11.2 - Naming Organic Compounds & Isomers
Naming a compound
A systematic name tells you how long the carbon chain is, which family the compound belongs to, and where its functional group sits.
Step 1 - Count the carbons for the stem
The number of carbons in the chain gives the stem, the first part of the name.
| Carbon atoms | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Stem | meth- | eth- | prop- | but- | pent- | hex- |
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 chain
- Number every carbon in turn (1, 2, 3...), starting from whichever end gives the functional group the lower number.
- Try both ends - only the lower number is used.
Step 4 - Write in the locant
- The number is called the locant - it says which carbon the functional group is joined to.
- Write it between the stem and suffix, with a hyphen on either side.
- A double bond is numbered by the lower of the two carbons it joins.
Step 5 - Assemble the parts
Put them together: stem, then locant, then suffix.
When the locant is skipped
Not every name needs a locant:
- An alkane has no functional group to place, so its name is stem + suffix only.
- A double bond in a two- or three-carbon chain has only one possible position, so no number is written.
- The number is only needed once there's more than one possible position for the group.
Worked example - Naming but-2-ene
Name the compound below.
Step 1 - Count the carbon atoms in the chain
Four carbon atoms joined in a row, so the stem is but-.
Step 2 - Find the functional group for the suffix
Two carbons are joined by a double bond (C=C), which puts the compound in the alkenes, so the suffix is -ene.
Step 3 - Number the chain
Numbering from either the left or the right puts the double bond between carbons 2 and 3. A double bond takes the lower number, so it's on carbon 2.
Step 4 - Write in the locant
The locant is 2, so the middle of the name is -2-.
Step 5 - Assemble the parts
but- + -2- + -ene
Systematic name = but-2-ene
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.
