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Current Question:

Nomenclature

Dear Dr. Shulgin:

In your book PIHKAL, in the synthesis of 2C-B, you state that the product of the 2,5-dimethoxybenzaldehyde and nitromethane is a beta-nitrostyrene. However, since the nitro is third out on the aliphatic chain, shouldn't it be called gamma? If not, then what does the beta stand for? Thanks from a confused novice chemist. -- I.C.

Dear I.C.:

The nomenclature that is used to give names to structures in the area of chemistry can follow some rather fuzzy rules. The first three Greek letters, alpha, beta, and gamma, are the ones most frequently encountered, but they are not necessarily interchangeable with the numbers, "1", "2" and "3." With a chain of carbon atoms, the number "1" indicates the first carbon atom, whereas the letter "alpha" indicates the first carbon atom that is capable of being substituted. A simple example would be bromopropionic acid. This is a three carbon chain molecule and, if the bromine was located at the end of this chain opposite the acid group it would have the structure Br-CH2-CH2-CO2H. Since it is called an acid, the carbons are numbered away from that group, and a correct name would be 3-bromopropionic acid. But that last carbon is the second of two that can be substituted with a bromo atom, so another correct name would be beta-bromopropionic acid. The bromo atom is "beta" to the carboxylic group.

In the example you mentioned in your question about the nitrostyrene, the parent structure of a styrene is a vinyl substituted benzene, and can be written as C6H5-CH=CH2. There are two carbon atoms in that olefinic (not aliphatic) chain and, counting out from the ring, these would be numbered "1" and "2." But here, either of these two atoms can be substituted with a nitro group, so they are "alpha" and "beta" in the same order.

The reaction you described puts the nitro at the end of the chain, so it should be called 2-nitrostyrene or beta-nitrostyrene. C6H5-CH=CH-NO2. There would be no meaning to a "gamma" as there is no third carbon atom in that chain.

Confusion often arises when the naming focus is changed. If this nitrostyrene were to be reduced to phenethylamine, C6H5-CH2-CH2-NH2, both the numbering and the Greek-lettering direction are reversed. It is now an amine, an ethylamine, and the identifying name of the carbon atom to which the nitrogen is attached must be changed from an "2" or "beta" to a "1" or "alpha." And the benzene ring (the "phenyl" group) is now on carbon "2" of the ethyl chain, and the correct name for this product is beta-phenylethylamine. This is usually contracted to beta-phenethylamine.

I hope this leaves you a little less confused!

--Dr. Shulgin
 

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