The difference between the four forms of tocopherol shown on Wikipedia are the number of methyl groups on the benzene ring and their relative
location(s). The alpha form has three methyl groups on its ring, two ortho to the hydroxyl group and one meta. The beta form has two methyl groups on
its ring, one ortho to the hydroxyl group and one meta. The gamma form also has two methyl groups on its ring, with one being ortho to the hydroxyl
group and the other being meta, only in this case the ortho methyl group is below the hydroxyl group instead of above it (in reference to the picture
shown on Wikipedia). And finally there's the delta form, which only has a single methyl group on its ring meta to the hydroxyl group.
Likewise, the related tocotrienols also follow this substitution and naming pattern, with the alpha form having three methyl groups on its ring, two
ortho to the hydroxyl group and one meta, the beta form having two methyl groups on its ring, one ortho to the hydroxyl group and one meta, and so on.
The difference, however, is that the tocotrienols have three carbon-carbon double bonds on their carbon chains, hence the name
tocotrienol (tri- for three, -ene for alkene).
And lastly, all four forms of tocopherol are chiral, each with three separate stereocenters. These stereocenters are indicated by the wedges and
dashes, which show the orientation of those bonds relative to one another in three-dimensional space (a wedge means the bond is pointing towards you,
a dash means the bond is pointing away from you, and a normal line means the bond is in the plane of the image/screen). This means that you can get
multiple different stereoisomers of the same tocopherol form simply by altering the relative orientations of those bonds. Since there are three chiral
centers in each form of tocopherol, there are a total of 8 (23) different possible stereoisomers for each. The tocotrienols, on the other
hand, only have a single stereocenter, giving only two (21) possible stereoisomers for each form.
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