CHAPTER 8: Question 12

Simple Answers

Trans-decalin has a rigid structure in which the axial and equatorial hydrogen atoms on each of the CH₂ groups are different. This results in there being five different types of hydrogen atoms in the molecule, which are labelled H_a to H_e in the diagram below, and hence results in a spectrum with five lines. The spectrum will not be
temperature dependent since the structure is rigid and so does not change with temperature.

Cis-decalin however, exists as an equilibrium mixture of the two degenerate conformations shown below. At room temperature, the rate of exchange between these two conformations is rapid, and the conformational exchange has the effect of converting the equatorial hydrogens in one structure into axial hydrogens in the other
structure and vice versa, as shown in the diagram below. Thus, at room temperature, only three signals are observed in the ¹H NMR spectrum corresponding to the hydrogens on C1 (H_a), C2 (H_b), and C3 (H_c) respectively. In this case however, the spectrum will be temperature dependent since at low temperatures, the rate at which the two conformations exchange will be reduced and it will be possible to observe separate signals for each axial and equatorial hydrogen.

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