Structural Isomerism & Stereoisomerism (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about structural isomerism and stereoisomerism.

Isomers have the same molecular formula but different structures, and they divide into two branches: structural isomers differ in how the atoms are connected, and stereoisomers have the atoms connected the same way but arranged differently in space.

Both branches start from the same place: two compounds share a molecular formula and are still different compounds. What separates the branches is what differs between them. In a structural isomer the connectivity differs, so the atoms are joined up in a different order. In a stereoisomer the connectivity is identical, and only the arrangement in space differs.

That distinction is the first decision you make with any pair of isomers, and the notes are built around it: given two structures, you determine whether it is a stereo or structural isomer before going any further.

We take four things in turn. First, what a structural isomer is. Then the three types of structural isomerism: chain, positional and functional group. Then what a stereoisomer is, and why a carbon-carbon double bond is what makes one possible. Then the E/Z system for naming them.

Structural isomers are compounds that have the same molecular formula but different structural formulae. Propene and cyclopropane are the example in the notes: both are three carbons and six hydrogens, C3H6, but propene has a double bond in an open chain and cyclopropane closes the three carbons into a ring.

That is the structural half of the split we opened with: the formula is fixed, and the connectivity has changed.

The wording matters here. It is the same molecular formula, not the same empirical formula and not the same chemical formula.

Every structural isomer falls into one of three types.

Chain isomerism is when compounds have the same molecular formula but their longest hydrocarbon chain is not the same, and branching is what causes this. Pentane and 2,2-dimethylpropane are both C5H12, but the longest chain in pentane is five carbons and in 2,2-dimethylpropane it is three, with two methyl branches. That is the pair on screen.

Positional isomers arise from differences in the position of a functional group, which sits on a different carbon in each isomer. Butan-1-ol and butan-2-ol are both C4H10O, with the alcohol group on the first carbon in one and the second in the other.

Functional group isomers arise when different functional groups give the same molecular formula. Butanal and butanone are both C4H8O, but one contains an aldehyde group and the other a ketone. Because the functional groups differ, the chemical properties differ too.

All three hold the molecular formula fixed and change the connectivity, which is what makes them structural.

Stereoisomers are compounds that have the same atoms connected in the same way, but the atoms are arranged differently in space. Nothing about the connectivity changes, which is what makes them different to structural isomers.

E/Z isomerism occurs as a result of restricted rotation about the planar carbon-carbon double bond. The pi bond prevents rotation, so the groups are locked in position.

There is a second condition: E/Z isomerism only occurs when each carbon of the double bond carries two different groups. The general alkene on screen, C2R4, has four groups, R1 to R4, attached to the double bond, and will only show E/Z isomerism if R1 and R3 are different, and R2 and R4 are different. The E/Z system is needed to tell the isomers apart.

You may see this referred to in other sources as cis/trans isomerism. That is a special case of E/Z: a cis isomer is essentially the same as a Z isomer, and a trans isomer essentially the same as an E isomer.

There is a second type of stereoisomerism called optical isomerism which will be covered in another video.

Assigning E or Z uses the Cahn-Ingold-Prelog priority rules. Look at the first atom attached to each carbon of the double bond: the higher the atomic number, the higher the priority.

If those first atoms are the same, the comparison settles nothing and you look at the second atoms attached. In the example in the notes, two carbons tie, and comparing what is attached to them gives CH2Br priority over CH3CH2, because bromine outranks carbon.

With priorities assigned, the rule is positional. Z isomers have the highest priority groups on the same side of the double bond, and E isomers have them on opposite sides. Z comes from the German word zusammen, meaning together, and E from entgegen, meaning opposite.

The two structures on screen are both 1-bromoprop-1-en-2-ol, with the methyl and the hydroxyl group placed differently. The heist priority group on the left-hand carbon is the bromine and on the right-hand carbon the hydroxyl. In compound A the two highest priority groups are on opposite sides, so it is the E isomer. In compound B they are on the same side, so it is Z.

Two things to get right, and two that lose marks.

Define a structural isomer as having the same molecular formula. Mark schemes penalise "same empirical formula" and "same chemical formula", and those score zero for that mark. On terminology, geometric isomerism was the older term for cis/trans isomerism, it is no longer in use, and it is not required in exams.

Two that cost marks. Do not use "arranged differently in space" or "different spatial arrangement" when defining a structural isomer, because those phrases define a stereoisomer and examiners are instructed to penalise it. And use full IUPAC names: butan-2-ol, not but-2-ol, 2-butanol or butane-2-ol.

Structural isomers have the same molecular formula and a different structural formula, and there are three types: chain, positional and functional group.

Stereoisomers have the same atoms connected in the same way, and differ only in how those atoms or groups of atoms sit in space. For E/Z isomerism that arrangement is locked by restricted rotation about a carbon-carbon double bond, and the CIP priority rules decide which isomer is E and which is Z.

Same molecular formula, two branches: connectivity for structural isomers, spatial arrangement for stereoisomers.

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