5.2 - Naming Organic Compounds & Isomers
Naming a compound
A systematic name tells you how long the main chain is, which family it belongs to, where the functional group sits, and what side chains are attached.
Step 1 - Find the longest chain for the stem
- The parent chain is the longest continuous chain of carbon atoms - its length gives the stem.
- A molecule drawn bent can have a longer chain than it looks, so count in every direction to find the longest continuous run.
- Any carbon left over sits on a side chain and isn't counted in the stem.
| Carbon atoms | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Stem | meth- | eth- | prop- | but- | pent- | hex- | hept- | oct- |
Step 2 - Find the functional group for the suffix
The functional group is the atom or group of atoms that decides how the compound reacts, and it tells you which family the molecule belongs to. The family gives the suffix.
| Family | What identifies it | Suffix |
|---|---|---|
| alkanes | every carbon-to-carbon bond is single | -ane |
| alkenes | a carbon-to-carbon double bond, C=C | -ene |
Step 3 - Number the parent chain
- Number the parent chain from whichever end gives the functional group the lower number.
- Where there's no functional group to place, number from the end that reaches a side chain first.
Step 4 - Name each side chain
- Name each side chain from its carbon count, ending in -yl, and give it the number of the parent-chain carbon it's joined to.
- One carbon = methyl, two = ethyl, three = propyl.
Step 5 - Count and order the side chains
- Where the same side chain appears more than once, use di-, tri- or tetra- (e.g. dimethyl) - each side chain still carries its own number.
- Where there's more than one kind of side chain, list them in alphabetical order (e.g. ethyl before methyl).
Step 6 - Assemble the parts
- Order: side chains with their numbers, stem, functional group locant, suffix.
- Numbers are separated from words by hyphens and from each other by commas (e.g. 2,4-dimethylpent-2-ene).
Extra points on numbering and shorthand
- The functional group takes priority over a side chain when numbering: choose the direction that gives the functional group the lower number, regardless of side chains.
- Not every name needs a locant - an alkane has none to place, and a double bond in a two- or three-carbon chain has only one possible position.
- In condensed structural formulae, a side chain goes in brackets after the carbon it's attached to: in CH3CH(CH3)CH2CH3, the bracketed CH3 is a methyl group on carbon 2 of a four-carbon chain.
Reading a branched name back into a structure
- Draw the parent carbon chain from the stem and number it.
- Add the functional group at the carbon named by its locant.
- Attach each side chain to the parent carbon given by its number.
- Add hydrogen atoms until every carbon has four bonds.
Worked example - Naming 2-methylbutane
Name the compound below.
Step 1 - Find the longest chain and count it
The longest continuous run of carbon atoms is four, so the parent chain gives the stem but-. The fifth carbon is on a side chain.
Step 2 - Find the functional group for the suffix
Every carbon-to-carbon bond is single, which puts the compound in the alkanes, so the suffix is -ane.
Step 3 - Number the parent chain
There's no functional group to place, so number from the end that reaches the side chain first. Numbering from the left puts the side chain on carbon 2; from the right puts it on carbon 3. The lower number wins, so number from the left.
Step 4 - Name the side chain
The side chain is a single carbon group (CH3-) - a methyl group - joined to carbon 2. That gives 2-methyl.
Step 5 - Count and order the side chains
There's only one side chain, so no prefixes (like di-) or alphabetical ordering are needed.
Step 6 - Assemble the parts
2-methyl + but- + -ane
Systematic name = 2-methylbutane
Drawing a structure from its name
A structure can be drawn directly from its systematic name by working through each part.
Step 1 - Draw the carbon chain
Read the stem and draw that many carbons in a row, then number them. The bonds between them go in next. For but-2-ene, the stem is but-, so draw four carbons in a row.
Step 2 - Read the suffix for the functional group
The suffix -ene means there is a carbon-to-carbon double bond (C=C).
Step 3 - Place the group at its locant
The locant 2 means the double bond starts at carbon 2, so put a double bond between carbons 2 and 3.
Step 4 - Add hydrogen atoms
Carbon always forms four bonds and hydrogen forms one. Add hydrogens until every carbon has four bonds (a double bond counts as two bonds):
- Carbon 1 has one bond → 3 hydrogens
- Carbon 2 has three bonds → 1 hydrogen
- Carbon 3 has three bonds → 1 hydrogen
- Carbon 4 has one bond → 3 hydrogens
Step 5 - Check and count for the formula
Check every carbon has four bonds and count the atoms: four carbons and eight hydrogens gives the molecular formula C4H8.
Naming alcohols
- An alcohol is an organic compound with a hydroxyl group (-OH) attached to a carbon atom.
- The hydroxyl group is the functional group of alcohols, so their names end in the suffix -ol.
- In methanol (1 carbon) and ethanol (2 carbons), the -OH group can only be on carbon 1, so no number is needed.
- From three carbons upwards, the -OH group can sit on different carbons, so a locant is needed to show its position.
Numbering the chain
- Number the carbon chain from the end that gives the -OH group the lowest possible number.
- Write that number between the stem and the suffix -ol.
Propan-1-ol and propan-2-ol
Attaching the -OH to an end carbon gives propan-1-ol; attaching it to the middle carbon gives propan-2-ol.
Both have the molecular formula C3H8O, but they are different compounds because their atoms are connected differently - they are structural isomers.
Longer chains and multiple -OH groups
A four-carbon chain gives butan-1-ol (-OH on carbon 1) and butan-2-ol (-OH on carbon 2), both C4H10O.
Molecules with two -OH groups use the suffix -diol with both numbers: a three-carbon chain with two -OH groups is a propanediol.
Structural isomers
- A molecular formula shows the number of each type of atom in a molecule (e.g. C4H10), but not how they're joined.
- A structural formula shows how the atoms are arranged.
Definition of structural isomers
Structural isomers are compounds with the same molecular formula but different structural formulae.
Position of functional group
The functional group can be attached at different points along the same carbon chain. For example, C4H8 gives but-1-ene (double bond between carbons 1 and 2) and but-2-ene (double bond between carbons 2 and 3).
Branching of the carbon chain
The arrangement of the carbon skeleton itself can differ. The steps below find every chain isomer systematically.
Step 1 - Draw the longest straight chain
Draw all carbons in a single row and fill in hydrogens. For C4H10, this is a straight four-carbon chain (butane).
Step 2 - Shorten and branch
Shorten the chain by one carbon and attach that carbon to a middle carbon of the chain, then fill in hydrogens. For C4H10, this gives a three-carbon chain with a methyl branch on carbon 2 (2-methylpropane).
Step 3 - Repeat for every position
Try moving the branch to any other non-end position, or shorten the chain further. For C4H10, no other arrangements exist, so there are two structural isomers.
Common traps
- Attaching a removed carbon to an end carbon does not create a branch - it just redraws the original straight chain.
- Bending a chain on paper does not make it a different molecule: what matters is which carbons are bonded together.
Properties of structural isomers
- Isomers can also have different functional groups, which puts them in different homologous series (for example, an alkene and a ring-shaped cycloalkane with the same molecular formula). Their chemical properties then differ as well.
- Isomers with the same functional group have similar chemical properties.
- Isomers have different physical properties (such as boiling point) because their molecular shapes differ.
Worked example - The isomers of C5H12
Draw all the structural isomers of C5H12.
Step 1 - Draw the longest straight chain
Draw all five carbons in a continuous row and fill in twelve hydrogens. This is pentane.
Step 2 - Shorten the chain by one carbon and add a branch
A four-carbon chain with one carbon branch attached to a middle carbon gives 2-methylbutane. Attaching the methyl group to the other middle carbon gives the exact same molecule numbered from the opposite end.
Step 3 - Shorten the chain by two carbons and add two branches
A three-carbon chain has only one middle carbon (carbon 2). Attaching both remaining carbons to it gives 2,2-dimethylpropane.
Shortening the chain to two carbons leaves no middle carbon to attach branches to, so no more isomers can be made.
Step 4 - Check each structure
Each structure has five carbons, twelve hydrogens, and every carbon has four bonds. All three are distinct: chains of 5, 4, and 3 carbons.
C5H12 has three structural isomers.
