Isomers are different compounds with the same molecular formula, differing in how the atoms are arranged. Ethanol and dimethyl ether are both C2H6O, yet ethanol is a water-soluble liquid and dimethyl ether is a gas at ordinary temperature. The arrangement, not the atom count, sets the properties, so a formula alone never identifies a substance.
Part of Organic Chemistry
Compounds with identical formulas but different atom connections, and why structure rather than composition determines behaviour.
Same Atoms, Different Connections, Different Substance
The carrier’s subtitle says it plainly: same atoms, different arrangements. Two kinds matter for a practical reader.
Chain isomers. The carbon skeleton is straight in one and branched in the other. Butane and isobutane, the carrier’s own example of a branch, both have four carbons and ten hydrogens (the count is worked out in Bonding Rules). They are different substances with different boiling points.
Functional-group isomers. The same atoms form a different group. Ethanol has a hydroxyl group (an alcohol), dimethyl ether has an oxygen bridging two carbons (an ether). Both are in the carrier’s functional-group table, the alcohol among the hydroxyls and the ether under diethyl ether and anisole. They differ so much that one is a reactive solvent-fuel and the other a gas.
Why It Matters In A Rebuilding Shop
Isomerism is the reason you identify compounds by properties, not formulas. The carrier’s field identification list (boiling point, solubility, smell, colour, flame test, pH) works because isomers differ on exactly those tests. It is also the reason a “similar” liquid cannot substitute for the one you need: a different isomer of a solvent or a fuel can have a different boiling point, a different flammability and a different toxicity.
Worked Example: Counting Isomers Of C4H10
Only two arrangements of four carbons with ten hydrogens exist.
- Put the four carbons in a straight line: butane.
- Take a three-carbon chain and hang the fourth carbon off the middle carbon: isobutane.
Check that the hydrogen count is the same in both. In butane, the two end carbons have 3 hydrogens each (6) and the two middle carbons 2 each (4), total 10. In isobutane, three carbons are methyl groups with 3 hydrogens each (9) and the central carbon holds one hydrogen (1), total 10. The formulas match (C4H10) and the structures do not.
Common Mistakes
- Assuming equal formula means equal hazard. It does not; see the Safety block.
- Treating “alcohol” as one thing. Ethanol and methanol are different compounds, not isomers of each other (their formulas differ), and one blinds at doses far smaller than the other. Methanol’s toxicity is the reason Methanol Hazards is the authoritative page on distillate safety.
Safety
Never substitute an isomer or a look-alike for a named solvent or fuel
A liquid that smells similar to your solvent may be a different compound with a different flash point and different toxicity. Keep a labelled reference sample of each material, use the identification tests in the carrier, and never infer safety from a shared formula or from a shared smell.
What’s Next
- Learn next: Functional Groups — the groups that distinguish functional isomers
- Combine with: Structural Formulas — the drawings that show the difference