity order : CH3OH > 1o > 2o > 3o (steric effects)
Esters can also be made from other carboxylic acid derivatives, especially acyl halides and
anhydrides, by reacting them with the appropriate alcohol in the presence of a weak base
If a compound contains both hydroxy- and carboxylic acid groups, then cyclic esters or lactones
can form via an intramolecular reaction. Reactions that form 5- or 6-membered rings are
particularly favourable.
Chapter 16 part 1 Carboxylic Acids
Mechanism of the Fischer Esterification
Addition of a proton (e.g.: p-TsOH, H2SO4) or a Lewis acid leads to a more reactive electrophile.
Nucleophilic attack of the alcohol gives a tetrahedral intermediate in which there are two
equivalent hydroxyl groups. One of these hydroxyl groups is eliminated after a proton shift
(tautomerism) to give water and the ester.
4. Preparation of Amides
Reaction type: Nucleophilic Acyl Substiution
In general, it is not easy to prepare amides directly from the parent carboxylic acid.
The acid will protonate the amine preventing further reaction since the carboxylate is a
poor electrophile and the ammonium ion is not nucleophilic.
It is much easier to convert the carboxylic acid to the more reactive acyl chloride first.
Chapter 16 part 1 Carboxylic Acids
5. a-Halogenation (Hell-Volhard-Zelinsky reaction)
Reaction type: Substitution
Reagents most commonly : Br2 and either PCl3, PBr3 or red phosphorous in catalytic amounts.
Carboxylic acids can be halogenated at the C adjacent to the carboxyl group.
This reaction depends on the enol type character of carbonyl compounds.
The product of the reaction, an a-bromocarboxylic acid can be converted via substitution
reactions to a-hydroxy- or a-amino carboxylic acids.
Reduction of Carboxylic Acids
Reaction usually in Et2O or THF followed by H3O+work-ups
Reaction type: Nucleophilic Acyl Substiution then NucleophilicAddition
Carboxylic acids are less reactive to reduction by hydride than aldehydes,ketones or
esters.
Carboxylic acids are reduced to primary alcohols.
As a result of their low reactivity, carboxylic acids can only be reducedby LiAlH4 and
NOT by the less reactive NaBH4
Chapter 16 part 1 Carboxylic Acids
Decarboxylation
Reaction type: Elimination
Loss of carbon dioxide is called decarboxylation.
Simple carboxylic acids rarely undergo decarboxylation.
Carboxylic acids with a carbonyl group at the 3- (or b-) position readily undergo thermal
decarboxylation, e.g. derivatives of malonic acid.
The reaction proceeds via a cyclic transition state giving an enol intermediate that
tautomerises to the carbonyl.