Monocarboxylic acids

MONOCARBOXLYIC ACID

Monocarboxylic acids, also known as monocarboxylic acids, are organic compounds that contain a single carboxylic acid functional group (-COOH) in their chemical structure. They are also called fatty acids when they are long-chain and present in fats and oils.

Monocarboxylic acids have the general formula of R-COOH, where R represents an alkyl or aryl group. The alkyl group can range from a single carbon atom to over 20 carbon atoms, with examples such as formic acid (HCOOH), acetic acid (CH3COOH), and palmitic acid (C15H31COOH).

Monocarboxylic acids are classified as weak acids because they only partially dissociate in water, releasing a proton (H+) and forming a carboxylate ion (RCOO-). The strength of the acid depends on the electron-withdrawing or electron-donating groups attached to the carboxyl group. Electron-withdrawing groups such as -Cl, -NO2, or -COOH increase the acidity of the carboxylic acid, while electron-donating groups such as -OH or -NH2 decrease the acidity.

Nomenclature of Carboxylic Acids

•    IUPAC names of carboxylic acids have been derived from the corresponding alkanes by replacing the letter ‘e’ of the parent hydrocarbon by ‘oic’ and adding suffix 'acid' at the end.
For example:

•    For compounds containing more than one carboxyl group, the number of carboxyl groups are indicated by adding the multiplicative prefix, di, tri, etc. to the term oic.
                   HOOC – CH2 – CH2 – CH2 –COOH
                              Pentanedioic acid

Structure of Carboxyl Group

The carboxyl carbon is sphybridised and is trigonal planar in shape just like aldehydes and ketones. All bonds to the carboxyl carbon lie in one plane and are separated by about 120°. The carboxylic carbon is less positive than carbonyl carbon because of the possible resonance structure shown below:

Methods of Preparation of Carboxylic Acids

Carboxylic acid from primary alcohols and aldehydes

 Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media or by potassium dichromate (K2Cr2O7) and chromium trioxide (CrO3) in acidic media (Jones reagent).

Carboxylic acid from alkylbenzenes

 Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain. Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected. Suitably substituted alkenes are also oxidised to carboxylic acids with these oxidising reagents.

 

Carboxylic acid from nitriles and amides

 Nitriles are hydrolysed to amides and then to acids in the presence of H+ or as a catalyst. Mild reaction conditions are used to stop the reaction at the amide stage.

Carboxylic acid from Grignard reagents

 Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which in turn give corresponding carboxylic acids after acidification with mineral acid.

Carboxylic acid from acyl halides and anhydrides

 Acid chlorides when hydrolysed with water give carboxylic acids or more readily hydrolysed with aqueous base to give carboxylate ions which on acidification provide corresponding carboxylic acids. Anhydrides on the other hand are hydrolysed to corresponding acid(s) with water.

Carboxylic acid from esters

 Acidic hydrolysis of esters gives directly carboxylic acids while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.

Physical Properties of Carboxylic Acids

•    Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax-like solids.

•    Carboxyl carbon is less positive due to resonance. Hence, the carboxylic acid group does not give nucleophilic substitution reactions.

•    Lower carboxylic acids are completely miscible in water due to their ability to form hydrogen bonds. Higher member of carboxylic acids are solids like waxes and are insoluble in water but soluble in oil.

•    Carboxylic acids have a higher boiling point than aldehydes, ketones or even alcohols of comparable molecular masses. This is due to an extensive network of intermolecular hydrogen bonding.

•    Melting points of aliphatic monocarboxylic acids shows alternation effect, i.e., the melting point of an acid with an even number of carbon atoms is higher than the next lower and next higher homologue containing an odd number of carbon atoms. This is because, in the case of acids with the even number of carbon atoms, the terminal -CHand -COOH groups lie on the opposite sides of the zig-zag chain. As a result, they get closely packed in the crystal lattice.

Chemical Reactions of Carboxylic Acids

Acidity of Carboxylic Acids

Carboxylic acids dissociate in water to give resonance stabilised carboxylate anions and hydronium ion.

The presence of the electron-withdrawing group (EWG) increases the acidic strength and presence of the electron-denoting group (EDG) decreases the acidic strength.

Effect of substituents on the acidity of carboxylic acids: Substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base. 

Electron withdrawing group (EWG) stabilises the carboxylate anion and strengthens the acid

Electron donating group (EDG) destabilises the carboxylate anion and weakens the acid

 

 The effect of the following groups in increasing acidity order is

Ph < I < Br < Cl < F < CN < NO2 < CF3

Thus, the following acids are arranged in order of increasing acidity (based on pKa values):

CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC-CH2COOH >


FCH2COOH > ClCH2COOH > BrCH2COOH > HCOOH > ClCH2CH2COOH >

(continue)

C6H5COOH > C6H5CH2COOH > CH3COOH > CH3CH2COOH

(continue )

 Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid, contrary to the decrease expected due to resonance effect shown below:

This is because of greater electronegativity of sp2 hybridised carbon to which carboxyl carbon is attached. The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity.


Reactions Involving Cleavage of C–OH Bond

Formation of anhydride

 Carboxylic acids on heating with mineral acids such as H2SO4 or with P2O5 give corresponding anhydride.

Esterification of Carboxylic Acids

Carboxylic acids are esterified with alcohols or phenols in the presence of a mineral acid such as concentrated H2SO4 or HCl gas as a catalyst.


 

Mechanism of esterification of carboxylic acids: The esterification of carboxylic acids with alcohols is a kind of nucleophilic acyl substitution. Protonation of the carbonyl oxygen activates the carbonyl group towards nucleophilic addition of the alcohol. Proton transfer in the tetrahedral intermediate converts the hydroxyl group into –+OHgroup, which, being a better leaving group, is eliminated as neutral water molecule. The protonated ester so formed finally loses a proton to give the ester.

 

Reactions of Carboxylic Acids with PCl5, PCl3 and SOCl2

The hydroxyl group of carboxylic acids, behaves like that of alcohols and is easily replaced by chlorine atom on treating with PCl5, PCl3 or SOCl2. Thionyl chloride (SOCl2) is preferred because the other two products are gaseous and escape the reaction mixture making the purification of the products easier.

Reaction of Carboxylic Acids with ammonia

 Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature give amides. For example:



 

Reactions Involving –COOH Group

Reduction of Carboxylic Acids

 Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane. Diborane does not easily reduce functional groups such as ester, nitro, halo, etc. Sodium borohydride does not reduce the carboxyl group.

 

 

Decarboxylation 

 Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime (NaOH and CaO in the ratio of 3 : 1). The reaction is known as decarboxylation.

Substitution Reactions in the Hydrocarbon Part

Halogenation of Carboxylic Acids

Hell-Volhard-Zelinsky reaction

 Carboxylic acids having an α-hydrogen are halogenated at the α-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give α-halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction.

 

Ring substitution of Carboxylic Acids

     –COOH group of aromatic carboxylic acids is a meta–directing group and is the deactivating group.

Uses of Carboxylic Acids

•    Formic acid is used in rubber, textile, dyeing, leather and electroplating industries

•    It is used as a reducing agent.

•    Ethanoic acid is used as a solvent and as vinegar in the food industry.

•    Benzoic acid and its salts are used as urinary antiseptics.

•    Benzoic acid is also used in the preparation of esters and in perfumery

•    Higher fatty acids are used for the manufacture of soaps and detergents

Haloform reaction

Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform. This oxidation does not affect a carbon-carbon double bond, if present in the molecule.



Iodoform reaction with sodium hypoiodite is also used for detection of CH3CO group or CH3CH(OH) group which produces CH3CO group on oxidation.

Aromatic side reaction of carboxylic acid

Reduction of carboxylic acid

Carboxylic acids, acid halides, esters, and amides are easily reduced by strong reducing agents, such as lithium aluminum hydride (LiAlH 4). The carboxylic acids, acid halides, and esters are reduced to alcohols, while the amide derivative is reduced to an amine.