Meta-substituted compounds, spectra

An example of an aromatic fluorine-containing compound can be found in Figure 6.7, where we have recorded the l9F spectra (both proton-coupled and decoupled) of fluorobenzene along with the H and l3C spectra. Once again we find a singlet for the fluorine atom in the ptoton-decoupled spectrum and a complex multiplet for the fluorine atom in the proton-coupled spectrum. The fluorine atom couples differently to the ortho-, meta-, and para-protons in this mono-substituted compound. Coupling constants for proton-fluorine can be found in Appendix F of Chapter 3. 

Chemical shift increments are the shifts from that of the protons of benzene (8 7.27). Thus for a CH C=0 substituent (line 26, Appendix Chart D.l), the ortho increment is +0.63, and the meta and para increments are both +0.28 (+ being at higher frequency than 8 7.27). The C-2 proton has two ortho substituents the C-4 and C-6 protons are equivalent and have ortho and para substituents the C-5 proton has two meta substituents. Thus the calculated increment for C-2 is + 1.26, for C-4 and C-6 is +0.91, and for C-5 is +0.56. The spectrum shows increments of +1.13, + 0.81, and +0.20, respectively. This agreement is adequate for determining the substitution pattern. Integration of the spectrum would show the expected ratio of 1 2 1. Furthermore, both the ortho- (with identical substituents) and para-substituted compounds would show the characteristic, symmetrical patterns of Figures... 

The spectrum shown in Figure 3.32c shows a complex second-order pattern for the protons. The same is true of meta-substituted rings. Figure 3.32d. Compounds such as o-dichlorobenzene and m-dichloro-benzene have a plane of symmetry, but as seen in Figure 3.32, the splitting patterns can be complex. 

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