The most potent and varied odors are aldehydes. Ketones are widely used as industrial solvents. Aldehydes and ketones contain the carbonyl group. Aldehydes are considered the most important functional group. They are often called the formyl or methanoyl group. Aldehydes derive their name from the dehydration of alcohols. Aldehydes contain the carbonyl group bonded to at least one hydrogen atom. Ketones contain the carbonyl group bonded to two carbon atoms.
Introduction to Naming Carbonyls
Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O. The carbon atom of this group has two remaining bonds that may be occupied by hydrogen, alkyl or aryl substituents. If at least one of these substituents is hydrogen, the compound is an aldehyde. If neither is hydrogen, the compound is a ketone.
Naming Aldehydes
The IUPAC system of nomenclature assigns a characteristic suffix -al to aldehydes. For example, H2C=O is methanal, more commonly called formaldehyde. Since an aldehyde carbonyl group must always lie at the end of a carbon chain, it is always is given the #1 location position in numbering and it is not necessary to include it in the name. There are several simple carbonyl containing compounds which have common names which are retained by IUPAC.
Also, there is a common method for naming aldehydes and ketones. For aldehydes common parent chain names, similar to those used for carboxylic acids, are used and the suffix –aldehyde is added to the end. In common names of aldehydes, carbon atoms near the carbonyl group are often designated by Greek letters. The atom adjacent to the carbonyl function is alpha, the next removed is beta and so on.
If the aldehyde group (-CHO) is attached to a ring the suffix –carbaldehyde is added to the name of the ring. The carbon attached to this group will get the #1 location number in naming the ring.
Summary of Aldehyde Nomenclature rules
- Aldehydes take their name from their parent alkane chains. The -e is removed from the end and is replaced with -al.
- The aldehyde funtional group is given the #1 numbering location and this number is not included in the name.
- For the common name of aldehydes start with the common parent chain name and add the suffix -aldehyde. Substituent positions are shown with Greek letters.
- When the -CHO functional group is attached to a ring the suffix -carbaldehyde is added, and the carbon attached to that group is C1.
Naming Ketones
The IUPAC system of nomenclature assigns a characteristic suffix of -one to ketones. A ketone carbonyl function may be located anywhere within a chain or ring, and its position is usually given by a location number. Chain numbering normally starts from the end nearest the carbonyl group. Very simple ketones, such as propanone and phenylethanone do not require a locator number, since there is only one possible site for a ketone carbonyl function
The common names for ketones are formed by naming both alkyl groups attached to the carbonyl then adding the suffix -ketone. The attached alkyl groups are arranged in the name alphabetically.
Summary of Ketone Nomenclature rules
- Ketones take their name from their parent alkane chains. The ending -e is removed and replaced with -one.
- The common name for ketones are simply the substituent groups listed alphabetically + ketone.
- Some simple ketones are known by their common names. Such as propanone which is commonly referred to as acetone.
Naming Carbonyls and Alkenes in the Same Molecule
When an aldehyde or ketone is present in a molecule which also contains an alkene functional group the carbonyl is given nomenclature priority by the IUPAC system. This means that the carbonyl is given the lowest possible location number and the appropriate nomenclature suffix is included.
When carbonyls are included with an alkene the following order is followed:
(Location number of the alkene)-(Prefix name for the longest carbon chain minus the -ane ending)-(an -en ending to indicate the presence of an alkene)-(the location number of the carbonyl if a ketone is present)-(either an –one or and -anal ending).
Remember that the carbonyl has priority so it should get the lowest possible location number. Also, remember that cis/tran or E/Z nomenclature for the alkene needs to be included if necessary.
Example \(\PageIndex{8}\)
4-penten-2-one |
trans-3-pentenal |
(E)-3-methyl-3-pentenal |
(Z)-5-bromo-4-hexen-3-one |
Aldehydes and Ketones as Fragments
- Alkanoyl is the common name of the R-C=O fragment, though the older naming, acyl, is still widely used.
- Formyl is the common name of the H-C=O fragment.
- Acetyl is the common name of the CH3-C=O- fragment.
Example \(\PageIndex{9}\)
acetyl chloride |
acetylaldehyde |
formyl fluoride |