Structural isomerism in the alkanes
4 marks · NormalHeptane (C7H16) is an alkane. Answer the following about its isomers.
Explain why heptane can form structural isomers.
Draw the skeletal formula of two different isomers of C7H16.
State one feature that all isomers of C7H16 share.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Heptane can form isomers because the seven carbon atoms can be joined in different arrangements (branches or straight chains).
(b) Correct skeletal formulae for two different isomers, e.g. straight-chain heptane and 2,2-dimethylpentane (drawn as lines of C atoms with correct branches and length). · must show First isomer: straight chain of 7 carbons, Second isomer: branched structure with 7 carbons, correct placement of branches
(c) All isomers have the same molecular formula: C7H16.
Must show: Two different skeletal formulae (straight chain, branched chain, correct C count)
Common mistakes: Drawing two versions of the same isomer rotated in space; Omitting correct number of carbons
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Explains that different arrangements/branching of the carbon chain leads to isomers
D1 (b) · 1 mark · Correct skeletal formula for straight-chain heptane
D2 (b) · 1 mark · Correct skeletal formula for a different branched isomer of heptane
B2 (c) · 1 mark · States that all isomers have same molecular formula
Worked solution
a) Heptane can form structural isomers since the seven carbon atoms can be arranged in a straight chain or with branches.
b) - Skeletal formula for straight-chain heptane: ——————— (7 connected lines) - Skeletal formula for 2,2-dimethylpentane (branched): | ———|—— (lines with a branch at the middle carbon, two branches attached)
c) All isomers of C7H16 have the same molecular formula.