Nomenclature:   Formula Substituent suffix = -oic acid  e.g. ethanoic acid Substituent prefix = carboxy   The root name is based on the longest chain including the carboxylic acid group. Since the carboxylic acid group is at the end of the chain, it must be C1. The carboxylic acid suffix is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -oic acid = -anoic acid etc.     Functional group is a carboxylic acid, therefore suffix = -oic acid Hydrocarbon structure is an alkane therefore -ane   The longest continuous chain is C4 therefore root = but butanoic acid   CH3CH2CH2C(=O)OH

The anion dervived by deprotonation of a carboxylic acids is the carboxylate.

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 π system of the carbonyl.

Acidity:

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

Reactivity:
 

Reactions of Carboxylic Acids

Preparation of Carboxylic Acid Derivatives

Reaction type:  Nucleophilic Acyl Substitution

Overview

• In principle, all carboxylic acids derivatives can be made from the parent carboxylic acid see above.
• In practice, there may be better methods, e.g. amides are more readily prepared from the more reactive acyl chlorides.
• However, appreciating the relationship between these groups is important and useful.

Preparation of Acyl Chlorides

Reaction type:  Nucleophilic Acyl Substiution

Summary

• Acyl chlorides are prepared by treating the carboxylic acid with thionylchloride, SOCl₂, in the presence of a base.
• Acyl chlorides are by far the most commonly encountered of the acyl halides. .

Back to the top 

Preparation of Acid Anhydrides

Reaction type:  Nucleophilic Acyl Substiution

Summary

• Symmetrical anhydrides can be are prepared by heating the carboxylic acid
• Symmetrical anhydrides are by far the most commonly encountered, e.g.acetic anhydride.

Preparation of Esters

Reaction type:  Nucleophilic Acyl Substiution

Preparation of Esters

Reaction type:  Nucleophilic Acyl Substiution

Summary

• 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 > 1^o > 2^o > 3^o (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.

Back to the top

Preparation of Amides

Reaction type:  Nucleophilic Acyl Substiution

Summary

• 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.

Reduction of Carboxylic Acids

Reaction usually in Et₂O or THF followed by H₃O⁺work-ups

Reaction type:  Nucleophilic Acyl Substiution then NucleophilicAddition

Summary

• 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

a-Halogenation (Hell-Volhard-Zelinsky reaction)

Reaction type: Substitution

Summary

• 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.

Decarboxylation

Reaction type: Elimination

Summary

• 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.

Spectroscopic Analysis

• IR - The -O-H and C=O should be obvious

• ¹H NMR - The -CO₂H proton is very deshielded

• ¹³C NMR
  CO₂H carbon  160 - 185 ppm (deshielded due to O, but not as much as aldehydes and ketones, 190-215 ppm)
   
• UV-VIS
  Simple carboxylic acids absorb at 210 nm, but this is too low to be particularly useful.
   
• Mass Spectrometry
  Peak for the molecular ion, M⁺, is usually prominent.
  Fragments due to loss of OH (M - 17)⁺ and then loss of CO (M - 45)⁺

                                                                                                                                                                                                                                         Alcohols

                                                                                                                                                                                                                                         Aldehydes & Ketones

                                                                                                                                                                                           Carboxylic Acid Derivatives