2019 H2 Chem P1 Q5 — Chemical bonding Solution

The answer is C (2 only).

Count the \(\pi\) bonds in each species (a double bond = 1 \(\sigma\) + 1 \(\pi\) bond):
• 1 \(\text{Al}_2\text{Cl}_6\): all single bonds, including the two dative \(\text{Cl} \rightarrow \text{Al}\) bonds in the bridges, so 0 \(\pi\) bonds.
• 2 \(\text{CO}_2\), O=C=O: two C=O bonds, so 2 \(\sigma\) and 2 \(\pi\) bonds.
• 3 \(\text{CH}_3\text{CHO}\): one C=O bond, so 6 \(\sigma\) and 1 \(\pi\) bond.
• 4 \(\text{CH}_2\text{=CHCH}_3\): one C=C bond, so 8 \(\sigma\) and 1 \(\pi\) bond.
Only species 2 contains two \(\pi\) bonds.

• A (3 and 4): each of these has only one \(\pi\) bond.
• B (1 only): \(\text{Al}_2\text{Cl}_6\) has no \(\pi\) bonds at all.
• D (4 only): propene has only one \(\pi\) bond.