0
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ 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 at 14:01
  • $\begingroup$ However the carbanion at gama position is stabilized by resonance. $\endgroup$ – anonymous Apr 16 at 14:13
  • $\begingroup$ Why space between keto and but? $\endgroup$ – Zenix Apr 16 at 17:35
4
$\begingroup$

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).
| improve this answer | |
$\endgroup$
  • $\begingroup$ @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 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 at 14:17
  • $\begingroup$ Thanks, I saw the reaction. But is there any reaction reported using OH- and I2 $\endgroup$ – anonymous Apr 16 at 14:26
  • 1
    $\begingroup$ 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 at 14:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.