# How to determine from the formula that there is a phenyl group present in the molecule?

Why is the organic compound $\ce{C6H5-CH=CH-CH2Cl}$ named 3-chloro-1-phenylprop-1-ene and not 9-chloronon-5,7-dien-1,3-yne even though there is no specification whatsoever that there is a phenyl group in the organic molecule? $$\ce{C6H5-CH=CH-CH2Cl}$$ can also be written as $$\ce{CH#C-C#C-CH=CH-CH=CH-CH2-Cl}$$ and the name for this organic compound would be 9-chloronon-5,7-dien-1,3-yne. How do we surely know that there exists a phenyl group in the molecule?
Also why is phenyl treated as a substituent and cannot be considered in the longest chain?

• In your linear molecule you have -2,4-yne , why not 1,3 - yne? – porphyrin Aug 1 '16 at 18:42
• Note that the name 3-chloro-1-phenylprop-1-ene is correct and may be used in general nomenclature (for example, the context may favour the chain, so that the unsaturated acyclic structure may be recognized or substituents may be treated alike); however the preferred IUPAC name (PIN) for this compound is (3-chloroprop-1-en-1-yl)benzene. – Loong Aug 2 '16 at 11:10
• @porphyrin my bad...made the edit – oshhh Aug 2 '16 at 11:56

It's a matter of convention.

$\ce{C6H5}$ is conventionally used to represent phenyl because it is a common moiety. Your long, highly unsaturated, chain is also $\ce{C6H5}$, but it wouldn't conventionally be written like that without expansion, unless you're setting out to confuse someone.

• And many chemists would simplify and disambiguate further by writing Ph- instead of C6H5-... – matt_black Aug 2 '16 at 9:26

The first molecule is cinnamyl chloride and has the structure

where the phenyl group appears.

The reason why phenyl is described as a substitutent is that there is no naming rule which encompasses both aliphatic chains and rings. So chains and rings are considered separately and are substituent of each other. If the highest priority moiety is on a ring, the ring becomes the parent chain. If it is on a chain, like in your case, the chain becomes the parent hydride and the phenyl group is considered a substituent.

• This may be helpful, but it doesn't really answer the question. Could you expand it to address what was asked? – jonsca Aug 1 '16 at 23:10
• Note that the given compound actually does not include a principal characteristic group (which is expressed as a suffix or implied by the name of the functional parent compound) that could determine the senior parent structure. Therefore, according to Subsection P-52.2.8 of the current version of Nomenclature of Organic Chemistry, the ring is selected as the parent hydride to construct the preferred IUPAC name (PIN), i.e. (3-chloroprop-1-en-1-yl)benzene. Nevertheless, the context may favour the chain and the name 3-chloro-1-phenylprop-1-ene may be used in general nomenclature. – Loong Aug 2 '16 at 11:29