Carboxylic Acid

Organic compounds having a carboxyl (-COOH) functional group are carboxylic acid. The carboxyl group is made of carbonyl (>C=O) and hydroxyl group (-OH).

Nomenclature
1. Monocarboxylic acid (IUPAC- Alkanoic acid):
StructureIUPAC NameCommon name
HCOOHMethanoic acidFormic acid
CH3COOHEthanoic acidAcetic acid
CH3CH2COOHPropanoic acidPropionic acid
CH3CH2CH2COOHButanoic acidButyric acid
2. Dicarboxylic acid- IUPAC name : Alkanedioic acid
Isomerism

Aliphatic monocarboxylic acid shows the following types of isomerism.

1. Chain isomerism
  • Chain isomers of C4H8O2
2. Functional isomerism

Carboxylic acid shows functional isomerism with ester.
Functional isomers of C2H4O2

Preparation of carboxylic acid

1. From aldehyde: Oxidation of aldehyde with acidified KMnO4 or K2Cr2O7 gives carboxylic acid.

\begin{align*} \underset{Aldehyde}{RCHO}+[O]&\xrightarrow[]{KMnO_{4}/H^{+}} \underset{Carboxylic\ acid}{RCOOH}\\ \underset{Acetaldehyde}{CH_{3}CHO} + [O]&\xrightarrow[]{KMnO_{4}/H^{+}} \underset{Acetic\ acid}{CH_{3}COOH} \end{align*}

2. From nitrile: Alkyl cyanide (alkanenitrile) on hydrolysis with dilute acid gives carboxylic acid.

3. From dicarboxylic acid: Dicarboxylic acid heated strongly produces monocarboxylic acid.

4. From sodium alkoxide: Sodium alkoxide passed with carbon monoxide gives carboxylic acid.

5. From trihaloalkane: Hydrolysis of trihaloalkane with aqueous NaOH followed by acidification gives carboxylic acid.


Physical properties:
  1. State, color, odour: The first three members are colorless liquid with a pungent odour. The next three members are oily liquids with a rancid butter odour. Higher members are colorless and odourless waxy solid.
  1. Solubility: First four members are soluble in water due to the formation of intermolecular hydrogen bonding with water and the higher members are insoluble in water.
  1. Boiling point: The boiling of the carboxylic acid is higher than those of other class of organic compounds of comparable molecular weight because of the greater extent of intermolecular hydrogen bonding in a carboxylic acid.

alkane < ether < aldehyde < ketone < alcohol < carboxylic acid
Increasing order of boiling point

Chemical properties of Carboxylic Acid

1. Acidic nature: Carboxylic acid dissociates in an aqueous medium to yield hydrogen ions which is responsible for its acidic nature.

\underset{\substack{Carboxylic\\ acid}}{RCOOH} + H_{2}O \rightleftharpoons \underset{\substack{Carboxylate\\ ion}}{RCOO^{-}} + \underset{\substack{Hydronium\\ ion}}{H_{3}O^{+}}

Carboxylic acids are stronger acids because the carboxylate ion (the conjugate base of carboxylic acid) is stabilized by resonance.


Effect of substituent on acidic strength of carboxylic acid:

i. The presence of electron releasing groups (alkyl groups) decreases the acidic strength of carboxylic acid.
Acidic order: HCOOH>CH3COOH>CH3CH2COOH.

ii. The presence of electron-withdrawing groups increases the acidic strength of carboxylic acid.
Cl3COOH > Cl2CHCOOH > ClCH2COOH > CH3COOH

FCH2COOH > ClCH2COOH > BrCH2COOH > ICH2COOH

NO2CH2COOH > ClCH2COOH > OHCH2COOH > NH2CH2COOH

1. Action with metals, metallic oxides and alkalis:

\begin{align*} \underset{Carboxylic\ acid}{RCOOH}+Na &\rightarrow \underset{\substack{Sodium\\ carboxylate}}{RCOONa}+H_{2}\\ \underset{Carboxylic\ acid}{RCOOH}+Na_{2}O &\rightarrow \underset{\substack{Sodium\\ carboxylate}}{RCOONa}+H_{2}O\\ \underset{Carboxylic\ acid}{RCOOH}+NaOH &\rightarrow \underset{\substack{Sodium\\ carboxylate}}{RCOONa}+H_{2}O\\ \end{align*}

2. Action with metal carbonates and bicarbonates (Test of carboxylic acid): Carboxylic acid reacts with sodium carbonate and sodium bicarbonate giving brisk formation of CO2 gas. This reaction is used as a test of carboxylic acid.

3. Action with P2O5 (Dehydration): Carboxylic acid on heating in presence of dehydrating agents like P2O5 to give acid anhydride.

4. α-halogenation of carboxylic acid (HVZ reaction): Carboxylic acid having α-hydrogen atom reacts with halogens (Cl2 or Br2) in presence of red phosphorous to give α-halo carboxylic acid. This reaction is called Hell-Volhard-Zelinsky (HVZ) reaction.

5. Action with LiAlH4: Carboxylic acid is reduced to primary alcohol with LiAlH4.

\begin{align*} RCOOH + 4[H] &\rightarrow RCH_{2}OH + H_{2}O\\ CH_{3}COOH + 4[H] &\rightarrow CH_{3}CH_{2}OH + H_{2}O \end{align*}

Abnormal behavior of formic acid

Formic acid differs from other monocarboxylic acids because besides -COOH group, it contains -CHO group. So, formic acid behaves both as an acid as well as an aldehyde.

1. Tollen’s test
2. Fehling’s test:
3. Action with mercuric chloride:
4. Action with acidified KMnO4:
5. Action of heat:
\underset{Formic\ acid}{HCOOH}\ \xrightarrow[]{\Delta}\ CO_{2}+H_{2}
6. Action with conc. H2SO4 or P2O5 :
\underset{Formic\ acid}{HCOOH}\ \xrightarrow[or\ P_{2}O_{5}]{conc.\ H_{2}SO_{4}}\ CO+H_{2}O

Benzoic Acid:

Benzoic acid is an aromatic organic compound that has a -COOH group attached to the benzene ring.

Preparation of benzoic acid

1. From alkyl benzene: Toluene is oxidized in presence of acidic KMnO4 to give benzoic acid.

Electrophilic substitution reaction

From the above resonating structure, it is clear that the -COOH group decreases the electron density at ortho and para positions. Consequently, incoming electrophile prefers to attack at the meta position. So, -COOH group is a meta directing group.

1. Bromination
2. Nitration
3. Sulphonation
Effect of substituent in acidic nature of benzoic acid

1. Presence of the electron-withdrawing group at the para position increases the acidic strength.

2. Presence of the electron releasing group at para position decreases the acidic strength.

3. Presence of any group at the ortho position increases the acidic nature.

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