Carboxylic Acids, Carboxylic Anhydrides, Esters, and Amides

Carboxylic acids are a significant class of organic compounds characterized by the carboxyl group, which gives rise to three primary classes of derivatives: anhydrides, esters, and amides. These derivatives are formed through the loss of water (H₂O) from the carboxyl group.

Carboxyl Derivatives

Anhydrides

An anhydride features two carbonyl groups bonded to a single oxygen atom, categorized as symmetrical or mixed. The nomenclature involves removing “acid” from the parent carboxylic acid name and appending “anhydride.”

Esters

Esters contain a carbonyl group linked to an –OR group, where R can be an alkyl or aryl group. The naming convention states the alkyl or aryl group first, followed by the acid name with the suffix -ic acid replaced by -ate. Lactones are cyclic esters.

Amides

Amides have a carbonyl group bonded to a nitrogen atom. To name an amide, the suffix -oic acid is dropped from the IUPAC name of the parent acid, replaced by -amide. If the nitrogen is bonded to additional groups, these are indicated with N- for one group and N,N-di- for two. Cyclic amides are called lactams, with penicillins and cephalosporins being notable examples.

Physical Properties of Carboxylic Acids

The carboxyl group comprises three polar covalent bonds: C=O, C–O, and O–H, crucial for determining the physical properties of carboxylic acids. They exhibit higher boiling points than other organic compounds of similar molecular weight due to polarity and hydrogen bonding, behaving as higher-molecular-weight compounds.

Carboxylic acids are more soluble in water than alcohols, ethers, aldehydes, and ketones of comparable molecular weight. For example, acetic acid (CH₃COOH) is infinitely soluble in water, while 1-propanol (C₃H₈O) is also infinitely soluble but has a lower boiling point.

Fatty Acids

Fatty acids are long-chain carboxylic acids derived from animal fats, vegetable oils, or phospholipids in biological membranes. Most contain between 12 and 20 carbon atoms in an unbranched chain, with unsaturated fatty acids predominantly in the cis configuration.

Common Fatty Acids

Carbon Atoms	Double Bonds	Common Name	Melting Point (°C)
12:0	0	Lauric acid	44
14:0	0	Myristic acid	58
16:0	0	Palmitic acid	63
18:0	0	Stearic acid	70
20:0	0	Arachidic acid	77
16:1	1	Palmitoleic acid	1
18:1	1	Oleic acid	16
18:2	2	Linoleic acid	-5
18:3	3	Linolenic acid	-11
20:4	4	Arachidonic acid	-49

Unsaturated fatty acids generally have lower melting points than saturated ones, which are solid at room temperature due to close packing. Unsaturated fatty acids remain liquid due to cis double bonds disrupting regular packing.

Soaps and Detergents

Natural soaps are sodium or potassium salts of fatty acids, produced from triglycerides through saponification with sodium hydroxide (NaOH). Soap molecules form micelles in aqueous solutions, encapsulating nonpolar substances for removal. However, they can form insoluble salts in hard water.

Synthetic detergents, using sulfonate groups instead of carboxylate groups, mitigate precipitate formation in hard water. Examples include sodium dodecyl sulfate (SDS), which serves as anionic detergents, often containing additives like foam stabilizers and bleaches.

Acidity of Carboxylic Acids

Carboxylic acids are weak acids with dissociation constants (K_a) ranging from 10^-4 to 10^-5, corresponding to pK_a values between 4.0 and 5.0. Electronegative substituents near the carboxyl group enhance acidity. The form of a carboxylic acid in solution is influenced by pH, affecting its dissociation.

Reactions of Carboxylic Acids

Carboxylic acids react with strong bases to form water-soluble salts and with ammonia and amines to yield salts. They also react with sodium bicarbonate and sodium carbonate, producing water-soluble sodium salts and carbonic acid, which decomposes into water and carbon dioxide.

Fischer Esterification

Fischer esterification synthesizes esters by reacting a carboxylic acid with an alcohol in the presence of an acid catalyst, typically concentrated sulfuric acid. This reversible reaction allows manipulation of equilibrium by adjusting conditions, forming a tetrahedral intermediate that loses water to yield an ester.

Decarboxylation

Decarboxylation eliminates carbon dioxide (CO₂) from a carboxyl group, typically occurring at high temperatures. The mechanism involves electron redistribution within a cyclic transition state. A notable example occurs in the tricarboxylic acid (TCA) cycle, where oxalosuccinic acid contains a carbonyl group β to one of its carboxyl groups.

Preparation of Amides

Amides are synthesized by treating a carboxylic acid with an amine, removing –OH from the acid and –H from the amine, typically forming an ammonium salt. Heating the salt eliminates water, forming an amide. More commonly, amides are prepared by reacting an anhydride with an amine.

Carboxylic Acids and Their Derivatives in Organic Chemistry

Carboxylic Acids and Their Derivatives in Organic Chemistry