Substituents benzene ring

No simple examples are known of electrophilic or radical substitution of either heterocyclic or homocyclic rings of benzopyrylium salts flavylium and l-phenyl-2-benzopyrylium salts nitrate in the substituent benzene ring. Having said this, the cyclisation of coumarin-4-propanoic acid may represent Friedel-Crafts type intramolecular attack on the carbonyl-O-protonated form i.e. on a 2-hydroxy-1-... 

In the absence of activating hydroxy or amino substituents, benzene rings are attacked only slowly by oxidizing agents such as chromic acid or permanganate,... 

A point in case is provided by the bromination of various monosubstituted benzene derivatives it was realized that substituents with atoms carrying free electron pairs bonded directly to the benzene ring (OH, NH2, etc) gave 0- and p-substituted benzene derivatives. Furthermore, in all cases except of the halogen atoms the reaction rates were higher than with unsubstituted benzene. On the other hand, substituents with double bonds in conjugation with the benzene ring (NO2, CHO, etc.) decreased reaction rates and provided m-substituted benzene derivatives. 

In such tables, typical chemical shifts are assigned to standard structure fragments (e.g., protons in a benzene ring). Substituents in these blocks (e.g., substituents in ortho, meta, or para positions) are assumed to make independent additive contributions to the chemical shift. These additive contributions are listed in a second series of tables. Once the tables are defined, the method is easy to implement, does not require databases, and is extremely fast. Predictions for a molecule with 50 atoms can be made in less than a second. On the other hand, it requires that the parent structure and the substituents are tabulated, and it considers no interaction... 

The free radical mechanism is confirmed by the fact that if a substituted aromatic hydrocarbon is used in this reaction, the incoming group (derived from the diazotate) may not necessarily occupy the position in the benzene ring normally determined by the substituent present—a characteristic of free radical reactions. 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

Coumarin is usually prepared by heating salicylaldehyde with acetic anhydride and sodium acetate (i.e., the Perkin cinnamic acid synthesis, p. 23 6), whereby the 0" hydroxy-cinnamic acid (I) undergoes cyclisation to coumarin. Coumarins having substituents in the benzene ring can often be similarly prepared. 

Since the original observations of Vorlander, it has been recognised that positively charged substituents directly attached to the benzene ring are dominantly m-directing. Vorlander examined the nitration... 

Substituents containing boron are of interest because of the possibility which the boron atom offers of conjugation of a vacant orbital with the 77-electrons of the benzene ring (—717). The case of phenylboronic acid has been discussed ( 5.3.4). 

Katritzky and Topsom have reviewed the information available, largely from infrared and n.m.r. studies, concerning the distortion of the tt-electron system in the benzene ring brought about in the ground state by substituents. Of particular interest is the observation that both n.m.r. studies (of m- F and chemical shifts) and infrared investigations (of the intensities of bands due to certain skeletal vibrations) suggest that the value of Taft s [Pg.226]

Laurino examined a similar method in which methanesulfonanilides were alkylated with bromoacetaldehyde diethyl acetal and then cyclized with TiCU[4J. 1 hese methods presumably involve generation of an electrophilic intermediate from the acetal functionality, followed by an intramolecular Friedel-Crafts reaction. As a consequence, the cyclization is favoured by ER substituents and retarded by EW groups on the benzene ring. 

As in the case of thiazole and the alkylthiazoles, cleavage of the thiazole ring takes place at the 1,2 and 3,4 bonds, confirmed by a metastable peak. The other important peaks result from fragmentation of thiirenium ion, in the case of 4- and 5-phenylthiazole and of the phenyl ring. These latter are generally present in the spectra of all comptmnds with benzene-ring substituents, they occur at m/e 77, 76, 75, 51, 50, 39 (124). The ion m/e 45 (HCS" ) is always present. 

The mass spectra of arylthiazoles with funcmonal substituents cm the benzene ring have also been studied (125, 126). They possess the fragmentation pattern of the aromatic derivative corresponding to the substituent together with that of the thiazole ring described previously (126). 

All compounds that contain a benzene ring are aromatic and substituted derivatives of benzene make up the largest class of aromatic compounds Many such compounds are named by attaching the name of the substituent as a prefix to benzene... 

When the benzene ring is named as a substituent the word phenyl stands for CgH5— Similarly an arene named as a substituent is called an aryl group A benzyl group IS CgHjCH,—... 

When a benzene ring bears two or more substituents both its reactivity and the site of further substitution can usually be predicted from the cumulative effects of its substituents In the simplest cases all the available sites are equivalent and substitution at any one of them gives the same product... 

Section 12 16 The order m which substituents are introduced onto a benzene ring needs to be considered m order to prepare the desired isomer in a multistep syn thesis... 

Substituents that are otherwise accessible only with difficulty such as fluoro lodo cyano and hydroxyl may be introduced onto a benzene ring... 

The position of substituents is indicated by numbers, with the lowest locant possible given to substituents. When a name is based on a recognized trivial name, priority for lowest-numbered locants is given to substituents implied by the bivial name. When only two substituents are present on a benzene ring, their position may be indicated by o- ortho-), m- meta-), and p- (para-) (and alphabetized in the order given) used in place of 1,2-, 1,3-, and 1,4-, respectively. 

Derivatives. Many derivatives of acylated phlorogluciaols that bear a benzene ring substituent or an ether or glycoside linkage occur ia aature. Examples are cotoia [479-21-0] (52) ia coto bark and conglomerone [480-25-1] (53) ia Eucalyptus conglomerata. 

Polymethylbenzenes (PMBs) are aromatic compounds that contain a benzene ring and three to sis methyl group substituents (for the lower homologues see Benzene Toluene Xylenes and ethylbenzene). Included are the trimethylbenzenes, (mesit iene (1), pseudocumene (2), and hernimeUitene (3)),... 

Replacement of the benzene ring by other ring systems, or the introduction of additional substituents, decreases or aboHshes activity. 

Carbon Substituents. Alkyl groups at positions 2 and 4 of a pyridine ring are more reactive than either those at the 3-position of a pyridine ring or those attached to a benzene ring. Carbanions can be formed readily at alkyl carbons attached at the 2- and 4-positions. This increased chemical reactivity has been used to form 2- and 4-(phenylpropyl)pyridines, eg, 4-(3-phenylpropyl)pyridine [2057-49-0] (21) (24). 

The entrance of a third or fourth substituent can be predicted by Beilstein s rule. If a substituent Z- enters into a compound C H XY, both X and Y exert an influence, but the group with the predominant influence directs Z- to the position it will occupy. Since all meta-directing groups are deactivating, it follows that ortho—para activating groups predominate when one of them is present on the benzene ring. 

FULLY CONJUGATED RINGS REACTIVITY OF SUBSTITUENTS 2.12.4.1 Fused Benzene Rings... 

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