Nomenclature of compounds with functional groups

NOMENCLATURE OF COMPOUNDS WITH FUNCTIONAL GROUPS

Key Notes

General rules

The main chain (or parent chain) must include the functional group. The presence of functional groups is indicated by adding the relevant suffix for that functional group. The position of the functional group must be defined and other substituents are identified as described for alkanes.

Alkenes and alkynes

Alkenes  and  alkynes  are  defined  by  adding  the  suffixes  -ene  and  -yne respectively. The stereochemistry of alkenes may need to be defined.

Aromatics

The simplest aromatic ring is benzene. Other important aromatic molecules include toluene, phenol, aniline, benzoic acid, and benzaldehyde. Any of these names can be used as parent names if other substituents are present. The posi- tion of substituents is determined by numbering round the ring, or in the case of disubstituted aromatic rings, the ortho, meta, para nomenclature.

Alcohols

Alcohols (or alkanols) are given the suffix -anol.

Ethers and alkyl halides

Ethers and alkyl halides are not identified with suffixes. Instead, these func-tional groups are considered to be substituents of the main alkane chain. The halogen of an alkyl halide is a halo substituent, while the ether is an alkoxy substituent.

Aldehydes and ketones

Aldehydes (or alkanals) are identified by the suffix -anal. Ketones (or alka-nones) are identified by the suffix -anone. Aldehydes must always be at position 1 of the main chain and do not need to be numbered.

Carboxylic acids and acid chlorides

Carboxylic acids and acid chlorides are identified by adding the suffix -anoic acid and -anoyl chloride respectively. Both these functional groups are always at the end of the main chain and do not need to be numbered.

Esters

Esters are named from the parent carboxylic acid and alcohol. The alkanoic acid is renamed alkanoate and the alkanol is treated as an alkyl substituent. The combined name is alkyl alkanoate. There must be a space between both parts of the name.

Amides

Amides are termed as alkanamides based on the parent carboxylic acid. If the amide nitrogen has alkyl groups, then these are considered as alkyl sub-stituents. The symbol N is used to show that the substituents are on the nitrogen and not some other part of the alkanamide skeleton.

Amines

Simple amines can be named by placing the suffix -ylamine after the root name. Other amines are named by considering the amino group as a sub-stituent of the main chain in the same way as alkyl halides and ethers.

Thiols and thioethers

Thiols are named by adding the suffix thiol to the name of the main alkane chain. Thioethers are named in the same way as ethers where the major alkyl substituent is considered to be the main chain with an alkylthio sub-stituent. Simple thioethers can be identified as dialkylsulfides.

General rules

Many of the nomenclature rules for alkanes hold true for molecules containing a functional group, but extra rules are needed in order to define the type of functional group present and its position within the molecule. The main rules are as follows:

i.The main (or parent) chain must include the functional group, and so may not necessarily be the longest chain (Fig. 1);

ii.The presence of some functional groups is indicated by replacing -ane for the parent alkane chain with the following suffixes:

iii.Numbering must start from the end of the main chain nearest the functional group. Therefore, the numbering should place the alcohol (Fig. 2) at position 1 and not position 4.

iv.The position of the functional group must be defined in the name. Therefore, the alcohol (Fig. 2) is a 1-butanol.

v.Other substituents are named and ordered in the same way as for alkanes. The alcohol (Fig. 2) has an ethyl group at position 2 and so the full name for the structure is 2-ethyl-1-butanol.

There are other rules designed for specific situations. For example, if the func-tional group is an equal distance from either end of the main chain, the number-ing starts from the end of the chain nearest to any substituents. For example, the alcohol (Fig. 3) is 2-methyl-3-pentanol and not 4-methyl-3-pentanol.

Alkenes and alkynes

Alkenes and alkynes have the suffixes -ene and -yne respectively (Fig. 4). With some alkenes it is necessary to define the stereochemistry of the double bond.

Aromatics

The best known aromatic structure is benzene. If an alkane chain is linked to a benzene molecule, then the alkane chain is usually considered to be an alkyl substituent of the benzene ring. However, if the alkane chain contains more than six carbons, then the benzene molecule is considered to be a phenyl substituent of the alkane chain (Fig. 5).

Note that a benzyl group consists of an aromatic ring and a methylene group

Benzene is not the only parent name which can be used for aromatic com- pounds (Fig. 7).

With disubstituted aromatic rings, the position of substituents must be defined by numbering around the ring such that the substituents are positioned at the lowest numbers possible, for example, the structure (Fig. 8) is 1,3-dichlorobenzene and not 1,5-dichlorobenzene.

Alternatively, the terms ortho, meta, and para can be used. These terms define the relative position of one substituent to another (Fig. 9). Thus, 1,3-dichlorobenzene can also be called meta-dichlorobenzene. This can be shortened to m-dichlorobenzene. The examples in Fig. 10 illustrate how different parent names may be used. Notice that the substituent which defines the parent name is defined as position 1. For example, if the parent name is toluene, the methyl group must be at position 1.

When more than two substituents are present on the aromatic ring, the ortho, meta, para nomenclature is no longer valid and numbering has to be used (Fig. 11).

Once  again,  the  relevant  substituent  has  to  be  placed  at  position  1  if  the parent name is toluene, aniline, et cetera. If the parent name is benzene, the num-bering is chosen such that the lowest possible numbers are used. In the example shown, any other numbering would result in the substituents having higher numbers (Fig. 12).

Alcohols

Alcohols or alkanols are identified by using the suffix -anol. The general rules described earlier can be used to name alcohols (Fig. 13).

Ethers and alkyl halides

The nomenclature for these compounds is slightly different from previous exam- ples in that the functional group is considered to be a substituent of the main alkane chain. The functional group is numbered and named as a substituent (Fig. 14).

Note that ethers have two alkyl groups on either side of the oxygen. The larger alkyl group is the parent alkane. The smaller alkyl group along with the oxygen is the substituent and is known as an alkoxy group.

Aldehydes and ketones

The suffix for an aldehyde (or alkanal) is -anal, while the suffix for a ketone (or alkanone) is -anone. The main chain must include the functional group and the numbering is such that the functional group is at the lowest number possible. If the functional group is in the center of the main chain, the numbering is chosen to ensure that other substituents have the lowest numbers possible (e.g. 2,2-dimethyl-3-pentanone and not 4,4-dimethyl-3-pentanone; Fig. 15). 3-Methyl-2-butanone can in fact be simplified to 3-methylbutanone. There is only one possible place for the ketone functional group in this molecule. If the carbonyl C=O group was at the end of the chain, it would be an aldehyde and not a ketone. Numbering is also not necessary in locating an aldehyde group since it can only be at the end of a chain (Fig. 16).

Carboxylic acids and acid chlorides

Carboxylic acids and acid chlorides are identified by adding the suffix -anoic acid and -anoyl chloride respectively. Both these functional groups are always at the end of the main chain and do not need to be numbered (Fig. 17).

Esters

To name an ester, the following procedure is carried out:

1.        identify the carboxylic acid (alkanoic acid) from which it was derived;

2.        change the name to an alkanoate rather than an alkanoic acid;

3.        identify the alcohol from which the ester was derived and consider this as an alkyl substituent;

4.        the name becomes an alkyl alkanoate.

For example, the ester (Fig. 18) is derived from ethanoic acid and methanol. The ester would be an alkyl ethanoate since it is derived from ethanoic acid. The alkyl group comes from methanol and is a methyl group. Therefore, the full name is methyl ethanoate. (Note that there is a space between both parts of the name.)

Amides

Amides are also derivatives of carboxylic acids. This time the carboxylic acid is linked with ammonia or an amine. As with esters, the parent carboxylic acid is identified. This is then termed an alkanamide and includes the nitrogen atom. For example, linking ethanoic acid with ammonia gives ethanamide (Fig. 19).

If the carboxylic acid is linked with an amine, then the amide will have alkyl groups on the nitrogen. These are considered as alkyl substituents and come at the beginning of the name. The symbol N is used to show that the substituents are on the nitrogen and not some other part of the alkanamide skeleton. For example, the structure in Fig. 20 is called N-ethylethanamide.

Amines

The nomenclature for amines is similar to alkyl halides and ethers in that the main part (or root) of the name is an alkane and the amino group is considered to be a substituent (Fig. 21). Simple amines are sometimes named by placing the suffix -ylamine after the main part of the name (Fig. 22).

Amines having more than one alkyl group attached are named by identifying the longest carbon chain attached to the nitrogen. In the example (Fig. 23), that is an ethane chain and so this molecule is an aminoethane (N,N-dimethylaminoethane).

Some simple secondary and tertiary amines have common names (Fig. 24).

Thiols and thioethers

Thiols are named by adding the suffix thiol to the name of the parent alkane (Fig. 25a). Thioethers are named in the same way as ethers using the prefixalkylthio, for example, 1-(methylthio)propane (Fig. 25c). Simple thioethers can be named by identifying the thioether as a sulfide and prefixing this term with the alkyl sub- stituents, for example, dimethyl sulfide (Fig. 25b).