Carboxylic Acids, Nomenclature, Acidity of Carboxylic Acids, Structure and Physical Properties (For CBSE, ICSE, IAS, NET, NRA 2022)

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Carboxylic Acids

  • It contains – COOH functional group
  • it՚s widely used and has many industrially important chemicals.
  • Acetic acids e. g. : Vinegar is produced in large quantities. It is very important building blocks in complex biological molecules.
  • Fatty acids have long chain aliphatic acids derived from hydrolysis of fats and oils.
  • Stearic acid contains long chain of 18 carbon atoms.

Nomenclature

  • Several carboxylic acids have long, and their common names are based on their sources.
  • It is named by choosing longest carbon chain containing – COOH group.
  • The final ‘e’ in alkane is replaced by – oic acid.
  • Numbering the carbon chain, -COOH carbon is given number 1 such as:
Carbon Chain

Some of the common carboxylic acids are:

Carboxylic Acids
Carboxylic Acids

Carboxylic acids with 2 carboxyl groups are called as dicarboxylic acids and named by adding dioic acid as suffix to the name of the corresponding hydrocarbon.

Preparation of Carboxylic Acids

  • Oxidation of Alkenes: Alkenes on oxidation with hot alkaline KMnO4 yields carboxylic acids.
Oxidation of Alkenes
  • Oxidation of Alcohols and Aldehydes: Alcohols and aldhydes can be oxidized to carboxylic acids using various oxidizing agents.
  • Oxidation of Alkyl benzenes: Primary and secondary alkyl groups attached to the benzene ring are oxidized using alkaline KMNO4 to carboxyl group. Acidified sodium dichromates are used in this oxidation.
Oxidation of Alkyl Benzenes
  • Carbonation of Grignard Reagents: It react with to give magnesium carboxylates which on acidification yields carboxylic acids. Here there is an increase of 1 carbon atom in the carboylic acid as compared to the starting alkyl halide.
Carbonation of Grignard Reagents
  • Hydrolysis of Nitriles and Cyanohydrins: On hydrolysis of alkyl halides can be converted to nitriles which yield carboxylic acids which have 1 extra carbon atoms than alkyl halide.
Dibromoethance

Cyanohydrins obtained from aldehyde also yield 2-hydroxycarboxylic acids on hydrolysis.

Structure and Physical Properties

  • It has hybridized carbon atom.
  • The 3 atoms attached to this carbon lies in the same plane with an bond angle of 120o between them.
  • Carboxylic acids form hydrogen bonds because of the presence of polar carbonyl and hydroxyl groups.
  • The carboxylic acids exist in dimeric form in which the molecules are held together by 2 hydrogen bonds.
Carboxylic Acids
Dimer of a Carboxylix Acid
  • Intermolecular hydrogen bonding is responsible for high melting and boiling points.
  • The lower members of carboxylic acids have appreciable water solubility because of the presence of hydrogen bonding between carboxylic acid and solvent water molecules.

Acidity of Carboxylic Acids

  • Carboxylic acids are acidic in nature, it mixes water in equilibrium to give a proton and carboxylate ions.
Carboxylic Acid
  • The carboxylic acids are more acidic than alcohols because lower the pKa leads to greater acidity.
  • The carboxylate ion obtained from dissociation of carboxylic acids are represented as resonance hybrid of these below 2 structure
Resonance Structures of Carboxylate Ion
  • The carboxylate ion gets stabilized due to delocalization of negative charge over 2 oxygen atoms.
  • The release of protons from – COOH group facilitates the stability of carboxylate ion.
  • The alkyl groups release electrons so it decreases in acidity eg: ethanoic acid is less acidic than methanoic acid. .
  • The electron-donating substituents decrease the acidity of carboxylic acids.
  • The pka value of choloroethanoic acid is 2.86 and it is stronger than ethanoic acid (pka – 4.76) . As halogen has – I effect and pulls the electron towards itself which facilitates the release of H + ions.
Ethanoic Acid
  • As the number of halogen groups increases in carboxylic acid its acidity increases because of the release of H + ion.
  • The inductive effect decreases with increase in the distance of the group in the carbon chain, thus the acidic nature of 2-chlorobutanoic acid (pka 2.86) is more than 3-chlorobutanoic acid (4.05) .
Chlorobutanoic Acid

Reaction of Carboxylic Acids

Formation of Salts

The carboxylic acids deprotonate by strong bases such as metal hydroxides to give salts.

Ethanoic Acid
  • Soaps are sodium salts of long chain carboxylic acids such as fatty acids.
  • The carboxylic acids are deprotonated by weak bases such as sodium bicarbonate.
Reduction of Carboxylic Acids

The carboxylic acids are reduced to primary alcohols by lithium aluminium hydride

Phenylethanoic Acid
Hell-Volhard – Zelinski Reaction

Carboxylic acids undergo halogenations at α-carbon atom using Br or in the presence of phosphorus or phosphorus trihalide.

Carboxylic Acids

The alpha-halo acids are useful intermediates in the sysnthesis of other organic compounds.

Synthesis of Acid Derivatives
  • It is very important reaction of carboxylic acids.
  • The nucleophilic addition to the carboxyl carbon of carboxylic acids is followed by elimination of leaving group to a substitution product.
  • The addition of nucleophile is followed by addition of proton to give addition product.
  • The substitution takes place at acyl carbon atom it is also known as nucleophilic acyl substitution.
Nucleophilic
  • Here in above picture, X indicates OH and is halide ion.
  • Carboxylic Acids
    Are carboxyl acid halide, anhydrides, esters and amide group as the substitution product is known as derivatives of carboxylic acids because they are derived from carboxylic acids.
Carboxylic Acids

Formation of acid chlorides: The carboxylic acids react with SOCl2, PCl3 or PCl5 to give carboxylic acid chlorides known as acyl chlorides.

Formation of Acid Chlorides

Formation of Acid Anhydrides: The carboxylic acid anhydrides are derived from carboxylic acids by loss of water. Their name is derived from corresponding acids by using word anhydride in place of acids. This method is used for the preparation of symmetrical anhydride. The unsymmetrical anhydride is prepared from reacting the carboxylic acid with acyl chlorides in the presence of pyridine to give carboxylic acid anhydrides.

Formation of Acid Anhydrides

Cyclic anhydrides are obtained by the dehydration of dicarboxylic acids at higher temperature.

Cyclic Anhydrides
  • Formation of Esters: The carboxylic acids react with alcohols to form esters.
Formation of Esters
  • Here in above equation, the acid is catalyzed esterification in equilibrium reaction.
  • The excess water in the product are used as solvent to carry out esterification.
Ethanoic Acid Methanol
  • Esters are also called as Alkyl alkanoates as alkyl group is derived from alcohol while alkanoate is derived from carboxylic acis.
  • Esters are also prepared by reaction of acid chlorides or acid anhydrides with alcohols.
Acid Chlorides or Acid Anhydrides
  • Formation of amides: The carboxylic acids react with ammonia or amines to give amides.
  • It involves in the formation of ammonium carboxylate salt as an intermediate which on heating gives amide.
Ammonium Carboxylate
  • Amides can also be obtained by the reaction of ammonia or amines with carboxylic acid halides, anhydrides and esters.
Amide
  • The order of reactivity of various carboxylic acid derivatives are as follows: Acid chloride > Acid anhydride > Ester > Amide.
  • The amide derivatives are prepared from its higher reactive substances. It is summarized as following:
Amide Derivatives

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