2017 H2 Chem P1 Q16 — Introduction to Organic Solution

The answer is B (1 and 2 only).
Write out each structure and count the H atoms (see the figure):
• Pair 1 is correct: 2-aminopropane, \(\text{CH}_3\text{CH(NH}_2)\text{CH}_3\) (\(\text{C}_3\text{H}_9\text{N}\)), has 9 H; 2-bromo-2-methylpropane, \((\text{CH}_3)_3\text{CBr}\) (\(\text{C}_4\text{H}_9\text{Br}\)), has 9 H.
• Pair 2 is correct: ethyl propanoate, \(\text{CH}_3\text{CH}_2\text{CO}_2\text{CH}_2\text{CH}_3\) (\(\text{C}_5\text{H}_{10}\text{O}_2\)), has 10 H; butane-1,2-diol, \(\text{HOCH}_2\text{CH(OH)CH}_2\text{CH}_3\) (\(\text{C}_4\text{H}_{10}\text{O}_2\)), has 10 H.
• Pair 3 is incorrect: butanenitrile, \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CN}\) (\(\text{C}_4\text{H}_7\text{N}\)), has 7 H (the nitrile carbon carries no H); 2-methylpropanal, \((\text{CH}_3)_2\text{CHCHO}\) (\(\text{C}_4\text{H}_8\text{O}\)), has 8 H.
• A is wrong: it includes pair 3, which has 7 and 8 H atoms.
• C is wrong: it leaves out pair 2, which has 10 H atoms in each compound.
• D is wrong: it leaves out pair 1, which has 9 H atoms in each compound.