Structural isomerism in the alkanes

3 marks · Normal

Explain how you would decide whether two structures for C5H12 drawn below are isomers or not.

Structure 1: CH3–CH2–CH2–CH2–CH3 Structure 2: CH3–CH(CH3)–CH2–CH3

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First, check that both have the same molecular formula (C5H12). Then, compare the way the carbon atoms are joined: if the connections are different (one straight chain, one branched), they are structural isomers.

Also accept: If they have the same number of C and H, but C atoms joined differently

Common mistakes: Just checking the names without counting atoms; Assuming a rotated structure is an isomer

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Marking points

M1 · 1 mark · States or shows that both have the same molecular formula (C5H12).

M2 · 1 mark · Compares how the atoms are connected in each structure.

A1 · 1 mark · Correctly concludes that these are different structural formulae and therefore are isomers.

Worked solution

First, count the atoms in both structures. Both have the same number and type of atoms (5 C, 12 H), so the same molecular formula. Next, look at how the carbon atoms are joined: Structure 1 is a straight chain, Structure 2 has a branch. Since the carbon skeleton is different, these are isomers.