Coupling Constants in Alkenes

Coupling constants in alkenes can also differ depending on whether the protons are cis or trans to each other. Note that in a terminal alkene (i.e., an alkene at the end of a carbon chain), the cis and trans protons are NOT equivalent. One is on the same side as the substituent, the other is on the opposite side. The coupling of trans protons to each other is typically very large, around 16 Hz, while the coupling of cis protons, while still large, is a little smaller, around 12 Hz. This leads to the pattern shown below, and an example of a molecule with this splitting pattern is shown on the next slide.

There are other times when protons on the same carbon are nonequivalent, which we'll see later.