Electrophilic aromatic substitution structure

Oxygen stabilized carbocations of this type are far more stable than tertiary carbocations They are best represented by structures m which the positive charge is on oxygen because all the atoms have octets of electrons m such a structure Their stability permits them to be formed rapidly resulting m rates of electrophilic aromatic substitution that are much faster than that of benzene... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

Reaction of benzamhde (C6H5NHCC6H5) with chlorine in acetic acid yields a mixture of two monochloro denvatives formed by electrophilic aromatic substitution Suggest reasonable structures for these two isomers... 

Cyclohexadienyl cation (Section 12 2) The key intermediate in electrophilic aromatic substitution reactions It is repre sented by the general structure... 

Electrophilic aromatic substitution reactions are important for synthetic purposes and also are one of the most thoroughly studied classes of organic reactions from a mechanistic point of view. The synthetic aspects of these reactions are discussed in Chapter 11 of Part B. The discussion here will emphasize the mechanisms of several of the most completely studied reactions. These mechanistic ideas are the foundation for the structure-reactivity relationships in aromatic electrophilic substitution which will be discussed in Section 10.2... 

The active electrophile is formed by a subsequent reaction, often involving a Lewis acid. As discussed above with regard to nitration, the formation of the active electrophile may or may not be the rate-determining step. Scheme 10.1 indicates the structure of some of the electrophihc species that are involved in typical electrophilic aromatic substitution processes and the reactions involved in their formation. 

Other matters that are important include the ability of the electrophile to select among the alternative positions on a substituted aromatic ring. The relative reactivity of different substituted benzenes toward various electrophiles has also been important in developing a firm understanding of electrophilic aromatic substitution. The next section considers some of the structure-reactivity relationships that have proven to be informative. 

In general, the reaction between a phenol and an aldehyde is classified as an electrophilic aromatic substitution, though some researchers have classed it as a nucleophilic substitution (Sn2) on aldehyde [84]. These mechanisms are probably indistinguishable on the basis of kinetics, though the charge-dispersed sp carbon structure of phenate does not fit our normal concept of a good nucleophile. In phenol-formaldehyde resins, the observed hydroxymethylation kinetics are second-order, first-order in phenol and first-order in formaldehyde. 

Both pyrimidine and purine aie planai. You will see how important this flat shape is when we consider the structure of nucleic acids. In tenns of their chemistry, pyrimidine and purine resemble pyridine. They are weak bases and relatively unreactive toward electrophilic aromatic substitution. 

Mechanistically it is an electrophilic aromatic substitution reaction. The electrophilic species (4—its exact structure is not known) is generated in a reaction of hydrogen cyanide and hydrogen chloride (gas) and a Lewis acid ... 

Substituent effect, additivity of, 570 electrophilic aromatic substitution and, 560-563 summary of. 569 Substitution reaction, 138 Substrate (enzyme), 1041 Succinic acid, structure of, 753 Sucralose, structure of. 1006 sweetness of, 1005 Sucrose, molecular model of. 999 specific rotation of, 296 structure of, 999 sweetness of, 1005 Sugar, complex, 974 d, 980 L, 980... 

The general approaches for the synthesis of poly(arylene ether)s include electrophilic aromatic substitution, nucleophilic aromatic substitution, and metal-catalyzed coupling reactions. Poly(arylene ether sulfone)s and poly(arylene ether ketone)s have quite similar structures and properties, and the synthesis approaches are quite similar in many respects. However, most of the poly(arylene ether sul-fone)s are amorphous while some of the poly(arylene ether)s are semicrystalline, which requires different reaction conditions and approaches to the synthesis of these two polymer families in many cases. In the following sections, the methods for the synthesis of these two families will be reviewed. 

In the discussion of electrophilic aromatic substitution (Chapter 11) equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are four or five hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases there is only one potential leaving group in a molecule. Therefore attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule. 

Scheme 17 Cyclic orbital interactions at the transition structures of electrophilic aromatic substitutions...

As a result of these substituent-induced polarizations, the complementary conjugative interactions at each ring site become somewhat imbalanced (so that, e.g., the donor-acceptor interaction from C3—C4 to C5—C(, is 23.1 kcal mol-1, but that in the opposite direction is only 16.4 kcal mol-1). From the polarization pattern in (3.133) one can recognize that excess pi density is accumulated at the ortho (C2, C6) and para (C4) positions, and thus that the reactivity of these sites should increase with respect to electrophilic attack. This is in accord with the well-known o, /(-directing effect of amino substitution in electrophilic aromatic substitution reactions. Although the localized NBO analysis has been carried out for the specific Kckule structure of aniline shown in Fig. 3.40, it is easy to verify that exactly the same physical conclusions are drawn if one starts from the alternative Kekule structure. 

These polarizations are seen to be in the opposite direction to those in aniline (3.133), so that higher pi density remains at the Ci (junction) and C3 and C5 (meta) positions. These polarity shifts are again consistent with the well-known m-directing effect of nitro substituents in electrophilic aromatic substitution reactions, and the results are again quite independent of which starting Kekule structure is selected for the localized analysis.63... 

The figure represents the chemical structure for paracetamol, which includes the N-(4-hydroxyphenyl) acetamide, derived from the interaction of p-aminophenol and an aqueous solution of acetic anhydride. The structure has two activating groups that make the benzene ring highly reactive toward electrophilic aromatic substitution. 

Nixon model, have been abandoned in all modern views of aromatic structure. Thus, given our present views of the structure of benzene, and the mechanism of electrophilic aromatic substitution, the Mills-Nixon hypothesis has no meaning. Nonetheless, the legacy of this hypothesis doggedly persists in our research discussions and should be laid to rest. ... 

The structural theory of organic chemistry was developed in the last half of the nineteenth century. It led to the concept that chemical, physical and biological properties of all kinds must be a function of structural change. The earliest structure-property relationships (SPR) were qualitative. Examples are the directional effect of substituents on the benzene ring with respect to electrophilic aromatic substitution and orientation in... 

This chapter is concerned with reactions that introduce or replace substituent groups on aromatic rings. The most important group of reactions is electrophilic aromatic substitution. The mechanism of electrophile aromatic substitution has been studied in great detail, and much information is available about structure-reactivity relationships. There are also important reactions which occur by nucleophilic substitution, including reactions of diazonium ion intermediates and metal-catalyzed substitution. The mechanistic aspects of these reactions were discussed in Chapter 10 of Part A. In this chapter, the synthetic aspects of aromatic substitution will be emphasized. 

An equally important general type of synthesis which proceeds via heterocyclization with formation of a ring bond y to the heteroatom involves acid-catalyzed intramolecular electrophilic aromatic substitution, especially those in which a carbonyl group functions as the electrophile. The most common structural requirements are summarized in (18)-(2l) ... 

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