8: Aldehydes and Ketones
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What is the difference between an aldehyde and a ketone?
Aldehyde groups are found at the end of the carbon chain ( C=O H) Ketones are found in the middle of the carbon chain (C=O)
Primary alcohols can be oxidised into what type of compound?
Aldehyde
After primary alcohols have been oxidised into aldehydes, what can the aldehydes be oxidised into?
Carboxylic Acids
Secondary alcohols can be oxidised into what type of compound?
Ketone
What can tertiary alcohols be oxidised into?
Nothing.
Why is oxidation of aldehydes into carboxylic acids easy?
Aldehydes have a hydrogen attached to the carbonyl group. This means the hydrogen can be lost easily, allowing for oxidation.
Why are ketones not oxidised as easily?
To oxidise a ketone, you need to break the C=O bond, which is more difficult.
Name 2 reagents that can be used to distinguish aldehydes and ketones.
Tollens Reagent Fehling’s Solution
Describe how to test for aldehydes using Tollens reagent.
Heat a test tube containing your solution with Tollens. If aldehydes are present, a silver mirror forms.
What is contained in a solution of Tollens? Name the complex ion.
Ammoniacal silver nitrate [Ag(NH3)2]
Describe the reduction/oxidation that takes place in a positive test for aldehydes using Tollens.
The aldehyde is oxidised (loss of hydrogen) The diamine silver ions in the Tollens are reduced, forming silver and ammonia.
What happens when Tollens is added to a ketone?
Nothing, ketones don’t react with Tollens reagent.
Describe how to test for aldehydes using Fehling’s solution.
Heat the Fehling’s with the unknown solution . If aldehydes are present, copper (II) ions are reduced to form a brick-red precipitate of copper (I) oxide.
Describe what you would see if you added ketones to Fehling’s solution.
Nothing, ketones don’t react with Fehling’s solution.
Name the compound used to reverse the oxidation of primary/secondary alcohols (reduction).
NaBH4
Give a general equation for the reduction of an aldehyde to a primary alcohol using a reducing agent (such as NaBH4)
RCOH + 2[H] -> RCH2OH
Give a general equation for the reduction of a ketone to a secondary alcohol using a reducing agent (such as NaBH4)
RC=OR + 2[H] -> RCHROH
Name the mechanism for aldehydes/ketones turning into alcohols.
Nucleophilic addition
During reduction of aldehydes/ketones into alcohols, where does the nucleophile come from? Name the nucleophile.
H- ions come from the reducing agent and act as a nucleophile.
On an aldehyde, where does the H- ion attack and why?
H- ions attack the carbon. The C=O bond is polar, so the carbon is slightly positive and the oxygen is slightly negative.
How does the H- ion form a bond with the carbon?
The H- donates is lone pair of electrons forming a bond with the carbon.
Why does the addition of H- cause one of the carbon oxygen bond to break? Where do electrons from the C=O bond go to?
Addition of the H- causes 5 bonds to form on the carbon atom. This can’t happen, so one of the bonds needs to break. This allows the carbon to have 4 bonds. A lone pair of electrons is forced from the C=O bond onto the oxygen.
The lone pair on the oxygen is then forced onto where? Where does this molecule come from?
H+ ion, which comes from water or a weak acid.
At the end of nucleophilic addition for aldehyde, what is produced?
Primary alcohol.