Introduction to Organic Chemistry
8 marks · HardHexanol is an organic compound containing 6 carbon atoms, 14 hydrogen atoms and 1 oxygen atom.
Explain what is meant by structural isomerism in the alkanes, using hexane (C6H14) as an example.
State the functional group present in all alcohols.
Draw the displayed formula for one possible structural isomer of hexanol.
The name of an alcohol ends with the suffix ‘-ol’. Explain how the suffix '-ol' is used in naming alcohols, and apply it to the compound you drew in part (c).
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) Structural isomerism is when compounds have the same molecular formula but different structural formulae. For hexane, C6H14, structural isomers have all the same atoms but arranged in different chains or branches (e.g. straight chain, branched chain).
(b) –OH or hydroxyl group
(c) Displayed formula for a hexanol isomer (e.g., 3-hexanol): · must show Six C atoms in a chain, –OH group attached to any one carbon. All bonds shown explicitly, correct number of H atoms for chosen isomer.
(d) The suffix ‘-ol’ in the name shows that the compound is an alcohol with an –OH group. For the structure drawn the name will be x-hexanol (with x being the position of the –OH, e.g. 3-hexanol).
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Marking points
M1 (a) · 1 mark · States that structural isomers have the same molecular formula but different structural formulae.
M2 (a) · 1 mark · Mentions arrangement of atoms or branching in structural isomers for hexane.
A1 (a) · 1 mark · Correctly links explanation to hexane by mentioning straight/branched chains or gives a named/structural example.
B1 (b) · 1 mark · States the correct functional group (–OH / hydroxyl group).
M3 (c) · 1 mark · Draws a correct displayed formula for a structural isomer of hexanol (chain with 6 Cs, 14 Hs, 1 –OH attached to any C).
A2 (c) · 1 mark · Shows all bonds and correct number of hydrogen atoms for chosen isomer.
M4 (d) · 1 mark · Explains that '-ol' indicates an alcohol with an –OH group.
A3 (d) · 1 mark · Gives correct name using the suffix (e.g., 3-hexanol) for the structure in (c). (follow-through)
Worked solution
a) Structural isomerism refers to the existence of molecules that have the same molecular formula but different structural formulae. This means the atoms are bonded together in different ways. In the case of the alkanes, such as hexane (C6H14), the six carbon atoms can be joined in a straight chain or arranged so that the chain is branched in different ways. Each arrangement gives a different structural isomer.
b) The functional group present in all alcohols is the hydroxyl group, written as –OH.
c) One possible isomer of hexanol is 3-hexanol. Its displayed formula is:
d) The suffix ‘-ol’ is used in naming alcohols to show that the molecule contains the –OH (hydroxyl) functional group. The position of the –OH group is numbered to give the lowest possible number. For the structure in (c), since the –OH is on the third carbon of the chain, the name is 3-hexanol.