9701 · 13.3

Shapes of organic molecules; σ and π bonds — practice questions

Practice and worked examples for 9701 Shapes of organic molecules; σ and π bonds. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

Determine the total number of sigma ( $\sigma$ ) and pi ( $\pi$ ) bonds in a molecule of but-2-ene, $CH_3CH=CHCH_3$ .

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Draw the displayed formula to see all the bonds clearly: H H H | | | H--C---C===C---C--H | | H H
Count the $\sigma$ bonds. Remember every bond (single, double, or triple) contains exactly one $\sigma$ $σ$ bond.
- C-H bonds: 3 + 1 + 1 + 3 = 8
- C-C bonds: 1 (single) + 1 (in double) + 1 (single) = 3
- Total $\sigma$ bonds = 8 + 3 = 11 $\sigma$ bonds.
Count the $\pi$ bonds. Only double or triple bonds contain $\pi$ $π$ bonds.
- The C=C double bond contains one $\pi$ bond.
- Total $\pi$ bonds = 1 $\pi$ bond.

Worked example 2

Predict the shape and the C-C-N bond angle in ethanenitrile, $CH_3CN$ .

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Identify the central atom for the angle in question. The angle is C-C-N, so the central atom is the carbon of the nitrile group (the one bonded to nitrogen).
Determine the bonding around this central carbon. It forms a single bond with the other carbon atom and a triple bond with the nitrogen atom ( $C \equiv N$ ).
Apply VSEPR theory. There are two regions of electron density around this carbon (the C-C single bond and the C≡N triple bond). There are no lone pairs on this carbon.
Predict the shape and angle. Two regions of electron density arrange themselves as far apart as possible, resulting in a linear shape with a bond angle of 180°.

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