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Presentation on theme: "CARBOXYLIC ACIDS AND DERIVATIVES. Naming of Carboxylic acids."— Presentation transcript:


2 Naming of Carboxylic acids

3 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.

4 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

5 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.

6 Formation of acyl chlorides from carboxylic acids Reagent: PCl 5 or SOCl 2 Condition: room temperature

7 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.

8 Write out the mechanism for hydrolysis of acyl chloride.

9 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.

10 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

11 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.

12 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)

13 3) Reaction with primary amines Condition: room temperature Product: N-substituted amides

14 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

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

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