alcohols

What are the three classes of alcohol?

Primary, secondary and tertiary alcohols. The classification depends on the number of carbon atoms directly attached to the carbon bearing the –OH group.

How is a primary alcohol identified structurally?

The carbon bonded to the –OH group is bonded to one other carbon atom (or no carbon atoms in methanol).

How is a secondary alcohol identified structurally?

The carbon bonded to the –OH group is bonded to two other carbon atoms.

How is a tertiary alcohol identified structurally?

The carbon bonded to the –OH group is bonded to three other carbon atoms.

What happens when a primary alcohol is oxidised?

A primary alcohol can be oxidised first to an aldehyde and then further to a carboxylic acid.

What happens when a secondary alcohol is oxidised?

A secondary alcohol is oxidised to a ketone.

What happens when a tertiary alcohol is treated with acidified potassium dichromate(VI)?

Tertiary alcohols are not easily oxidised.

What oxidising agent is used to oxidise alcohols?

Acidified potassium dichromate(VI), K₂Cr₂O₇.

What colour change occurs when acidified potassium dichromate(VI) oxidises an alcohol?

The solution changes from orange to green as dichromate(VI) ions are reduced to chromium(III) ions.

What is the oxidation equation for a primary alcohol to an aldehyde?

RCH₂OH + [O] → RCHO + H₂O.

What is the oxidation equation for an aldehyde to a carboxylic acid?

RCHO + [O] → RCOOH.

What is the overall oxidation equation for a primary alcohol to a carboxylic acid?

RCH₂OH + 2[O] → RCOOH + H₂O.

What is the oxidation equation for a secondary alcohol to a ketone?

R₂CHOH + [O] → R₂CO + H₂O.

What is the oxidation sequence for a primary alcohol?

Primary alcohol → aldehyde → carboxylic acid.

What is the oxidation sequence for a secondary alcohol?

Secondary alcohol → ketone.

Why can a primary alcohol give either an aldehyde or a carboxylic acid when oxidised?

The aldehyde is an intermediate and can be oxidised further. The conditions determine whether the aldehyde is removed before further oxidation or remains in contact with the oxidising mixture.

How is an aldehyde prepared from a primary alcohol?

Heat the primary alcohol with acidified potassium dichromate(VI) and distil the aldehyde off as it forms.

Why is distillation used when preparing an aldehyde from a primary alcohol?

The aldehyde is distilled off as it forms, separating it from the oxidising mixture and preventing further oxidation.

Why must the aldehyde be distilled off as it forms?

To minimise its contact with the oxidising agent and prevent it being further oxidised to a carboxylic acid.

How is a carboxylic acid prepared from a primary alcohol?

Heat the primary alcohol with acidified potassium dichromate(VI) under reflux so the aldehyde remains in the reaction mixture and is further oxidised.

Why is reflux used when preparing a carboxylic acid from a primary alcohol?

Reflux allows prolonged heating while condensing organic vapours back into the reaction flask, so the aldehyde remains and can be further oxidised.

What is reflux?

Heating a reaction mixture while a condenser continuously condenses vapours and returns them to the reaction flask, allowing prolonged heating without significant loss of volatile substances.

What is the purpose of a reflux condenser?

It condenses vapours and returns the liquid to the reaction flask, preventing volatile substances from escaping while the mixture is heated.

How is a reflux condenser positioned?

Vertically, with the top left open to the atmosphere.

Why must the top of a reflux condenser be open?

To prevent pressure build-up in the apparatus.

How should cooling water enter a condenser?

Cooling water enters at the bottom and leaves at the top.

Why does cooling water enter at the bottom and leave at the top of a condenser?

It keeps the water jacket full and provides efficient cooling.

What is a Liebig condenser used for in this experiment?

It is used during distillation to condense the aldehyde vapour into a liquid product.

What is distillation?

A separation technique in which a liquid is vaporised and then condensed, allowing a component with a suitable boiling point to be collected.

Why is the distillation condenser angled downward?

So the condensed liquid flows by gravity into the receiving vessel.

How should the thermometer be positioned during distillation?

The bulb should be level with the entrance to the side arm of the distillation apparatus.

What does the thermometer measure during distillation?

The temperature of the vapour entering the condenser, which corresponds approximately to the boiling point of the component being distilled.

Why should anti-bumping granules be added before heating?

They provide nucleation sites, promoting smooth boiling and preventing sudden, vigorous bumping.

What problem do anti-bumping granules prevent?

They prevent vigorous, uneven boiling and sudden formation of large bubbles, known as bumping.

Why should anti-bumping granules not be added to a hot liquid?

They should be added before heating because adding them to a hot liquid can cause sudden, vigorous boiling.

Why is a naked flame avoided when heating organic substances such as ethanol?

Ethanol and many organic compounds are flammable, so a naked flame presents a fire risk.

What is Required Practical 5 in AQA A-level Chemistry?

Distillation of a product from a reaction.

What is the key difference between distillation and reflux when oxidising a primary alcohol?

Distillation removes the aldehyde as it forms; reflux returns vapours to the flask so the aldehyde remains and is further oxidised to the carboxylic acid.

What is an aldehyde?

An organic compound containing a carbonyl group with the carbonyl carbon bonded to a hydrogen atom; general structure RCHO.

What is a ketone?

An organic compound containing a carbonyl group with the carbonyl carbon bonded to two carbon-containing groups; general structure R₂CO.

Which class of alcohol forms aldehydes when oxidised?

Primary alcohols.

Which class of alcohol forms ketones when oxidised?

Secondary alcohols.

Which class of alcohol forms carboxylic acids via an aldehyde intermediate?

Primary alcohols.

How can an aldehyde be distinguished from a ketone using Fehling's solution?

Warm the substance with Fehling's solution. An aldehyde produces an opaque red/brick-red precipitate of Cu₂O; a ketone gives no reaction.

What is Fehling's solution?

An alkaline solution containing copper(II) ions, which acts as the oxidising agent in the test for aldehydes.

What happens to Cu²⁺ ions in Fehling's test?

Cu²⁺ ions are reduced to Cu⁺ ions, which form copper(I) oxide, Cu₂O, as an opaque red/brick-red precipitate.

What happens to an aldehyde in Fehling's test?

The aldehyde is oxidised to a carboxylic acid while Cu²⁺ is reduced to Cu⁺, producing Cu₂O.

What is the positive observation for an aldehyde with Fehling's solution?

The clear blue solution forms an opaque red/brick-red precipitate when warmed.

What is the observation for a ketone with Fehling's solution?

No positive reaction occurs; the solution remains blue and no red Cu₂O precipitate forms.

How can an aldehyde be distinguished from a ketone using Tollens' reagent?

Warm the substance with Tollens' reagent. An aldehyde produces a silver mirror; a ketone gives no reaction.

What is Tollens' reagent?

An aqueous alkaline solution of silver nitrate in excess ammonia solution, also called ammoniacal silver nitrate solution.

What happens to silver ions in Tollens' test?

Ag⁺ ions are reduced to silver atoms, Ag.

What happens to an aldehyde in Tollens' test?

The aldehyde is oxidised to a carboxylic acid while Ag⁺ is reduced to silver atoms.

What is the positive observation for an aldehyde with Tollens' reagent?

A silver mirror forms on the inside of the test tube.

What is the observation for a ketone with Tollens' reagent?

No positive reaction occurs; no silver mirror forms.

Why do aldehydes give positive Fehling's and Tollens' tests but ketones do not?

Aldehydes are readily oxidised to carboxylic acids under these test conditions, whereas ketones are not readily oxidised.

What is Required Practical 6 in AQA A-level Chemistry?

Tests for alcohol, aldehyde, alkene and carboxylic acid.

What two tests can distinguish aldehydes from ketones?

Fehling's solution and Tollens' reagent.

What is the redox relationship in Fehling's and Tollens' tests?

The aldehyde is oxidised while the metal ion is reduced: Cu²⁺ → Cu⁺ in Fehling's test and Ag⁺ → Ag in Tollens' test.

What is an oxidising agent?

A substance that oxidises another substance and is itself reduced. It accepts electrons.

What is a reducing agent?

A substance that reduces another substance and is itself oxidised. It donates electrons.

What is formed when ethanol is oxidised to an aldehyde?

Ethanal, CH₃CHO.

What is formed when ethanol is completely oxidised?

Ethanoic acid, CH₃COOH.

What happens when ethanol is heated with acidified potassium dichromate(VI) and the product is distilled off?

Ethanol is oxidised to ethanal, which is distilled off as it forms.

What happens when ethanol is heated with acidified potassium dichromate(VI) under reflux?

Ethanol is oxidised via ethanal to ethanoic acid.

What is the difference between preparing ethanal and preparing ethanoic acid from ethanol?

Ethanal is prepared by distilling it off as it forms; ethanoic acid is prepared by refluxing so the ethanal remains and is further oxidised.