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It is told often that only compounds having a methyl ketone and corresponding alcohols show positive iodoform test.

I tried to sketch a mechanism to show that 1-phenyl-1-keto-but-2-ene also gives iodoform test. However I haven't found any source that confirms that 1-phenyl-1-keto-but-2-ene shows iodoform test. Please check if there is any mistake in the mechanism:

My proposed mechanism

This mechanism is very similar to the classical iodoform reaction mechanism.

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  • $\begingroup$ C-C double bond group not have larger -Ieffect as compared to C=O group and abstraction of H+ from C H bond is not so easy by OH-. This fact can be used here. $\endgroup$ – Fish92003 Apr 16 '20 at 14:01
  • $\begingroup$ However the carbanion at gama position is stabilized by resonance. $\endgroup$ – anonymous Apr 16 '20 at 14:13
  • $\begingroup$ Why space between keto and but? $\endgroup$ – Zenix Apr 16 '20 at 17:35
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Enones react with with iodine and catalytic base under aqueous conditions to give $\alpha$-iodoenones (Ref.1). Pyridines are most often used for this, the reference quoted also uses $\ce{K2CO3/DMAP}$. They further show that reaction works for a range of enones including those with the $\ce{RCOCH=CH-CH3}$ grouping and no mention is made of $\gamma$-iodination. A "background" reaction was run with $\ce{K2CO3/I2}$ in aq. THF and no iodination was observed.

Proposed mechanism from the referenced paper:

enter image description here

References:

  1. Marie E. Krafft, John W. Cran, "A Convenient Protocol for the α-Iodination of α,β-Unsaturated Carbonyl Compounds with $\ce{I2}$ in an Aqueous Medium," Synlett 2005, (8), 1263-1266 (DOI: 10.1055/s-2005-868495).
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  • $\begingroup$ @waylander the document in your link requires paid subscription. Could you please provide the link of some other source which shows the reaction? $\endgroup$ – anonymous Apr 16 '20 at 14:14
  • $\begingroup$ I got to it directly through Google Scholar by searching "alpha iodination of enones" The paper is by Krafft and Cran and is the top hit on the Google Scholar $\endgroup$ – Waylander Apr 16 '20 at 14:17
  • $\begingroup$ Thanks, I saw the reaction. But is there any reaction reported using OH- and I2 $\endgroup$ – anonymous Apr 16 '20 at 14:26
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    $\begingroup$ The closest is the K2CO3 in aq THF without added pyridine which gives no reaction, I think that is a significant result with respect to your question. I would expect I2/OH- to similarly give no reaction $\endgroup$ – Waylander Apr 16 '20 at 14:37

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