Reactions with Alcohols

What reagent is used to oxidise alcohols, and what is the observation when oxidation occurs?

Acidified potassium dichromate(VI) solution (K₂Cr₂O₇ in dilute H₂SO₄), represented as [O]. The solution changes colour from orange to green as the dichromate(VI) ions (Cr₂O₇²⁻) are reduced to chromium(III) ions (Cr³⁺), confirming that oxidation of the alcohol has taken place.

What is produced when a primary alcohol is oxidised under different conditions, and how is each product obtained?

A primary alcohol can be oxidised in two stages:

• To an aldehyde — heat gently with acidified dichromate and distil the product off immediately as it forms. Distillation prevents further oxidation because the aldehyde (with its lower boiling point) is removed from the oxidising agent before it can react further. Example: CH₃CH₂OH + [O] → CH₃CHO + H₂O (ethanol → ethanal)

• To a carboxylic acid — heat under reflux with excess acidified dichromate. Reflux keeps the aldehyde in contact with the oxidising agent so it is fully oxidised. Example: CH₃CHO + [O] → CH₃COOH (ethanal → ethanoic acid)

What is the equation for butan-1-ol to butanoic acid

• CH₃CHO + 2[O] → CH₃COOH + H₂O

• under excess potassium dichromate

What is produced when a secondary alcohol is oxidised, and what are the conditions?

A secondary alcohol is oxidised to a ketone when heated under reflux with acidified dichromate(VI). Ketones cannot be oxidised further under these conditions, so reflux is used simply to ensure the reaction goes to completion rather than to prevent over-oxidation. Example: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O (propan-2-ol → propanone)

What happens when you try to oxidise a tertiary alcohol with acidified dichromate?

Nothing — tertiary alcohols do not undergo oxidation under normal conditions. The dichromate solution remains orange. This is because the carbon bearing the –OH group has no hydrogen atom attached to it, and a C–H bond on that carbon is required for the oxidation to proceed.

What is dehydration of an alcohol, and what type of reaction is it?

Dehydration is the removal of water (H₂O) from an alcohol to form an alkene. It is an elimination reaction. The –OH group and a hydrogen atom from an adjacent carbon are removed together as water.

What are the two methods and conditions for dehydrating an alcohol?

• Heat the alcohol with excess concentrated sulfuric acid (H₂SO₄) or concentrated phosphoric acid (H₃PO₄) — both act as acid catalysts.

• Pass the alcohol vapour over hot aluminium oxide (Al₂O₃) catalyst.

Give the equation for the dehydration of ethanol and name the product.

CH₃CH₂OH → CH₂=CH₂ + H₂O. The product is ethene. The reaction requires heat and a concentrated acid or Al₂O₃ catalyst.

What is produced when an unsymmetrical alcohol such as butan-2-ol is dehydrated, and why?

Two alkenes can be formed depending on which adjacent carbon loses a hydrogen — but-1-ene and but-2-ene (which can also show E/Z isomerism). This is because the –OH and an H can be eliminated from either of the two adjacent carbon atoms.

How can an alcohol be converted into a haloalkane, and give an example reaction?

The –OH group of the alcohol is replaced by a halogen atom in a substitution reaction. One method is to react the alcohol with a sodium halide (e.g., NaBr) in the presence of concentrated sulfuric acid. The acid reacts with the sodium bromide to produce hydrogen bromide (HBr) in situ, which then reacts with the alcohol.

EQ1: NaBr + H₂SO₄ → NaHSO₄ + HBr

EQ2 CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O (ethanol → bromoethane)

What is the overall equation for the reaction of propane-2ol with H₂SO₄ and NaBr

CH₃CHOHCH₃ + NaBr +H₂SO₄ → CH₃CHBrCH₃₊ NaHSO_4. +H₂O