# Which has the higher chemical shift E/Z alkene?

If you have a simple alkene for example 1-methyl-But-2-ene

then you will have two methyl groups on one side of the double bond, which one of these will have the highest. Will the one on the same side as the rest of the chain of butene have the higher ppm or will it be the one in trans with the butene.

By the way, on my spectra, these two methyl groups are not equivalent. I need to know if the E or Z will have a higher chemical shift and why, thanks.

• “1-Methyl-but-2-ene” is not a valid name. Do you mean “pent-2-ene”? – Faded Giant Jan 1 '16 at 19:53

## 2 Answers

Are you talking about 1H NMR? In the general case, it is not so easy to say - it depends on all substituents, with geminal substitution usually contributing most to change in chemical shift. trans and cis substitution will affect chemical shifts differently, but will depend on the electronegativities of all groups concerned.

For your example, there will be very little difference between chemical shift for these two methyl groups, but the methyl trans to the alkyl chain is probably slightly downfield shifted. Coupling constants for the two methyl groups would be slightly different. In acyclic systems, 4J couplings from a methyl to a cis or trans proton can range from about -3 to +2.5 (but usually 0.8 to 2), and usually |Jtrans|>|Jcis|. Of course, nOe information would also be useful.

If you are talking about 13C NMR, a methyl peak trans to a carbon chain will be downfield shifted from a methyl group trans to a proton.

cis-butene: $\delta$(H2): 5.37 ppm [1]

trans-butene: $\delta$(H2): 5.55 ppm [2]

There are lists of increments (Schoolery, Tobey-Simon...) out there to predict chemical shifts in systems such as there, but for methyl groups in vinylic these are very similar. In those cases it really doesn't bear to assign isomers through chemical shifts when you can get the correct assignment through coupling constants.