JEE Main & Advanced Chemistry Carboxylic Acids Chemical Properties Of Monocarboxylic Acids

(1) Reaction involving removal of proton from \[-OH\] group

 

(i) Action with blue litmus : All carboxylic acids turn blue litmus red.

 

(ii) Reaction with metals

 

\[2C{{H}_{3}}COOH+2Na\to \underset{\text{Sodium acetate}}{\mathop{2C{{H}_{3}}COONa}}\,+{{H}_{2}}\]

 

\[2C{{H}_{3}}COOH+Zn\to (\underset{\text{Zinc acetate}}{\mathop{C{{H}_{3}}COO{{)}_{2}}}}\,Zn+{{H}_{2}}\]

 

(iii) Action with alkalies 

 

\[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,+NaOH\to \underset{\text{Sodium acetate}}{\mathop{C{{H}_{3}}COONa}}\,+{{H}_{2}}O\]

 

(iv) Action with carbonates and bicarbonates

 

\[2C{{H}_{3}}COOH+N{{a}_{2}}C{{O}_{3}}\to \underset{\text{Sod}\text{. acetate}}{\mathop{2C{{H}_{3}}COONa}}\,+C{{O}_{2}}+{{H}_{2}}O\]

 

\[C{{H}_{3}}COOH+NaHC{{O}_{3}}\to \underset{\text{Sod}\text{. acetate}}{\mathop{C{{H}_{3}}COONa}}\,+C{{O}_{2}}+{{H}_{2}}O\]

 

• Reaction of carboxylic acid with aqueous sodium carbonates solution produces bricks effervescence. However most phenols do not produce effervescence. Therefore, this reaction may be used to distinguish between carboxylic acids and phenols.

 

(2) Reaction involving replacement of –OH group              

 

(i) Formation of acid chloride

 

\[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,+PC{{l}_{5}}\to \underset{\text{Acetyl chloride}}{\mathop{3C{{H}_{3}}COCl}}\,+POC{{l}_{3}}+HCl\]

 

\[\underset{\text{Acetic acid}}{\mathop{3C{{H}_{3}}COOH}}\,+PC{{l}_{3}}\to \underset{\text{Acetyl chloride}}{\mathop{3C{{H}_{3}}COCl}}\,+{{H}_{3}}P{{O}_{3}}\]

 

\[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,+SOC{{l}_{2}}\to \underset{\text{Acetyl chloride}}{\mathop{C{{H}_{3}}COCl}}\,+S{{O}_{2}}+HCl\]

 

(ii) Formation of esters (Esterification)

 

 

\[\underset{\begin{smallmatrix} \text{Ethyl acetate} \\ \text{(Fruity smelling)}\end{smallmatrix}}{\mathop{C{{H}_{3}}COO{{C}_{2}}{{H}_{5}}}}\,+{{H}_{2}}O\]

 

(a) The reaction is shifted to the right by using excess of alcohol or removal of water by distillation.

 

(b) The reactivity of alcohol towards esterification.

 

tert-alcohol < sec-alcohol < pri-alcohol < methyl alcohol

 

(c) The acidic strength of carboxylic acid plays only a minor role.

 

\[{{R}_{3}}CCOOH<{{R}_{2}}CHCOOH<RC{{H}_{2}}COOH<C{{H}_{3}}COOH<HCOOH\]

 

When methanol is taken in place of ethanol. then reaction is called trans esterification.

 

(iv) Formation of amides

 

\[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,+N{{H}_{3}}\xrightarrow{\text{heat}}\underset{\text{Amm}\text{. acetate}}{\mathop{C{{H}_{3}}COON{{H}_{4}}}}\,\xrightarrow{\Delta }\]

 

\[\underset{\text{Acetamide}}{\mathop{C{{H}_{3}}CON{{H}_{2}}}}\,+{{H}_{2}}O\]

 

(v) Formation of acid anhydrides

 

 

(vi) Reaction with organo-metallic reagents

 

\[R'C{{H}_{2}}MgBr+RCOOH\xrightarrow{\text{ether}}\underset{\text{Alkane}}{\mathop{R'C{{H}_{3}}}}\,+RCOOMgBr\]

 

(3) Reaction involving carbonyl \[(>C=O)\] group:

 

 Reduction : \[R-\underset{O}{\mathop{\underset{|\,|}{\mathop{C}}\,}}\,-OH\xrightarrow{LiAl{{H}_{4}}}R-C{{H}_{2}}-OH\]

 

Carboxylic acid are difficult to reduce either by catalytic hydrogenation or \[{Na}/{{{C}_{2}}{{H}_{5}}OH}\;\]

 

(4) Reaction involving attack of carboxylic group \[(-COOH)\]

 

(i) Decarboxylation : \[R-\overset{O}{\mathop{\overset{|\,|}{\mathop{C}}\,}}\,-OH\xrightarrow{(-C{{O}_{2}})}R-H\]

 

When anhydrous alkali salt of fatty acid is heated with sodalime then :

 

\[\underset{\text{Sodium salt}}{\mathop{RCOONa}}\,+NaOH\underset{\text{heat}}{\mathop{\xrightarrow{CaO}}}\,\underset{\text{Alkane}}{\mathop{R-H}}\,+N{{a}_{2}}C{{O}_{3}}\]

 

• When sodium formate is heated with sodalime H₂ is evolved. (Exception)

 

\[HCOONa+NaOH\xrightarrow{CaO}{{H}_{2}}+N{{a}_{2}}C{{O}_{3}}\]

 

 

(ii) Heating of calcium salts

 

\[\underset{\text{Sodium salt}}{\mathop{{{(RCOO)}_{2}}Ca}}\,\xrightarrow{\text{heat}}\underset{\text{Ketone}}{\mathop{RCOR}}\,+CaC{{O}_{3}}\]

 

(iii) Electrolysis : (Kolbe's synthesis) 

 

\[RCOONa\] ? \[RCO{{O}^{-}}+N{{a}^{+}}\]

 

At anode \[2RCO{{O}^{-}}\to R-R+2C{{O}_{2}}+2{{e}^{-}}\]

 

At cathode \[2N{{a}^{+}}+2{{e}^{-}}\to 2Na\xrightarrow{2{{H}_{2}}O}2NaOH+{{H}_{2}}\]

 

\[\underset{\text{Potassium acetate}}{\mathop{2C{{H}_{3}}COOK}}\,+2{{H}_{2}}O\xrightarrow{\text{Electrolysis}}\]

 

\[\underset{\text{Ethane}}{\mathop{C{{H}_{3}}-C{{H}_{3}}}}\,+2C{{O}_{2}}+2KOH+{{H}_{2}}\]

 

(iv) Formation of Alkyl halide (Hunsdiecker's reaction) 

 

\[\underset{\text{Silver acetate}}{\mathop{C{{H}_{3}}COOAg}}\,+B{{r}_{2}}\underset{CC{{l}_{4}}}{\mathop{\xrightarrow{\text{heat}}}}\,\underset{\text{Methyl bromide}}{\mathop{C{{H}_{3}}Br}}\,+AgBr+C{{O}_{2}}\]

 

• In Hunsdiecker reaction, one carbon atom less alkyl halide is formed from acid salt.

 

(v) Formation of amines (Schmidt reaction) 

 

\[\underset{\text{Acid}}{\mathop{RCOOH}}\,+\underset{\begin{smallmatrix} \text{Hydrazoic} \\ \text{acid}\end{smallmatrix}}{\mathop{{{N}_{3}}H}}\,\xrightarrow{{{H}_{2}}S{{O}_{4}}(conc.)}\underset{\begin{smallmatrix} \text{Primary} \\ \text{amine}\end{smallmatrix}}{\mathop{RN{{H}_{2}}}}\,+C{{O}_{2}}+{{N}_{2}}\]

 

In Schmidt reaction, one carbon less product is formed.

 

(vi) Complete reduction

 

\[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,+6HI\xrightarrow{P}\underset{\text{Ethane}}{\mathop{C{{H}_{3}}C{{H}_{3}}}}\,+2{{H}_{2}}O+3{{I}_{2}}\]

 

In the above reaction, the – COOH group is reduced to a \[C{{H}_{3}}\] group.

 

(5) Reaction involving hydrogen of a-carbon

 

Halogenation

 

(i) In presence of U.V. light

 

\[-\overset{H}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}\,-COOH+C{{l}_{2}}\xrightarrow{U.V.\Delta }\underset{\alpha \text{-chloro acid}}{\mathop{-\overset{Cl}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,-}}\,COOH+}}\,HCl\]

 

(ii) In presence of Red P and diffused light [Hell Volhard-zelinsky reaction]

 

Carboxylic acid having an a-hydrogen react with \[C{{l}_{2}}\] or \[B{{r}_{2}}\] in the presence of a small amount of red phosphorus to give chloro acetic acid. The reaction is known as Hell Volhard-zelinsky reaction.

 

\[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,\underset{-HCl}{\mathop{\xrightarrow{C{{l}_{2}},\text{red }{{P}_{4}}}}}\,\underset{\text{Chloro acetic acid}}{\mathop{ClC{{H}_{2}}COOH}}\,\underset{-HCl}{\mathop{\xrightarrow{C{{l}_{2}},\text{red}\,{{P}_{4}}}}}\,\]

 

\[\underset{\text{Dichloro acetic acid}}{\mathop{C{{l}_{2}}CHCOOH}}\,\underset{-HCl}{\mathop{\xrightarrow{C{{l}_{2}}\text{, red }{{P}_{4}}}}}\,\underset{\text{Trichloro acetic acid}}{\mathop{C{{l}_{3}}CCOOH}}\,\]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(a) The reaction is shifted to the right by using excess of alcohol or removal of water by distillation.           

 

(b) The reactivity of alcohol towards esterification.            tert-alcohol < sec-alcohol < pri-alcohol < methyl alcohol            (c) The acidic strength of carboxylic acid plays only a minor role.                       When methanol is taken in place of ethanol. then reaction is called trans esterification.            (iv) Formation of amides            \[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,+N{{H}_{3}}\xrightarrow{\text{heat}}\underset{\text{Amm}\text{. acetate}}{\mathop{C{{H}_{3}}COON{{H}_{4}}}}\,\xrightarrow{\Delta }\] \[\underset{\text{Acetamide}}{\mathop{C{{H}_{3}}CON{{H}_{2}}}}\,+{{H}_{2}}O\]            (v) Formation of acid anhydrides             (vi) Reaction with organo-metallic reagents            \[R'C{{H}_{2}}MgBr+RCOOH\xrightarrow{\text{ether}}\underset{\text{Alkane}}{\mathop{R'C{{H}_{3}}}}\,+RCOOMgBr\]            (3) Reaction involving carbonyl (>C = O) group:                  Reduction :            Carboxylic acid are difficult to reduce either by catalytic hydrogenation or \[{Na}/{{{C}_{2}}{{H}_{5}}OH}\;\]            (4) Reaction involving attack of carboxylic group (? COOH)            (i) Decarboxylation : \[R-\overset{O}{\mathop{\overset{|\,|}{\mathop{C}}\,}}\,-OH\xrightarrow{(-C{{O}_{2}})}R-H\]            When anhydrous alkali salt of fatty acid is heated with sodalime then :            \[\underset{\text{Sodium salt}}{\mathop{RCOONa}}\,+NaOH\underset{\text{heat}}{\mathop{\xrightarrow{CaO}}}\,\underset{\text{Alkane}}{\mathop{R-H}}\,+N{{a}_{2}}C{{O}_{3}}\]            q When sodium formate is heated with sodalime H2 is evolved. (Exception)            \[HCOONa+NaOH\xrightarrow{CaO}{{H}_{2}}+N{{a}_{2}}C{{O}_{3}}\]            (ii) Heating of calcium salts            \[\underset{\text{Sodium salt}}{\mathop{{{(RCOO)}_{2}}Ca}}\,\xrightarrow{\text{heat}}\underset{\text{Ketone}}{\mathop{RCOR}}\,+CaC{{O}_{3}}\]            (iii) Electrolysis : (Kolbe's synthesis)             \[RCOONa\rightleftharpoons RCO{{O}^{-}}+N{{a}^{+}}\]            At anode \[2RCO{{O}^{-}}\to R-R+2C{{O}_{2}}+2{{e}^{-}}\]            At cathode \[2N{{a}^{+}}+2{{e}^{-}}\to 2Na\xrightarrow{2{{H}_{2}}O}2NaOH+{{H}_{2}}\]            \[\underset{\text{Potassium acetate}}{\mathop{2C{{H}_{3}}COOK}}\,+2{{H}_{2}}O\xrightarrow{\text{Electrolysis}}\] \[\underset{\text{Ethane}}{\mathop{C{{H}_{3}}-C{{H}_{3}}}}\,+2C{{O}_{2}}+2KOH+{{H}_{2}}\]            (iv) Formation of Alkyl halide (Hunsdiecker's reaction)             \[\underset{\text{Silver acetate}}{\mathop{C{{H}_{3}}COOAg}}\,+B{{r}_{2}}\underset{CC{{l}_{4}}}{\mathop{\xrightarrow{\text{heat}}}}\,\underset{\text{Methyl bromide}}{\mathop{C{{H}_{3}}Br}}\,+AgBr+C{{O}_{2}}\]            q In Hunsdiecker reaction, one carbon atom less alkyl halide is formed from acid salt.            (v) Formation of amines (Schmidt reaction)             \[\underset{\text{Acid}}{\mathop{RCOOH}}\,+\underset{\begin{smallmatrix}  \text{Hydrazoic} \\  \text{acid} \end{smallmatrix}}{\mathop{{{N}_{3}}H}}\,\xrightarrow{{{H}_{2}}S{{O}_{4}}(conc.)}\underset{\begin{smallmatrix}  \text{Primary} \\  \text{amine} \end{smallmatrix}}{\mathop{RN{{H}_{2}}}}\,+C{{O}_{2}}+{{N}_{2}}\]            In Schmidt reaction, one carbon less product is formed. (vi) Complete reduction \[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,+6HI\xrightarrow{P}\underset{\text{Ethane}}{\mathop{C{{H}_{3}}C{{H}_{3}}}}\,+2{{H}_{2}}O+3{{I}_{2}}\] In the above reaction, the ? COOH group is reduced to a \[C{{H}_{3}}\] group. (5) Reaction involving hydrogen of a-carbon Halogenation (i) In presence of U.V. light \[-\overset{H}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}\,-COOH+C{{l}_{2}}\xrightarrow{U.V.\Delta }\underset{\alpha \text{-chloro acid}}{\mathop{-\overset{Cl}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,-}}\,COOH+}}\,HCl\] (ii) In presence of Red P and diffused light [Hell Volhard-zelinsky reaction] Carboxylic acid having an a-hydrogen react with Cl2 or Br2 in the presence of a small amount of red phosphorus to give chloro acetic acid. The reaction is known as Hell Volhard-zelinsky reaction. \[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,\underset{-HCl}{\mathop{\xrightarrow{C{{l}_{2}},\text{red }{{P}_{4}}}}}\,\underset{\text{Chloro acetic acid}}{\mathop{ClC{{H}_{2}}COOH}}\,\underset{-HCl}{\mathop{\xrightarrow{C{{l}_{2}},\text{red}\,{{P}_{4}}}}}\,\] \[\underset{\text{Dichloro acetic acid}}{\mathop{C{{l}_{2}}CHCOOH}}\,\underset{-HCl}{\mathop{\xrightarrow{C{{l}_{2}}\text{, red }{{P}_{4}}}}}\,\underset{\text{Trichloro acetic acid}}{\mathop{C{{l}_{3}}CCOOH}}\,\]