Section / Topic Revision Notes

Carbonyl compounds

What is a carbonyl compound? 

A carbonyl compound is an organic molecule that contains a C=O group.

This is a carbon atom with a double bond attached to an oxygen atom, referred to as a carbonyl group. The carbon atom is sp² hybridised forming a trigonal planar molecule.

The carbon atom can be bonded to an R group (alkyl, alkenyl or alkynyl) or other functional groups.

The carbon atom in the C=O group has a δ+ charge and the O atom has a δ– group.

Carbonyl compounds include aldehydes, ketones, carboxylic acids, esters, amides, acyl chloride and acid anhydrides.

In an infrared spectrum the –C=O group will give a peak between 1630 and 1820 cm^-1

How are carbonyl compounds formed?

One method of forming a carbonyl compound is via the oxidation of alcohols.

Primary alcohols can be oxidised to form aldehydes (which can then undergo further oxidation to form carboxylic acids).

Secondary alcohols can be oxidised to form ketones.

The oxidising agents used are acidified potassium or sodium dichromate(VI) (K₂Cr₂O₇ or Na₂Cr₂O₇) which will change colour from orange to green. Alternatively, acidified potassium manganate(VII) (KMnO₄) can be used, the colour change is from purple to colourless. 

In order to form the aldehyde, distillation apparatus should be used otherwise the aldehyde will undergo further oxidation. The aldehyde product has a lower boiling point than the alcohol reactant so it can be distilled off as soon as it forms.

Using [O] to represent the oxidising agent the oxidation of ethanol (the primary alcohol) to ethanal (the aldehyde) can be written as:

CH₃CH₂OH + [O] → CH₃CHO + H₂O

Ethanoic acid can also form from ethanal:

CH₃CHO + [O] → CH₃COOH

To form a ketone, which can not undergo further oxidation, reflux apparatus can be used.

Using [O] to represent the oxidising agent the oxidation of propanone (the ketone) can be written as:

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

How do you test for the presence of a carbonyl group?

Aldehydes and ketones can be tested as they contain the carbonyl group. In order to confirm the presence of a carbonyl group a simple chemical test can be performed.

The compound is reacted with 2,4-dinitrophenylhydrazine, also known as 2,4-DNPH. Compounds containing -C=O groups will undergo a condensation reaction with 2,4-dinitrophenylhydrazine. A condensation reaction is a reaction in which two molecules join together and a small molecule (such as water or hydrogen chloride is eliminated.

The positive result for this reaction test is a deep orange precipitate being formed. This precipitate can be purified by recrystallisation. The melting point of the formed precipitate can then be measured and compared to literature values to find out which specific aldehyde or ketone had reacted with 2,4-DNPH. The 2,4-DNPH test is especially useful as other carbonyl compounds such as carboxylic acids and esters do not give a positive result.

Aldehydes and ketones have different chemical properties which means they can be identified.

The carbonyl group in an aldehyde is always situated at the end of 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 situated in the middle of 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.

Aldehydes can undergo oxidation, ketones can not.

In order to determine whether a compound is an aldehyde or a ketone two chemical tests can be performed: 

1. Fehling’s solution is a blue alkaline solution which contains copper(II) ions, which act as the oxidising agent. When Fehling’s solution is warmed with the aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu(II) ions are reduced to Cu(I) ions. The colour change is from blue to a red precipitate. The red precipitate contains copper(I) oxide, Cu₂O. Ketones do not react with Fehling’s solution as they can not be oxidised.
    
    
2. Tollens’ reagent is a solution that contains aqueous silver nitrate in excess ammonia. It is also known as ammoniacal silver nitrate. When warmed with an aldehyde, the aldehyde again is oxidised to a carboxylic acid and the silver(I) ions are reduced to silver metal. The silver metal formed will create a mirror on the inside of the test tube. Ketones do not give a silver mirror as they can not be oxidised. This test is often referred to as the silver mirror test.
    
   
   

What are the reactions of carbonyl compounds?

As the C atom in the C=O group has a δ+ charge and the O atom has a δ- group carbonyl compounds can undergo nucleophilic addition. For example, cyanide ions can act as nucleophiles to form hydroxynitrile compounds. 

For an aldehyde, the general mechanism is:

For a ketone, the general mechanism is:

Reduction of carbonyl compounds can also occur. Sodium borohydride, NaBH₄, can reduce aldehydes and ketones, while lithium aluminium hydride, LiAlH₄, which is a much more reactive reducing agent can reduce carboxylic acids, amides and esters. LiAlH₄ reacts violently with water and so the reaction is carried out in a solution in dry ethoxyethane (dry ether). The reaction happens at room temperature so no heating is required. 

Reduction products are:

• Aldehydes form primary alcohols
• Ketones form secondary alcohols
• Amides form amines
• Carboxylic acids form primary alcohols 
• Esters form primary alcohols

What keyword definitions do I need to know for carbonyl compounds? 

Some keyword definitions you need to know are:

• Aldehyde - a compound with the carbonyl group at the end of the molecule
• Ketone - a compound with the carbonyl group in the middle of the molecule
• Oxidation - the process by which oxygen is added to a molecule or hydrogen removed
• Reduction - the process by which oxygen is removed from a molecule or hydrogen is added
• 2,4-dinitrophenylhydrazine - also known as 2,4-DNPH which can test for the presence of the carbonyl group
• Fehling’s test - test to distinguish between an aldehyde and ketone
• Tollens’ test -  test to distinguish between an aldehyde and ketone

This is a quick summary of some key concepts on carbonyl groups - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!