Aldehydes & Ketones (AQA A Level Chemistry): Revision Note

Oxidation of Aldehydes

• Aldehydes and ketones contain the carbonyl functional group, C=O, which is why they are also known as carbonyl compounds

• The difference between aldehydes and ketones lies in the groups attached to the carbon atom of the carbonyl group

• In aldehydes, the carbonyl group is always located at the end of the carbon chain

  • When naming aldehydes, the number 1 is not included in the name because the carbonyl carbon is always carbon 1

  • The simplest aldehyde is methanal, HCHO, in which the only carbon present is the carbonyl carbon.

• In ketones, the carbonyl group is always located within the carbon chain

  • The simplest ketone is propanone, CH₃COCH₃, because a ketone must have an alkyl group on both sides of the carbonyl carbon

Preparation of Aldehydes & Ketones

• Aldehydes and ketones can be prepared by oxidising primary and secondary alcohols, as shown below

Further Oxidation

• During the oxidation of a primary alcohol to an aldehyde, the apparatus must be set up for distillation so that the aldehyde can be distilled off as soon as it is formed

  • This prevents further oxidation from taking place

• Aldehydes are easily oxidised to form carboxylic acids

• To oxidise a primary alcohol directly to a carboxylic acid, the reaction mixture is heated under reflux

  • Although an aldehyde is still formed as an intermediate, it evaporates, condenses, and returns to the reaction mixture, where it is further oxidised to the carboxylic acid

• The oxidising agent used for these oxidation reactions is acidified potassium dichromate(VI), K₂Cr₂O₇ , in the presence of sulfuric acid, H₂SO₄

• Ketones are very resistant to oxidation, so secondary alcohols do not undergo further oxidation beyond the ketone stage

  • This is because ketones do not have a readily available hydrogen atom bonded to the carbonyl carbon, unlike aldehydes or alcohols

  • Oxidising a ketone would require an extremely strong oxidising agent, and this would likely result in destructive oxidation, breaking carbon–carbon bonds

Distinguishing Between Aldehydes & Ketones

Distinguishing Between Aldehydes & Ketones

• Mild oxidising agents can be used to distinguish between aldehydes and ketones

  • An aldehyde will be oxidised to a carboxylic acid, whereas a ketone will not undergo oxidation

• Several tests can be used to tell aldehydes and ketones apart

• You are specifically required to know the following methods:

  • Tollens’ reagent (the most commonly used test)

  • Fehling’s solution

Using Tollens' Reagent - The Silver Mirror Test

• Tollens’ reagent contains the silver(I) complex ion, [Ag(NH₃)₂]⁺, which is formed when aqueous ammonia is added to a solution of silver nitrate

  • For this reason, Tollens’ reagent is also known as ammoniacal silver nitrate

• The [Ag(NH₃)₂]⁺ solution is colourless

  • When gently warmed with an aldehyde, the aldehyde is oxidised, and the silver(I) complex ions are reduced to solid metallic silver, Ag

• This deposition of silver on the inside of the test tube produces the characteristic silver mirror, which indicates a positive result

Positive Test Result:

• When Tollens' reagent is gently warmed with an aldehyde, a silver mirror is formed

• When gently warmed with a ketone, no silver mirror will be seen, as the ketone cannot be oxidised by Tollens' reagent, so no reaction takes place

Using Fehling's Solution 

• Fehling’s solution is an alkaline solution containing copper(II) ions, which act as the oxidising agent

• When an aldehyde is warmed with Fehling’s solution, the aldehyde is oxidised, and a colour change occurs.

  • The solution is initially blue due to the presence of copper(II) complex ions

• As the aldehyde is oxidised to a carboxylic acid, the blue Cu²⁺ ions are reduced to Cu⁺ ions, forming a brick-red precipitate of copper(I) oxide

• If a ketone is warmed with Fehling’s solution, no reaction occurs because ketones are not oxidised.

  • The solution, therefore, remains blue.

• Heating with acidified potassium dichromate could also be used to distinguish between an aldehyde and a ketone

  • The aldehyde would be oxidised, and you would see an orange to green colour change

  • The ketone would not be oxidised, so you would see no colour change

Summary of the Oxidation Reactions Table