🧫Organic Chemistry II Unit 4 – Carboxylic Acids and Derivatives

Key Concepts and Definitions

• Carboxylic acids contain a carboxyl functional group (-COOH) consisting of a carbonyl group (C=O) bonded to a hydroxyl group (-OH)
• Derivatives of carboxylic acids include esters, amides, acid anhydrides, and acid chlorides, formed by replacing the -OH group with other functional groups
• Nomenclature of carboxylic acids follows IUPAC rules, with the suffix "-oic acid" added to the longest carbon chain containing the carboxyl group
  • Common names of carboxylic acids often derive from their natural sources (acetic acid, butyric acid)
• Acidity of carboxylic acids is influenced by the stability of the conjugate base, with electron-withdrawing groups increasing acidity
• Reactivity of carboxylic acids and their derivatives depends on the electrophilicity of the carbonyl carbon and the leaving group ability of the substituent

Structure and Nomenclature

• General formula of carboxylic acids is R-COOH, where R represents an alkyl group, aryl group, or hydrogen atom
• IUPAC nomenclature prioritizes the carboxyl group, with the longest carbon chain containing it receiving the "-oic acid" suffix
  • Substituents on the carbon chain are indicated with prefixes and their positions (2-methylpropanoic acid)
• Common names of carboxylic acids often reflect their natural origins or historical discovery (formic acid from ants, citric acid from citrus fruits)
• Cyclic carboxylic acids are named by adding "carboxylic acid" to the name of the corresponding cycloalkane (cyclopentanecarboxylic acid)
• Aromatic carboxylic acids are named as derivatives of benzoic acid, with substituents indicated by prefixes (4-hydroxybenzoic acid)

Physical Properties

• Carboxylic acids are polar molecules due to the presence of the carboxyl group, which forms strong hydrogen bonds
• Lower molecular weight carboxylic acids (up to C4) are miscible with water, while higher molecular weight acids have limited solubility
• Carboxylic acids have higher boiling points compared to alcohols of similar molecular weight due to intermolecular hydrogen bonding
  • Boiling points increase with increasing carbon chain length and the presence of additional polar functional groups
• Carboxylic acids are weak acids, with pKa values typically ranging from 4 to 5, depending on the structure of the R group
• Solid carboxylic acids often have strong, pungent odors (butyric acid in rancid butter, valeric acid in sweaty socks)

Acidity and Reactivity

• Carboxylic acids are weak acids that partially dissociate in aqueous solutions, forming carboxylate anions and hydronium ions
  • Dissociation constant (Ka) and pKa values indicate the relative strength of carboxylic acids
• Acidity of carboxylic acids is influenced by the stability of the conjugate base (carboxylate anion), which is affected by resonance and inductive effects
  • Electron-withdrawing groups (halogens, nitro groups) stabilize the conjugate base and increase acidity
  • Electron-donating groups (alkyl groups) destabilize the conjugate base and decrease acidity
• Reactivity of carboxylic acids is determined by the electrophilicity of the carbonyl carbon and the nucleophilicity of the attacking species
  • Carboxylic acids undergo nucleophilic acyl substitution reactions, where the -OH group is replaced by a nucleophile
• Carboxylic acid derivatives (esters, amides, acid anhydrides, acid chlorides) have varying reactivity based on the leaving group ability of the substituent
  • Reactivity order: acid chlorides > acid anhydrides > esters > amides

Synthesis Methods

• Oxidation of primary alcohols or aldehydes using strong oxidizing agents (chromic acid, potassium permanganate)
  • Aldehydes are oxidized under milder conditions compared to primary alcohols
• Hydrolysis of nitriles (R-CN) under acidic or basic conditions, followed by protonation of the resulting carboxylate anion
• Carbonation of Grignard reagents (R-MgX) or organolithium compounds (R-Li) with carbon dioxide, followed by acidic workup
• Oxidative cleavage of alkenes using strong oxidizing agents (ozone, potassium permanganate) and subsequent hydrolysis
• Hydrolysis of esters, amides, or acid anhydrides under acidic or basic conditions
  • Acid-catalyzed hydrolysis proceeds via protonation of the carbonyl oxygen, making it more electrophilic
  • Base-promoted hydrolysis involves nucleophilic attack of hydroxide ion on the carbonyl carbon

Important Reactions

• Esterification: reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst (sulfuric acid, hydrochloric acid) to form an ester
  • Mechanism involves protonation of the carbonyl oxygen, nucleophilic attack by the alcohol, and dehydration
• Amide formation: reaction of a carboxylic acid with an amine, often using coupling reagents (DCC, EDC) to activate the carboxylic acid
  • Mechanism involves nucleophilic attack of the amine on the activated carboxylic acid derivative
• Reduction of carboxylic acids to primary alcohols using strong reducing agents (lithium aluminum hydride, borane)
  • Mechanism involves hydride transfer to the carbonyl carbon, followed by protonation and hydrolysis
• Decarboxylation: loss of carbon dioxide from a carboxylic acid, often induced by heat or catalysts (copper, silver)
  • Mechanism involves formation of a carbanion intermediate, which is stabilized by adjacent electron-withdrawing groups
• Hell-Volhard-Zelinsky (HVZ) reaction: α-halogenation of carboxylic acids using a halogen (bromine, chlorine) and a catalytic amount of phosphorus tribromide or thionyl chloride
  • Mechanism involves formation of an acyl halide intermediate, enolization, and electrophilic halogenation of the enol

Derivatives and Their Uses

• Esters: formed by condensation of carboxylic acids and alcohols, used as solvents (ethyl acetate), fragrances (methyl butyrate), and plasticizers (phthalates)
  • Biodiesel is a mixture of fatty acid methyl esters (FAMEs) derived from vegetable oils or animal fats
• Amides: formed by condensation of carboxylic acids and amines, found in proteins (peptide bonds), polymers (nylon), and pharmaceuticals (acetaminophen)
  • Primary amides (R-CONH2) are neutral compounds with high melting points due to extensive hydrogen bonding
• Acid anhydrides: formed by dehydration of two carboxylic acid molecules, used in the synthesis of esters, amides, and other carboxylic acid derivatives
  • Cyclic anhydrides (maleic anhydride, phthalic anhydride) are important intermediates in organic synthesis
• Acid chlorides: formed by reaction of carboxylic acids with thionyl chloride or phosphorus trichloride, highly reactive electrophiles used in the synthesis of esters, amides, and ketones
  • Benzoyl chloride is a common reagent for introducing the benzoyl protecting group (Bz) in organic synthesis

Biological and Industrial Applications

• Fatty acids: long-chain carboxylic acids found in natural fats and oils, used in the production of soaps, detergents, and cosmetics
  • Omega-3 fatty acids (EPA, DHA) have anti-inflammatory properties and are essential for human health
• Amino acids: building blocks of proteins, containing both carboxyl and amino groups, used in the synthesis of peptides and pharmaceuticals
  • Essential amino acids (lysine, threonine) cannot be synthesized by the human body and must be obtained through diet
• Citric acid: a tricarboxylic acid found in citrus fruits, used as a food additive (acidulant, preservative) and in the production of biodegradable polymers
• Acetic acid: the main component of vinegar, used in the production of cellulose acetate (textiles, cigarette filters), polyvinyl acetate (adhesives), and ethyl acetate (solvent)
• Salicylic acid: a beta-hydroxy acid used in the synthesis of aspirin (acetylsalicylic acid) and other anti-inflammatory drugs
  • Salicylic acid is also used as a topical medication for the treatment of acne and warts
• Terephthalic acid: a dicarboxylic acid used in the production of polyethylene terephthalate (PET), a thermoplastic polymer used in beverage bottles and textile fibers