An Improved Experiment To Illustrate the Effect of Electronegativity on Chemical Shift
Downloaded by UNIV OF PRINCE EDWARD ISLAND on August 31, 2015 | http://pubs.acs.org Publication Date: September 1, 1988 | doi: 10.1021/ed065p819
RobM K. Boggess Radford University, Radford, VA 24142 In an article by Greever' and in an experiment in the laboratory manual by Sawyer et aL2,nuclear magnetic resonance (NMR) is used to ohserve the effect of electronegativity on the chemical shift of protons in similar compounds. In the article in this Journal, the NMR spectra of several 1propane derivatives are obtained, and the correlation between electronegativity of the substituent and the chemical shift of the C-1 protons is observed. In the experiment described by Sawyer et al.=, the NMR spectra of the ethyl halides are recorded, and the chemical shift values and coupling constants for both sets of protons are obtained. The electronegativity of the halide is plotted versus the chemical shifts of both sets of protons, and one can observe the expected effect of electronegativity on the chemical shift of the methylene protons, but no trend is apparent for the methyl protons. Onedifficulty with thisret of halides is the fact that ethyl chloride is a gas a t room temperature, and, while a solution can be prepared, it is not convenient todo this; thus. the data set is reduced to two samples. Our experience has shown that good substitutes are the 1,3-dihalopropanes. All are liquids soluble in common organic solvents (we use dCHCId. ,.. and are available commerciallv in hieh - .~ u r i-t va t a reasonable cost. The same trpnds in chemical shifts evident frum the ethyl halides are also ohserved with the 1,3-dihalopropanes (see the tableand Fig. 1 J. Additionally, l-hromo-3chloroorouane. which also has the desirable features above, provides an interesting spectrum for interpretation hy stu: dents. Upon careful inspection of this spectrum (Fig. 2), the signal near 3.6 ppm can be seen to be two, overlapping triplets corresponding to the nonequivalent protons on the 1.3-carbons. Inspection of the data in the table shows that the two triplets in the l-bromo-3-chloropropanecan be identified by their chemical shift and that the chemical shift of the C-2 protons has a value intermediate between 1,3-dihromopropane and 1,3-dichloropropane; however, the appearance of the signal for the C-2 protons is nearly identical regardless of the compound. Data obtained with our 60-MHz instrument and data treatment and sample questions follow.
bromo-3-chloiopropane relative to those of the 1.3-dibromopropane and 1,3-dichloropropane. Is there any correlation between the electronegativity of the suhstituent and the coupling constants? Do the integration ratios agree with
Figure 1. A plot of the chemical shin (ppm) versus me electronegativltyof (he halogen for me 13dihalapropanes.(0:C-1.3 protons; 0: G Z protons1
Treatment of Data From the spectra, obtain the delta, tau, and coupling constants for each set of protons for each compound, and tabulate the results. For each set of protons, plot chemical shift (ppm) versus electronegativity of the halide. Samole Questions What conelusionscan he made with respect Lo theeffect of electronegativity of the suhstituents on the chemical shifts of the adjacent protons? Is the effect of electronegativity transmitted to the protons on the adjoining carhon? Kxplain the appearance of the spectra for the 1,s-dihalopropanes. Why is the suectrum ofthe 1-hromo-3-chlorouropane diff'erentfrom thk others near 3.5 ppm but verisimilar to the others near 2.2 ppm? Comment on the delta values of the 1-
' Greever, J. C . J. Chm. Educ
1978. 55. 538.
Sawyer. D. S.; Heineman. W. R.: Beebe, J. M. ChemistvExperC ments for lnslrumental Melhods: Wiley: New York, 1984.
Figure 2. The NMR spectrum of -3.5 ppm are lndlcated.
Volume 65
l-bromP3-chloropropane. The two triplets at
Number 9
September I986
818
Br, CI
+H2X + H r
3.69 3.55 2.25
6.31 6.45 7.75
6-7 6-7 6-7
3.8 (total)
2.0
Huhsey. Jams E. mwgegek Chomlshy. RI11~1plos of SlruclursandReact~itx3rd sd.:H w r and Row: New Y a k , 1983 a values campare favotably to nose reported by ~riedrich,E. ~unkle,K. G. J. Cham. ~ d u c1984. . 61,830. Normalized to 2.0 for G2 protons.
Downloaded by UNIV OF PRINCE EDWARD ISLAND on August 31, 2015 | http://pubs.acs.org Publication Date: September 1, 1988 | doi: 10.1021/ed065p819
me
c.;
those predicted from a consideration of the structure of the molecules? Partial support for this work has been provided by the
820
Journal of Chemical Education
National Science Foundation's College Science Instrumentation Program, Grant #CSI-8551905. The author thanks Anna Sequeira for help in recording the NMR spectra.
