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Hydrogen bonding dominates the physical properties of alcohols and phenols.
Boiling points: Alcohols >> ethers > alkanes (for similar MW). Among isomeric alcohols: 1° > 2° > 3° because branching reduces surface area and H-bonding efficiency. Example: 1-butanol (118°C) > 2-butanol (100°C) > 2-methylpropan-2-ol (82°C).
Solubility in water: Lower alcohols (C1-C3) are miscible (strong H-bonding). Solubility decreases as alkyl chain length increases (hydrophobic effect dominates). The rule of thumb: solubility is appreciable up to 4-5 carbons. Phenol is partially soluble (complete above 66°C).
Key comparison: ethanol (b.p. 78°C) vs dimethyl ether (b.p. -24°C) — same molecular formula C2H6O but 102°C difference! Ethanol has O-H for H-bonding; dimethyl ether cannot donate H-bonds. This isomer comparison is a frequently tested concept demonstrating how functional group determines physical properties.