Filemon
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iodation of double bond benzalacetone
Markownikoff's rule doesn't work in this case. That position is preferably iodide benzalacetone?
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Sauron
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Hmmm, you mean addition to PhCH=CH-C(=O)-Me?
A tug of war between the benzylic H and the alpha-carbonyl H.
I'd expect the benzylic to win and get iodated, (iodinated?) but only the lit. knows for sure.
Sic gorgeamus a los subjectatus nunc.
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chemrox
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So.. I'm going to stick my neck out here a bit to try and learn something .. I mentioned I've been away from this stuff for awhile? So here goes and
cut me to shreds if you will but please explain as you go.
2-iodo-4-phenyl-1,3-butadiene by way of the enol -> abstracted hydroxyl -> conjugated pi system stabilized carbocation at C2
2nd order SN2 reax
or is this total organic phantasy?
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Nicodem
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Filemon: That double bond is too electrophilic to getting iodinated to a stable product. Only very nucleophilic C=C double bonds add iodine. Besides,
what does the Markownikoff's rule have to do with iodinations? The addition of I2 on a double bond can only give one product (except for side products
arrising from the proton elimination in the intermediate halonium ion or similar side ractions). You can only talk about the Markownikoff's rule in
electrophilic additions where the constitution of the nucleophile differs from the electrophile (which is obviously not the case in the additions of
I2 and other halogens).
You can consider Markownikoff's rule, for example, in hydroiodinations (additions of HI on double bonds). In the case of benzalacetone the addition of
HI would proceed in a normal fashion for a 1,4-nucleophilic addition (Michael addition). The product is not stable and can probably only exist in
equilibrium conditions (it eliminates HI at the first chance it gets).
Chemrox: I would not know since I really don't know what you are talking about. Please explain using chemical terminology.
[Edited on 12/10/2007 by Nicodem]
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Filemon
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Excuseme, I have explained bad. I have misled. I wanted to say hydroiodation with HI.
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chemrox
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OK the compound is: 4-phenyl-3-buten-2-one right? HI is the reagent that is reacting with it. In solution, 4-phenyl-3-buten-2-one undergoes
keto-enol tautomerism with the enol tautomer resonance stabilized in a highly conjugated system, double bonds at C1-C2, C3-C4 easily shift to C2-C3
and C4-to the rings system. The carbocation at C2 forms when the enol hydroxyl (nomenclature please) is pulled off by a proton from HI. Once formed,
the carbocation easily undergoes nucleophilic attack by the iodine ion. Rate is determined by the removal of OH. That part is the higher peak on the
energy diagram. A second, shorter peak represents the substitution by the nucleophile, I-minus.
The part I'm shakiest about is the resonance thing. Why would that stabilize the carbocation at C2? Is a resonance structure that places a double
bond on the carbon alpha to the ring between it and the ring, all that big a contributer?
(Edited to add the second paragraph)
[Edited on 12-10-2007 by chemrox]
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Nicodem
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There are no carbocations or tautomers involved (except for the tautomerisation in the last step). I have no idea where from you got such complicated
explanations. The reaction is just a most simple 1,4-nucleophilic addition or a general reaction of what is more commonly known as Michael addition
when the nucleophile is a carboanion. Its mechanism is thought in every basic organic course and can be found in any basic organic chemistry book.
Searching for "nucleophilic conjugate addition" or "1,4-nucleophilic addition" gives numerous explanations and depictions of the mechanism. Linked are
just two the Wiki entries and the picture:
http://en.wikipedia.org/wiki/Conjugate_addition
http://en.wikipedia.org/wiki/Michael_reaction
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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chemrox
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Well shit! @Nicodem: Thanks- How I got to that? I was sitting in my living room, watching a movie on TV with all my books at the office and wanted
to see if my first hit was on the mark. NOT. Thanks for reviewing it and correcting the work. Here's what I've decided. I have a good text on
retrosynthetic methods with lots of reactions diagrammed out. I will work my way through the illustrated reactions writing out the mechanisms until I
feel like I can push e's again. If I don't do this I'm affraid I won't get past being a technician.
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