Part 3: Dehydration mechanisms and required practical 5

Make an alkene from an alcohol, explain the role of the acid and connect each purification step to the impurity it removes.

Remove water to create C=C

Heating an alcohol with concentrated phosphoric or sulfuric acid can eliminate water and form an alkene. The acid acts as a catalyst and dehydrating agent. Depending on the preparation, product may be distilled off as it forms; use the arrangement specified rather than assuming every dehydration is performed under reflux.

For propan-2-ol the product is propene. Butan-2-ol can give but-1-ene and but-2-ene; the latter has E and Z forms. The two atoms removed to make water are associated with the OH group and a neighbouring carbon-bound hydrogen.

Protonate OH before it leaves

OH⁻ is a poor leaving group in this acid-catalysed pathway. First, an oxygen lone pair attacks H⁺, making an –OH₂⁺ group. Water can now leave: the C–O bond pair moves to O. For the secondary-alcohol example, this gives a carbocation.

Next, the electron pair from a C–H bond on an adjacent carbon forms the extra bond of C=C as H⁺ is lost. If a water molecule is shown accepting this proton, add an arrow from its oxygen lone pair to H. The acid is regenerated. The carbon that becomes part of the double bond must be the one neighbouring the original OH-bearing carbon.

The mechanism name is elimination. “Dehydration” describes the loss of water; include the requested mechanism name when the question asks for it.

RP5: distillation of a reaction product

A common preparation is cyclohexene from cyclohexanol with an acid catalyst. Their formulas are C₆H₁₂O and C₆H₁₀, so the change is loss of H₂O. Gentle controlled heating and distillation remove the volatile product from the reaction mixture.

Collect the crude distillate, then use a separating funnel to separate organic and aqueous layers. A prescribed carbonate wash can neutralise acidic impurities, releasing CO₂; vent the funnel regularly and point it away from people. Identify the correct layer from the procedure or density evidence.

Use a suitable anhydrous drying agent to remove residual water, remove the drying agent by decanting or filtration, and redistil if the procedure requires it. Collect the specified boiling range. A narrow range supports purity but does not on its own establish identity; a chemical test or spectrum adds evidence.

Calculate theoretical product before percentage yield

A product mass must be compared with the theoretical mass of that same product. If a liquid reactant is measured by volume, first use mass = density × volume. Then convert to moles and apply the equation ratio.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources