3.4.4 Nomenclature and Isomerism in Organic Chemistry - Naming organic compounds

Specification

Homologous series and functional groups

An homologous series is a family of compounds that all contain the same functional group(s) and differ in successive members by one -CH2- unit.

The following is a list of the common groups:

homologous series	functional group	description
alkanes	Default (no functional group)	only carbons, hydrogens and single bonds
alkenes	C=C	1 double bond, unsaturated
alkynes	C C	1 triple bond
alcohols	-OH	hydroxy or alcohol group
halogenoalkanes	-Cl, -Br, -I	one halogen atom attached to chain
aldehydes	-CHO	terminates a chain with a carbonyl group and hydrogen
ketones	-CO-	carbonyl group in the middle of a chain
carboxylic acids	-COOH	terminates a chain with a carbonyl and a hydroxyl group
acid chlorides	-COCl	terminates a chain with a carbonyl and chlorine
nitriles	-C N	terminates a chain with a triple bond N
amines	-NH2	may be 1º, 2º or 3º
amides	-CONH2	carbonyl and amine groups
ethers	R-O-R	alkyl groups bridged by oxygen
esters	RCOOR	alkyl groups linked by -COO-

Nomenclature

Compounds should be named using a rigorous system.

• 1 Identify the root
• 2 Prioritise the functional groups and obtain the suffix to the root
• 3 Add in other functional group and branches including locants if needed as prefixes

The root of the name comes from the number of carbons in the longest chain, unless there is a terminating group or a multiple bond on one of the chains in which case this becomes the root.

• 1 carbon chain = meth-
• 2 carbon chain = eth-
• 3 carbon chain = prop-
• 4 carbon chain = but-
• 5 carbon chain = pent-
• 6 carbon chain = hex-

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Functional group priority and names

The group with the most oxidised carbon takes priority in case of competition. The functional group usually gives the suffix to the root. However, in the case of halogens these become a prefix.

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Locants (positioning numbers)

These are numbers that are used with branches or functional groups to avoid ambiguity. They are placed in front of the part of the name to which they refer, separated by a dash.

For example, propanol. The name does not tell us whereabouts in the structure the -OH group is located. A locant is needed, which is placed in the suffix of the name, indicating alcohol, propan-1-ol, or propan-2-ol

propan-1-ol	propan-2-ol

The locants are always numbered from the end of the chain that produces the lowest numbers.

2-chloropentane

A locant is used for every attached heteroatom or group.

2,2-dichlorohexane

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Aromatic chemistry

This deals with the chemistry of benzene and its compounds. The benzene ring is a hexagon of six carbon atoms each of which is attached to a hydrogen atom. There are the equivalent of three double bonds within the ring, their electrons delocalised around the ring. This is drawn in shorthand form as a hexagon with a ring inside.

Benzene showing atoms and bonds	representation of benzene

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Naming aromatic systems

The benzene ring may take one of two names in its compounds, a suffix or a prefix:

• -benzene
• Phenyl- or phen-

There is no logical way to ascertain which is used and often both are acceptable. It's just a matter of learning the different structures.

phenol	phenylamine or aminobenzene	benzaldehyde

There are many more trivial names still associated with aromatic systems and these are in common use, particularly in research establishmetns and industry. For instance, phenylamine (above) is usually called aniline. Methylbenzene is called toluene, etc..

Where there are two (or more) substituents attached to the benzene ring, the substituent with the highest priority (most oxidised group) is assigned position 1 on the ring and the ring carbons are then numbered clockwise.

2-nitromethylbenzene	3-nitromethylbenzene	2,4,6-trinitromethylbenzene (TNT)

NOTE The initials TNT come from the old (trivial) name for methylbenzene, toluene.

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