CARBOXYLIC ACIDS AND DERIVATIVES

Naming of Carboxylic acids

Acidity of carboxylic acidsWeak acids (pH ≈ 4)Partially dissociates in water.

Carboxylate ion formed is stabilised by delocalisation of the negative charge over the C atom and both O atoms.

Order of acid strength : ethanol < water < phenol < ethanoic acid

acid strength increaseEthanoic acid is stronger than phenol.The O–H bond dissociates more readily to give H+ ions, and

the carboxylate ion formed, CH3COO-, is further stabilised by delocalisation of the negative charge over the C atom and both O atoms.

The equilibrium shift to the right.

The strength of carboxylic acid is affected by the nature of the substituent group.

a)Electron-donating group (EDG) decreases acid strength.EDG reduces polarisation of the O–H bond

strengthening it proton loss is more difficult.EDG intensifies the negative charge on the O atom

carboxylate ion less stable.

* The greater the electron-donating effect, the weaker the acid.

e.g :Arrange the following acids according to increasing acid strength.

1 2 3

a) Electron-withdrawing group (EWG) increases acid strength. EWG increases polarisation of the O–H bond weakens

the bond dissociates readily. EWG reduces the negative charge on the O atom

carboxylate ion stabilised.* The greater the electron-withdrawing effect, the stronger

the acid.

The acidity of halogen-substituted ethanoic acid increase when the halogen is more electronegative.

As substituent group is further away from the –COOH group, its effect on acidity decreases.

Similar reasoning for aromatic acids.

Formation of acyl chlorides from carboxylic acids

Reagent : PCl5 or SOCl2 Condition : room temperature

Reaction of acyl chloridesAcyl chlorides are the most reactive derivatives if

carboxylic acids.Due to electron-withdrawing Cl atom makes the

carboxyl C atom more positively charged.

1)Hydrolysis (Nucleophilic substitution)Condition : room temperature

Observation : heat and white fumes of HCl evolvedGives immediate white ppt with AgNO3(aq)

solution.

Write out the mechanism for hydrolysis of acyl chloride.

Ease of hydrolysis :

Chloroethane and chlorobenzene have no reaction with water.

Ethanoyl chloride reacts vigourously with cold water.Relative ease of hydrolysis can be followed by adding

AgNO3(aq) white ppt of AgCl formed.

Ag+(aq) + Cl-(aq) AgCl(s)When warm NaOH(aq) react with ethanoyl chloride

followed by excess dilute HNO3 and AgNO3(aq), white ppt forms immediately.

Reaction is more vigourous than water.

Reactivity of the compounds increases in going from water to NaOH(aq).

Warm NaOH(aq) addded followed by excess dilute HNO3 and AgNO3(aq).

Compound Observation Reaction

Ethanoyl chloride White ppt forms immediately.

CH3COCl + OH- CH3COO- + H+ + Cl-

Chloroethane White ppt forms after several minutes.

CH3CH2Cl + OH- CH3CH2OH+ Cl-

Chlorobenzene No ppt No reaction

Chlorobenzene has no reaction because :

a)C–Cl bond is strengthened by overlapping of the p-orbital of Cl with the -orbitals of the benzene ring.

b)High electron density on the benzene ring tends to repel the approaching nucleophile, OH-.

Ethanoyl chloride undergoes hydrolysis faster than chloroethane.

because the C atom carries a considerably larger positive charge since it is attached to 2 very electronegative atoms, Cl and O.

2) Reaction with alcohols and phenolsCondition : room temperatureProduct : esters

Reaction is rapid and does not require heating.It is a complete reaction useful in making esters

(espcially phenate esters)

3) Reaction with primary aminesCondition : room temperatureProduct : N-substituted amides

Formation of PolyestersCondensation polymerisation monomer

molecules join together to form polymer molecule and other small molecules (H2O, HCl) are eliminated.

Condensation polymerisation polyesters nylons

e.g : TeryleneMonomers :

Condition : heat

Reaction :

Polyesters can be weakened by dilute acid and alkalis (acid or base hydrolysis).