Alcohols are produced industrially by hydration of alkenes in the presence…

The idea

Industrially, an alkene is hydrated directly: ethene is mixed with steam and passed over a catalyst of concentrated phosphoric acid at about 300 °C and 60 to 70 atmospheres. The acid is a catalyst, so it is regenerated and does not appear in the overall equation CH₂=CH₂ + H₂O → CH₃CH₂OH. The process is continuous, fast and gives a pure product with an atom economy of one hundred per cent, but only a small fraction of the ethene reacts on each pass, so unreacted gas is recycled and the overall yield stays high. With an unsymmetrical alkene the water adds to give the more substituted alcohol, so hex-1-ene gives mainly hexan-2-ol, and yield calculations in exams are usually built on that product.

What earns the marks

• Steam as the reagent and concentrated phosphoric acid as the catalyst.
• Conditions of roughly 300 °C and 60 to 70 atmospheres.
• The equation, with the acid shown as a catalyst rather than a reactant.
• Identifying the major alcohol from an unsymmetrical alkene, such as hexan-2-ol from hex-1-ene.

Common mistakes and how to put them right

• Writing liquid water as the reagent rather than steam.

• Giving hexan-1-ol from hex-1-ene by adding the water the wrong way round.

• Including the acid catalyst among the reactants in the overall equation.

• Concluding that a low conversion on each pass means a low overall yield, when the alkene is recycled.

Exam tips

• In yield calculations, find moles of alkene first from mass and density, then apply the percentage.
• Quote both temperature and pressure; one alone often does not score the conditions mark.
• Say the catalyst is regenerated if asked why it is not in the equation.