🥼Organic Chemistry Unit 20 – Carboxylic Acids and Nitriles

Key Concepts and Definitions

• Carboxylic acids contain a carbonyl group (C=O) bonded to a hydroxyl group (OH) and have the general formula RCOOH
• Nitriles are organic compounds that contain a carbon-nitrogen triple bond (C≡N) and have the general formula R-C≡N
• The carbonyl carbon in carboxylic acids is sp2 hybridized, resulting in a planar geometry around the carbon atom
• Carboxylic acids are weak acids that can donate a proton (H+) from their hydroxyl group, forming a carboxylate anion (RCOO-)
  • The acidity of carboxylic acids is influenced by the electron-withdrawing effect of the carbonyl group and the nature of the R group
• Nitriles are relatively polar due to the electronegativity difference between carbon and nitrogen atoms in the triple bond
• The carbon-nitrogen triple bond in nitriles consists of one σ (sigma) bond and two π (pi) bonds, resulting in a linear geometry

Structure and Nomenclature

• Carboxylic acids are named by replacing the -e ending of the corresponding alkane with -oic acid (ethanoic acid)
  • For branched carboxylic acids, the longest carbon chain containing the carboxyl group is used as the base name
  • Substituents are named and numbered according to their position relative to the carboxyl carbon
• Common names for carboxylic acids include formic acid (methanoic acid), acetic acid (ethanoic acid), and benzoic acid (benzenecarboxylic acid)
• Nitriles are named by replacing the -e ending of the corresponding alkane with -nitrile (ethanenitrile)
  • For branched nitriles, the longest carbon chain containing the nitrile group is used as the base name
  • Substituents are named and numbered according to their position relative to the nitrile carbon
• The common name for the simplest nitrile is hydrogen cyanide (HCN), while other examples include acetonitrile (ethanenitrile) and benzonitrile (benzenecarbonitrile)

Physical Properties

• Carboxylic acids have higher boiling points compared to alcohols and aldehydes of similar molecular weight due to intermolecular hydrogen bonding between the hydroxyl groups
  • The strength of hydrogen bonding increases with the polarity of the O-H bond and the electronegativity of the oxygen atom
• Lower molecular weight carboxylic acids (up to C4) are miscible with water due to their ability to form hydrogen bonds with water molecules
  • As the carbon chain length increases, the solubility of carboxylic acids in water decreases due to the increasing hydrophobic nature of the alkyl group
• Carboxylic acids are soluble in organic solvents, such as ethers, alcohols, and hydrocarbons
• Nitriles have higher boiling points compared to alkanes of similar molecular weight due to the polarity of the carbon-nitrogen triple bond
  • The polarity of nitriles allows for dipole-dipole interactions between molecules, increasing their attractive forces
• Lower molecular weight nitriles are soluble in water due to their polarity and ability to form hydrogen bonds with water molecules
  • As the carbon chain length increases, the solubility of nitriles in water decreases due to the increasing hydrophobic nature of the alkyl group

Synthesis and Preparation Methods

• Carboxylic acids can be prepared by the oxidation of primary alcohols or aldehydes using strong oxidizing agents such as potassium permanganate (KMnO4) or chromic acid (H2CrO4)
  • The oxidation of primary alcohols proceeds through an aldehyde intermediate, which is further oxidized to the carboxylic acid
• The hydrolysis of nitriles in the presence of a strong acid (H+) or base (OH-) yields carboxylic acids
  • This reaction proceeds through an amide intermediate, which is further hydrolyzed to the carboxylic acid
• Carboxylic acids can be synthesized by the carbonation of Grignard reagents (RMgX) followed by an acidic workup
  • The Grignard reagent reacts with carbon dioxide (CO2) to form a magnesium carboxylate, which is protonated during the acidic workup to yield the carboxylic acid
• Nitriles can be prepared by the dehydration of amides using dehydrating agents such as phosphorus pentoxide (P2O5) or thionyl chloride (SOCl2)
  • The dehydration reaction eliminates a water molecule from the amide, forming the carbon-nitrogen triple bond of the nitrile
• The nucleophilic substitution reaction between an alkyl halide (R-X) and a metal cyanide salt (NaCN or KCN) yields nitriles
  • This reaction, known as the Kolbe nitrile synthesis, proceeds through an SN2 mechanism, with the cyanide anion acting as the nucleophile

Reactivity and Chemical Behavior

• Carboxylic acids undergo nucleophilic acyl substitution reactions, where the hydroxyl group is replaced by a nucleophile
  • Examples of nucleophiles include water (hydrolysis), alcohols (esterification), amines (amide formation), and halides (acid halide formation)
• The carbonyl carbon in carboxylic acids is electrophilic due to the electron-withdrawing effect of the oxygen atom, making it susceptible to nucleophilic attack
• Carboxylic acids can be reduced to primary alcohols using strong reducing agents such as lithium aluminum hydride (LiAlH4) or sodium borohydride (NaBH4)
  • The reduction occurs through a tetrahedral intermediate, which collapses to form the primary alcohol upon protonation
• Nitriles undergo nucleophilic addition reactions, where a nucleophile adds to the electrophilic carbon atom of the nitrile group
  • Examples of nucleophiles include water (hydrolysis to amides or carboxylic acids), alcohols (iminoether formation), and Grignard reagents (ketone formation)
• The carbon atom in the nitrile group is electrophilic due to the electron-withdrawing effect of the nitrogen atom, making it susceptible to nucleophilic attack
• Nitriles can be reduced to primary amines using strong reducing agents such as lithium aluminum hydride (LiAlH4) or catalytic hydrogenation (H2 with a metal catalyst)
  • The reduction occurs through an imine intermediate, which is further reduced to the primary amine

Important Reactions and Mechanisms

• Fischer esterification: the acid-catalyzed reaction between a carboxylic acid and an alcohol to form an ester
  • The reaction proceeds through a protonated carboxylic acid intermediate, which undergoes nucleophilic attack by the alcohol, followed by the elimination of water
• Nucleophilic acyl substitution: the replacement of the hydroxyl group in a carboxylic acid with a nucleophile (water, alcohols, amines, or halides)
  • The reaction proceeds through a tetrahedral intermediate, which collapses to form the substituted product (ester, amide, or acid halide) and water
• Reduction of carboxylic acids: the conversion of carboxylic acids to primary alcohols using strong reducing agents (LiAlH4 or NaBH4)
  • The reaction proceeds through a tetrahedral intermediate, which collapses to form an aldehyde, followed by further reduction to the primary alcohol
• Hydrolysis of nitriles: the acid- or base-catalyzed addition of water to a nitrile to form an amide or carboxylic acid
  • Under acidic conditions, the reaction proceeds through a protonated nitrile intermediate, which undergoes nucleophilic attack by water, followed by tautomerization to the amide and further hydrolysis to the carboxylic acid
  • Under basic conditions, the reaction proceeds through the formation of a carboxylate anion, which is protonated to yield the carboxylic acid
• Reduction of nitriles: the conversion of nitriles to primary amines using strong reducing agents (LiAlH4) or catalytic hydrogenation (H2 with a metal catalyst)
  • The reaction proceeds through an imine intermediate, which is further reduced to the primary amine

Applications in Industry and Research

• Carboxylic acids are used in the production of polymers, such as polyesters (polyethylene terephthalate, PET) and polyamides (nylon)
  • Dicarboxylic acids (acids with two carboxyl groups) are commonly used as monomers in step-growth polymerization reactions
• Acetic acid, a common carboxylic acid, is used in the production of vinyl acetate (a precursor to polyvinyl acetate, PVA), cellulose acetate (a synthetic fiber), and as a food additive (vinegar)
• Nitriles are used as solvents, such as acetonitrile, which is commonly used in high-performance liquid chromatography (HPLC) and battery electrolytes
• Acrylonitrile, a nitrile monomer, is used in the production of polyacrylonitrile (PAN), a precursor to carbon fibers and a copolymer in acrylonitrile-butadiene-styrene (ABS) plastics
• Carboxylic acids and nitriles are important intermediates in the synthesis of pharmaceuticals, agrochemicals, and other fine chemicals
  • For example, ibuprofen, a nonsteroidal anti-inflammatory drug (NSAID), is synthesized from isobutylbenzene via a nitrile intermediate

Common Lab Techniques and Safety

• Handling carboxylic acids: wear appropriate personal protective equipment (PPE) such as gloves, lab coats, and safety glasses to avoid skin and eye contact
  • Use a fume hood when working with volatile carboxylic acids to minimize inhalation exposure
  • Neutralize spills with a weak base, such as sodium bicarbonate or sodium carbonate, before cleaning up
• Handling nitriles: wear appropriate PPE and work in a well-ventilated area or fume hood to avoid inhalation of vapors
  • Some nitriles, such as hydrogen cyanide and acetonitrile, are highly toxic and require special precautions
  • In case of skin contact, immediately flush the affected area with water and seek medical attention if necessary
• Purification techniques for carboxylic acids: recrystallization, distillation, and extraction
  • Recrystallization involves dissolving the impure carboxylic acid in a hot solvent, filtering off impurities, and allowing the pure compound to crystallize upon cooling
  • Distillation is used to separate carboxylic acids based on their boiling points, with the pure compound being collected at its specific boiling temperature
  • Extraction involves separating the carboxylic acid from an aqueous solution using an organic solvent, followed by evaporation of the solvent to obtain the pure compound
• Purification techniques for nitriles: distillation, chromatography, and extraction
  • Distillation is used to separate nitriles based on their boiling points, with the pure compound being collected at its specific boiling temperature
  • Chromatography, such as column chromatography or HPLC, is used to separate nitriles from impurities based on their interaction with a stationary phase and a mobile phase
  • Extraction involves separating the nitrile from an aqueous solution using an organic solvent, followed by evaporation of the solvent to obtain the pure compound