Carboxylic Acid Derivatives and Their Reactions

Carboxylic Acid Derivatives

  • Definition: All carboxylic acid derivatives have a heteroatom (O, N, or halide) bonded to the carbonyl carbon.

    • Example: Aldehydes and ketones only have C or H bonded to the carbonyl carbon.

  • Importance in Industries:

    • Esterases clarify fruit juices.
    • Peptides/proteins, including insulin fragments, are hormones in our body.
    • Ester hydrolysis is used as a screening test for infections (e.g., UTIs).
    • Pectin and its derivatives play roles in food and biological systems.

Nomenclature of Carboxylic Acid Derivatives

  • Suffixes:
    • Carboxylic acid: –oic acid
    • Ester: –oate
    • Acid chloride: –oyl halide
    • Acid anhydride: –oic anhydride
    • Amide: –amide
  • Example Names:
    • N-propylpropanamide (for substituted amides)
    • Ethyl pentanoate (for esters, group on O at the beginning).

Acyl Substitution Reaction

  • Mechanism:

    1. Nucleophilic Attack: A nucleophile attacks the carbonyl carbon.
    2. Elimination of Leaving Group: Unlike aldehydes/ketones, where the tetrahedral intermediate leads to an alcohol, carboxylic acid derivatives have leaving groups (LGs) that are eliminated, reforming the carbonyl.

  • Good Leaving Groups:

    • Good LGs are often electronegative atoms or groups that can leave with a stable electronic configuration.
    • Best leaving groups are weak bases.

Examples of Good Leaving Groups:

  • I−, Br−, Cl− (conjugate acids: HI, HBr, HCl)
  • H2O (conjugate acid: H3O+)

Relative Reactivity in Acyl Substitutions

  • Ranking of Reactivity (Most to Least):

    • Acid chloride > Acid anhydride > Carboxylic acid > Ester > Amide

  • Electronic Effects:

    • Electron-withdrawing groups increase carbonyl reactivity by enhancing electrophilicity at carbonyl C.
    • Poor electron donors (excellent electron withdrawing) make hydrides more reactive.

Interconversion of Carboxylic Acid Derivatives

  • More reactive acid derivatives can be converted into less reactive ones.

Synthesis Methods:

  • Acid Chlorides from Carboxylic Acids:
    • Synthesized by reaction with thionyl chloride (SOCl2).
  • Ester and Amide Synthesis:
    • Acid chlorides and anhydrides can react with neutral alcohols and amines.

Fischer Esterification:

  • Direction: Acid-catalyzed reaction of carboxylic acids with alcohols.
  • Equilibrium Considerations: Needs excess alcohol (Le Chatelier’s principle).

Hydrolysis Reactions

  • Ester Hydrolysis:
    • Acidic Conditions: Reverse of Fischer esterification using H2O as nucleophile.
    • Basic Conditions: Enzymatic hydrolysis in the body via esterases; also known as saponification.

Amide Hydrolysis

  • Requires strong conditions (e.g., strong acid, heat); amides are the least reactive acid derivative and often resistant to hydrolysis without these conditions.

β-Lactam Antibiotics (e.g., Penicillin)

  • Hydrolysis is more feasible due to ring strain.
  • Resistance is often due to bacterial production of β-lactamases.

Organometallic Reactions

  • Grignard reagents react with esters and amides to give tertiary alcohols, needing two equivalents to react completely.