Alkanes
5 marks · HardThis question is about unbranched alkanes.
Draw the displayed (structural) formula of the straight-chain alkane with eight carbon atoms.
Write the name of this alkane.
State the molecular formula for this compound.
How many single carbon–carbon bonds are present in one unbranched molecule of this alkane? Show your reasoning.
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) — · must show 8 carbon atoms in a straight chain, 18 hydrogens correctly attached
(b) octane · must show octane
(c) C8H18 · must show C8H18
(d) 7 · must show States 7 single C–C bonds, Reasoning: one fewer C–C bond than number of carbon atoms
Must show: a straight C8 chain, all hydrogens shown, name: octane, formula: C8H18, bond count: 7
Also accept: For the displayed formula, zig-zag or linear structure accepted as long as all atoms/bonds correct
Common mistakes: Branched structure; Wrong count for number of bonds
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Draws 8 carbons in a single straight chain, all C–C bonds shown
B2 (a) · 1 mark · All 18 hydrogens correctly attached to carbons; no extra/missing H
B3 (b) · 1 mark · Correct name: octane
B4 (c) · 1 mark · Correct molecular formula: C8H18
B5 (d) · 1 mark · Correctly states 7 single C–C bonds and gives logical reasoning (i.e., 8 carbons linked = 7 bonds) (follow-through)
Worked solution
a) The displayed formula for the straight-chain alkane with eight carbon atoms shows 8 carbons joined in a row with single bonds, each carbon attached to enough hydrogens to make 4 bonds in total per carbon, for example:
H–C–C–C–C–C–C–C–C–H, with hydrogens on all free valencies (each end carbon with 3 hydrogens, inner carbons with 2 each).
b) Name: octane
c) Molecular formula: C8H18
d) There are 7 single carbon–carbon bonds present. Reason: Each pair of adjacent carbons forms one C–C bond; for 8 carbons in a line, that is 8–1 = 7 bonds.