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CARBOXYLIC ACIDS AND DERIVATIVES

CARBOXYLIC ACIDS AND DERIVATIVES. Naming of Carboxylic acids. 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.

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CARBOXYLIC ACIDS AND DERIVATIVES

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  1. CARBOXYLIC ACIDS AND DERIVATIVES

  2. Naming of Carboxylic acids

  3. Acidity of carboxylic acids • Weak acids (pH ≈ 4) • Partiallydissociates 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, CH3COO-, 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. • Electron-donating group (EDG) decreases acid strength. • EDG reduces polarisationof the O–H bond strengthening it proton loss is more difficult. • EDG intensifies the negativecharge 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

  5. Electron-withdrawing group (EWG) increases acid strength. • EWG increases polarisationof the O–H bond weakens the bond dissociates readily. • EWG reduces the negativecharge 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 furtheraway from the –COOH group, its effect on acidity decreases. • Similar reasoning for aromatic acids.

  6. Formation of acyl chlorides from carboxylic acids • Reagent : PCl5or SOCl2 • Condition : room temperature

  7. Reaction of acyl chlorides • Acyl chlorides are the most reactive derivatives if carboxylic acids. • Due to electron-withdrawing Clatom makes the carboxyl C atom more positively charged. • Hydrolysis (Nucleophilic substitution) • Condition : room temperature • Observation : heat and white fumes of HCl evolved • Gives immediate white ppt with AgNO3(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 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.

  10. Reactivity of the compounds increases in going from water to NaOH(aq). • Warm NaOH(aq) addded followed by excess dilute HNO3 and AgNO3(aq).

  11. Chlorobenzene has no reaction because : • C–Cl bond is strengthened by overlapping of the p-orbital of Cl with the -orbitals of the benzene ring. • 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 (H2O, 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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