Carboxylic Acids: Structure, Properties, Nomenclature, Preparation, and Reactions

Carboxylic Acids: Structure and Nomenclature

Definition and General Structure

Carboxylic acids are organic compounds containing both a carbonyl group (C=O) and a hydroxyl group (OH) attached to the same carbon atom, forming the carboxyl functional group (-COOH).

• General formula: R-COOH, where R is an alkyl or aryl group.

• They are key intermediates and products in organic and biological chemistry.

Naming Carboxylic Acids

The IUPAC system provides systematic rules for naming carboxylic acids:

• Replace the -e ending of the parent alkane with -oic acid.

• Identify the longest carbon chain containing the carboxyl group.

• Numbering begins at the carboxyl carbon (C as #1).

• For cyclic acids, add "carboxylic acid" to the parent ring name.

• For acids with double bonds, use the ending -enoic acid and indicate stereochemistry with cis or trans prefixes.

• For compounds with two carboxyl groups, use -dioic acid or dicarboxylic acid.

Example: CH3CH2CH2COOH is named butanoic acid.

Common Names of Carboxylic Acids and Acyl Groups

Many carboxylic acids have traditional common names, especially those found in nature. Their corresponding acyl groups are used in naming derivatives.

Structure	Name	Acyl group
HCOOH	Formic	Formyl
CH3COOH	Acetic	Acetyl
CH3CH2COOH	Propionic	Propionyl
CH3(CH2)2COOH	Butyric	Butyryl
HOOC-CH2-COOH	Malonic	Malonyl
HOOC-(CH2)4-COOH	Adipic	Adipoyl
C6H5COOH	Benzoic	Benzoyl
HOOC-CH=CH-COOH	Maleic	Maleoyl
HOOC-CH(OH)-CH2-COOH	Malic	Maloyl
CH3CH(OH)COOH	Lactic	Lactyl

Physical Properties of Carboxylic Acids

Boiling and Melting Points

Carboxylic acids have higher melting and boiling points than hydrocarbons and other oxygen-containing compounds of similar size due to strong intermolecular forces, especially hydrogen bonding.

Compound	Boiling Point (°C)
2-methyl butene (alkene)	31
2-butanone (ketone)	80
2-butanol (alcohol)	99
Propanoic acid (carboxylic acid)	141

Structure and Bonding

• Carboxylic acids are planar molecules.

• Bond angles are approximately 120°, indicating sp2 hybridization of the carboxyl carbon.

• Resonance structures exist, delocalizing electrons between the carbonyl and hydroxyl oxygens.

Polarity

• Carboxylic acids are strongly polar due to the presence of both hydroxyl (-OH) and carbonyl (C=O) groups.

• This polarity contributes to their solubility and reactivity.

Solubility in Water

• Carboxylic acids can form hydrogen bonds with water.

• Acids with 1–4 carbon atoms are very soluble in water.

• Solubility decreases as the carbon chain length increases (≥5 C atoms).

IUPAC Name	Condensed Structural Formula	Solubility in Water
Methanoic acid	HCOOH	Soluble
Ethanoic acid	CH3COOH	Soluble
Propanoic acid	CH3CH2COOH	Soluble
Butanoic acid	CH3CH2CH2COOH	Soluble
Pentanoic acid	CH3CH2CH2CH2COOH	Slightly soluble
Hexanoic acid	CH3CH2CH2CH2CH2COOH	Slightly soluble

Preparation of Carboxylic Acids

Oxidation of Primary Alcohols and Aldehydes

Carboxylic acids can be synthesized by oxidizing primary alcohols or aldehydes, commonly using chromium trioxide (CrO3).

• General reaction: $RCH_2OH \xrightarrow{[O]} RCOOH$

• Example: Ethanol oxidized to ethanoic acid.

Hydrolysis of Nitriles

Nitriles can be hydrolyzed by heating with acid or base to yield carboxylic acids.

• General reaction: $R-CN + 2H_2O + H^+ \rightarrow R-COOH + NH_4^+$

• Industrial application: Synthesis of ibuprofen involves nitrile hydrolysis.

Reactions of Carboxylic Acids

Acidity and Acid-Base Reactions

Carboxylic acids are stronger acids than water and alcohols, but weaker than inorganic acids. Their acidity is due to the resonance stabilization of the carboxylate ion.

• Ionization in water: $RCOOH + H_2O \rightleftharpoons RCOO^- + H_3O^+$

• Neutralization: Carboxylic acids react with strong bases to form salts and water.

• Example: $CH_3COOH + NaOH \rightarrow CH_3COO^-Na^+ + H_2O$

Resonance Structure of Carboxylic Acids

The carboxylate ion is stabilized by resonance, which delocalizes the negative charge over both oxygen atoms.

• Resonance forms: $RCOO^- \leftrightarrow RCOO^-$ (delocalized charge)

Substituent Effects on Acidity

• Electron-withdrawing groups (e.g., F, NO2) increase acidity by stabilizing the carboxylate anion.

• Electron-donating groups (e.g., OH, alkyl) decrease acidity by destabilizing the carboxylate anion.

• Aromatic substituents: p-Nitrobenzoic acid is more acidic than benzoic acid due to the electron-withdrawing nitro group.

Carboxylic Acid Salts: Properties and Applications

• Carboxylic acid salts are formed by neutralization with strong bases.

• Long-chain carboxylic acid salts (12–18 carbons) have amphiphilic properties, forming micelles in water.

• Example: Sodium stearate (sodium octadecanoate) is a common soap.

Micelles and Cleaning Action of Soaps

• Micelles are spherical aggregates formed by amphiphilic molecules in water.

• Structure: Carboxylate ions on the outside, nonpolar tails on the inside.

• Action: Nonpolar interior dissolves grease and oils, which are then washed away.

Soaps vs Detergents

• Soaps are salts of long-chain carboxylic acids; detergents are synthetic surfactants.

• Detergents often work better in hard water due to reduced precipitation.

Real-life Applications of Carboxylic Acid Salts

• Preservatives: Sodium propionate (cheese, bread), sodium benzoate (fruit juices, jams).

• Flavor enhancers: Monosodium glutamate (MSG) in foods.

Compound	Application
Sodium propionate	Preservative in cheese and bread
Sodium benzoate	Antibacterial preservative
Monosodium glutamate	Flavor enhancer

Reduction Reaction

• Carboxylic acids can be reduced to alcohols using strong reducing agents such as lithium aluminum hydride (LiAlH4).

• General reaction: $RCOOH \xrightarrow{LiAlH_4} RCH_2OH$

Esterification (Fischer Esterification)

• Carboxylic acids react with alcohols in the presence of acid to form esters.

• General reaction: $RCOOH + R'OH \xrightarrow{H^+} RCOOR' + H_2O$

Reaction with Thionyl Chloride

• Treating carboxylic acids with thionyl chloride (SOCl2) produces acyl chlorides.

• General reaction: $RCOOH + SOCl_2 \rightarrow RCOCl + SO_2 + HCl$

Industrial Applications

• Adipic acid is used in nylon production.

• Ibuprofen synthesis involves carboxylic acid chemistry.

• Synthetic esters are used in the food industry.

• Soap industry relies on carboxylic acid salts.

Special Types of Carboxylic Acids

Alpha Hydroxy Acids (AHAs)

AHAs are carboxylic acids with a hydroxyl group on the carbon adjacent to the carboxyl group. They are widely found in nature and have commercial applications.

Alpha Hydroxy Acid	Source	Structure
Glycolic acid	Sugar cane	HOCH2COOH
Lactic acid	Milk	CH3CH(OH)COOH
Tartaric acid	Grapes	HOOCCH(OH)CH(OH)COOH
Citric acid	Citrus fruits	HOOCCH2C(OH)(COOH)CH2COOH
Malic acid	Apples	HOOCCH2CH(OH)COOH

• Applications: Used in skin care (dry skin, sun damage, acne scars, pigmentation), and in treatment of fibromyalgia.

• Side effect: Increased UV sensitivity.

Fatty Acids

Fatty acids are long-chain carboxylic acids (typically 12–18 carbons), insoluble in water, and can be saturated or unsaturated.

• Saturated fatty acids: No double bonds (e.g., stearic acid).

• Unsaturated fatty acids: One or more double bonds (e.g., palmitoleic acid).

Dicarboxylic Acids

• Contain two carboxyl groups.

• Ionize in two steps, each carboxyl group can lose a proton.

• Example: Oxalic acid, malonic acid, adipic acid.

Carboxylic Acids in Biological Systems

• Carbonic acid: Important in blood pH regulation.

• Buffer system: $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

• Normal blood pH is 7.4; arterial pH varies between 7.35 and 7.45.

Summary Table: Key Reactions of Carboxylic Acids

Reaction	Reagents	Product
Oxidation	CrO3, KMnO4	Carboxylic acid
Hydrolysis of nitriles	Acid/base, heat	Carboxylic acid
Reduction	LiAlH4	Alcohol
Esterification	Alcohol, acid	Ester
Reaction with SOCl2	Thionyl chloride	Acyl chloride

Additional info:

• Micelle formation is crucial for the cleaning action of soaps and detergents.

• Substituent effects on acidity are a classic example of structure-reactivity relationships in organic chemistry.

• Carboxylic acids play vital roles in metabolism, industrial synthesis, and daily life applications.