Multisubstituted benzene rings

Moving on to multisubstituted aromatic systems, the real value of Table 5.4 soon becomes apparent. In dealing with such systems, it will not be long before you encounter a 1,4 di-substituted benzene ring. This substitution pattern (along with the 1,2 symmetrically di-substituted systems) gives rise to an NMR phenomenon that merits some explanation - that of chemical and magnetic equivalence and the difference between them. Consider the 1,4 di-substituted aromatic compound shown in Structure 5.1. 

In terms of chemical equivalence, (or more accurately, chemical shift equivalence) clearly, Ha is equivalent to Ha. But it is not magnetically equivalent to Ha because if it was, then the coupling between Ha and Hb would be the same as the coupling between Ha and Hb. Clearly, this cannot be the case since Ha is ortho to Hb but Ha is para to it. Such spin systems are referred to as AA BB systems (pronounced A-A dashed B-B dashed). The dashes are used to denote magnetic non-equivalence of the otherwise chemically equivalent protons. What this means in practise is that molecules of this type display a highly characteristic splitting pattern which would be described as a pair of doublets with a number of minor extra lines and some broadening at the base of the peaks (Spectrum 5.6). 

These extra lines are often mistakenly thought to be impurity peaks. An in-depth understanding of how they may arise is not really necessary for the purpose of interpretation. What is important is that you instantly recognise the appearance of such spin systems. Check that the system integrates correctly and check that the two halves of the system are symmetrical. Note This phenomenon has nothing whatsoever 

Spectrum 5.6 also shows a good example of roofing , which we touched on earlier. If you imagine the simple case of a pair of doublets well separated from each other, then all four of their lines will be of almost equal intensity. But when coupled doublets get closer together, they become distorted so that their inner lines become more intense, and their outer lines less intense. This is the onset of non-first-ordemess . The closer a pair of coupled doublets are to each other, the more extreme the effect becomes. It is worth noting that the phenomenon can sometimes be a useful interpretive tool, as the roofing can indicate which doublet is coupled to which other one, in spectra where you encounter two or more systems of this type doublets which are coupled to each other, always roof towards a point between them, as shown. 

Obviously, there are too many possible combinations of groups for us to show a comprehensive collection of them but Spectrum 5.7 shows a nice example of a 1,3 di-substituted pattern featuring two strongly deshielding groups (a nitro group and a methyl ester) and serves to demonstrate the limitations of Table 5.4. 

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Substituent increments Z, obtained from 13C shifts of numerous monosubstituted benzenes according to eq. (4.15) have been tabulated [383]. They permit prediction of benzene ring carbon shifts in multisubstituted benzenes according to eq. (4.16). These increments and their practical application will be summarized in Section 4.16. 

Friedel Crafts type alkylations of benzene by alkenes involve the initial formation of a lattice associated carbenium ion, formed by protonation of the sorbed olefin. The chemisorbed alkene is covalently bound to the zeolite in the form of an alkoxy group and the carbenium ion formed exists only in the transition state. As would be expected fixjm conventional Friedel Crafts alkylation, the reaction rate over acidic molecular sieves also increases with the degree of substitution of the aromatic ring (tetramethyl > trimethyl > dimethyl > methyl > unsubstituted benzene). The spatial restrictions induced by the pore size and geometry frequently inhibit the formation of large multisubstituted products (see also the section on shape selectivity). 

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