tag:blogger.com,1999:blog-6010505890506526002.post7610627940648120195..comments2024-03-28T07:04:52.697-04:00Comments on Just Like Cooking: Cryptic Retraction, UncoveredSee Arr Ohhttp://www.blogger.com/profile/09464185815368499346noreply@blogger.comBlogger6125tag:blogger.com,1999:blog-6010505890506526002.post-36933315993208674442013-08-12T22:25:21.541-04:002013-08-12T22:25:21.541-04:00Oops, the chromatography has actually been discuss...Oops, the chromatography has actually been discussed on the reddit link above at the end of the article. Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-6010505890506526002.post-91635430742259842112013-08-12T22:03:34.005-04:002013-08-12T22:03:34.005-04:00The compound in JACS is definitely related to the ...The compound in JACS is definitely related to the one in OM. I am still not convinced that it is as simple as the contamination by the ammonium salt. The procedure says:<br /><br />"After filtration and evaporation of the solvent, the residue was purified by flash chromatography on silica gel (ethyl acetate / hexane = 3 / 7) affording the compound 1 in 86 % yield (3.3 g)"<br /><br />I would expect this to remove the ammonium salt. Right? There must be more to it. I am still wondering what went wrong.<br /><br />Could the student have cut the corners and skipped the chromatography? After all the conversion was spot-to-spot on TLC, hum, expect maybe for that little spot with an rf of 0.000...Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-6010505890506526002.post-24247599938938274732013-08-10T21:59:41.237-04:002013-08-10T21:59:41.237-04:00A few of the results note "TE 999.9 K"....A few of the results note "TE 999.9 K". What does this mean? And, is it applicable to the current discussion?<br />This appears to be a measurement of temperature in degrees Kelvin, after looking at other results in the SI. But they are all about room temperature, and 999.9 K is well above room temperature.<br /><br />See pages 36 and 37 of the SI at http://pubs.acs.org/doi/suppl/10.1021/ja807685r/suppl_file/ja807685r_si_001.pdf .<br /><br />I am not familiar with the instrument, and couldn't locate a description of this at http://www.bruker.com .<br /><br />Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-6010505890506526002.post-2291955090449491892013-08-09T16:09:55.789-04:002013-08-09T16:09:55.789-04:00So it is just a claim of restricted rotation in th...So it is just a claim of restricted rotation in this case then. Okay. I was thinking there was something more subtle going on. I guess I just need to break out my modeling kit and make this guy to see the crowdedness of the central stereocenter.Andrehttps://www.blogger.com/profile/12140011313455957356noreply@blogger.comtag:blogger.com,1999:blog-6010505890506526002.post-72546850081167987062013-08-09T15:13:24.271-04:002013-08-09T15:13:24.271-04:00Andre,
It all comes down to time (and kinetics, I...Andre,<br /><br />It all comes down to time (and kinetics, I guess). We think of free rotation as something that a molecule either has, or doesn't have, but it is not an either/or situation, it is more of a continuum. For NMR, it basically comes down to "can the two protons (or carbons) switch positions multiple times in the time between pulses in the NMR. <br /><br />My favorite example of this phenomenon is the proton NMR of cyclohexane. At room temperature, you get 1 signal, because the ring can flip many times during the course of the pulse (1 secondish). However, if you cool the sample, the ring flip slows (kinetics and all), and the axial and equatorial protons resolve, giving two signals. This powerpoint deck has a good explanation and some nice spectra to explain the effect. <br /><br />http://www.chem.queensu.ca/facilities/NMR/nmr/chem805/PPT09/5-NMR-conversion.ppt<br /><br />Coming back to the original question, the benzyl group is probably pinned in by the groups surrounding it, so although the individual bonds have free rotation, the benzyl group gets "stuck" and can't rotate very fast at room temperature. Depending on the environment, this effect will probably go away if the sample were heated, and the two signals will coalesce into a single signal between the two observed signals.<br /><br />Hope this helps!Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-6010505890506526002.post-2955994872855099492013-08-09T11:28:51.197-04:002013-08-09T11:28:51.197-04:00I've been following this since all of you were...I've been following this since all of you were discussing it on twitter, and I just never got the argument for C2 and C6 having different resonances. Can you explain it in really basic terms?<br /><br />Is there some hindered rotation of the ring that organic chemists know about that I don't? Because if not, C2 and C6 are completely equivalent (as are C3 and C5). <br /><br />I get that the chiral centers in the molecule make the benzylic protons inequivalent, but I don't see how that translates to the planar ring with free rotation which will put the carbons in the *exact* same magnetic environment and should make them identical.Andrehttps://www.blogger.com/profile/12140011313455957356noreply@blogger.com