VINYLIC PROTONS

 

Vinyl protons (protons directly attached to a C=C double bond) are typically found in the 5.5 – 6.5 PPM range

Coupling patterns can be very complicated for vinyl protons and generally don’t follow the ‘simplified’ N+1 rule that saturated protons follow.  We generally won’t try to interpret these coupling patterns, but rather simply assign them as “vinyl” protons

Nearby groups can have a large impact on the chemical shift of vinyl protons.  Compare the difference in the compound below with that of cyclohexene above.  Notice that the vinyl proton closest to the electronegative oxygen is pulled downfield (i.e., higher PPM) than the one further from the oxygen:

Now consider 2-cyclohexenone below.  Notice that the proton closest to the carbonyl group is at a higher chemical shift than the proton in cyclohexene [6.05 ppm for cyclohexenone vs. 5.66 for cyclohexene].  This is not surprising since the proton is not only vinylic, but it is also alpha to a carbonyl group.  We know that a proton alpha to a carbonyl group is pulled downfield.  How come the other proton is at an even HIGHER chemical shift even though it is further from the carbonyl??

The answer is because of a resonance interaction of the carbonyl group with the neighboring C=C double bond. The resonance interaction pulls electron density onto the electronegative oxygen, leaving considerable positive charge character on the lower carbon atom.  This increased positive charge has the expected effect of pulling the chemical shift of the attached proton downfield (i.e., higher PPM)