Aldehydes and Ketones
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What does the carbonyl group consist of?
C=O
What compounds have a C=O group?
Aldehydes and ketones
How are aldehydes named?
With the suffix -al.
How are ketones named?
With the suffix -one.
Describe the carbonyl group.
Strongly polar. Difference in electronegativity between O and C. Permanent dipole-dipole forces between molecules. Higher boiling points than alkanes/alkenes.
Describe the solubility of aldehydes and ketones.
Short chain aldehydes and ketones mix completely with water because hydrogen bonds can form with the O in the aldehyde/ketone and H in the water molecule. As the chain length of aldehydes and ketones increases, carbonyl compounds become less soluble in water.
Describe benzenecarbaldehyde.
Smells of almonds. Used to scent soaps and flavour foods.
What kind of reactions can aldehydes and ketones undergo?
Nucleophilic addition reactions (due to the C=O group and double bond)
Why are hydroxynitriles useful in synthesis?
The -OH and -CN groups are both reactive and can be converted into other functional groups.
What reagent is used in the nucleophilic addition of an aldehyde or ketone?
KCN or NaCN
Why does the nucleophilic addition of an aldehyde or unsymmetrical ketone form a racemic mixture of two optical isomers?
The :CN- may attack from above or below the flat C=O group.
What are aldehydes oxidised into?
Carboxylic acids
What is used to represent an oxidising agent?
[O]
What is an example of an oxidising agent?
Acidified (with dilute sulfuric acid) potassium dichromate
Why can’t ketones be oxidised easily to carboxylic acids?
A C-C bond must be broken. Stronger oxidising agents break the hydrocarbon chain of the ketone molecule resulting in a shorter chain molecule, carbon dioxide and water.
What are the two tests used to distinguish aldehydes and ketones?
Fehling’s test Tollens’ reagent
What is Fehling’s solution made up of?
Fehling’s solution A (contains Cu^2+ ions and is thus blue) and Fehling’s solution B (contains alkali and complexing agent)
Describe the test with Fehling’s solution.
Warm an aldehyde with Fehling’s solution. Brick red precipitate of copper oxide is produced as copper (II) oxidises the aldehyde to a carboxylic acid and itself is reduced to copper (I). Ketone’s give no reaction to this test.
Describe the Tollens’ Reagent test.
Warm an aldehyde with Tollens’ reagent. Aldehydes are oxidised into carboxylic acids. Ag+ reduced to metallic silver. Silver mirror forms. Ketone’s give no reaction to this test.
What does Tollens’ reagent contain?
The complex ion [Ag(NH3)2] which is formed when aqueous ammonia is added to an aqueous solution of silver nitrate.
What do reducing agents reduce aldehydes and ketones into?
Alcohols
Give an example of a reducing agent.
NaBH4 in aqueous solution.
What does NaBH4 generate?
The nucleophile :H-, the hydride ion.
Why does NaBH4 reduce C=O but not C=C?
It is repelled by the high electron density in the C=C bond but is attracted to the slightly positive carbon in C=O.