4.1.1(a)-(b) - Organic nomenclature and formula representations

Systematic Names

A systematic organic name is designed to identify one structure clearly. Everyday names can be useful, but they are often ambiguous; IUPAC nomenclature gives chemists a shared framework for communicating structures.

The first part of many organic names is the carbon-chain stem. OCR expects you to know the first ten straight-chain alkanes and the corresponding alkyl groups.

An alkyl group is formed by removing one hydrogen atom from an alkane, so an alkyl group has formula $C_nH_{2n+1}$. For example, a methyl group is CH₃ and an ethyl group is CH₃CH₂. In a name, alkyl groups are usually branches attached to a longer parent chain.

The middle of a name gives information about bonding in the carbon chain:

Later OCR Chemistry lessons add more functional groups, but the same idea stays the same: the name should locate the important group and describe the carbon framework unambiguously.

Building a Name

A good naming method is a sequence, not a guess.

1. Find the parent carbon chain. This is usually the longest chain containing the important functional group.
2. Choose the suffix or main ending, such as -ane, -ene or -ol.
3. Number the chain from the end that gives the important group the lowest possible number.
4. Name and number any branches or prefixes.
5. Assemble the name with locants, hyphens and commas where needed.

Worked example: naming CH₃CH₂CH(CH₃)CH₂OH

First find the chain that contains the OH group. Starting from the CH₂OH end gives a four-carbon parent chain, so the parent is based on butane. The OH group is on carbon 1, so the suffix is butan-1-ol. There is a CH₃ branch on carbon 2. The name is:

2-methylbutan-1-ol

This is more informative than a casual description such as "a methyl butanol", because the numbers say exactly where the branch and the OH group are.

Worked example: interpreting 3-methylpentane

The parent chain has five carbons, so start with pentane. The name says there is a methyl branch on carbon 3:

CH₃CH₂CH(CH₃)CH₂CH₃

The branch is written in brackets in the structural formula to show that it is attached to the middle carbon rather than extending the main chain.

Choosing between two numbering directions

The principal suffix gets priority over a branch: an alcohol is numbered to give OH its lowest locant. If that locant is tied, compare the branch locants at the first point of difference. For example, in CH₃CH(CH₃)CH(OH)CH₂CH₂CH₃ the OH is on carbon 3 rather than 4, so the name is 2-methylhexan-3-ol. If OH is central in an odd-length chain and has the same number from either end, choose the direction giving the branch the lower number.

For several substituents, use di-, tri- and so on for repeats, commas between numbers, and hyphens between a number and a word. List different prefixes alphabetically, ignoring di-/tri-: 3-ethyl-2,2-dimethylhexane. For an alkene without a higher-priority suffix, number to give the C=C the lowest locant before considering branches: CH₂=CHCH(CH₃)CH₃ is 3-methylbut-1-ene.

Common naming traps are easy to spot:

• Do not choose a parent chain that leaves the important functional group outside the chain.
• Do not forget the number for a branch or for -ol when more than one position is possible.
• Do not number from the end that gives a higher number to the suffix group.
• Do not use names for functional groups outside the OCR Chemistry specification.

Formula Representations

Formulae are different levels of detail for the same molecule. OCR does not require you to recite formal definitions of these terms, but you must be able to interpret and use them.