Coaching Centre
Isomers, formulae and shape
Molecular, structural, condensed and skeletal formulae; chain, position and functional group isomers and how to tell an isomer from the same molecule drawn differently; and the tetrahedral, trigonal planar and linear shapes around single, double and triple bonded carbon.
C₈H₁₈ is a single formula, but it describes 18 different compounds. The straight-chain one, octane, boils at 126 °C and knocks in a car engine; its bushiest isomer, 2,2,3,3-tetramethylbutane, is a waxy solid that does not melt until about 100 °C. Same atoms, different arrangement, different substance. By C₂₀H₄₂ there are more than 366 000 possible isomers, which is why chemists never use a molecular formula on its own to identify an organic compound.
Four ways to write the same molecule
- Molecular formula (C₄H₁₀O) counts atoms. It is shared by every isomer.
- Structural formula shows every atom and every bond. In the HSC, "draw the structural formula" means this, with all the H atoms.
- Condensed structural formula (CH₃CH(OH)CH₂CH₃) lists each carbon with what is attached to it, using brackets for branches.
- Skeletal formula draws the carbon chain as a zigzag. Each corner and line end is a carbon, H atoms on carbon are left out, and all other atoms are written in. It is quick and shows the real zigzag shape.
Structural isomers
Structural isomers have the same molecular formula but different structural formulae. They are different compounds with different physical properties, and sometimes different chemical properties.
| Type | What changes | Examples |
|---|---|---|
| Chain isomers | The carbon skeleton (how the chain branches) | C₅H₁₂: pentane, 2-methylbutane, 2,2-dimethylpropane |
| Position isomers | Same skeleton and functional group, but the group or multiple bond sits in a different place | but-1-ene and but-2-ene; 1-chloropropane and 2-chloropropane; butan-1-ol and butan-2-ol |
| Functional group isomers | A different functional group altogether | C₃H₆O: propanal and propanone. C₃H₆O₂: propanoic acid and methyl ethanoate. C₆H₁₂: hex-1-ene and cyclohexane |
The number of possible isomers grows fast. Alkanes have 2 isomers at C₄, 3 at C₅, 5 at C₆, 9 at C₇ and 18 at C₈. Chain isomers of alkanes have similar chemical reactions, but the more branched an isomer is, the lower its boiling point (7D): pentane boils at 36 °C, 2-methylbutane at 28 °C and 2,2-dimethylpropane at 10 °C.
Draw and name every structural isomer with formula C₄H₉Cl.
Same molecule, or an isomer?
The commonest mistake is "finding" an isomer that is really the same molecule drawn another way. Before you count a structure, check:
- Is it just flipped or rotated? Pent-1-ene drawn with the double bond on the right is still pent-1-ene.
- Is the chain just bent on paper? A "branch" at the end of a chain is part of the chain.
- Name it. If two drawings have the same correct IUPAC name, they are the same compound.
- Work through the pairs. For each one, decide whether the two drawings are the same compound, chain isomers, position isomers, functional group isomers or not isomers at all.
- Pair 3 looks like two different molecules. Name both of them. What does that tell you?
- Find a pair with different numbers of hydrogen atoms. Which quick check would have caught it straight away?
- Aim for 12 out of 12, then press New round to see the pairs in a different order.
The shape around each carbon
Electron pairs around an atom repel each other and spread as far apart as possible (VSEPR theory). Each single, double or triple bond counts as one region of electron density, so the shape around a carbon depends on what kind of bonds it has.
| Bonds on the carbon | Regions of electron density | Shape | Bond angle | Example |
|---|---|---|---|---|
| Four single bonds | 4 | Tetrahedral | 109.5° | every carbon in ethane |
| One double + two single | 3 | Trigonal planar | 120° | both carbons in ethene; the whole molecule is flat |
| One triple + one single | 2 | Linear | 180° | ethyne, H–C≡C–H, is a straight line |
Two consequences matter later. Tetrahedral carbons make alkane chains zigzag and let them rotate freely about each C–C bond, so long chains are floppy. A C=C bond cannot rotate, which locks part of the molecule flat. That rigidity is why but-2-ene exists as two different stereoisomers, and it changes how well molecules pack together, and so their melting points.
Test yourself
Which pair are position isomers?
Same five-carbon chain, same group (C=C), different position.
How many structural isomers does C₅H₁₂ have?
Pentane, 2-methylbutane and 2,2-dimethylpropane.
The H–C–H bond angle in ethene is closest to
Three regions of electron density around each carbon give a trigonal planar shape.
Ethanol, C₂H₅OH, and methoxymethane, CH₃OCH₃, are
Both are C₂H₆O, but one is an alcohol and the other an ether.
Label the shapes and bond angles.
How is each pair related?
Compounds with the same molecular formula but different structures are structural . Isomers with different carbon skeletons are isomers. A carbon atom with four single bonds is , with angles of 109.5°. A carbon in a triple bond is . A formula that shows the carbon chain as a zigzag without the H atoms on carbon is a formula.
C₄H₈ can exist as several open-chain alkenes. Draw or describe and name the open-chain structural isomers of C₄H₈, and identify the type of isomerism between each pair. Explain why "but-3-ene" is not a separate isomer.
There are three open-chain isomers. But-1-ene, CH₂=CHCH₂CH₃, has the double bond at the end of a four-carbon chain. But-2-ene, CH₃CH=CHCH₃, has it in the middle of the same chain. These two have the same skeleton and the same functional group in different places, so they are position isomers. 2-Methylpropene, CH₂=C(CH₃)CH₃, has a three-carbon chain with a methyl branch, a different carbon skeleton, so it is a chain isomer of both but-1-ene and but-2-ene. "But-3-ene" would be CH₃CH₂CH=CH₂, which is simply but-1-ene drawn the other way round. IUPAC rules number the chain from the end that gives the double bond the lowest number, so its correct name is but-1-ene and it is not a separate isomer.