Most alcohols undergo dehydration to form an alkene when heated with a strong acid.
The reaction is an Elimination and favoured at higher temperatures. The most commonly used acids in the laboratory are Bronsted- proton donors such as sulfuric acid and phosphoric acid. Lewis acids such as alumina are often used in industrial gas-phase dehydrations.
The temperature and concentration of the acid required to hydrate an alcohol depend on the structure of the alcohol substrate.
1. Primary alcohols are most difficult to dehydrate. eg dehydration of ethanol requires conc sulfuric acid and temp of 180 degrees Celsius.
2.Secondary alcohols usually dehydrate under milder conditions. Cyclohexanol for example dehydrate in 85% phosphoric acid at 165-170 degrees Celsius.
3. Tertiary alcohols are usually so easily dehydrated that relatively mild conditions can be used. Tert-Butylalcohol dehydrates at in 20% aqueous sulfuric acid at a temperature 85 degrees Celsius.
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