Question

Correspond each of the peaks to the H's in the structure. Is it the Z or E isomer? Show how you know.

1H NMR (CDCI, 300 MHz) 9-(2-phenylethenyl)anthracene 7.9424 - 7.8874 - 6.9760 6.9207 1H 7.0 8.6 8.4 8.2 8.0 7.8 7.6 7.4 7.2 6.8 6.6

Answer 1

In order to know the geometry of alkene from NMR spectroscopic data, we have to calculate couplings constants of the corresponding alkene protons.

Cis alkenes have coupling constants in the range of 6-14 Hz ( typically 10 Hz)

Trans alkenes have coupling constants in the range of 11-18 Hz ( typically 16 Hz)

A coupling constant is a difference between peaks in Hz which are formed due to coupling with the neighbor hydrogens. Here alkene gives doublet due to the presence of single neighbor hydrogen.

from the plot, we can calculate the difference between the doublet line in ppm. we just need to multiply the frequency at which NMR was recorded to know coupling constants in Hz.

7.9424 – 7.8874 = 0.055 ppm

J= coupling constant  = 0.055 X 300 = 16.5 Hz

for another alkene C-H

6.9760 – 6.9207 = 0.0553 ppm

J= coupling constant = 0.0553  X 300 = 16.59 Hz

From these coupling constants, we can tell that these alkene hydrogens are trans to each other or this is a trans alkene.

So, this is trans-9-(2-phenethynyl)anthracene or -9-[(E)-2-phenethynyl]anthracene. This product is an E isomer.

Thanks and regards.