Unsaturated hydrocarbons

5 marks · Hard

Butene ( C4H8 ) has several isomers.

(a)

Draw the displayed formula of the branched-chain isomer of butene.

[2 marks]

(b)

Draw the displayed formula of one straight-chain isomer of butene where the double bond is at the end of the carbon chain.

[1 mark]

(c)

Explain why these structures are isomers of each other.

[2 marks]

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.

Show answer

(a) The displayed formula of the branched-chain isomer of butene: (CH3)2C=CH2 · must show Two methyl groups (CH3) attached to a central C=C bond: (CH3)2C=CH2, All bonds shown between all atoms

(b) The displayed formula of but-1-ene: H2C=CH–CH2–CH3 · must show Straight chain of four carbons with double bond between C1 and C2, All bonds shown between all atoms

(c) They are isomers because both have the same molecular formula (C4H8) but different structural arrangements of their atoms. · must show Same molecular formula stated (C4H8), Different structural arrangements explained or stated

Show mark scheme and worked solution

Marking points

D1 (a) · 2 marks · Displayed formula of (CH3)2C=CH2: correct structure for a branched-chain isomer; all bonds shown.

D2 (b) · 1 mark · Displayed formula of straight-chain but-1-ene: H2C=CH–CH2–CH3, all bonds shown.

E1 (c) · 1 mark · States both structures have the same molecular formula.

E2 (c) · 1 mark · States or explains that the structures are different because their atoms are arranged differently (different structural or branched/straight chain).

Worked solution

a) The branched-chain isomer of butene:

b) The straight-chain isomer, but-1-ene:

c) These are isomers because both have the same molecular formula (C4H8) but their atoms are joined in different ways, giving different structural formulas.