Edexcel IGCSE Chemistry · Spec 4.5-4.6
Isomerism & Types of Organic Reaction
Isomerism and the main types of organic reaction.
Chemistry revision video
Isomerism & Types of Organic Reaction
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Same formula, different molecule
Structural isomers are compounds with the same molecular formula but different structural formulae. The same atoms, joined together in a different arrangement.
Butane and methylpropane are both C4H10. Butane is a straight chain of four carbons; methylpropane is a chain of three with the fourth branching off. Same atoms, different molecules, different boiling points.
Finding the isomers of a formula
Work systematically rather than by inspiration. Draw the longest possible straight chain first, then shorten it by one carbon and attach the spare as a branch, trying each position along the chain.
Check each new structure is genuinely different. Rotating a molecule or drawing the same branch at the other end of the chain gives you the same compound drawn twice, and that is the commonest mistake. Count the carbons in the longest chain and note where the branch sits; if those two facts match a structure you already have, it is not a new isomer.
For alkenes there is a second source of isomers: the position of the double bond. But-1-ene and but-2-ene are both C4H8.
The types of organic reaction
Four names cover almost everything in this part of the course.
Addition: something adds across a double bond, which opens, and nothing is lost. Alkenes do this, with bromine, hydrogen or steam.
Substitution: one atom is swapped for another. Alkanes do this with halogens, but only in ultraviolet light, and a small molecule such as hydrogen bromide is released.
Combustion: reaction with oxygen, releasing energy.
Polymerisation: many small molecules join into one large one. Addition polymerisation loses nothing; condensation polymerisation releases a small molecule, usually water, at every link.
What the examiners say about this topic
A principal examiner report for Edexcel International GCSE Chemistry discusses a question comparing the reactions of ethane and ethene with bromine, and its observations are unusually practical.
It explains that a maximum of three marks was allowed per reaction, so writing everything you know about one substance cannot earn full marks. Some low scoring answers dealt only with the observations, and some high scoring ones omitted the observations altogether. A complete answer covers both substances and both kinds of detail.
It then records a common chemical error: giving the product of the addition reaction as bromoethane instead of dibromoethane. Bromine adds across the double bond as two atoms, so two bromines end up in the product, and the di is not optional.
The most striking remark is about handwriting. The report says that in many responses it was difficult for examiners to distinguish between ene and ane. Those three letters are the entire difference between an alkane and an alkene, and an examiner who cannot read which one you wrote cannot award the mark.
It is worth writing those endings deliberately clearly, and worth using the full name rather than relying on the ending alone where the sentence allows.
Why isomers matter
Different structures give different properties. A branched isomer has a lower boiling point than its straight chain equivalent, because the branching keeps the molecules further apart, so the intermolecular forces between them are weaker and less energy is needed to separate them.
That reasoning is worth having ready, because it connects isomerism to bonding, and a question asking why two isomers boil at different temperatures is really a question about intermolecular forces.
Spec 4.5-4.6
What you need to know
- Explain what structural isomers are
- Recognise isomers of a molecule
- Name the main types of organic reaction
Active recall
Quick check
Answer each question before opening the answer.
What are isomers?
Molecules with the same molecular formula but a different arrangement of atoms (structure).
Name a type of organic reaction.
Combustion, addition or substitution (for example).
Chemistry revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.