A case in point is embelin. I would appreciate it if experts in this area could share their views.
Its an aromatic compound and also phenolic. Even if an additional OH is added it remains phenolic.
I beg to disagree, 1,4-benzoquinone is NOT aromatic even if it could satisfy Huckel's (4n+2) pi electron rule. The carbonyl pi electrons do not resonate within the ring = no proper conjugation, just cross-conjugation.
Furthermore, X-ray and electronic calculations have predicted a "diketone" geometry; the different bond lengths (determined by electron diffraction) show that 1,4-benzoquinone is not aromatic and its bond lengths actually resemble the bond lengths in acrolein (attachment).
At Jan-Erik Raitanen: So you mean that 1,4-benzoquinone is not aromatic. Hence, can it be said that embelin which has two hydroxyl groups and an aliphatic chain in the ring is not a phenolic? Correct me if I am wrong.
Thanks for your answers.
Yes, 1,4-benzoquinone as such (i.e., without any other substituents) is not aromatic. A substituted 1,4-benzoquinone (or possibly its anion) could be aromatic, though.
Moving on to Embelin:
In a way, Embelin can be considered as a phenolic lipid, which is a hybrid class of phenols. Such compounds are derived from hydroxyphenols, for example catechols (1,2-dihydroxybenzene), resorcinols (1,3-dihydroxybenzene) or hydroquinones (1,4-dihydroxybenzene). These are all aromatic and phenolic from the beginning, it's just that the plant/animal/fungi/etc. metabolizes them as lipids for some purpose (e.g., protection against infections, wounds or UV radiation).
The definition of a phenolic compound is "...a compound where a hydroxyl group is directly bonded to an aromatic hydrocarbon group", though, so I can understand the confusion! Nevertheless, as stated above, Embelin could be seen as as phenolic lipid.
I hope this helped a bit.
Regards,
JR
1,4-Benzoquinone is not aromatic as it contains 4 pi electrons & if hydroxy group attached to it it can not be phenolic ,it will be enolic.
At Madhukar Baburao Deshmukh: So, I could not say Embelin as a phenolic compound even though it is derived from phenols as said by Jan-Erik Raitanen.
Just trying to understand it better.
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