# Percentage of trans-stilbene traces in cis-stilbene H NMR

This is H NMR spectrum of cis-stilbene. There are some traces of trans stilbene. I have to determine how many trans-stilbene is present, the exact percentage and I have problem with that.

I guess that it has to do with integration of peaks but I'm still not sure is trans-cis isomer ratio 1:10 or am I wrong?

• Step 1: identify which peaks correspond to cis and which to trans. Feb 8 '18 at 11:13
• Small peaks at 7,35 ppm and 7,51 ppm correspond to trans isomer and peaks at 7,27-7,15 ppm and 6,59 ppm correspond to cis isomer. Feb 8 '18 at 11:20
• For accurate integration you also need to set the recycle delay to a longer value than usual (e.g. ~20 s), although I appreciate this may not be possible in your case. Feb 8 '18 at 12:22

You have to focus on the $\ce{^1H}$ of the double bond. They are singlets because both molecules are completely symmetrical. As per the tables, the chemical shift of the cis isomer is $5.25+1.38-0.07=6.56$ ppm. On the other hand, the chemical shift of the trans isomer is $5.25+1.38+0.36=6.99$ ppm.
$\frac{0.1571}{0.1571+1}100=14\%$