Carbonyl compounds

A carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a carbonyl group is often referred to as a carbonyl compound.

Physical properties of carbonyl compounds

The boiling point of carbonyl compounds is higher than the alkanes with similar melting point.
The boiling point increases with the increasing number of carbon atoms. This is because there are more electrons, hence more energy is required to break the forces
Besides temporary dipoles, permanent dipole-dipole forces are also present due to the carbonyl compounds being polar
Methanal and ethanal are gases at room temperature, while others are liquids

Preparation of aldehydes and ketones

Aldehydes and ketones can be made by oxidising primary and secondary alcohols respectively. The oxidising agent used is either acidified potassium dichromate(K2Cr2O7) or potassium manganate(VII) ,(KMnO4)

To make an aldehyde:

-The primary alcohol used must be in excess and heated under reflux with acidified K2Cr2O7/KMnO4. The aldehyde must be distilled as soon as possible

-Under these conditions, a primary alcohol is oxidised to an aldehyde. Take ethanol as an example, ethanal is produced

CH3CH2OH + [O] = CH3CHO + H2O

To make a ketone:

-The secondary alcohol is heated under reflux with acidified K2Cr2O7/KMnO4.

-Under these conditions, a secondary alcohol is oxidised to a ketone. Take propan2-ol as an example

CH3CH(OH)CH3 + [O] = CH2COCH3 + H2O

Reactions of Aldehydes and Ketones

The C=O bond of the carbonyl group is highly polarised due to oxygen atom being more electronegative
This causes the slightly positive carbon atom to be susceptible to nucleophilic attacks. Nucleophiles are something that carries a negative charge
Therefore, carbonyl compounds will undergo nucleophilic addition

Reduction

Reagent: Lithium tetrahydriodoaluminate, LiAlH4 or sodium tetrahydriodoaluminate, NaBH4

Condition: For LiAlH4 - dry ether and room temperature

For NaBH4 - in aqueous alkaline solution, cannot be used to reduce carboxylic acid

Product: Aldehyde - primary alcohol

Ketone - secondary alcohol

LiAlH4 and NaBH4 are acting as reducing agents as well as providing the nucleophile, H-. This is a redox reaction as well as a nucleophilic addition reaction
For aldehydes, primary alcohols are formed upon reduction

For Ketones, secondary alcohols are formed upon reduction

Note: Due to the reactivity of LiAlH4, it cannot be used in the presence of water or alcohol and it must be carried out in solution in a carefully dried ether such as ethoxyethane

Oxidation

Reagent: Acidified potassium dichromate, K2Cr2O7 or acidified potassium manganate(VII), KMnO4

Condition: Heat under reflux

Product: Aldehyde - carboxylic acid

Ketone - will not be oxidised

Aldehydes will be oxidised to carboxylic acids

Ketones will not be oxidised

Nucleophilic addition with hydrogen cyanide, HCN

Reagent: Sodium/potassium cyanide, NaCN and a little sulfuric acid, H2SO4

Condition: Room temperature and in situ

Product: Hydroxynitriles

Hydrogen cyanide is not used alone because it is a poisonous gas. Instead, it is produced from the reaction between the reaction between sodium/potassium cyanide and sulfuric acid. The solution will contain hydrogen cyanide and some free cyanide ions
For both aldehydes and ketones, hydroxynitriles are produced.

The mechanism of this reaction is nucleophilic addition:

NaCN + H2SO4 = HCN + NaHSO4

-The electron-deficient carbon atom is attacked by the nucleophile, CN-

-The negative ion formed then picks up a hydrogen ion from hydrogen cyanide, or from water

Testing for Aldehydes and Ketones

2,4-dinitrophenylhydrazine or 2,4-DNPH can be used to detect the presence of carbonyl group, C=O. The structure of 2,4-DNPH is like this =>
When aldehyde or ketone is present, an orange-yellow precipitate is formed
A condensation reaction occurs when a carbonyl compound is added to 2.4-DNPH. During this reaction, a water molecule is lost and the final compound will be seen as an orange-yellow precipitate. No need to be heated

Testing for aldehydes using Tollens reagent( silver mirror test)

Tollens reagent contains diamminesilver(I) ions, [Ag(NH3)2]+. Aldehydes will reduce the diamminesilver(I) ions to metallic silver, aldehyde itself is oxidised to a salt of carboxylic acid
Since ketones cannot be oxidised, it will not reduce it to metallic silver
Therefore few drops of aldehyde is added freshly prepared Tollens reagent and warmed in a water bath for a few minutes. A grey precipitate or a silver mirror is observed only if aldehyde is present

Test for aldehydes using Fehling's solution

Fehling's solution contains copper(II) ions complexed with tartrate ions in sodium hydroxide solution. Complexing the copper(II) ions with tartrate ions prevents precipitation of copper(I) hydroxide that is a blue solution
Only aldehydes will reduce the complexed copper(II) ion to copper(I) oxide. Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid
When a few drops of the aldehyde is added to the reagent. and the mixture is warmed gently in a hot water bath for a few minutes, a red precipitate observed only if aldehydes is present

Reactions to form tri-iodomethane

Tri-iodomethane forms as a yellow precipitate with methyl ketones (R-CO-CH3) or ethanal(R-C-CH3), methyl ketone is a ketone in which a methyl group is attached to the carbonyl group. Chemists use this appearance of this yellow precipitate as evidence. The reagent used in an alkaline solution of iodine is warmed together with the substance
The reactions involve 2 steps

Halogenation, the carbonyl compound is halogenated - the three hydrogen atoms in the CH3 group are replaced by iodine atoms

RCOCH3 + I2 = RCOCI3, in NaOH(aq)

Hydrolysis, the intermediate is hydrolysed to form the yellow precipitate of Tri-iodomethane

RCOCI3 + NaOH = RCOO-Na+ +CHI3

Infra-red spectroscopy

The sample is irradiated with electromagnetic waves in the ir region of the spectrum
Spectrophotometer detects the intensity of the wavelengths that passes the sample
All organic molecules absorb ir radiation
Energy absorbed corresponds to the changes of vibration of the bonds between the atoms (stretching, bending and twisting)
They have natural frequency, and if we irradiate the molecules with energy corresponding to this frequency, larger vibration energy is absorbed( resonance)

Absorbances: