P position

The most important reaction of the diazonium salts is the condensation with phenols or aromatic amines to form the intensely coloured azo compounds. The phenol or amine is called the secondary component, and the process of coupling with a diazonium salt is the basis of manufacture of all the azo dyestuffs. The entering azo group goes into the p-position of the benzene ring if this is free, otherwise it takes up the o-position, e.g. diazotized aniline coupled with phenol gives benzeneazophenol. When only half a molecular proportion of nitrous acid is used in the diazotization of an aromatic amine a diazo-amino compound is formed. 

The rearranging entity has been shown to be the bivalent cation the adjacent charges may so weaken the N—N link that charges of nearly integral size may be built up in the 4 and 4 positions. In the bent, but strainless, cation the minimum separation of the two p-positions would suffice for the establishment of a lai ely electrostatic bond, which could pass smoothly into the covalent rearrangement product (benzidine). 

In unsaturated molecules electronic effects can be transmitted by mesomerism as well as by inductive effects. As with the latter, the mesomeric properties of a group are described by reference to hydrogen. Groups which release electrons to the unsaturated residue of the molecule are said to exert a +Af effect, whereas groups which attract electrons are said to exert a —Af effect. In aromatic structures the important feature of an M-substituent is that it influences the 0- and p-positions selectively. 

An alkyl group activates all nuclear positions, the 0- and p-positions more than the r-position. The activation is not very strong. 

COMPARISON BETWEEN ir NET CHARGES OF NITROGEN AND CARBON ATOMS IN THE a- AND p-POSITIONS IN PYRIDINE AND THIAZOLE (PPP-ir METHOD)... 

The direct reaction of other alkyl chlorides, such as butyl chloride, results in unacceptably low overall product yields along with the by-product butene resulting from dehydrochlorination. AH alkyl haHdes having a hydrogen atom in a P- position to the chlorine atom are subject to this complication. 

The di- or tettahydto-l,4-thiazine skeleton is obtained if mercaptans which have a carbonyl group in the P-position react with ethyleneimine (120—124)... 

Maleic anhydride [108-31 -6] (1), maleic acid [110-16-7] (2), and fumaric acid [110-17-8] (3) are multiftmctional chemical iatermediates that find appHcations in nearly every field of industrial chemistry. Each molecule contains two acid carbonyl groups and a double bond in the a, P position. Maleic anhydride and... 

The two carbons that bear no numbers are common to both rings and carry no hydrogen atoms. From the symmetrical configuration of the naphthalene molecule, it should be possible for only two isomers to exist when one hydrogen atom is replaced by another atom or group. Therefore, positions 1, 4, 5, and 8 are identical and often are designated as "a" positions likewise, positions 2, 3, 6, and 7 are identical and are designated as "P" positions, as shown in (2). 

The contributions from the canonical forms have been calculated, and the contributions from the equivalent polar stmctures (2) and (3) dominate those of (4) and (5) (6). Thus, electrophilic substitution is predicted to occur in the a-position, and this is proven experimentally in most cases. Nitrosation and selenocyanation occur at the P-position (7,8). 

The following rules pertain to electrophilic substitution in pyrroles (35) (/) an electron-withdrawing substituent in the a-position directs substitution to the P and a -positions, (2) an electron-releasing substituent in the a-position directs substitution to the neighboring -position or to the a -position, (J) an electron-withdrawing substituent in the -position leads to substitution in the a -position, and (4) an electron-releasing substituent in the P-position tends to direct substitution into the neighboring a-position. 

Analytical and Test Methods. In addition to the modem spectroscopic methods of detection and identification of pyrroles, there are several chemical tests. The classical Runge test with HCl yields pyrrole red, an amorphous polymer mixture. In addition, all pyrroles with a free a- or P-position or with groups, eg, ester, that can be converted to such pyrroles under acid conditions undergo the Ehrlich reaction with p-(dimethylamino)henzaldehyde to give purple products. 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

The 8 Hz coupling indicates a proton in the y-position (B) the 5 Hz coupling locates a vicinal proton in position a (C), the additional 0.9 Hz coupling locates the remaining proton in position a (D) and thereby the p-position of the substituent. 

Protons of substrueture B and C are assigned by means of the mesomerie effeet of the aldehyde group whieh deshields the protons in o-position of the attaehed p-disubstituted benzenoid ring and in p-position of the eentral CC double bond ort/io-protons of the monosubstituted benzenoid ring D split into a doublet beeause of one ortho eoupling ( 7.5 Hz) while the meta-protons split into a triplet beeause of two ortho eouplings. 

In general it is considered essential that the bulk of the phenol used initially should not be substituted, i.e. should be reactive, at the o- and p-positions and is thus trifunctional with respect to the reaction with formaldehyde. 

In addition to the above possible mechanisms the possibility of reaction at w-positions should not be excluded. For example, it has been shown by Koebner that o- and p-cresols, ostensibly difunctional, can, under certain conditions, react with formaldehyde to give insoluble and infusible resins. Furthermore, Megson has shown that 2,4,6-trimethylphenol, in which the two ortho- and the one para-positions are blocked, can condense with formaldehyde under strongly acidic conditions. It is of interest to note that Redfam produced an infusible resin from 3,4,5,-trimethylphenol under alkaline conditions. Here the two m- and the p-positions were blocked and this experimental observation provides supplementary evidence that additional functionalities are developed during reaction, for example in the formation of quinone methides. 

Resorcinol (1,3-dihydroxybenzene) is more reactive to formaldehyde than phenol itself, the two hydroxyl groups reinforcing each other in activating the o-and p-positions. This allows faster reactions to occur than with phenol, when compared at the same temperature, and under appropriate conditions curing can occur at normal room temperatures. 

Dibromoquinone-4-chloroimide, in whose stead 2,6-dichloroquinone-4-chlo-roimide (q.v.) may also be employed, reacts primarily with phenols and anilines which are not substituted in the p-position. 

Alternatively, the pyruvic acid can first condense with the aldehyde. Addition of the aniline to the P-position of 18 provides the same intermediate (13), as above. The mechanism could be substrate dependent. 

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