Alcohols

Definition and Classification

Alcohol Functional Group

Alcohols: organic compounds with hydroxyl group (-OH) attached to saturated carbon. General formula: R–OH, where R = alkyl or aryl group.

Primary, Secondary, and Tertiary Alcohols

Classified by carbon bonding to –OH carbon: primary (1°) attached to one carbon, secondary (2°) to two carbons, tertiary (3°) to three carbons.

Simple and Polyhydric Alcohols

Simple alcohols: one –OH group. Polyhydric alcohols: multiple –OH groups, e.g., diols (two), triols (three), with distinct properties.

Aromatic Alcohols

Aromatic alcohols (phenols) differ by –OH attached directly to aromatic ring; distinct acidic and chemical behavior; excluded from classical alcohols.

Nomenclature

IUPAC Naming

Identify longest chain containing –OH; suffix "-ol" replaces "-e" of alkane name. Number chain to give –OH lowest possible number.

Common Names

Often named alkyl alcohols: "ethyl alcohol" for ethanol, "isopropyl alcohol" for 2-propanol; widely used in industry and commerce.

Polyhydric Alcohols Naming

Use prefixes di-, tri- before "ol" suffix, e.g., ethylene glycol (1,2-ethanediol), glycerol (1,2,3-propanetriol).

Substituent Naming

When –OH is substituent, use “hydroxy-” prefix on parent compound, e.g., hydroxybenzene (phenol).

Structure and Bonding

Hydroxyl Group Geometry

–OH bonded to sp³ carbon; oxygen has two lone pairs; tetrahedral geometry around carbon; bond angle ~109.5°; O–H bond length ~0.96 Å.

Hydrogen Bonding

Strong intermolecular hydrogen bonding via –OH groups; affects boiling points, solubility, and viscosity.

Electronic Effects

Oxygen electronegativity induces polarity; partial negative charge on oxygen, partial positive on hydrogen and carbon; influences reactivity.

Conformational Flexibility

Free rotation about C–O bond; intramolecular hydrogen bonding possible in diols; affects stability and physical properties.

Physical Properties

Boiling and Melting Points

Elevated boiling points compared to hydrocarbons due to hydrogen bonding; melting points vary with molecular size and symmetry.

Solubility

Low molecular weight alcohols soluble in water due to hydrogen bonding; solubility decreases with increasing hydrocarbon chain length.

Density and Viscosity

Generally denser than water for low molecular weight alcohols; viscosity influenced by hydrogen bonding and molecular weight.

Refractive Index and Polarity

Alcohols exhibit moderate polarity; refractive index increases with chain length and aromatic substitution.

Acid-Base Behavior

Acidity of Alcohols

Weak acids; pKa typically ~16–18; acidity influenced by substituents, hydrogen bonding, and solvent effects.

Basicity and Coordination

Oxygen lone pairs can act as Lewis bases; coordinate with metal ions to form complexes; important in catalysis and biochemistry.

Deprotonation and Alkoxide Formation

Strong bases (e.g., NaH, Na metal) deprotonate alcohols to form alkoxides (RO⁻); reactive intermediates in synthesis.

Comparison with Phenols

Phenols more acidic (pKa ~10) due to resonance stabilization of phenolate ion; classical alcohols less acidic, lack resonance stabilization.

Synthesis Methods

Hydration of Alkenes

Acid-catalyzed addition of water to alkenes; Markovnikov addition predominates; method for primary, secondary, tertiary alcohols.

Reduction of Carbonyl Compounds

Aldehydes and ketones reduced by hydrides (LiAlH4, NaBH4) to primary and secondary alcohols, respectively.

Grignard Reaction

Organomagnesium halides react with aldehydes/ketones to form alcohols after acidic workup; versatile for carbon chain extension.

Other Methods

Fermentation for ethanol; substitution reactions of alkyl halides with hydroxide; hydroboration-oxidation of alkenes yields alcohols.

Chemical Reactions

Oxidation

Primary alcohols oxidize to aldehydes then carboxylic acids; secondary to ketones; tertiary resist oxidation under mild conditions.

Esterification

Reaction with carboxylic acids or acid derivatives to form esters; acid catalysis common; reversible equilibrium process.

Substitution Reactions

Alcohols converted to alkyl halides via HX, SOCl2, or PBr3; substitution proceeds via SN1 or SN2 depending on structure.

Dehydration

Acid-catalyzed elimination of water forms alkenes; mechanism depends on alcohol type; carbocation intermediate in tertiary alcohols.

Spectroscopic Identification

Infrared Spectroscopy (IR)

Characteristic broad O–H stretch ~3200–3600 cm⁻¹; intensity and shape affected by hydrogen bonding.

Nuclear Magnetic Resonance (NMR)

¹H NMR: singlet or broad signal for –OH proton; ¹³C NMR: shifts for carbon bearing –OH; chemical shifts influenced by hydrogen bonding.

Mass Spectrometry

Fragmentation patterns include loss of water (M-18), cleavage at alpha carbon; aids structural elucidation.

Ultraviolet-Visible Spectroscopy (UV-Vis)

Generally weak absorption; relevant for conjugated or aromatic alcohols.

Industrial Applications

Solvents

Ethanol, isopropanol widely used as polar solvents in pharmaceuticals, cosmetics, and paints.

Fuel and Fuel Additives

Ethanol as biofuel and octane enhancer; butanol and other alcohols explored for fuel applications.

Chemical Intermediates

Alcohols serve as precursors for esters, ethers, aldehydes, and plastics; crucial in industrial syntheses.

Pharmaceuticals and Personal Care

Glycerol and derivatives used in formulations; antiseptic properties of certain alcohols exploited in medicine.

Environmental and Safety Considerations

Toxicity and Exposure

Low molecular weight alcohols toxic at high doses; inhalation and ingestion risks; safety protocols mandatory.

Biodegradability

Generally biodegradable; used in green chemistry; ethanol produced renewably by fermentation.

Flammability

Highly flammable liquids; proper storage and handling necessary; flash points vary with chain length.

Environmental Fate

Volatile and water-soluble; can contaminate water sources; regulated discharge and disposal practices.

Summary Tables

Classification of Alcohols

Common Reactions of Alcohols

References

• Smith, J. G. "Organic Chemistry," 5th ed., McGraw-Hill, 2016, pp. 234-278.
• Clayden, J., Greeves, N., Warren, S., Wothers, P. "Organic Chemistry," Oxford University Press, 2012, vol. 1, pp. 150-190.
• March, J. "Advanced Organic Chemistry," 4th ed., Wiley, 1992, pp. 120-165.
• Carey, F. A., Sundberg, R. J. "Advanced Organic Chemistry Part A," 5th ed., Springer, 2007, vol. 1, pp. 320-370.
• McMurry, J. "Organic Chemistry," 9th ed., Cengage Learning, 2016, pp. 210-260.

Representative Chemical Formulas

Primary alcohol: CH3CH2OH (ethanol)Secondary alcohol: CH3CHOHCH3 (2-propanol)Tertiary alcohol: (CH3)3COH (tert-butanol)Oxidation of primary alcohol:R–CH2–OH + [O] → R–CHO → R–COOHEsterification:R–OH + R'–COOH ⇌ R'–COOR + H2O (acid catalyst)Grignard synthesis:R–MgX + R'–CHO → R–CH(OH)–R' (after acidic workup)