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General Methods of Preparation of Carboxylic Acids

Oxidation is a direct method most commonly used for preparation of carboxylic acids.  Direct oxidation of alcohols, ketons. aldehydes, alkenes and alkyl benzene results in formation of carboxylic acids

  • Oxidation of Primary Alcohols & Aldehydes

Primary alcohols on oxidation give aldehydes, the reaction does not stop here and the oxidation continues to give carboxylic acids as final product.

              R-CH_3OH\overset{[O]}{\rightarrow}R-CHO\overset{[O]}{\rightarrow}R-COOH

Primary alcohol          Aldehyde                Carboxylic Acid

or

Example: 

CH_3CH_2CH(CH_3)CH_2OH\overset{KMnO_4}{\rightarrow} CH _3CH_2CH(CH_3)COOH

2-Metyl-1-butanol                                            2-Metyl-1-butanoic acid

  • Oxidation of Alkyl Benzenes

Strong oxidation of alkyl benzenes also result in formation of carboxylic acids

           Ar-R \overset{KMnO_4}{\rightarrow} Ar-COOH

Example

NO_2-C_6H_5- CH_3 \overset{KMnO_4}{\rightarrow}NO_2-C_6H_5- COOH

  • Grignard Synthesis of Ccarboxylic Acid?

?Grignard synthesis has the advantage of increasing the length of carbon chain  and thus it is useful for extending the range of available materials.

R- X\overset{Mg}{\rightarrow}R-Mg X \xrightarrow[CO_2]{Dry Ether}R- COOMgX\overset{H^{+}}{\rightarrow} R- COOH

CO2(g) is bubbled into ether solution of grignard reagent which results in addition of grignard reagent to the C=O bond to form magnesium slats of the carboxylica cids from which carboxylic acids are produced by treatement with mineral acids

This method is useful for converting alkyl halides into corresponding carboxylic acids having one carbon atom more than that present in alkyl halides (ascending the series)

  • Oxidation of Alkenes 

Strong oxidation of alkenes results in formation of carboxylic acids. Alkenes can be oxidized to carboxylic acids with hot alkaline KMnO4

RCH=CHR'\overset{KMnO_4, OH^-, Heat}{\rightarrow}RC O_2H +R'C O_2H}

Example:

CH_3CH=CHCH_3 \overset{[O]]}{\rightarrow}2 CH_3COOH

2-Butene                                         Ethanoic Acid

  • Hydrolysis of Nitriles (Cyanides)

?Just like grignard synthesis , nitrile synthesis also increases the length of carbon chain.  Aliphatic nitriles are prepared by treatment of alkyl halides with sodium cyanide in a solvent that will dissolve both reactants. In dimethyl sulfoxide (DMSO), reaction occurs rapidly and exothermically at room temperature. The resulting nitrile is then hydrolysed to the acid by boiling with aqueous alkali or acid.

Example:

This synthetic method is generally limited to the use of primary alkyl halides. Aryl halides (except for those with ortho and para nitro groups) do not react with sodium cyanide.

  • Hydrolysis of Trihalogen Derivatives

Trihalogen derivatives in which the three halogen atoms are all attached to the same carbon atom yields carboxylic acid on hydrolysi

 CH3CCl+ KOH →  CH3CO2  H2O

  • Oxidation of Methyl Ketone

Methyl ketone can be converted to carboxylic acids via the haloform reaction.

  • Koch Reaction

A recent method for manufacturing fatty acids is to heat an olefin with carbon monoxide and steam under pressure at 300-400° C in the presence of a catalyst, e.g. phosphoric acid.

CH2 = CH2 + CO + H2O → CH3 – CH2 – COOH

  • Heating Gem Dicarboxylic Acids

The most convenient laboratory preparation for formic acid is to heat glycerol with oxalic acid at 100 – 110° C.

  • Hydrolysis of  acyl halides and anhydrides

Acid chlorides on hydrolysis give carboxylate ions which on acidification provide corresponding carboxylic acids.

Acid Anhydrides on the other hand give corresponding acid on hydrolysis.

R-COCl+ H_2O\rightarrow R-COOH + Cl^-

Acid Chloride                               Carboxylic acid

R-COCl+ H_2O\overset{OH^-/H_2O}{\rightarrow}R-COO^- + Cl^-\overset{H_3O^+}{\rightarrow}R-COOH

Acid Chloride                                                                                               Carboxylic acid

(C_6H_5CO)_2O\overset{H_2O}{\rightarrow}C_6H_5COOH

Benzoic anhydride             Benzoic Acid

  • Hydrolysis of esters

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

C_6H_5-COOC_2H_5\overset{H_3O^+}{\rightarrow}C_6H_5-COOH + C_2H_5OH

Ethyl Benzoate                                        Benzoic Acid

CH_3CH_2CH_2COOC_2H_5+ NaOH\rightleftharpoons CH_3CH_2CH_2COONa \overset{H_3O^+}{\rightarrow}CH_3CH_2CH_2COOH

Ethyl butanoate                                                                                                                                 Butanoic Acid

You can also refer to  Organic Chemistry Revision Notes and IIT JEE Chemistry Syllabus
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