Compounds containing the carbonyl group
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List the key organic compounds that the carbonyl group is present in
• Aldehydes • Ketones • Carboxylic acids • Esters • Acyl Chlorides • Acid anhydrides
Describe the polarity of carbonyl compounds and how this affects melting and boiling points
• The carbonyl group is strongly polar • This means that permanent dipole-dipole forces form between molecules. • These forces mean that boiling points are higher than those alkanes of comparable relative molecular mass, but not as high as alcohols, where hydrogen bonding can occur between the molecules.
Describe the solubility of aldehydes and ketones
• Shorter chain aldehydes and ketones mix completely with water because hydrogen bonds form between the oxygen of the carbonyl compound and water. • As the length of the carbon chain increases, the carbonyl compound becomes less soluble in water due to the part of the molecule that can hydrogen bond being a smaller percentage of the larger molecule.
What are the most common reactions involving carbonyl compounds
Nucleophilic additions
Why are nucleophilic addition reactions the most common reaction of the carbonyl groups
• The polarity of the C=O bond means that nucleophilic reagents can attack the delta positive carbon atom. • Also, since they contain a double bond, carbonyl compounds are unsaturated which means addition reactions are possible.
Describe the conditions required for the nucleophilic addition of hydrogen cyanide to an aldehyde or ketone
• Sodium/potassium cyanide are used as the source of cyanide ions. • Dilute hydrochloric acid is then added to provide the H+ ion.
What are the products of the nucleophilic addition of hydrogen cyanide to aldehydes or ketones
Hydroxynitriles
Why are hydroxynitriles and the nucleophilic addition reaction that forms them useful
• They are useful in synthesis because both the -OH and -CN groups are reactive and can be converted into other functional groups. • The reaction is also useful in organic synthesis because it increases the length of the carbon chain by one carbon.
In terms of optical isomerism, what does the nucleophilic addition of hydrogen cyanide to an aldehyde or unsymmetrical ketone produce and why
• A racemic mixture • This is because the CN- ion can attack from above or below the flat C=O group.
Can Ketones be oxidised to carboxylic acids
• No • Ketones cannot be oxidised easily to carboxylic acids because it would require a C-C bond to be broken due to the oxygen being in the middle of the molecule. • Even if this does happen, the result is a shorter chain molecule, carbon dioxide and water, not a carboxylic acid.
What is the reagent that oxidised aldehydes to carboxylic acids
Acidified (with dilute sulfuric acid) potassium dichromate
Describe Fehlings test
• Fehlings contains the blue Cu2+ ion • when an aldehyde is warmed with Fehlings solution, a brick red precipitate of copper (I) oxide is produced as the copper (II) oxidised the aldehyde to a carboxylic acid and is itself reduced to copper (I). • Ketones give no reaction to this test.
Describe the silver mirror test
• The silver mirror test uses Tollens reagent which is formed when aqueous ammonia is added to an aqueous solution of silver nitrate. • when an aldehyde is warmed with Tollens reagent, metallic silver is formed as the aldehyde is oxidised to a carboxylic acid. • a silver mirror on the inside of the test tube forms. • Ketones give no reaction to this test
What are the two chemical tests that distinguish between an aldehyde and a ketone
Fehlings and the silver mirror test
What reaction do Fehlings and the silver mirrr test use to distinguish between aldehydes and ketones
• The oxidation of aldehydes. • the fact that ketones cannot be oxidised
What happens when a reducing agent reacts with an aldehyde or ketone
The aldehydes and ketones will be reduced to alcohols
Give an example of a reducing agent that will reduce aldehydes and ketones, what it does and how it works
• Sodium tetrahydridoborate (III), NaBH4 in aqeous solution • This generates the nucleophile :H-, the hybride ion. • This works because it is attracted to the delta positive C of the C=O bond
Why does sodium tetrahydroborate (III) NaBH4 reduce the C=O but not C=C
It is repelled by the high electron density in the C=C bond but is attracted to the delta positive C of the C=O bond
Where does the H+ ion used in the reduction of an aldehyde or ketone come from
The solvent (acidic solvent)
Where does the :H- ion involved in the reduction of an aldehyde or ketone come from
The reducing agent (eg NaBH4)
What type of reaction is the reduction of aldehydes and ketones back into alcohols and why
• Nucleophilic addition • This is because the H- ion is a nucleophile
What two functional groups does a carboxylic acid contain
• The carbonyl group (C=O) • The hydroxyl group -OH
Is the carbon from the COOH is carboxylic acids included in the acids name
Yes. This carbon counts towards the number used for naming the acid.
What is the main difference in the properties of the -OH group in alcohols compared to carboxylic acids
The -OH group in carboxylic acids is much more acidic than the -OH group in alcohols