Summary: Alcohols Phenols And Ethers

Table of Contents
1. Alcohols and Phenols
2. Definition - Alcohols
3. Definition - Phenol
4. Classification
5. Nomenclature of Alcohols
6. Nomenclature of Phenols
7. Physical Properties
8. Chemical Properties
9. Key Differences
View more

Alcohols and Phenols

Alcohols and Phenols are organic compounds that contain the hydroxyl (-OH) functional group. The -OH group controls many physical and chemical properties. Alcohols are widely used as solvents, fuels, disinfectants and in chemical manufacture. Phenols are important in plastics, pharmaceuticals, antiseptics and some personal-care products.

Definition - Alcohols

Alcohols are compounds with one or more hydroxyl groups attached to a saturated carbon. General formula: R-OH. Properties vary with chain length, branching and the carbon bearing the -OH (primary, secondary, tertiary).

Definition - Phenol

Phenol (formula C6H5OH) is an aromatic compound where the hydroxyl group is directly bonded to a benzene ring. It is a corrosive white crystalline solid, sparingly soluble in cold water, and dissolves in alkali to give the phenoxide ion. Historical extraction from coal tar was reported in 1834.

Classification

• By carbon bearing -OH: Primary (1°), Secondary (2°), Tertiary (3°) - defined by how many carbons are attached to the carbon with -OH.
• By number of -OH groups: Monohydric, Dihydric (glycols), Trihydric - according to how many hydroxyls are present.
• Aromatic alcohols and phenols: -OH attached to an aromatic ring; phenols classified as mono-, di- or trihydric with possible ortho, meta, para arrangements.
• Other types: Allylic alcohols (-OH next to C=C) and benzyl alcohols (-OH on a benzylic carbon).

Nomenclature of Alcohols

• Select the longest chain containing the hydroxyl group and change the alkane suffix -ane to -ol.
• Number the chain to give the -OH the lowest possible locant and indicate positions of substituents.
• IUPAC systematic names are preferred; older common or carbinol names exist historically.

Nomenclature of Phenols

• Phenols are named as benzene derivatives bearing hydroxyl groups; use di-, tri- prefixes for multiple -OH groups.
• Locate substituents relative to the hydroxy group; positional descriptors ortho, meta, para are commonly used.

Physical Properties

• Hydrogen bonding between -OH groups raises boiling points compared with similar hydrocarbons.
• Boiling point trends depend on chain length and branching (longer chains ↑ bp; more branching ↓ bp); less-branched isomers often have higher bp.
• Solubility: Short-chain alcohols are highly soluble in water; solubility decreases with larger hydrophobic parts. Phenols hydrogen-bond with water but are only moderately soluble; they become highly soluble when converted to phenoxide in alkali.
• Density: Common alcohols are slightly less dense than water and float; phenol's density is greater than water (≈ 1.07 g cm^-3) and sinks.
• Odour, volatility and flammability vary with molecular mass; low-molecular alcohols are more volatile and are flammable.

Chemical Properties

• Oxidation: Primary alcohols → aldehydes → carboxylic acids; secondary alcohols → ketones; tertiary alcohols resist oxidation under mild conditions.
• Acid-base behaviour: Alcohols are weak acids (typical pKa ≈ 16) and are deprotonated only by strong bases to give alkoxide salts. Example reaction: 2 R-OH + 2 Na → 2 R-O-Na⁺ + H₂↑.
• Phenols vs alcohols: Phenols are more acidic (phenol pKa ≈ 10) because the phenoxide ion is resonance-stabilised; phenols are deprotonated by bases like NaOH.
• Substituent effects on phenol acidity: Electron-withdrawing groups increase acidity (especially at ortho/para positions); electron-donating groups decrease acidity.
• Dehydration: Alcohols undergo acid-catalysed dehydration to alkenes (E1/E2 mechanisms); phenols do not dehydrate to give alkenes because removal of -OH would destroy aromaticity.
• Esterification: Alcohols react with carboxylic acids (Fischer esterification) to form esters. Phenols are less nucleophilic and form esters more readily with acid chlorides or anhydrides under activation.
• Reaction with metals and acid chlorides: Alcohols/phenols react with reactive metals to give alkoxides/phenoxide salts; both react with acid chlorides to form esters (R-OH + R'COCl → R'COOR + HCl).
• Nucleophilic substitution and aromatic behaviour: Alcohols can be converted into better leaving groups for SN1/SN2 substitutions. The -OH group on phenols activates the ring toward electrophilic aromatic substitution, directing ortho/para positions.

Key Differences

• Acidity: Phenols ≫ aliphatic alcohols (phenol pKa ≈ 10; aliphatic alcohols pKa ≈ 16).
• Reactivity: Alcohols undergo typical alcohol reactions (oxidation, dehydration, esterification); phenols favor reactions of the aromatic ring and form phenoxide salts readily.
• Solubility: Small alcohols are miscible with water; phenols are moderately soluble but dissolve in alkali as phenoxide.
• Boiling point: Both show elevated boiling points from hydrogen bonding; detailed trends depend on molecular structure.