Acidity of carboxylic acids Weak 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 increase Ethanoic acid is stronger than phenol. The O–H bond dissociates more readily to give H + ions, and the carboxylate ion formed, CH 3 COO -, 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. 123
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: PCl 5 or SOCl 2 Condition: room temperature
Reaction of acyl chlorides Acyl 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 evolved Gives immediate white ppt with AgNO 3 (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 AgNO 3 (aq) white ppt of AgCl formed. Ag + (aq) + Cl - (aq) AgCl(s) When warm NaOH(aq) react with ethanoyl chloride followed by excess dilute HNO 3 and AgNO 3 (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 HNO 3 and AgNO 3 (aq). CompoundObservationReaction Ethanoyl chlorideWhite ppt forms immediately.CH 3 COCl + OH - CH 3 COO - + H + + Cl - ChloroethaneWhite ppt forms after several minutes. CH 3 CH 2 Cl + OH - CH 3 CH 2 OH+ Cl - ChlorobenzeneNo pptNo 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 phenols Condition : room temperature Product: 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 amines Condition: room temperature Product: N-substituted amides
Formation of Polyesters Condensation polymerisation monomer molecules join together to form polymer molecule and other small molecules (H 2 O, HCl) are eliminated. Condensation polymerisation polyesters nylons e.g : Terylene Monomers : Condition : heat
Reaction : Polyesters can be weakened by dilute acid and alkalis (acid or base hydrolysis).