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

Oxidation of Aldehydes

• Aldehydes and ketones contain the carbonyl functional group, C=O

  • This is why aldehydes and ketones are also known as carbonyls

• The difference between aldehydes and ketones is the groups bonded to the carbon of the carbonyl group

The aldehyde and ketone functional groups

• The carbonyl group in an aldehyde is always situated at the end of the chain

  • When naming aldehydes, you do not include the '1' in the name as the carbonyl carbon is always number 1 on the chain

  • The simplest aldehyde is methanal, HCHO, with the only carbon being that of the carbonyl group

• The carbonyl group in a ketone is always located within the chain

  • The simplest ketone is propan-2-one, CH₃COCH₃, as you need an alkyl group either side of the carbonyl carbon in a ketone

Preparation of Aldehydes & Ketones

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

Oxidation of alcohols

Further Oxidation

• To oxidise a primary alcohol to an aldehyde, the apparatus must be set up to distil off the aldehyde as it is produced

• Further oxidation can then take place

  • Aldehydes are easily oxidised to carboxylic acids

• To oxidise a primary alcohol straight to a carboxylic acid, you heat the reaction mixture under reflux

  • The aldehyde is still produced, but as it evaporates it condenses back into the reaction mixture where it is further oxidised to the carboxylic acid

Oxidation of an aldehyde

• The oxidising agent used for all of the oxidation reactions is acidified potassium dichromate

  • K₂Cr₂O₇ with sulfuric acid, H₂SO₄

• Ketones are very resistant to being oxidised, so no further oxidation reaction will take place with secondary alcohols

  • This is because ketones do not have a readily available hydrogen atom, in the same way that aldehydes (or alcohols) do

  • An extremely strong oxidising agent would be needed for oxidation of a ketone to take place

  • The oxidation will likely oxidise a ketone in a destructive way, breaking a C-C bond

Distinguishing Between Aldehydes & Ketones

• Weak oxidising agents can be used to distinguish between an aldehyde and a ketone

  • The aldehyde will be oxidised to a carboxylic acid, but the ketone will not undergo oxidation

• There are a number of tests that can be used to distinguish between aldehydes and ketones

• You specifically need to know the following methods to distinguish between an aldehyde and a ketone:

  • Tollens' reagent (this is the most commonly used method)

  • Fehling's solution

Using Tollens' Reagent - The Silver Mirror Test

• Tollens' reagent contains the silver(I) complex ion [Ag(NH₃)₂]⁺

• This is formed when aqueous ammonia is added to a solution of silver nitrate

  • Tollens' reagent is also known as ammoniacal silver nitrate

• If gently warmed with Tollens' reagent, an aldehyde will become oxidised

• The silver(I) complex ion solution, [Ag(NH₃)₂]⁺, is colourless

• As the aldehyde is oxidised, it causes the [Ag(NH₃)₂]⁺ ions to become reduced to solid metallic silver, Ag

• This is why a positive test result is called a "silver mirror"

Positive Test Result:

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

  • This is the positive test result

• When Tollens' reagent is 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

Positive result for the silver mirror test

Using Fehling's Solution 

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

• If an aldehyde is warmed with Fehling's solution, the aldehyde will be oxidised and a colour change will take place

• Fehling's solution is blue, because of the copper(II) complex ions present

• During the reaction, as the aldehyde is oxidised to a carboxylic acid, the blue Cu²⁺ ions are reduced to Cu⁺ ions and a brick red precipitate is formed

  • The brick red precipitate is copper(I) oxide

• If a ketone is warmed with Fehling's solution, no reaction takes place as the ketone will not be oxidised, so the solution will remain blue

Positive result for Fehling's solution