Carboxylic acid derivatives: reactions

Hydrolysis of acyl chlorides to form carboxylic acids

RnCs: water

Observation: Rapid reaction forming whtie fumes of HCl

Hydrolysis of alkyl chloride(halogenoalkane)

RnCs: NaOH (aq), heat under reflux

OR KOH (aq), heat under reflux

Condensation of acylchloride to form esters

RnCs: alcohol, room temperature OR phenol, room temperature

Observations: white fumes of HCl

Condensation of acylchloride to form amides

RnCs: primary amides in excess

Note:

• in excess to ensure complete reaction as amine may react with HCl to form an ammonium salt via acid-base reaction

• carboxylic acids cannot be used in place of acylchloride as they will react with amines in acid-base reaction instead

• acyl chloride can also react with secondary amines or ammonia (but not tertiary amines as they lack H atom to be replaced with N atom)

rank the rate of hydrolysis in increasing order

• acyl chlorides

• aryl chlorides

• alkyl chlorides

aryl chlorides(no reaction) < alkyl chlorides < acyl chlorides

Use electronic factor to explain why can’t aryl chlorides undero hydrolysis under normal lab conditions

• lone pair of electron on Cl are delocalised into the benzene ring

• imparting partial double bond character to the C-Cl bond

• makiing it difficult to cleave the C-Cl bond

• Carbon in C-Cl is least electron deficient and attracts nucleophile(OH^-) minimally

Use steric factor to explain why can’t aryl chlorides undergo hydrolysis under normal lab conditions

• large aromatic ring gives more steric hindrance to approaching nucleophilic attack

• electron-rich benzene ring repels nucleophile

Use electornic reason to explain why is rate of hydrolysis for acyl chloride faster than alkyl chloride.

in acyl chloride,

• carbon on C-Cl is attached directly to 2 electronegative atoms Cl and O

• carbon is highly electron deficient and attracts nucleophile(OH^-) strongly

In alkyl chloride,

• carbon on C-Cl is attached directly to only 1 electronegative Cl atom

• carbon is less electron deficient and attracts the nucleophile(OH^-) less strongly

Use steric reason to explain why is rate of hydrolysis for acyl chloride faster than alkyl chloride.

In acyl chloride,

• planar acyl carbon is less sterically hindered for apporaching nucleophile to attack

In alkyl chloride,

• tetrahedral geometry around carbon is more sterically hindered for approaching nucleophile to attack

Acid hydrolysis of ester

RnCs: dilute H₂SO₄, heat

Base hydrolysis (Saponification) of esters

RnCs: dilute NaOH, heat

followed by acidification with diulte H₂SO₄