Tuesday, February 5, 2008

Aldehydes - Ketones - Introduction and Nomenclature

Aldehydes contain carbonyl group C=O as functional group and the carbonyl atom carries at least one H atom.


In ketones, also carbonyl group C=O is the functional group. But the carbonyl carbon does not contain any H atoms, but it is attached to two alkyl or aryl groups.

Getting an aldehyde from methylbenzene - by oxidation

Getting ketone from alcohols - By oxidation of secondary alcohols

Aldehydes and ketones are polar molecules because the C=O bond has a
dipole moment:

• Their polarity makes aldehydes and ketones have higher boiling points than
alkenes of similar molecular weight.

Aldehydes and Ketones – Nomenclature

Nomenclature of Aliphatic Aldehydes

Common system
Aldehydes are named according to the name of the corresponding acid which they form on oxidation.
The suffix –ic acid of the name of the acid is replaced by aldehyde.
Ex: The aldehyde that gives acetic acid is termed as acetaldehyde.

Braching in the aldehyde chain, is indicated by carbon atom positions α, β, γ, δ.

The carbon atom next to the carbonyl carbon is assigned the letter α. The carbon next to α-carbon is the β-carbon. The carbon next to β-carbon is the γ-carbon. The carbon next to γ-carbon is the δ Carbon.

δ - γ- β- α carbons
Ex: α-Methyl butyraldehyde

IUPAC system

Aldehydes are termed alkanals. The terminal ‘e’ of the name of corresponding alkane is replaced by ‘al’.
Ex: methanal, ethanal, propanal.
Nomenclature for Aldehydes with Branches
1. The longest chain containing –CHO group is considered as the parent chain and the name is derived as an alkanal.
2. To determine the number of the carbon where a substituent is attached to the aldehyde chain, the carbons in the chain are numbered in such a way that the aldehydic group carbon gets lowest number (i.e 1). In other words number of the aldehydic chain carbons is started from the carbon in the carbonyl group.

Nomenclature of Aromatic Aldehydes

The simplest aromatic aldehyde is benzaldehyde. In aromatic aldehydes, -CHO group is directly attached to the benzene ring.
In case of substituted aromatic aldehydes, the positions of the substituents in benzene ring with respect to –CHO group are indicated either by suffixes ortho, meta or para or by numbers 1,2,3… etc. with the carbon bearing the –CHO group as number 1.
Ex; 2-Hydroxybenaldehyde – OH is the substituent at the 2 carbon from CHO group.
The aldehydic group (CHO) can be a part of the side chain. In other words, an aldehydic group may be attached to benzene ring.
The name will be as an example 2-Phenylethanal. In this compound CH2CHO is attached to a benzene ring. The substituent is ethanal group, and the numbering of carbon atoms still starts from CHO group carbon and the terminology indicates that a benzyl group is attached to ethanal at 2nd carbon.

Nomenclature of Ketones

Common system
Ketones are named by using the names of alkyl groups present in the molecule.
Ex: Dimethyl ketone, Methyl isopropyl ketone

IUPAC system

Ketones are termed as alkanones.

Rules for arriving at names
1. The longest chain carrying the carbonyl groupis considered as the parent chain and the name is derived by replacing the terminal ‘e’ of the name of the corresponding alkane by letters ‘one’.
Ex: Propanone
2. In case of substituted ketones, theparent chain is numbered in such a way that the ketone group carbon gets the lowest number (but the numbering does not start from ketone group carbon – caron attached with a double bond to oxygen).
3. The position of carbonyl group and the substituents is indicated by numbers.
Ex: 3-methylbutan-2-one (indicated that there is methyl group at 3rd carbon and carbonyl group at 2nd carbon on butanone.

Compounds having both aldehyde and ketone groups.

For compounds having both aldehyde and ketone groups, the aldehyde group is considered as the principal functional group and ketonic group is regarded as substituent. It is named as prefix oxo- along with a number to indicate its position.
Ex: 2-Methyl-4-oxohexanal

Aromatic Ketones

Purely aromatic or mixed aromatics ketones are know by their common names.
Acetophenone – Methyl phenyl ketone
Propiophenone – Ethyl phenyl ketone
Benzophenone - Diphenyle ketone

IUPAC 1993 recommendation for 3 same functional groups

If an unbranched chain is directly bonded to more than two same functional groups, the organic compound is named as a derivative of parent alkane which does not include the carbon atoms of the functional groups. These compounds are named by use of suffix tricarboaldehyde (for three –CHO groups).
Ex: Butane-1,2,4-tricarbaldehyde

If three groups are not directly bonded to the unbranched carbon chain, the two like groups are considered in the parent chain and are named by using di before the name of the functional group. The third group forming the side chain is considered as a substituent group.

Ex: 3-Formylmethylhexane-1,6-dial (three –CHO groups are there. But at 1 and 6 they are directly bonded. At three there is a branching of the alkane chain and –CHO is attached to the methyl group of the branch).

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