Joe C. G r e e v e r Delta State University Cleveland, Mississippi 3 8 7 3 3
I
Group Electronegativity by NMR
I t is recognized t h a t as t h e electronegativity of a n atom or group increases, t h e n m r signal for protons attached t o t h e carbon atom bearing the electronegative atom or group shifts t o lower field (higher 6 values) ( I ) . T h e methyl group of chloromethane is found a t 6 = 3.06 compared t o 6 = 2.69 for the methyl group in hromwrhane ( ~ ) . ~ h i s c n r r e l a t i oislinear n in man). systems. When 6 values fur protons attached to carbon atoms hearing a n electroneeativiatom are d o t t e d versus - - ~~electronegativity, a straight line curve is obtained. Barring a n i s o t r o ~ i effects. c t h e 6 values for n-nrotons of similar com~~~~pounds with different electronegative atoms or groups should fall along this line. T h e purpose of this experiment is to allow t h e student t o observe t h e correlation between chemical shift and electronegativity. T h i s is accomplished by obtaining t h e n m r spectra of several readily available organic compounds and plotting the 6 values of the n-protons against the literature value of electronegativity. I n addition, "unknowns" will be run, group electronegativity found and checked aeainst t h e
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T h e correlation between t h e electronegativity of a group (-OH, -SH, etc.) and t h e chemical shift of t h e a-proton does n o t always work because 1) The electronegativity of most groups is not well established. 2) Certain groups ( e N ) possess the so called anisotropic effect, and will not fit a plot hased solely on electronegativity. Barring t h e anisotropic effects one may be able t o observe a t least a qualitative correlation between group electronegativity and chemical shift. Since there is little agreement on t h e "best" method of calculating group electronegativity (3).this terhnique provider a n experime&l mrtho2 of e;aluation which may he satisfactory. Kmpirical relationships between group electronegativity and chemical ihift have been proposed by Dailey and Shoolery (4) and by Muller (5);however, these relationships are n o t completely satisfactory. The samples to be run are all liquids at 25", the m a t volatile biling at 45'; thug they can be run as neat liquids in the nmr spectrometer. The neat liquids can be run in an nmr instrument sensitive enough to differentiate absorptions off 0.05 6 units. The signal to be studied will be the triplet at the lowest field (largest 6 value); for example, in l-bromopropane the triplet of the a-CHI-pups appears at 6 = 3.35 while the M H z - is found at 6 = 1.89 and the CH3 triplet absorbs a t 6 = 1.06 (3). The bvaluerepresented by the center of mass ofthelow field triplet is taken as the 6 value to be plotted as the X-axis in the graph. The Pauling electronegativity values for the various halogen atoms are plotted as the Y-axis. The compounds used in the analysis are: 1-chloropropane, l-bromopropane, 1-iodopropane, l-aminopropane, 1-propanol, l-nitropropane. All of these compounds are easily obtained, and the mast expensive is l-bromopropane ($8.851100g);' therefore, the experiment is relatively inexpensive to run (providing one has the nmr machine). For establishment of astandard curve the chloride, bromide, and iodide are used, since the chemical shift of all @-CHIgroups will be based only on electronegativity differences. The standard curve is obtained bv dottine the 6 value for each of the low field triolets of the alkyl hal& as the-s axiiand therlertronegativity of &h hnlogrn as The Y axls For the I.propyl halidrs the curve is linear. Once the rtandard curve i i estnl,lirhed, the chemical shifts of a-protons in This paper is based on one presented a t the Southeast Regional Meeting of the American Chemical Society in Gatlinburg, Tennessee, on Octoher 28,1976. Fisher Scientific Ca., 1976 Price. 538 1 Journal of Chemical Education
Estimates of the Electronegativnles of Some Common Groups Omup CHa CCls C8r3 NH2 OH CN COOH NO2 CsHs
Experimental methods "Best" a Otherb 2.3 3.0
3.3 2.85 3.4 3.0
Ca1culations X g4
X cC
2.34. 2.63 2.76. 3.03 2.85 1.7.2.99. 3.40 2.3. 3.51, 3.6. 3.89 3.11.3.17.3.22.3.27 2.84,2.88,2.97,3.50 3.2. 3.45. 3.92 3.01, 3.13. 3.18
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2.28 2.83 2.59 2.50 2.86 4.17
2.30 3.19 3.10 2.78 3.42 3.76 3.36 4.32 2.58
2.50
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see Huheey. J. E.. J Phyh. Chem.. 69,3264 (1965); 70. 2086 (1966)fordetails. cCllfffd, A. F.. J Phys Chem.. 63. 1227 (1959). dPa~llngscale value calculated by eledronegatlvny equalization.
"unknowns" such as l-aminopropane can be found and the electronegativity of the group determined in this way cam be checked against the literature values for the groups studied (3)(see table). The electronegativity value obtained from such compounds as l-cyclopropane, obtained in this manner, will not correlate well with the eleetronegativity values found in the literature. This is due to the anisotropic effect on the CN group and can be explained by the instructor. Experimental Using a Pasteur pipet place 0.5 ml of the liquid to be examined in an nmr sample tube and add 2 drops of the TMS standard. Place the sample in the nmr instrument and set the TMS signal at 6 = 0.Adjust the spectrum amplitude so that none of the peaks go off the scale. Run the spectrum of the sample. Repeat this procedure using: l-chloropropane, l-bromopropane, 1-iodopropane, l-propanol, l-aminopropane, and l-nitropropane. From your spectra determine the 6 value for the n-CHI group of each compound. This will be the center of mass of the triplet at the lowest field (highest 6 value). Plot a graph with the electronegativity of each halogen atom as the Y-axis and the chemical shift of the a-protons as the X-axis. Draw the best straight line through the three paints on the graph. After obtaining the standard curve, put the 6 values of the a-protons of l-propanol, l-aminopropane and l-nitropropane on the line and determine the electronegativity of each group by reading the electronegativity on the Y-axis. Compare the value for the eleetronegativity of each group obtained from the curve to that listed in the table. Conclusion This experiment is easily done. inexoensive. and shows the direct correlation of chemical shift and &ectr&eeati&v. .. . It can be used for either organic chemistry when nmr is discussed or in physical rhemistry when structure and elertronegativity are being studied. ~~
LRerature Cited
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