SolutionsAlkanes and Cycloalkanes
1. 2-Methylbutane
For 2-methylbutane, (CH3)2CHCH2CH3:
- Write its molecular formula and that of its other two isomers.
- Classify its carbons as primary, secondary and tertiary.
- Will it boil at a higher or lower temperature than pentane? Why?
- Balance its complete combustion.
- C5H12. Its isomers are pentane and 2,2-dimethylpropane.
- Three primary (the CH3 groups), one secondary (the CH2) and one tertiary (the CH).
- At a lower temperature (27.8 °C versus 36.1 °C): branching reduces the contact surface and the dispersion forces.
- C5H12 + 8 O2 → 5 CO2 + 6 H2O
2. Its chlorination
The reactivity of hydrogens towards chlorine is primary : secondary : tertiary = 1 : 4 : 5. Calculate the proportions of the four monochlorinated derivatives of 2-methylbutane.
| Product | Hydrogens × reactivity | Proportion |
|---|---|---|
| 1-Chloro-2-methylbutane (on one of the CH3 groups of C2) | 6 × 1 = 6 | 6/22 ≈ 27% |
| 2-Chloro-2-methylbutane (tertiary) | 1 × 5 = 5 | 5/22 ≈ 23% |
| 2-Chloro-3-methylbutane (secondary) | 2 × 4 = 8 | 8/22 ≈ 36% |
| 1-Chloro-3-methylbutane (on the CH3 of C4) | 3 × 1 = 3 | 3/22 ≈ 14% |
This matches the experimental results (27, 23, 36 and 14%). With bromine, in contrast, almost only the tertiary derivative is obtained.
3. Conformations
Looking along the C2–C3 bond of 2-methylbutane, how many gauche methyl–methyl interactions do its staggered conformations have?
One of the staggered conformations has two CH3/CH3 gauche interactions (≈1.8 kcal/mol) and the other two have only one (≈0.9 kcal/mol). The latter are the most stable.
4. Methylcyclohexane
Which chair of methylcyclohexane is the most stable? What is the energy difference and what causes it?
The chair with the methyl group equatorial, by 1.8 kcal/mol. When axial, the methyl group suffers two gauche-type interactions (0.9 + 0.9 kcal/mol) with carbons 3 and 5 of the ring, the so-called 1,3-diaxial interactions.