Which have vinylic protons, and which have allylic methyl groups?

Question

carvone

citronellal

citronellol

p-cymene

geranial

geraniol

limonene

menthone

b-myrcene

a-terpinene

Answer 1

You need to understand the difference between the vinylic and allylic positions to go through the question.

 

A group is said to be in the vinylic position when it is  attached to the carbon in a double bond, such as:

 

X–C=C

 

A group is said to be in the allylic position when it is attached to a carbon adjacent to another carbon in a double bond, such as:

 

X–C–C=C

 

 

A vinylic proton is a proton attached to a carbon in a double bond. So you will see a structure like this:

 

H–C=C

 

An allylic methyl group will look like this:

 

CH₃–C–C=C

 

If you look at the structure of carvone, you have three double bonds. If you focus on the double bond in the ring, you will see that:

 

At the vinylic positions (one on each side), you have a proton on one side, and a methyl group on the other

At the allylic positions, you have two protons on one side (away from the C=O), and the protons of the CH₃ group, and the C=O group on the other side.

This means that, around the double bond located in the ring, you have one vinylic proton and no allylic methyl groups. You now have to repeat the process for each double bond for each molecule.

 

However keep in mind that a proton on a benzene ring is aromatic, not vinylic, and that the position one carbon off from a benzene ring is benzylic, not allylic.