Carboxylic Acids and Esters Study Guide

Structure and Nomenclature of Carboxylic Acids

Definition of Carboxylic Acids: A carboxylic acid contains a carboxyl group, which is a functional group consisting of a hydroxyl group ( $-OH$ ) attached to the carbon atom in a carbonyl group ( $C=O$ ).
IUPAC Naming Conventions: * The naming system replaces the "-e" in the corresponding alkane name with "-oic acid." * Examples include: * Methane ( $CH_4$ ) → Methanoic acid * Ethane ( $CH_3-CH_3$ ) → Ethanoic acid * Substituents are numbered starting from the carboxyl carbon, which is always assigned as carbon 1.
Aromatic Carboxylic Acids: For benzoic acid, the positions of substituents on the ring are indicated using the prefixes: * Ortho * Meta * Para
Common Naming System: * Many carboxylic acids maintain traditional common names using specific prefixes: * 1 Carbon: form * 2 Carbons: acet * 3 Carbons: propion * 4 Carbons: butyr * When common names are utilized, Greek letters are used to designate carbons adjacent to the carboxyl carbon: alpha ( $\alpha$ ), beta ( $\beta$ ), and gamma ( $\gamma$ ).

Common Carboxylic Acids and Preparation

Methanoic Acid (Formic Acid): Found in red ant stings; it is the substance responsible for skin irritation.
Ethanoic Acid (Acetic Acid): Responsible for the sour taste of vinegar.
Naming Case Study (Propanoic Acid): * STEP 1: Identify the longest chain (3 carbons). IUPAC name is propanoic acid; common name is propionic acid. * STEP 2: Name substituents. For a 3-carbon chain with a methyl group on the second carbon, the IUPAC name is 2-methylpropanoic acid and the common name is 2-methylpropionic acid.
Preparation via Oxidation: * Carboxylic acids are prepared from primary alcohols or aldehydes. * The process involves the oxidation of a primary alcohol to an aldehyde, which is then further oxidized to a carboxylic acid. * Example: Ethanol in wine is oxidized by atmospheric oxygen into acetaldehyde, and subsequently into acetic acid (vinegar).
Chemistry Link to Health: Alpha Hydroxy Acids (AHAs): * Found naturally in fruits, milk, and sugar cane. * Structure: These are carboxylic acids with a hydroxyl group ( $-OH$ ) on the carbon atom adjacent to the carboxyl group ( $\alpha$ -carbon). * Medical Use: Dermatologists use high concentrations ( $20-70\%$ ) for acne scar removal and skin peels to treat irregular pigmentation and age spots.

Physical Properties of Carboxylic Acids

Polarity: Carboxylic acids are strongly polar because they contain two polar groups: a hydroxyl group ( $-OH$ ) and a carbonyl group ( $C=O$ ).
Boiling Points: * Carboxylic acids have higher boiling points than alcohols, ketones, and aldehydes of similar molar mass. * This is due to the carboxyl group's ability to form multiple hydrogen bonds with other carboxylic acid molecules.
Dimer Formation: The elevated boiling points are also attributed to the formation of dimers. Two hydrogen bonds form between the carboxyl groups of two acid molecules, effectively doubling the molecular mass and increasing the energy required to boil the substance.
Solubility in Water: * Carboxylic acids with 1 to 5 carbon atoms are very soluble in water because they form hydrogen bonds with water molecules. * Solubility decreases significantly as the number of carbon atoms increases.

Table 16.2: Selected Carboxylic Acid Data

Methanoic acid: BP $101\,^\circ C$ ; Soluble; $K_a = 1.8 \times 10^{-4}$ .
Ethanoic acid: BP $118\,^\circ C$ ; Soluble; $K_a = 1.8 \times 10^{-5}$ .
Propanoic acid: BP $141\,^\circ C$ ; Soluble; $K_a = 1.3 \times 10^{-5}$ .
Butanoic acid: BP $164\,^\circ C$ ; Soluble; $K_a = 1.5 \times 10^{-5}$ .
Pentanoic acid: BP $187\,^\circ C$ ; Soluble; $K_a = 1.5 \times 10^{-5}$ .
Hexanoic acid: BP $205\,^\circ C$ ; Slightly soluble; $K_a = 1.4 \times 10^{-5}$ .
Benzoic acid: BP $250\,^\circ C$ ; Slightly soluble; $K_a = 6.4 \times 10^{-5}$ .

Acidity and Neutralization

Weak Acid Dissociation: Carboxylic acids are weak acids that dissociate in water to produce carboxylate ions and hydronium ions ( $H_3O^+$ ). The negative charge is stabilized across two oxygen atoms in the carboxylate ion.
Neutralization Reaction: A carboxylic acid reacts with a strong base (e.g., $NaOH$ or $KOH$ ) to produce a carboxylate salt and water.
Naming Carboxylate Salts: Replace the "-ic acid" ending with "-ate."
Properties of Carboxylate Salts: * Ionic compounds with strong attractions between metal ions ( $Li^+$ , $Na^+$ , $K^+$ ) and the carboxylate ion. * Solids at room temperature. * High melting points. * Usually soluble in water.

Carboxylic Acids as Preservatives and in Metabolism

Preservatives: * Sodium propionate: Added to bread, cheeses, and bakery items to inhibit microorganisms. * Sodium benzoate: Added to juices, margarine, relishes, salads, and jams to inhibit mold and bacteria. * Monosodium glutamate (MSG): Used in meats, fish, and vegetables to enhance flavor; known to cause headaches in some individuals.
Chemistry Link to Health: Metabolism: * At cellular pH, carboxylic acids exist primarily in their dissociated carboxylate ion forms. * Glycolysis: Glucose is broken down into two molecules of pyruvate (the salt of pyruvic acid). * Anaerobic Exercise: Under low oxygen levels, pyruvate is reduced to lactic acid or the lactate ion. * Citric Acid Cycle (Krebs Cycle): Di- and tricarboxylates are oxidized and decarboxylated (losing $CO_2$ ) to produce cellular energy. * Citrate (6C) is oxidized to $\alpha$ -ketoglutarate (5C). * $\alpha$ -ketoglutarate loses $CO_2$ to become succinate (4C). * Succinate is converted into oxaloacetate through a series of reactions.

Esters: Synthesis and Properties

Historical Context: Chemists in the 1800s found salicin in willow bark reduced pain. The body converted it to salicylic acid (stomach irritant). Bayer developed acetylsalicylic acid (aspirin), an ester that is less irritating but effective.
Synthesis (Esterification): * Reaction between a carboxylic acid and an alcohol. * Requires an acid catalyst and heat. * The $-H$ from the hydroxyl group of the acid is replaced by an alkyl group from the alcohol. * Excess alcohol is used to shift equilibrium toward the ester.
Characteristics and Sources: * Found in fats and oils. * Responsible for aromas and flavors in fruits and flowers.
Naming Esters: * STEP 1: Name the carbon chain from the alcohol as an alkyl group (e.g., methyl from methanol). * STEP 2: Change the "-ic acid" ending of the acid name to "-ate" (e.g., ethanoic acid becomes ethanoate). * Example: Methanol + Ethanoic acid → Methyl ethanoate (IUPAC) or Methyl acetate (Common).

Health and Environmental Links: Esters and Plastics

Oil of Wintergreen (Methyl Salicylate): * Pungent, minty odor/flavor. * Used in skin ointments; can pass through the skin. * Acts as a counter-irritant to soothe muscles by producing heat.
Plastics (Polyesters): * Dacron: Permanent-press fabric; used for artificial blood vessels/valves (biologically inert, non-clotting). * Mylar: A film form of polyester. * PETE (polyethyleneterephthalate): Used for plastic drink bottles.

Table 16.3: Esters in Fruits and Flavorings

Propyl ethanoate (propyl acetate): Pears.
Pentyl ethanoate (pentyl acetate): Bananas.
Octyl ethanoate (octyl acetate): Oranges.
Ethyl butanoate (ethyl butyrate): Pineapples.
Pentyl butanoate (pentyl butyrate): Apricots.
Ethyl heptanoate: Grapes (referenced in text).

Final Comparison of Ester Properties

Boiling Points: * Higher than alkanes and ethers. * Lower than alcohols and carboxylic acids of similar mass (because esters lack hydroxyl groups and cannot hydrogen bond to each other).
Solubility: * Soluble in water if they contain 2 to 5 carbon atoms. * The carbonyl oxygen (partially negative) forms hydrogen bonds with water hydrogens (partially positive).
Hydrolysis Reactions: * Acid Hydrolysis: Ester + Water + Acid Catalyst → Carboxylic Acid + Alcohol. * Base Hydrolysis (Saponification): Ester + Strong Base → Carboxylate Salt + Alcohol. * Note: Old aspirin can undergo acid hydrolysis if it reacts with water and heat over time.