SME 6.2 Carbonyl Compounds

What is a carbonyl compound?

an organic molecule that contains a C=O group

Carbonyl compounds include

aldehydes, ketones, carboxylic acids, esters, amides, acyl chloride and acid anhydrides

Primary alcohols can be oxidised to form 

aldehydes then further oxidation to carboxylic acids

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

To form a ketone, which can not undergo further oxidation,

reflux apparatus can be used

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

The pos result 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

The carbonyl group in a ketone is always situated

in the middle of the chain

The simplest ketone is

is propan-2-one, CH3COCH3, as you need an alkyl group either side of the carbonyl carbon in a ketone

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

1. Fehling’s solution

2. Tollens’ reagent

Fehling’s solution is

a blue alkaline solution which contains copper(II) ions, which act as the oxidising agent