Alcohols and Carboxylic Acids

Welcome to this comprehensive lesson on Alcohols and Carboxylic Acids for CXC Chemistry (2024-2025 syllabus). This module covers the structure, properties, preparation, and reactions of these important organic compounds, as well as their significance in everyday life.

Learning Objectives:

Describe the structure and properties of alcohols and carboxylic acids
Explain the methods of preparation of these compounds
Illustrate and explain key chemical reactions
Discuss the applications and significance in various fields
Understand the environmental impact and safety considerations

Section 1: Alcohols

1.1 Structure and Classification of Alcohols

Alcohols are organic compounds containing a hydroxyl group (-OH) attached to a carbon atom. The general formula for alcohols is R-OH, where R represents an alkyl group.

Alcohols are classified into three types based on the number of carbon atoms attached to the carbon bearing the hydroxyl group:

Primary (1°) alcohols: The carbon atom bearing the -OH group is attached to only one carbon atom
Secondary (2°) alcohols: The carbon atom bearing the -OH group is attached to two carbon atoms
Tertiary (3°) alcohols: The carbon atom bearing the -OH group is attached to three carbon atoms

1.2 Nomenclature of Alcohols

Alcohols are named according to IUPAC rules:

Identify the longest carbon chain containing the hydroxyl group
Number the carbon chain to give the carbon with the -OH group the lowest possible number
The name ends with the suffix "-ol"
Indicate the position of the -OH group by a number prefix

Examples:

CH₃CH₂OH: Ethanol (not 1-ethanol as the position is understood)
CH₃CH(OH)CH₃: Propan-2-ol (2-propanol)
CH₃CH₂CH(OH)CH₃: Butan-2-ol
(CH₃)₃COH: 2-methylpropan-2-ol

1.3 Physical Properties of Alcohols

The physical properties of alcohols are influenced by the presence of the polar hydroxyl group and hydrogen bonding:

Boiling Points: Higher than corresponding alkanes due to hydrogen bonding
Solubility: Lower alcohols (C₁-C₃) are completely soluble in water; solubility decreases as chain length increases
Density: Generally less dense than water
State: Lower alcohols (C₁-C₄) are liquids at room temperature; higher alcohols are waxy solids

Alcohol	Formula	Boiling Point (°C)	Water Solubility
Methanol	CH₃OH	64.7	Completely miscible
Ethanol	CH₃CH₂OH	78.4	Completely miscible
Propan-1-ol	CH₃CH₂CH₂OH	97.2	Completely miscible
Butan-1-ol	CH₃(CH₂)₃OH	117.7	7.7 g/100 mL
Pentan-1-ol	CH₃(CH₂)₄OH	138.0	2.3 g/100 mL

1.4 Preparation of Alcohols

Alcohols can be prepared by several methods including:

1.4.1 Hydration of Alkenes

CH₂=CH₂ + H₂O → CH₃CH₂OH

(H₂SO₄ catalyst, 300°C, 70 atm)

1.4.2 Reduction of Carbonyl Compounds

Aldehyde: R-CHO + 2[H] → R-CH₂OH

Ketone: R-CO-R' + 2[H] → R-CHOH-R'

(Using NaBH₄ or LiAlH₄ as reducing agents)

1.4.3 Fermentation

C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂

(Catalyzed by yeast in the absence of oxygen)

1.4.4 Grignard Reaction

R-MgX + R'-CHO → R-CHOH-R'

(Followed by hydrolysis)

1.5 Chemical Reactions of Alcohols

1.5.1 Combustion

Alcohols burn in air to produce carbon dioxide and water with the release of heat:

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat

1.5.2 Oxidation Reactions

Alcohols can be oxidized to form various compounds depending on their class:

Primary alcohols: Oxidized to aldehydes and then to carboxylic acids
Secondary alcohols: Oxidized to ketones
Tertiary alcohols: Resistant to oxidation under normal conditions

Common oxidizing agents include:

Potassium dichromate(VI) in acidic conditions (K₂Cr₂O₇/H₂SO₄)
Potassium permanganate (KMnO₄)
Copper(II) oxide (CuO)

1.5.3 Dehydration

When heated with concentrated sulfuric acid or phosphoric acid, alcohols undergo dehydration to form alkenes:

CH₃CH₂OH → CH₂=CH₂ + H₂O

(conc. H₂SO₄, 170°C)

1.5.4 Esterification

Alcohols react with carboxylic acids in the presence of a concentrated acid catalyst to form esters:

R-OH + R'-COOH ⇌ R'-COO-R + H₂O

(conc. H₂SO₄ catalyst)

1.5.5 Reaction with Sodium

Alcohols react with sodium metal to produce sodium alkoxide and hydrogen gas:

2R-OH + 2Na → 2R-ONa + H₂

1.5.6 Halogenation (Lucas Test)

Alcohols react with hydrogen halides to form alkyl halides:

R-OH + HX → R-X + H₂O

(where X = Cl, Br, or I)

Lucas Test: A test for classifying alcohols based on their rate of reaction with Lucas reagent (conc. HCl and ZnCl₂):

Tertiary alcohols: Immediate turbidity
Secondary alcohols: Turbidity within 5 minutes
Primary alcohols: No immediate reaction, requires heating

1.6 Industrial and Everyday Applications of Alcohols

Alcohols have numerous applications in various industries and everyday life:

Ethanol: Beverages, solvent, antiseptic, fuel (gasohol)
Methanol: Solvent, fuel, production of formaldehyde
Propanol and Isopropanol: Antiseptics, solvents
Ethylene glycol: Antifreeze, production of polyester
Glycerol: Cosmetics, food additive, pharmaceuticals

Section 2: Carboxylic Acids

2.1 Structure and Classification of Carboxylic Acids

Carboxylic acids are organic compounds containing the carboxyl functional group (-COOH). The general formula is R-COOH, where R represents an alkyl or aryl group.

Carboxylic acids are classified based on the nature of the R-group:

Aliphatic carboxylic acids: R is an alkyl group (e.g., ethanoic acid)
Aromatic carboxylic acids: R is an aryl group (e.g., benzoic acid)
Dicarboxylic acids: Contains two carboxyl groups (e.g., oxalic acid)

2.2 Nomenclature of Carboxylic Acids

Carboxylic acids are named according to IUPAC rules:

Identify the longest carbon chain containing the carboxyl group
The name ends with the suffix "-oic acid"
The carboxyl carbon is always numbered as carbon-1

Examples:

HCOOH: Methanoic acid (common name: formic acid)
CH₃COOH: Ethanoic acid (common name: acetic acid)
CH₃CH₂COOH: Propanoic acid (common name: propionic acid)
C₆H₅COOH: Benzoic acid
HOOC-COOH: Ethanedioic acid (common name: oxalic acid)

2.3 Physical Properties of Carboxylic Acids

The physical properties of carboxylic acids are influenced by hydrogen bonding and the polar nature of the carboxyl group:

Boiling Points: Higher than corresponding alcohols due to stronger hydrogen bonding
Solubility: Lower carboxylic acids (C₁-C₄) are soluble in water; solubility decreases as chain length increases
State: Lower carboxylic acids (C₁-C₃) are liquids at room temperature; higher members are solids
Odor: Lower carboxylic acids have distinctive pungent odors

Carboxylic Acid	Formula	Melting Point (°C)	Boiling Point (°C)	Water Solubility
Methanoic acid (Formic)	HCOOH	8.4	100.8	Completely miscible
Ethanoic acid (Acetic)	CH₃COOH	16.6	118.1	Completely miscible
Propanoic acid (Propionic)	CH₃CH₂COOH	-20.5	141.1	Completely miscible
Butanoic acid (Butyric)	CH₃(CH₂)₂COOH	-5.1	163.5	Partially soluble
Benzoic acid	C₆H₅COOH	122.4	249.2	Slightly soluble

2.4 Acidity of Carboxylic Acids

Carboxylic acids are weak acids that partially dissociate in water to form H⁺ ions:

R-COOH + H₂O ⇌ R-COO⁻ + H₃O⁺

The acidity of carboxylic acids is due to:

The polarity of the O-H bond
The stabilization of the carboxylate anion by resonance
The inductive effect of the R group

Effect of Substituents on Acidity:

Electron-withdrawing groups (e.g., halogens) increase acidity
Electron-donating groups (e.g., alkyl groups) decrease acidity

Order of acidity: Cl₃C-COOH > Cl₂CH-COOH > ClCH₂-COOH > CH₃-COOH

2.5 Preparation of Carboxylic Acids

Carboxylic acids can be prepared by several methods including:

2.5.1 Oxidation of Primary Alcohols or Aldehydes

R-CH₂OH → R-CHO → R-COOH

(Using K₂Cr₂O₇/H₂SO₄ or KMnO₄)

2.5.2 Oxidation of Alkyl Benzenes

C₆H₅-CH₃ + 3[O] → C₆H₅-COOH + H₂O

(Using KMnO₄ under heat and pressure)

2.5.3 Hydrolysis of Nitriles

R-CN + 2H₂O + H⁺ → R-COOH + NH₄⁺

(Acid or base catalyzed)

2.5.4 Carbonation of Grignard Reagents

R-MgX + CO₂ → R-COO⁻MgX⁺

R-COO⁻MgX⁺ + H⁺ → R-COOH + Mg²⁺ + X⁻

2.6 Chemical Reactions of Carboxylic Acids

2.6.1 Salt Formation

Carboxylic acids react with bases (metal hydroxides, carbonates, or bicarbonates) to form salts:

R-COOH + NaOH → R-COONa + H₂O

2R-COOH + Na₂CO₃ → 2R-COONa + H₂O + CO₂

2.6.2 Esterification

Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters:

R-COOH + R'-OH ⇌ R-COO-R' + H₂O

(conc. H₂SO₄ catalyst)

2.6.3 Acid Chloride Formation

Carboxylic acids react with thionyl chloride (SOCl₂) or phosphorus pentachloride (PCl₅) to form acid chlorides:

R-COOH + SOCl₂ → R-COCl + SO₂ + HCl

2.6.4 Amide Formation

Acid chlorides react with ammonia or amines to form amides:

R-COCl + NH₃ → R-CONH₂ + HCl

2.6.5 Reduction

Carboxylic acids can be reduced to primary alcohols using strong reducing agents:

R-COOH + 4[H] → R-CH₂OH + H₂O

(Using LiAlH₄)

2.6.6 Decarboxylation

Some carboxylic acids undergo decarboxylation when heated strongly:

R-COOH → R-H + CO₂

(Heat, sometimes with catalyst)

2.7 Industrial and Everyday Applications of Carboxylic Acids

Carboxylic acids have numerous applications in various industries and everyday life:

Ethanoic acid (Acetic acid): Production of vinegar, esters, cellulose acetate, and as a solvent
Methanoic acid (Formic acid): Textile dyeing, leather tanning, rubber coagulation
Benzoic acid: Food preservative, production of phenol
Citric acid: Food additive, flavoring agent, cleaning products
Salicylic acid: Production of aspirin, cosmetics
Fatty acids: Production of soaps, detergents, cosmetics

Section 3: The Relationship Between Alcohols and Carboxylic Acids

3.1 Interconversion of Functional Groups

Alcohols and carboxylic acids are connected through a series of oxidation-reduction reactions:

3.2 Biochemical Significance

The oxidation of alcohols to carboxylic acids is important in many biochemical processes:

Ethanol metabolism in the liver (ethanol → acetaldehyde → acetic acid)
Fatty acid metabolism (oxidation of alcohols to carboxylic acids)
Citric acid cycle (involves various carboxylic acids)

Section 4: Derivatives of Carboxylic Acids

4.1 Esters

Esters have the general formula R-COO-R' and are formed by the reaction between carboxylic acids and alcohols.

4.1.1 Properties of Esters

Pleasant, fruity odors
Lower boiling points than corresponding carboxylic acids
Insoluble in water but soluble in organic solvents

4.1.2 Reactions of Esters

Hydrolysis: Esters are hydrolyzed to carboxylic acids and alcohols in the presence of acid or base.

R-COO-R' + H₂O → R-COOH + R'-OH (acid catalyzed)

R-COO-R' + OH⁻ → R-COO⁻ + R'-OH (base catalyzed)
Reduction: Esters can be reduced to alcohols.

R-COO-R' + 4[H] → R-CH₂OH + R'-OH

4.1.3 Applications of Esters

Flavoring agents in food and beverages
Perfumes and fragrances
Solvents (e.g., ethyl acetate)
Plasticizers in polymers
Waxes and oils

4.2 Acid Anhydrides

Acid anhydrides have the general formula R-CO-O-CO-R' and are formed by the dehydration of carboxylic acids.

Self-Assessment Questions

1. What is the pH range of acids?

2. Name a strong acid and a weak acid.

3. What is the reaction between an acid and a base called?

4. What type of salt is formed from sulfuric acid and sodium hydroxide?