21.3 Reactions of Carboxylic Acids
2 min read•may 7, 2024
Carboxylic acids are versatile organic compounds with a wide range of reactions. From to conversion into and , these molecules undergo various transformations. Their reactivity stems from the carbonyl group and acidic proton.
reactions can convert carboxylic acids to alcohols using reagents like or . Understanding these reactions is crucial for synthesizing important organic compounds and predicting chemical behavior in different environments.
Carboxylic Acid Reactions
Fischer esterification reaction mechanism
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- Acid-catalyzed condensation reaction forms ester from carboxylic acid and alcohol
- Protonation of carboxylic acid activates carbonyl carbon for nucleophilic attack by alcohol
- Proton transfer from intermediate to hydroxyl group
- Dehydration step eliminates water molecule forming ester product
- Equilibrium favors ester formation by removing water ( with toluene, using drying agents like )
- Excess of either carboxylic acid or alcohol reactant shifts equilibrium towards products
- Increasing reaction temperature favors endothermic ester formation
Conversion of carboxylic acids
- () or () convert carboxylic acids to acid chlorides via mechanism with chloride leaving group
- Dehydrating agents ( DCC) couple two carboxylic acids forming anhydrides through nucleophilic attack of one carboxylate on another activated carbonyl
- Ammonia or primary/secondary amines react with acid chlorides or anhydrides yielding by nucleophilic acyl substitution expelling chloride or carboxylate
- High temperatures required for direct amidation of carboxylic acids with amines
Reduction of carboxylic acids
- Lithium aluminum hydride () reduces carboxylic acids to primary alcohols
- Hydride transfer from to carbonyl carbon followed by protonation in aqueous workup
- Also reduces esters, aldehydes and ketones lacking selectivity
- Borane () selectively reduces carboxylic acids to primary alcohols
- coordinates to carbonyl oxygen facilitating hydride transfer and protonation
- Reduces aldehydes and ketones more slowly than
- Does not reduce esters or amides under standard conditions
Reactivity and Properties
- Carboxylic acids exhibit high due to resonance stabilization of the conjugate base (carboxylate anion)
- The carbonyl carbon acts as an in many reactions, susceptible to nucleophilic attack
- of carboxylic acids is not common as they are already in a high oxidation state
- Reduction (as discussed above) converts carboxylic acids to less oxidized species like aldehydes or alcohols
Key Terms to Review (22)
Acid anhydrides: Acid anhydrides are organic compounds formed by the removal of water between two carboxylic acid molecules. They play a significant role in nucleophilic acyl substitution reactions, where they act as electrophiles ready to react with nucleophiles.
Acid Chlorides: Acid chlorides, also known as acyl chlorides, are a class of organic compounds derived from carboxylic acids. They are characterized by the presence of a carbonyl carbon bonded to a chlorine atom, making them highly reactive and versatile in organic synthesis.
Acidity: Acidity refers to the ability of a substance to donate protons (H+ ions) or the concentration of H+ ions in a solution. It is a fundamental concept in organic chemistry that governs the properties and reactivity of various functional groups, including alcohols, phenols, and carboxylic acids.
Acidity constant, Ka: The acidity constant, Ka, measures the strength of an acid in a solution by quantifying its tendency to donate a proton (H+) to a base. It is expressed as the equilibrium constant for the dissociation of the acid into its conjugate base and a proton in water.
Amides: Amides are a class of organic compounds that contain a carbonyl group (C=O) bonded to a nitrogen atom. They are derived from carboxylic acids and can be considered the result of replacing the hydroxyl group (-OH) of a carboxylic acid with an amino group (-NH2). Amides are important functional groups in many organic molecules, including proteins, and play a crucial role in various chemical reactions and processes.
Anhydrides: Anhydrides are organic compounds derived from carboxylic acids, characterized by a carbonyl-oxygen-carbonyl functional group. They play a crucial role in various reactions and transformations involving carboxylic acid derivatives, including naming, nucleophilic acyl substitution, and the chemistry of acid halides.
Azeotropic Distillation: Azeotropic distillation is a technique used to separate azeotropic mixtures, which are binary or multicomponent mixtures that exhibit a constant boiling point and composition during distillation. This process is particularly relevant in the context of the reactions of carboxylic acids, as it can be employed to purify or isolate specific products.
Borane: Borane is a chemical compound with the formula BH3, consisting of a boron atom bonded to three hydrogen atoms. It is a highly reactive and flammable gas that serves as a key intermediate in organic chemistry, particularly in reactions involving alkenes, alkynes, and carboxylic acids.
Dicyclohexylcarbodiimide: Dicyclohexylcarbodiimide (DCC) is a reagent commonly used in organic chemistry, particularly in the context of reactions involving carboxylic acids and acid anhydrides. It serves as a coupling agent, facilitating the formation of new carbon-carbon or carbon-heteroatom bonds.
Electrophile: An electrophile is a species that is attracted to electron-rich regions and seeks to form new bonds by accepting electron density. Electrophiles play a crucial role in many organic reactions, including polar reactions, electrophilic aromatic substitution, and nucleophilic acyl substitution, among others.
Fischer Esterification: Fischer esterification is a chemical reaction that involves the formation of an ester compound from a carboxylic acid and an alcohol, typically in the presence of an acid catalyst. This reaction is a key process in the synthesis and interconversion of various organic compounds, particularly esters, which have a wide range of applications in the fields of chemistry, biology, and industry.
Fischer esterification reaction: A Fischer esterification reaction is a chemical process where a carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester and water. It is an equilibrium process that is often driven to completion by removing the water formed during the reaction.
Lithium Aluminum Hydride: Lithium aluminum hydride (LiAlH4) is a powerful reducing agent used in organic chemistry for the selective reduction of various functional groups. It is a white, crystalline solid that reacts violently with water and other protic solvents, making it an important reagent in many synthetic transformations.
Molecular Sieves: Molecular sieves are a type of adsorbent material composed of microporous aluminosilicate minerals that can selectively adsorb molecules based on their size and shape. They are commonly used in various chemical processes, including the reactions of carboxylic acids, to remove water and other impurities.
Nucleophile: A nucleophile is a species that donates a pair of electrons to form a covalent bond with another atom or molecule. Nucleophiles are central to understanding many organic reactions, including polar reactions, electrophilic addition reactions, and nucleophilic substitution reactions.
Nucleophilic Acyl Substitution: Nucleophilic acyl substitution is a type of organic reaction where a nucleophile attacks the carbonyl carbon of a carboxylic acid derivative, such as an acid chloride, anhydride, or ester, leading to the replacement of the leaving group with the nucleophile. This process is central to the reactivity and transformations of carboxylic acid derivatives.
Nucleophilic acyl substitution reaction: A nucleophilic acyl substitution reaction is a type of chemical reaction where a nucleophile replaces the leaving group in an acyl compound. This reaction is fundamental in organic chemistry for modifying carboxylic acid derivatives into other functional groups.
Oxidation: Oxidation is a fundamental chemical process in which a substance loses electrons, resulting in an increase in its oxidation state. This term is central to understanding various reactions and transformations in organic chemistry, from the hydration of alkenes to the oxidation of alcohols and aldehydes.
Oxonium Ion: An oxonium ion is a positively charged species that contains an oxygen atom with three covalently bonded substituents, giving it a formal positive charge. This reactive intermediate plays a crucial role in various organic chemistry reactions, including the acidic cleavage of ethers, the ring-opening of epoxides, the formation of acetals, and reactions of carboxylic acids.
Phosphorus Trichloride: Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. It is a colorless, fuming liquid that is widely used in organic synthesis, particularly in the preparation of various phosphorus-containing compounds.
Reduction: Reduction is a chemical process that involves the gain of electrons by a molecule or atom, resulting in a decrease in its oxidation state. This term is particularly important in the context of various organic chemistry reactions and transformations.
Thionyl Chloride: Thionyl chloride (SOCl2) is a highly reactive organic compound commonly used as a reagent in various chemical reactions, particularly in the context of preparing alkyl halides from alcohols, as well as in the reactions and chemistry of carboxylic acids and their derivatives.