In many of the sites in online, acetophenone is mentioned as an unsaturated compound because of the presence of C=C bond in the phenyl group. But, many of my teachers say that it is a saturated compound because it does not decolourise Bromine water and pink colour of Bayer's reagent. Which is true and why?
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4$\begingroup$ You need better teachers! Acetophenone, as you correctly say, is unsaturated due to the benzene ring $\endgroup$– WaylanderCommented Feb 13 at 14:54
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$\begingroup$ @Waylander But, why Acetophenone does not decolourise neither Bromine water nor the pink colour of Bayer's reagent? $\endgroup$– Vinay5101Commented Feb 13 at 15:11
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1$\begingroup$ Because of the extra stability of the conjugated double bonds compared to an isolated double bond. Acetophenone does react with bromine under basic conditions in a bromoform reaction $\endgroup$– WaylanderCommented Feb 13 at 15:21
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$\begingroup$ @Waylander Thanks a lot $\endgroup$– Vinay5101Commented Feb 13 at 15:26
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1 Answer
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This case explains a problem with using "saturated" versus "unsaturated". The boundary in real chemistry is not as distinct as the words would seem to imply because the behavior of the compound may depend on the conditions to which the compound is exposed.
In acetophenone both the carbonyl group and the phenyl group can undergo addition, and thus show unsaturation, under appropriate conditions. Carbonyl groups do not readily add hydrogen as $\ce{H2}$, but they can be hydrogenated through reaction with a hydride-source nucleophile such as $\ce{NaBH4}$ or "alkanated" with alkyl nucleophiles such as a Grignard reagent. As for the phenyl group, aromatic rings are resistant to addition, but for example benzene can be hydrogenated at elevated temperatures on platinum catalysts. For instance, Lin and Vannice[1] study this catalytic reaction between 44 and 83°C.
Reference
- S.D. Lin, M.A. Vannice, "Hydrogenation of Aromatic Hydrocarbons over Supported Pt Catalysts .I. Benzene Hydrogenation", Journal of Catalysis, Volume 143, Issue 2, 1993, Pages 539-553, ISSN 0021-9517, https://doi.org/10.1006/jcat.1993.1297.
