Naming organic compounds with IUPAC rulesIB MYP Chemistry: Revision notes

An IUPAC name is built from a stem, which gives the number of carbon atoms in the longest chain, and an ending (suffix), which gives the family.

Stems: 1 meth-, 2 eth-, 3 prop-, 4 but-, 5 pent-, 6 hex-, 7 hept-, 8 oct-, 9 non-, 10 dec-

Endings:

• -ane: alkane (C-C single bonds only)
• -ene: alkene (a C=C double bond)
• -ol: alcohol (-OH group)
• -oic acid: carboxylic acid (-COOH group)
• -oate: ester (-COO- group), as the second word of the name

Put the stem and ending together. When the position of the group could vary, add a number, with hyphens, to show which carbon atom it is on. Number from the end that gives the lowest number to the functional group.

• CH₃CH₂CH₃ is propane
• CH₂=CHCH₂CH₃ is but-1-ene and CH₃CH=CHCH₃ is but-2-ene
• CH₃CH₂CH₂OH is propan-1-ol and CH₃CH(OH)CH₃ is propan-2-ol
• CH₃CH₂COOH is propanoic acid (the carbon in -COOH is carbon 1, so it needs no number)
• CH₃COOCH₂CH₃ is ethyl ethanoate: the alkyl group on the O gives the first word, and the acid part gives -oate

A side chain is called an alkyl group. Methyl is -CH₃ (one carbon) and ethyl is -C₂H₅ (two carbons).

Rules for naming a branched alkane:

1. Find the longest continuous carbon chain. This gives the stem.
2. Number the chain from the end that gives the lowest number to the branch.
3. Write the number, a hyphen and the branch name, then the stem: 2-methylbutane.
4. For two identical branches use di-, with a number for each: 2,2-dimethylpropane.

Worked example: CH₃CH₂CH(CH₃)CH₂CH₃. The longest chain has 5 carbons (pentane). The CH₃ branch is on carbon 3 from either end. The name is 3-methylpentane.

The same compound can be shown in different ways:

• Molecular formula: the number of each atom, such as C₄H₁₀
• Structural (condensed) formula: the atoms in each carbon's group written in order, such as CH₃CH₂CH₂CH₃ or CH₃CH(CH₃)CH₃
• Displayed formula: every atom and every bond drawn, with each bond as a line (C-H, C-C, C=C, C-O, O-H). Each carbon makes 4 bonds.

A molecular formula does not show the arrangement of atoms, so it cannot tell isomers apart. A structural or displayed formula can.

Structural isomers have the same molecular formula but different structural formulae, so they have different names.

C₄H₁₀: butane (CH₃CH₂CH₂CH₃) and 2-methylpropane (CH₃CH(CH₃)CH₃).

C₅H₁₂: three isomers:

• pentane, CH₃CH₂CH₂CH₂CH₃
• 2-methylbutane, CH₃CH(CH₃)CH₂CH₃
• 2,2-dimethylpropane, CH₃C(CH₃)₂CH₃

Other families have isomers too: C₄H₈ gives but-1-ene and but-2-ene, and C₃H₇OH gives propan-1-ol and propan-2-ol.

Name to structure: (1) draw the longest chain from the stem; (2) add the functional group or branch at the numbered carbon; (3) fill each carbon up to four bonds with hydrogen.

Example: 3-methylpentane: five carbons in a chain, a CH₃ on carbon 3, giving CH₃CH₂CH(CH₃)CH₂CH₃.

Structure to name: (1) find the functional group for the ending; (2) find the longest chain for the stem; (3) number from the end that gives the lowest numbers; (4) add branches and positions.

Example: CH₃CH₂CH(CH₂CH₃)CH₂CH₂CH₃. The longest chain is 6 carbons, with an ethyl group on carbon 3, so it is 3-ethylhexane.