Why does ethanol mix with water in all proportions, but oil-like higher alcohols don't?
Ethanol's small O–H group hydrogen-bonds strongly with water, and its tiny carbon chain barely gets in the way, so the two mix completely.
Ethanol's small O–H group hydrogen-bonds strongly with water, and its tiny carbon chain barely gets in the way, so the two mix completely. As the carbon chain grows (butanol, hexanol...), the greasy hydrocarbon part dominates and can't hydrogen-bond with water, so solubility falls off. It's a tug-of-war between the water-loving O–H and the water-hating carbon chain.
Key point
Solubility drops as the hydrophobic carbon chain outgrows the hydrogen-bonding O–H group.
Common mistake
thinking all alcohols are equally miscible with water.
Memory tip
short chain → mixes (O–H wins); long chain → doesn't (carbon tail wins).
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