What a learner can do afterwards
- Draws and names every chain and position isomer of a given molecular formula
- Recognises functional group isomerism between families such as alcohols and ethers
- Explains why a double bond prevents rotation and what that produces
- Assigns E or Z by comparing the priority of the groups on each carbon of the double bond
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A molecular formula only counts atoms, it never shows how they join up. Butane and methylpropane both have the formula C4H10, yet they are different substances with different properties. Molecules with the same formula but different connections are called isomers, and learning to spot them starts with naming hydrocarbons with care.
Chain isomers connect their carbon skeletons differently, like butane and methylpropane. Position isomers keep the same skeleton and functional group but sit the group on a different carbon, like propan-1-ol and propan-2-ol. To test a formula such as C5H12, draw and name every chain and position isomer you can find.
Functional group isomers arrange the same atoms as different families. Ethanol and dimethyl ether share the formula C2H6O but behave nothing alike, because one is an alcohol and the other is an ether. One formula can describe more than one molecule, which is why chemists also draw structural formulas that show the bonds.
A double bond locks the molecule because the pi bond stops rotation, freezing different arrangements in place and giving stereoisomers. To assign E or Z, rank the two groups on each carbon of the double bond by priority. If the top ranked groups sit on the same side you call it Z, and if they sit on opposite sides you call it E. For C4H8, work out which pairs differ by E-Z isomerism and which differ by structure.
One formula can hide several molecules, so draw the connections before you decide.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.