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Just wondering why the name of this compound is 3-phenylpropene? I understand the concept of phenyl groups but I'm confused where the 3 came from, because if it is referring to the double bond in propene, then shouldn't it be numbered 2?

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    $\begingroup$ 3 is referring to phenyl. The double bond is at 1. In propene it can't be otherwise. $\endgroup$ Nov 2 '16 at 9:06
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Obviously, there are three carbons in propene to which a phenyl group might be attached. Let's have a look and name the compounds:

phenylpropenes

1 with the phenyl group at C-2 of propene is 2-phenylpropene, also known as $\alpha$-methylstyrene.

2 with the phenyl group at C-1 of propene is 1-phenylpropene. To be exact, the compound drawn is (E)-1-phenylpropene. E is for entgegen, that is on opposite sides of the double bond.

3 is 3-phenylpropene, also known as allylbenzene.

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According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the ring or the chain can be the senior parent structure. For the preferred IUPAC name, the ring is selected as the senior parent structure. In general nomenclature, however, the context may favour the ring or the chain.

For the name that is given in the question, the propene chain is taken as the senior parent structure. Accordingly, the benzene ring is considered to be a substituent (i.e. a phenyl group).

The propene chain is numbered from one end to the other, the direction being chosen so as to give the lower locants to the double bond. (Only if a choice remains, low locants are given to prefixes that indicate substituent groups.) Thus, the numbering of the propene chain leads to prop-1-ene:

prop-1-ene

Accordingly, in this case, the substituent is assigned the locant ‘3’:

3-X-prop-1-ene

Since this substituent is a phenyl group, the complete systematic name is 3-phenylprop-1-ene.

3-X-prop-1-ene

However, note that this is not the preferred IUPAC name. As mentioned above, the ring is selected as the senior parent structure for the preferred name. Therefore, the preferred name in this case is (prop-2-en-1-yl)benzene.

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  • $\begingroup$ Note that the standalone name “prop-1-ene” for propene (PIN) does not follow P-14.3.4.2 (d). $\endgroup$
    – mykhal
    Dec 5 '18 at 12:25
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When a simple substituent like, $\ce{-OH}$, $\ce{-NO2}$ is attached to the benzene ring then benzene is considered as the parent chain. But when a complex substituent like a long ($>6$) carbon chain, or a group with double bond/triple bond or another substituent is attached to the benzene ring, then benzene is considered the substituent, and named as phenyl group.

Here, the propene part is the main chain. Double bond gets more priority than phenyl substituent, so numbering is started from that end. Phenyl group gets number 3. So the more correct name should be 3-phenyl-1-propene. However, writing 3-phenylpropene does not cause any ambiguity (as the numbering is started from the double bond end)

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