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In alkenes and other structures, chemical shifts can be predicted using procedures that assign "substituent constants" to different groups which are oriented in a particular way (cis, trans, geminal) to the hydrogen being calculated. See for example http://tonga.usip.edu/gmoyna/NMR_lectures/NMR_lecture24/sld012.htm. The alkene hydrogen closest to the phenyl group (Hc in your drawing) has a chemical shift of 7.82 d and the other alkene hydrogen closest to the COOH group (Hb) in your drawing) is at 6.47 d. Thus, the hydrogen closest to the more electronegative COOH group has the lesser downfield shift. The phenyl group is probably exerting its "ring current" effect on Hc, shifting it downfield to a greater degree. To predict chemical shifts in general, you need acess to a handbook of actual chemical shift data or to the procedures and their associated substituent constants. I'm looking in "The Chemist's Companion" at the moment. The slide at the above link uses "Shoolery constants" for the phenyl and COOH groups to calculate the chemical shifts of Hb and Hc.
The -CH=CHCOOH group can behave as a ring deactivator. Resonance structures place a positive charge on the ortho and para ring carbons. The signals of the hydrogens attached to these carbons should appear a little further downfield than the signals of the meta hydrogens.
You can view a mass spectrum, 1H-NMR spectrum, and IR spectrum of trans-cinnamic acid at http://www.rod.beavon.clara.net/cinnamic.htm. For some reason, the COOH hydrogen is not evident (its 1H-NMR signal would be expected to appear in the 10-12 d region). These were part of a small collection of spectra at http://www.rod.beavon.clara.net/spectra.htm. Other carboxylic acids in this collection show the COOH hydrogen signal clearly in the 11-12 d region. Steve |
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