Structural isomerism in the alkanes
4 marks · NormalButane (C4H10) has two structural isomers.
State the names of the two isomers of C4H10.
Draw the displayed (full structural) formula for each isomer. Show all atoms and bonds.
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) Butane and 2-methylpropane · must show Both correct names, or names/accepted synonyms
(b) Displayed formulae of butane and 2-methylpropane with all atoms and bonds shown · must show First: Straight-chain (butane): CH3–CH2–CH2–CH3 (structure with all bonds shown), Second: Branched chain (2-methylpropane): CH3 | CH3—C—CH3 (drawn with all hydrogens; central C attached to three CH3 groups)
Must show: Displayed formulae: all atoms and bonds correctly shown; correct number of hydrogens
Also accept: 1st: n-butane or butane 2nd: 2-methylpropane or isobutane
Common mistakes: Omitting some hydrogens; Drawing the same structure twice by rotation
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct name for n-butane (butane or n-butane)
B2 (a) · 1 mark · Correct name for 2-methylpropane (or isobutane)
B3 (b) · 1 mark · Displayed formula of butane: all atoms and bonds shown, correct structure
B4 (b) · 1 mark · Displayed formula of 2-methylpropane: all atoms and bonds shown, correct structure
Worked solution
a) The two isomers of C4H10 are butane (n-butane) and 2-methylpropane (isobutane).
b) Displayed formulae:
Butane: H H H H | | | | H–C–C–C–C–H | | | | H H H H
2-methylpropane: H H H | | | H–C–C–C–H | | H H With a CH3 group attached to the middle carbon, or (drawn as): CH3 | CH3–C–CH3 | H (Each C has 4 bonds; all H must be shown.)