Summary notes for AQA A-Level Chemistry Unit 3.3.8 - Aldehydes and ketones (A-level only)
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Unit 3.3.8 - Aldehydes and ketones
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AQA A Level Chemistry Student Book 2
Summary notes for AQA A-Level Chemistry Unit 3.3.8 - Aldehydes and ketones (A-level only) by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: The Carbonyl Group, Chemical Test to Distinguish Between Aldehydes and Ketones, Physical Properties of Aldehyde...
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Unit 3.3.8 - Aldehydes and ketones
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Section 3 : Organic Chemistry
Aldehydes and Ketones
The Carbonyl Group
The carbonyl group is C=O.
• As expected for a double bond, C=O undergoes addition reactions.
• However, the carbonyl double bond does not react in the same way as the C=C.
• This is because C=O is polar - O is more electronegative than C + draws the electrons in the covalent
bond towards itself resulting in a δ+ C + a δ- O. The δ+ C is readily attacked by nucleophiles + hence
the carbonyl group undergoes nucleophilic addition reactions.
Aldehydes, carboxylic acids + ketones all contain a carbonyl group.
• Aldehydes have a functional group shown as CHO in structural formulae + are named using the -al
suf x.
- Aldehydes are readily oxidised to carboxylic acids. Carboxylic acids have a functional group
shown as COOH in structural formulae + are named using the -oic acid suf x.
• Ketones have a functional group shown as CO in structural formulae + are named using the -one
suf x (or oxo- pre x if another functional group is present).
Chemical Test to Distinguish Between Aldehydes and Ketones
• Aldehydes are much easier to oxidise than ketones.
• This is the basis for the chemical tests to distinguish between aldehydes + ketones, including
Fehling’s solution + Tollens’ reagent (both of which are weak oxidising agents). The weak oxidising
agents will oxidise aldehydes but they will not oxidise ketones + this difference enables the two to be
distinguished.
Fehling’s Solution
Warm the unknown compound w/ Fehling’s solution. Aldehyde = blue sol. ➜ brick red ppt.
• Fehling’s solution is an alkaline solution of copper(II) sulfate. This contains Cu2+ ions ∴ blue colour.
• In the presence of an aldehyde, Cu2+ is reduced (Cu2+ + e- → Cu ) to form a brick red precipitate of
copper(I) oxide which has the formula Cu2O.
• The aldehyde is itself oxidised by Fehling’s solution to form a carboxylic acid.
∴ Fehling's solution = [O]
• A ketone will not be oxidised further ∴ no reaction/change occurs in the presence of a ketone.
Tollens’ Reagent (silver mirror test)
Warm the unknown compound w/ Tollens’ reagent. Aldehyde = colourless solution ➜ silver mirror
• Tollens’ reagent contains the complex ion [Ag(NH3)2]+.
• In the presence of an aldehyde, [Ag(NH3)2]+ is reduced to form a precipitate of metallic silver (which
will coat the inside of the apparatus, forming a silver mirror) as per the half equation:
Ag+(aq) + e- → Ag(s)
• The aldehyde is itself oxidised by Tollens’ reagent to form a carboxylic acid. ∴ Tollens’ reagent = [O]
• A ketone will not be oxidised further ∴ no reaction/change occurs in the presence of a ketone.
Physical Properties of Aldehydes and Ketones
Boiling Points
• Van der Waals’ forces + permanent dipole-dipole forces exist between one aldehyde molecule +
another - the same is true of ketones.
- This means that the b.p. of aldehydes + ketones are higher than that of alkanes w/ a similar Mr.
• As the length of the carbon chain ↑, the b.p. of the aldehydes/ketones ↑. This is due to a greater no. of
electrons + hence stronger van der Waals’ forces of attraction between the molecules.
• As branching ↑, the b.p. of the aldehydes/ketones ↓ due to less effective permanent dipole-dipole
forces. The branching allows less interaction between the molecules.
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