Introductions
Aromatic Carboxylic acid
Organic compounds containing carboxyl group (-COOH) as a functional group are called
carboxylic acid. The carboxyl group is the combination of the carbonyl & hydroxyl groups.
General representation:
R─ COOH or Ar ─ COOH
Where, R= alkyl groups (in aliphatic carboxylic acids) and Ar = aryl group usually Phenyl group (in
aromatic carboxylic acid)
Nomenclature
IUPAC format : Alkanoic Acid
Aliphatic Carboxylic Acids
Aromatic Carboxylic acid
Isomerism
2) Functional Isomerism:
Example: Isomers of C2H4O2
Carboxylic acid shows following types of isomerism.
1) Chain Isomerism:
Example: Isomers of C4H8O2
2) From Nitriles (or Cyanides)
Alkane nitrile undergoes acidic hydrolysis to give carboxylic acid.
Question: Write the functional isomer of ethanoic acid & its IUPAC names.
2) From Nitriles (or Cyanides)
Alkane nitrile undergoes acidic hydrolysis to give carboxylic acid.
Question: Write the functional isomer of ethanoic acid & its IUPAC names.
General Methods of Preparation of Monocarboxylic acids
1) From Oxidation of Aldehydes
Aldehydes are easily oxidized into carboxylic acids by using oxidizing agents under normal conditions.
3) From Grignard’s reagent (Carboxylation reaction)
Grignard reagent reacts with carbon dioxide in the presence of dry ether to form an addition product, which
on acid hydrolysis gives carboxylic acid.
4) From Dicarboxylic Acid
When dicarboxylic acid is heated with glycerol, forms monocarboxylic acid.
5) From Sodium Alkoxide
When sodium alkoxide reacts with carbon monoxide in presence of temperature and pressure, gives
sodium alkanoate, which on acidification efnrdil. HCl forms carboxylic acid.
6) From Trihaloalkanes
Question. What is meant by carboxylation reaction? Write an example of it.
Question. Give the chemical reaction for the preparation of ethanoic acid from
i) Sodium ethoxide ii) Ethane nitrile iii) Methyl magnesium iodide
iv) 1,1,1-Trichloroethane
Preparation of Benzoic acid from Alkylbenzene
By Oxidation of Toluene
By Oxidation of Benzaldehyde
Physical Properties of Monocarboxylic Acid
a)State, color, and odor: Lower members of carboxylic acids up to C10 are colorless unpleasant smelling
liquid while higher members are colorless, odorless waxy solids.
b) Solubility: The first four members are completely soluble in water because they can form
intermolecular hydrogen bonds with water molecules whereas higher members are insoluble in water due
to an increase in the hydrophobic nature of the hydrocarbon portion
Question: The boiling point of Methanoic acid is higher than ethanol though they have same molecular
mass. Explain
Question: The boiling point of Methanoic acid is higher than ethanol though they have same molecular
mass. Explain
Question: The boiling point of Methanoic acid is higher than ethanol though they have same molecular
mass. Explain
Chemical Properties of Aliphatic and Aromatic carboxylic acids
1)Acidic nature:
Carboxylic acids are more acidic in nature than aliphatic alcohols because the carboxylate ion forme
after the ionization of carboxylic acid is resonance stabilized whereas alkoxide ion formed after the
ionization of alcohol is not resonance stabilized. More the stability of ions in an aqueous solution higher
will be the acid strength.
Example: Acetic acid (Ethanoic acid) is a stronger acid than Ethanol
Effect of Substituent on acidic strength of Carboxylic Acid
i) Electron donating groups such as −CH3, −OH, −NH2, etc. donates the electrons and shows + I effect
which destabilizes the carboxylate anion by intensifying the negative charge on the oxygen atom & decrease
the acid strength of carboxylic acid.
Example:
Question: Why is ethanoic acid (acetic acid) weaker acid than Methanoic acid (Formic acid)?
ii) Electron withdrawing groups such as −F, −Cl, −Br, −I, −CN, −NO2, etc. withdraw the electrons and
shows the − I effect, which stabilizes the carboxylate anion by dispersing the negative charge on the oxygen atom
& increase the acid strength of carboxylic acid.
Example:
Question: Why is Chloroacetic acid stronger acid than acetic acid?
Effect of Substituent on acidic strength of Benzoic Acid
Benzoic acid is less acidic than formic acid but more acidic than acetic acid because the phenyl group (−C6H
is a weak electron-releasing group than the −CH3 group of acetic acid. −CH3 group intensify the negative char
more in acetate ion and destabilize than the −C6H5 group in benzoate ion.
Example:
Decreasing Order of acidic strength: HCOOH > C6H5COOH> CH3COOH
Electron donating substituent (-OH, -NH2, -CH3, etc.) decreases the acidic strength of benzoic acid
whereas electron-withdrawing substituent (-NO2, -Cl, -Br, -I, -CN, etc.) increases the acidic strength
benzoic acid.
For example:
Reactions to show the acidic nature of carboxylic acid
Laboratory Test of Carboxylic Acid
Carboxylic acid reacts with sodium carbonate or sodium bicarbonate & produces brisk effervescence of
carbon dioxide gas whereas other weak acids like alcohol & phenols do not react with sodium
carbonate or sodium bicarbonate.
Other Important Reactions:
Electrophilic Substitution Reaction of Benzoic Acid
−COOH group is meta directing group. Due to resonance in benzoic acid, electron density
decreases more at the ortho and para positions as compared to the meta positions. Hence, the incoming
electrophile will prefer to attack the meta position during the electrophilic substitution
reactions.
Some examples of electrophilic substitution reactions are as follows:
Uses of Benzoic acid
Salt of benzoic acid i.e. sodium benzoate is used as a food preservative.
Esters of benzoic acid are used in perfumery.
Used in organic synthesis (dyes, drugs).
Formic acid (Methanoic acid)
Formic acid differs from other carboxylic acid because it contains both carboxyl group (−COOH)
and aldehyde (−CHO) group. So, it behaves as both acid as well an aldehyde
Acidic Nature:
Abnormal behavior of Methanoic acid (or Formic acid):
Difference between properties of HCOOH & CH3COOH
USES:
- Formic acid is used in leather tanning, the textile industry for the dying process,
- It is used in the preparation of carbon monoxide.
- It is used in medicine for the treatment of gout.
- Acetic acid is used in plastic and rubber industries, as a coagulant in the manufacture of rubber from
latex and casein from milk, as vinegar, and as a solvent.
Aliphatic and Aromatic Derivatives of Carboxylic Acid
IUPAC Nomenclature:
O
Functional Group: CCl
Aliphatic Derivative: “Alkanoyl halide”
Aromatic derivative: “Benzoyl halide”
Examples:
The organic compounds formed by the replacement of the –OH group of carboxylic acid by electron
rich species like -X (halo), -NH2 (Amino), -OR (Alkoxy), and –OCOR group are known as derivatives
of
carboxylic acid.
These derivatives are either aliphatic or aromatic in nature.
i.e
[A] Acid Halide or Acyl halide
IUPAC Nomenclature:
O
Functional Group: CCl
Aliphatic Derivative: “Alkanoyl halide”
Aromatic derivative: “Benzoyl halide”
Examples
Aliphatic Acid halide
Aliphatic Acid halide
[B] Acid Amides
IUPAC Nomenclature: Functional Group:
Aliphatic Derivative: “Alkanamide”
Aromatic derivative: “Benzamide”
Examples:
Aliphatic Acid Amide
Aromatic Acid Amide
[C] Esters
Aliphatic Esters
IUPAC
Nomenclature:
Functional Group:
Secondary suffix: -oate
Aliphatic Derivative: “….. alkanoates”
Aromatic derivative: “……benzoate”
Examples:
Aliphatic Esters
Aromatic Esters
[D] Acid Anhydride
IUPAC Nomenclature: Functional
Group: Secondary suffix: -oic anhydride
IUPAC Name: “….. oic anhydride”
Examples:
Aliphatic Acid Anhydride
Aromatic Acid Anhydride
Claisen Condensation
Hofmann-Bromamide reaction(Decarbonylation reaction)
This reaction is used to decrease the carbon chain by one carbon atom.
When α-hydrogen containing esters undergoes condensation reaction in the presence of a strong base
like sodium ethoxide, then β-keto ester is formed. This reaction is called the Claisen condensation
reaction.
For example:
Hofmann-Bromamide reaction(Decarbonylation reaction)
When primary amide is heated with bromine in the presence of aqueous NaOH or KOH solution, then
Example:
Note: This reaction is used to decrease the carbon chain by one carbon atom.
Comparative Study of Physical Properties of Acid Derivatives
Relative reactivity of Acid derivatives
The reactivity order of acid derivatives toward nucleophilic substitution follows the order;