Polysubstituted benzene rings

Since chemical shift increments are approximately additive, it is possible to calculate the ring proton shifts in polysubstituted benzene rings from the monosubstituted values in Appendix Chart D.l. The chemical shift increments for the ring protons of m-diacetylbenzene ... 

Shifts from benzene for polysubstituted benzene ring carbon atoms can, be approximated by applying the principle of substituent additivity. For example, the shift from benzene for C-2. of the disubstituted compound 4-chlorobenzonitrile is calculated by adding the effect for an ortho CN group (+3.6) to that for a meta Cl group (+1.3) ... 

Substituents of polysubstituted benzene rings can be numbered according to their distance from the substituent that imparts a special name to the compound. (9.3)... 

When naming a polysubstituted benzene ring, we will follow the same four-step process used for naming alkanes, alkenes, aUtynes, and alcohols. 

When designing a synthesis for a polysubstituted benzene ring, it is often most efficient to utilize a retrosynthetic analysis, as discussed in Section 12.5. The following example illustrates the process. 

IDENTIFYING DIRECTING EFFECTS FOR DISUBSTITUTED AND POLYSUBSTITUTED BENZENE RINGS... 

Identifying Directing Effects for Disubstituted and Polysubstituted Benzene Rings... 

The second structural property described by the 4ypc index is the substitution pattern on the benzene ring. The value of the 4ypc index increases sharply with the degree of substitution, while in the isomeric classes of substituted benzenes it increases with the proximity of substituents. Thus, this structural parameter has also been found to be very useful in describing activities and properties of polysubstituted benzenes [103], chlorinated benzenes [279], and polychlorinated biphenyls [286]. 

Indene Dihydroindene under Disubstituted and Polysubstituted Benzene Hydrocarbons Indene under Polycyclic Hydrocarbons, Nonaltemant Compounds with Two or Three Fused Rings Indeno(l,2,3-C(i0pyrene under Poiycyciic Hydrocarbons, Nonaltemant Compounds with More Than Five Fused Rings Iodine Iron... 

Unlike alkylation, acylation is controlled easily to give monosubstitution, because once an acyl group is attached to a benzene ring, it is not possible to introduce a second acyl group into the same ring. Because of this, a convenient synthesis of alkylbenzenes starts with acylation, followed by reduction of the carbonyl group with zinc and hydrochloric acid (Section 16-6). For example, propylbenzene is prepared best by this two-step route because, as we have noted, the direct alkylation of benzene with propyl chloride produces considerable amounts of isopropylbenzene and polysubstitution products ... 

The quantum-chemical calculation of charge-transfer states as possible intermediates in electrophilic aromatic substitution reactions, making allowance for solvation effects, has been reviewed.6 It has been shown that a simple scaled Hartree-Fock ab initio model describes the ring proton affinity of some polysubstituted benzenes, naphthalenes, biphenylenes, and large alternant aromatics, in agreement with experimental values. The simple additivity rule observed previously in smaller... 

The effect of one substituent on the reactivity of another is generally similar to that observed in the corresponding polysubstituted benzenes. However, the partial bond fixation in an azole can lead to differential effects in the mutual interactions of substituents, similar to those found in naphthalene where the benzene ring fusion induces bond fixation. 

Benzene. The earliest work in directed metalation was done on the benzene system in the early 1930s (3). That the benzene ring was the most promising system for the exploitation of this reaction appears logical since it was by far the most examined aromatic system at that time. Not only have the largest number of directing substituents been successfully demonstrated for this system, but in all probability the demand for a convenient route to a specific polysubstituted aromatic compound will be highest for this system. In all cases examined, directed metalation of a monosubstituted benzene has yielded almost exclusively ortho metalation, with only ortho-disubstituted products obtained (Reaction 1). With... 

Fig. 3a-b. Distribution of Ring Energy Content for benzenoid hydrocarbons and polysubstituted benzene derivatives. The interquartile ranges for the distribution are 25.9 kcal/mol and 16.1 kcal/mol,... 

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