Rank the following four isomeric hydrocarbons in order of decreasingstability: (a)

Chapter 11, Problem 1-4

(choose chapter or problem)

Rank the following four isomeric hydrocarbons in order of decreasingstability: (a) 1-cyclopropylpentane; (b) cis-1-ethyl-2-propylcyclopropane; (c) ethylcyclohexane;(d) cis-1,4-dimethylcyclohexane.What does the problem ask? To establish an approximate order of energy content forthe four molecules and thus estimate relative stability.How to start? Careful structural drawings will help you recognize what features may stabilizeor destabilize each hydrocarbon relative to the others. Notice that this problem includesa nomenclature component: Each name must be converted to a structure. Finally, to make thecomparison fair, we need to draw the most stable possible conformation of each one.Information: The lower the energy content of a structure, the more stable it is. Stericcrowding raises energy content and decreases stability (Section 2-8). So, a staggered conformationis more stable than an eclipsed one; anti is more stable than gauche; equatorialis more stable than axial (Section 4-4). In each case, the more stable option has less stericcrowding. Note any other destabilizing effects (such as bond-angle strain).Proceed: As shown above, we identify the structural features that affect stability. Howimportant is each one? Find the quantitative information: Bond-angle strain in cyclopropanesis the largest destabilizing factor (27.6 kcal mol21; Section 4-2). Next we add the enforcedeclipsing in (b), which is at least as bad as the 4.9 kcal mol21 in the highest-energy conformationof butane (Figure 2-13). Cyclohexanes do not suffer from bond-angle strain, but(d) cannot avoid one axial substituent (1.7 kcal mol21, Table 4-3). With these data in hand,apply the logic: Higher energy content 5 less stable. The stability order is therefore (c) moststable, then (d), then (a), and fi nally (b) least stable. (If needed, you could even give approximateenergy differences by adding the energy-content increments for each compound.)(a) (b) (c) (d)Steric crowding