Meta position

Table 3-1. Substituent constants for various groups to be used in Eqs. (1) and (2) (

These and other studies of the relative substituent effects of X and CH X in nitration were considered in terms of the transmission factor a of the methylene group. To avoid complications from conjugative interactions, attention was focussed mainly on substitution at the meta-position, and ct was defined in terms of partial rate factors by the equation ... 

This additivity of incremental indices has enabled the identification of a large number of arylthiazoles variously substituted in the ortho, para, and meta positions of the aromatic ring (174, 175). 

A methyl group is an electron releasing substituent and activates all of the ring carbons of toluene toward electrophilic attack The ortho and para positions are activated more than the meta positions The relative rates of attack at the various positions m toluene compared with a single position m benzene are as follows (for nitration at 25°C)... 

None of the three major resonance forms of the intermediate formed by attack at the meta position has a positive charge on the carbon bearing the —CF3 group... 

Attack at the meta position leads to a more stable intermediate than attack at either the ortho or the para position and so meta substitution predominates Even the inter mediate corresponding to meta attack however is very unstable and is formed with dif ficulty The trifluoromethyl group is only one bond farther removed from the positive charge here than it is m the ortho and para intermediates and so still exerts a significant although somewhat diminished destabilizing inductive effect... 

All the ring positions of (trifluoromethyl)benzene are deactivated compared with benzene The meta position is simply deactivated less than the ortho and para positions The partial rate factors for nitration of (trifluoromethyl)benzene are... 

Figure 12 11 compares the energy profile for nitration of benzene with those for attack at the ortho meta and para positions of (trifluoromethyl)benzene The presence of the electron withdrawing trifluoromethyl group raises the activation energy for attack at all the ring positions but the increase is least for attack at the meta position... 

Since substituent effects in aliphatic systems and in meta positions in aromatic systems are essentially inductive in character, cr and cr values are often related by the expression cr = 0.217cr — 0.106. Substituent effects fall off with increasing distance from the reaction center generally a factor of 0.36 corresponds to the interposition of a —CHj— group, which enables cr values to be estimated for R—CHj— groups not otherwise available. 

The inertness of chlorine in the meta position in Halex reactions is of commercial value. For example, 3,4-dichloronitroben2ene [99-54-7] forms 3-chloro-4-duoronitroben2ene [350-30-1/, which is then reduced to 3-chloro-4-duoroaniline [367-21-5] for incorporation in the herbicide damprop—isopropyl or the duoroquinolone antibacterials, nordoxacin and pedoxacin. 

Many friction material formulations contain 5—15 wt % of friction particles, the granulated cross-linked products of the reaction of CNSL, a phenol substituted at the meta position with a unsaturated side chain, and formaldehyde. Friction particles range in size from 50 to 500 p.m. They reduce frictional wear and increase pedal softness (86). 

A lower alkyl or alkoxy group substituted in the ortho or meta position to an amino group may promote coupling. Good couplers are obtained from dimethylaruline when lower alkyl, lower alkoxy, or both groups are present in the 2- and 5-position. 

Sandmeyer Reaction. This general reaction allows the phenol function to be introduced. The technique complements chlorination insofar as it makes it possible to produce chlorophenols chloriaated in the meta position from the corresponding meta-chlotinated anilines. 

Phenylisothiazole is nitrated predominantly in the meta position of the phenyl group, whereas 4-phenylisothiazole is nitrated ortho and para in the phenyl group <72AHC(14)1). Nitration of 3-phenyl-l,2,4-oxadiazole gives a mixture of m- and p-nitrophenyl derivatives <6301196). 

Under carefully controlled conditions, nitration of 3,5-diphenylisoxazole in AC2O/HNO3 at 20°C gave 4-nitro-3,5-diphenylisoxazole. However, 3,5-diphenylisoxazole in HNO3 underwent nitration first at the para position of the 5-phenyl substituent and then at the meta position of the 3-phenyl group (74KGS597). 

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. 

An electron-accepting substituent A (see above) induces the reverse deshielding in ortho and para positions (-M effect, larger //and shifts ), again with no significant effect on meta positions. 

Anisotropy of molecular motion monosubstituted benzene rings, e.g. phenyl benzoate (44), show a very typical characteristic in the para position to the substituents the CH nuclei relax considerably more rapidly than in the ortho and meta positions. The reason for this is the anisotropy... 

The signal at 5m = 6.79 splits into a doublet of doublets. The larger eoupling (8.7 Hz) indieates a proton in the ortho position, the smaller (2.5 Hz) a further proton in a meta position, and in sueh a way that the ortho proton 5m = 7.97) does not show any additional ortho eoupling. 

Irradiation of solutions of alkenes in benzene or substituted benzenes gives primarily 1 1 adducts in which the alkene bridges meta positions of the aromatic ring. ... 

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