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  Aldehydes and Ketones, Chemistry tutorial

Introduction:

Aldehydes and ketones are the simple organic compounds known as carbonyl compounds. They encompass a carbonyl group.

Structures of Aldehyde and Ketones:

In aldehydes, the carbonyl group consists of a hydrogen atom attached to it altogether by either a second hydrogen atom or a hydrocarbon group which might be an alkyl group or one having a benzene ring.

For illustration:

2462_Structures of Aldehyde and Ketones.jpg

Fig: Structures of Aldehyde and Ketones

All the structures above encompass precisely the similar end to the molecule; all that differs is the complexity of the other group attached.

In ketones the carbonyl group consists of two hydrocarbon groups attached. These could either be alkyl group or one having alkyl group and one aromatic ring or two aromatic rings.

For illustration:

1756_Ketones having carbonyl group.jpg

Fig: Ketones having carbonyl group

IUPAC Nomenclature of Aldehydes and Ketones:

Aldehydes:

The term Aldehyde are derived from the root hydrocarbon by adding the suffix '-al' to substitute '-e' from the corresponding alkane. For illustration:

871_Aldehydes.jpg

Fig: Aldehydes

Where there are other substituents on the carbon chain, they are named by using the prefix (es) with the numbering of carbon from the carbonyl carbon.

Ketones:

Ketones are named as derivatives of the corresponding alkane by the suffix '-one' substituting the '-e' from the corresponding alkane. For illustration,

644_Ketones.jpg

Fig: Ketones

The position of the carbonyl group on the carbon chain for the isomeric ketones is pointed out by numbers, for illustration: Pentan-3-one

Bonding and reactivity:

The carbon-oxygen double bond is extremely highly polar. This is due to the reason that oxygen is far more electronegative than carbon and therefore consists of a strong tendency to pull electrons in a carbon-oxygen bond towards itself making it partly negative and the carbon partly positive.       

1542_Bonding and reactivity.jpg

Fig: Bonding and reactivity

Physical properties of aldehydes and ketones:

The simple aliphatic aldehydes and ketones are all colourless liquid at 20oC by the exception of methanal which is a gas (that is, boiling point: 21oC). Ethanal consists of a boiling point of 20oC that means, it boils at close to room temperature. The boiling points rise as the molecules get bigger.

The size of the boiling point is governed through the strength of the Van der Waal's force of attraction that becomes stronger as the molecules get longer and encompass more electrons. This describes why the boiling point increases as the number of carbon atoms in the chain increases.

The functional group (that is, carbonyl group) is polar because of an important difference in the electronegativity of the carbon oxygen atoms. Because of the high polarity in their molecules, carbonyl compounds have higher boiling points than alkanes of alike molecular masses, however much lower than those of alcohols of similar molecular masses due to carbonyl compounds unlike alcohols don't encompass intermolecular hydrogen bonding.

Table: Boiling Point of Propane, Ethanal and Ethanol

Molecule           Type     Boiling Point (oC)

CH3CH2CH3     alkane           -42

CH3CHO          aldehyde        -21

CH3CH2OH       alcohol           78

It will be noted that in the above table, the aldehyde (by dipole-dipole attractions and also Van der Waals forces) consists of a boiling point higher than the likewise size alkane which only consists of forces of attraction.

The density of simple aldehydes and ketones are lower than that of water and rises as the relative molecular mass increases. Aromatic aldehydes and ketones are somewhat denser than water.

Aldehydes and ketones having short hydrocarbon chains are miscible with water while those by long hydrocarbon chains are insoluble. Lower members of the carbonyl compounds can form hydrogen bonds with water molecule and therefore they have reasonably high solubility in water compared by hydrocarbons of similar molecular masses. Carbonyl compound having carbon atoms of five and above and aromatic compounds which takes place in nature are insoluble in the water as hydrophobic character of the alkyl chains and aromatic rings outweighs the polar character of the carbonyl group.

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