Carboxylic Acid

Carboxylic acids have an important role in plant and animal life processes. Many organic acids are used for the production of food products, soft drinks, soaps, drugs such as aspirin, phenacetin etc. The huge usage of Aldehydes, ketones and carboxylic acid in making dyestuff, perfumes and rayon. The class of organic compounds which contains >C=O group are known as the carbonyl group. This group is present in a number of families of organic compounds. Few common out of them are:

Carboxylic Acid Functional Group

Carboxylic acids are the organic compounds which contain one or more carboxyl (-COOH) groups. It is a combination of the carbonyl group (>C=O) and hydroxyl group (-OH) linked to each other.Carboxylic AcidThe name carboxyl has been derived from carbo from the carbonyl group and oxy from the hydroxyl group. But it is interesting to note that although carboxylic acids contain a carbonyl group, they do not give the characteristic reactions of this group as we have studied in aldehydes and ketones. Aliphatic (open chains) -COOH obtained by the hydrolysis (breakdown due to reaction with water) of naturally occurring fats and oils are known as fatty acids. For example, palmitic acid, oleic acid, stearic acid etc.

The possible resonance structure is shown below:

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Carboxylic Acid Nomenclature

The IUPAC names have been derived from the corresponding alkanes by replacing the letter ‘e’ of the alkane with ‘oic’ and adding suffix ‘acid’ at the end. The common and IUPAC names of -COOH are listed as follows:

Carboxylic Acid Derivatives

Compounds containing the carboxyl group are distinctly acidic and are called carboxylic acids. The general formula is CnH2nO2. Their derivatives are compounds with functional groups that can be converted to carboxylic acids by a simple acidic or basic hydrolysis. The most important acid derivatives are esters, amides, nitriles, acid halides, and anhydrides.

Carboxylic Acid

Test for Carboxylic Acid

The presence of carboxyl group in a can be detected by the following tests.

(i) Sodium bicarbonate test (effervescence test). A -COOH reacts with an aqueous solution of sodium bicarbonate to evolve CO2 gas with a brisk effervescence.

(ii) Ester test. A -COOH reacts with an alcohol in the presence of small amount of conc. H2SO4 to form an ester with a fruity smell.

(iii) Litmus test. -COOH, in general, turn blue litmus red.

Properties of Carboxylic Acid

  • Boiling point: The boiling point of -COOH acids is higher than that of alcohols, ketones or aldehydes of similar molecular weight.

The high boiling points of carboxylic acids are the result of the formation of a stable hydrogen-bonded dimer.

  • Melting points: There is no regular pattern in the melting point of Carboxylic acid (up to 10 carbon atoms) having an even number of C atoms are higher than neighboring members having an odd number of C atoms. It is because -COOH acid and methyl group in even members lie in opposite side of zig-zag carbon chain hence they fit better into Crystal lattice resulting in higher melting points.Vice-versa is observed in case of a -COOH having an odd number of carbon atoms.

Amides have Surprisingly high boiling points and melting points compared with other compounds of similar molecular Weight. Primary and Secondary amides participate in strong hydrogen bonding.

  • Solubility: -COOH form hydrogen bonds with water and the lower molecular – weight carboxylic acids (up to 4 carbon atoms) are miscible with water. Acid derivatives (esters, acid chlorides, anhydrides, nitriles, and amides) are soluble in common organic solvents such as alcohols, ethers, chlorinated alkanes and aromatic hydrocarbons. Acid chlorides and anhydrides cannot be used in nucleophilic solvents such as HO and alcohols because they react with these Solvents.

Alcohol to Carboxylic Acid

Primary alcohols first oxidized to the aldehyde and then monocarboxylic acids with the same number of carbon atoms with normal oxidizing agents such as acidified K2Cr2O7  or KMnO4. For Example:

Aldehyde to Carboxylic acid

An Aldehyde can be oxidized to corresponding acids with mild oxidizing agents such as ammoniacal silver or Tollen’s reagent [Ag(NH3)+2OH].

Benzaldehyde gets oxidized to benzoic acid even with atmospheric oxygen.

C6H5CHO +  [O] -→  C6H5COOH

Nitrile to Carboxylic Acid

Cyanides can be hydrolyzed by the acids by a boiling point with mineral acids (HCl or H2So4) or alkali (NaOH OR KOH). for example:

Ester to Carboxylic Acid

Esters are hydrolyzed with mineral acids or alkalis to give carboxylic acids. Mineral acids give directly output. The basic hydrolyzed gives the carboxylates first when further acidified it gives -COOH compounds. For example:

Carboxylic Acid to Alcohol

All these reagents can reduce acid to alcohol. Reagents may be:

  • LiAlH/ Ether
  • B2H6
  • H3O+

Carboxylic Acid to Ketone

The following reaction shows the reagent helps in the reduction of -COOH to ketones with organolithium compounds.

 

Carboxylic to Ester

Acids are treated with alcohols using mineral acid (Con.H2SO4) or HCl gas as a catalyst. It is a kind of nucleophilic acyl substitution. Protonation of the carbonyl oxygen activates the carbonyl group towards the nucleophilic addition of the alcohol.

Carboxylic Uses

  • Methanoic acid: In rubber, textile, dyeing, leather and electroplating industries.
  • Ethanoic acid:  As a solvent and as a vinegar in the food industry.
  • Hexanoic acid: In the manufacture of nylon-6, 6.
  • Higher fatty acids: For the manufacture of soaps and detergents.
  • Esters of benzoic acid: In perfumery.
  • Sodium benzoate: As a food preservative.

Carboxylic Acid Reactions

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