Q13.46E - Organic Chemistry (Mcmurry)

Question

How many ₁₃C NMR absorptions would you expect for cis-1,3-dimethylcyclohexane?For trans-1,3-dimethylcyclohexane? Explain.

Step-by-Step Solution

Answer

cis-1,3-Dimethylcyclohexane and the racemic mixture of trans-1,3-dimethylcyclohexane shows five absorptions in its ₁₃C NMR spectrum.  

Step 1: NMR

₁₃C NMR is an application of NMR spectroscopy and uses carbon atoms to determine the structure of the molecule. This technique comprise less sensitivity than ₁H NMR spectroscopy. Several organic and organometallic compounds are characterized using ₁₃C NMR spectroscopy. 

Step 2: Number of 13 C NMR absorptions for cis-1,3-dimethylcyclohexane and trans-1,3-dimethylcyclohexane

  cis-1,3-dimethylcyclohexane and trans-1,3-dimethylcyclohexane

cis-1,3-Dimethylcyclohexane is a meso compound. Due to the symmetry it exhibits five absorptions in the ₁₃C NMR spectra. Trans-1,3-Dimethylcyclohexane exists as a pair of enantiomers. They undergo ring-flip at room temperature that average the absorptions due to the non equivalent carbons. Like the cis isomer, the racemic mixture of the trans enantiomers shows five absorptions in its ₁₃C NMR spectrum.