Carboxylic Acids
Definition:
“A homologous series of organic compounds containing the carboxyl (–COOH)
functional group.”
Note:
★ The carboxyl group itself is made up of a carbonyl group (> C = 0) and a hydroxyl group
(-OH) is called a carboxyl group (‘Carb’ from carbonyl and ‘oxyl’ from hydroxyl).
★ Carboxylic acids are a class of organic acids
★ Carboxylic acids are weak acids (undergo incomplete ionization in water)
★ The name of a carboxylic acid ends with ‘-oic acid’
★ The dilute aqueous solution of ethanoic acid (acetic acid) is vinegar
★ Carboxylic acids and esters exhibit functional group isomerism
General Formula
CₙH₂ₙ₊₁COOH or CₙH₂ₙ₊₁CO₂H or CₙH₂ₙO₂
Nomenclature
❖ Common Names
I.
II.
III.
Common names may be derived from the source from which they are isolated
e.g., some ants spray formic acid (methanoic acid) from their abdomens as a
defence mechanism. The Latin word formica means ‘ant’.
Ethanoic acid was first isolated from vinegar. Hence, it is also known as acetic
acid. Acetum is a Latin word meaning ‘vinegar’.
Likewise, butyric acid (butanoic acid) is named after butyrum (Latin for butter)
❖ IUPAC Names
I.
II.
The IUPAC nomenclature is a standardized naming system that is followed by
chemists from around the world.
It was established to give each compound a unique and unambiguous name.
The following table lists the common/trivial and the IUPAC names for the first four
carboxylic acids:
Formula
Structure
Common Name
IUPAC Name
HCOOH
Formic acid
Methanoic acid
CH₃COOH
Acetic acid
Ethanoic acid
CH₃CH₂COOH
Propionic acid
Propanoic acid
CH₃CH₂CH₂COO
H
Butyric acid
Butanoic acid
CH₃CHCOOH
|
CH₃
Isobutyric acid
2-Methylpropanoic acid
Ethanoic Acid (CH₃COOH):
❖ It is the second simplest carboxylic acid, after methanoic acid
❖ It has a characteristic vinegar smell
❖ An aqueous solution of ethanoic acid is a weak acid
❖ Ethanoic acid is completely miscible with (soluble in) water
❖ Its dilute aqueous solution (5% acetic acid) is known as vinegar
Preparation of ethanoic acid:
The oxidation of ethanol yields ethanoic acid. This can be done in 2 ways:
1.
Bacterial fermentation
Ethyl alcohol can be oxidized to acetic acid by atmospheric oxygen in the presence of
certain bacteria
CH₃CH₂OH
Ethanol
2.
+
O₂
Air
→
CH₃COOH
Ethanoic Acid
Catalytic oxidation
In laboratories, ethanoic acid is produced by heating a mixture of ethanol and acidified
potassium manganate (VII). Acidified potassium dichromate (VI) can also be used for this
purpose.
CH₃CH₂OH
Ethanol
+
[O]
→
Nascent Oxygen
Physical Properties of Carboxylic Acids
As we move down the series,
● m.p & b.p increases
● viscosity increases
● flammability decreases
● density increases
● solubility in water decreases
● solubility in organic solvents increases
CH₃COOH
Ethanoic Acid
Chemical Properties of Carboxylic Acids
Ionization In Water
Carboxylic acids are weak acids as they only partially dissociate/ionise in water to release H⁺
ions (or protons)
CH₃COOH ⇌ CH₃COO⁻ + H⁺
Ethanoic acid Ethanoate ion Proton
Note
★ The acidic properties of carboxylic acids are due to the presence of H⁺ ions
★ Compared to mineral acids, carboxylic acids show a slower rate of reaction
Reactions with Reactive Metals
Carboxylic acids react slowly with reactive metals (i.e. those that are placed above hydrogen in
the reactivity series) to form salt and hydrogen gas. To write the name of the salt, the
ending ‘-oic acid’ of the carboxylic acid is changed to ‘-oate’.
2CH₃COOH(aq) + Zn(s) → (CH₃COO)₂Zn(aq) + H₂(g)
Ethanoic acid
Zinc
Zinc ethanoate
Hydrogen
Reactions with Metal Carbonates and Metal Bicarbonates
Metal bicarbonates are also known as metal hydrogen carbonates. Carboxylic acids react slowly
with metal carbonates and metal bicarbonates to give salt, carbon dioxide gas as well as
water.
2CH₃COOH(aq) + Na₂CO₃(aq) → 2CH₃COONa(aq) + CO₂(g) + H₂O(l)
Ethanoic acid
Sodium carbonate
Sodium ethanoate Carbon dioxide Water
CH₃COOH(aq) + NaHCO₃(aq) →
Ethanoic acid Sodium bicarbonate
CH₃COONa(aq) + CO₂(g) + H₂O(l)
Sodium ethanoate Carbon dioxide Water
Reactions with Bases (Metal Oxides and Metal Hydroxides)
Carboxylic acids will neutralize bases to form organic salts and water.
2CH₃COOH(aq) + CuO(s) → (CH₃COO)₂Cu(aq) + H₂O(l)
CH₃COOH(aq) + NaOH → CH₃COONa + H₂O(l)