Carbon atoms in the parent chain are numbered sequentially from one end to the other. The direction of numbering is chosen so that the substituents (side chains or functional groups) receive the lowest possible numbers, known as locants.
If there are multiple substituents, the numbering direction is determined by the 'first point of difference' rule, ensuring the set of locants is as low as possible.
In the final name, numbers are separated from words by hyphens (e.g., 2-methylpentane) and from other numbers by commas (e.g., 2,2-dimethylbutane).
Alkyl groups are hydrocarbon side chains formed by removing one hydrogen from an alkane. They are named by replacing the '-ane' suffix with '-yl' (e.g., methane becomes methyl, ethane becomes ethyl).
If multiple identical substituents are present, prefixes such as di- (2), tri- (3), or tetra- (4) are used. These prefixes do not affect the alphabetical ordering of the substituents.
When different types of alkyl groups are present, they must be listed in alphabetical order in the final name, regardless of their numerical position (e.g., ethyl comes before methyl).
Functional groups determine the chemical properties of the molecule and are often indicated by a specific suffix at the end of the name. For example, alcohols end in -ol, aldehydes in -al, and ketones in -one.
Carboxylic acids use the suffix -oic acid, while nitriles use -nitrile. In some cases, such as halogenoalkanes, the functional group is indicated by a prefix (e.g., chloro-, bromo-, fluoro-).
The position of the functional group is indicated by a number placed before the suffix or prefix, unless the group is always at the end of a chain (like aldehydes or carboxylic acids), in which case the carbon it is attached to is automatically C1.
It is vital to distinguish between the parent chain and side chains. The parent chain is the longest continuous path, even if it 'turns a corner' in a structural drawing.
| Feature | Alkyl Group | Functional Group |
|---|---|---|
| Composition | Carbon and Hydrogen only | Often contains O, N, or Halogens |
| Naming | Suffix '-yl' | Specific suffixes (e.g., -ol, -oic acid) |
| Priority | Lower priority in numbering | Higher priority (usually C1 or lowest locant) |
Note that punctuation is strict: commas separate numbers, while hyphens separate numbers from letters. No spaces are used except in specific cases like carboxylic acids.
Always verify the longest chain: Examiners often draw molecules in a 'bent' shape to trick students into picking a shorter horizontal chain instead of the true longest continuous chain.
Check the numbering direction: Before finalizing a name, number the chain from both ends. Choose the direction that results in the lowest number at the first point of difference.
Alphabetical Order: Ensure substituents are listed alphabetically (e.g., '3-ethyl-2-methyl...') rather than numerically ('2-methyl-3-ethyl...'). Remember that prefixes like 'di-' or 'tri-' are ignored for alphabetization.
Punctuation Check: A common way to lose marks is by misplacing commas and hyphens. Double-check that every number-word boundary has a hyphen and every number-number boundary has a comma.
