Chemistry Corner

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I need to do an NMR analysis on this molecule.. (We are supposed to know this from last year but our ONE we did last year was way easier.) In our class we do a 5 step analysis.

The first step is to draw the structure.

The second step is to label the H's (a,b,c,d, etc.)

(See attached file dssd.bmp)

The third step is to order the (a/b/c/...):

1. first by resononce effects where + is deshielded and - is shielded

2.second by indctive effects(closer to electonegetive is deshielded)

The 3rd step is the step I'm having lots of trouble with.

By using the resonance and inductive effects we are supose to put the a/b/c/d.... in order starting with the one that is most electronegetive.

The fourth step is too number the H's...whcih I have no problem with.

The fith step requires us to find the coupling constants. I have looked on the interent for help but have found that they only find it of A or B or C, not Ab or Ac or Bc or Ba.....etc.

If you, steve, or anyone else who knows basics to nmr can you please help me understand at least step 3....

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.

There are also tables of coupling constants; my Chemist's Companion gives Jtrans for vinylic hydrogens H2C=CHY where Y = Ph or COOR at 17-18 Hz. The ring hydrogens should exhibit coupling constants in the range of 0-10 Hz (see for example http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/couplingaromatic.html).

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.

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