Nitro group charge separation

A good illustration of this concept is seen in a series of nitrophenols. The nitro group itself has to be drawn with charge separation to accommodate the electrons and our rules of bonding. However, resonance structures suggest that there is electron delocalization within the nitro group. 

Analytical Properties Selectivity is based on charge transfer interactions nitroaromatic compounds are separated essentially according to the number of nitro groups, the higher number compounds being most strongly retained when using methanol/water mobile phases Reference 46... 

Suppose it is decided that the valence 5, noncharge-separated representation of the nitro group is to be used throughout the database. The SMIRKS [0 2]=[N+ 1][0- 3] [0 2]=[N+0 1]=[0+0 3], when applied to any charge-separated nitro group will transform it into the proper form. This is accomplished by creating another new SQL function, xform(smiles, smarts). As with the cansmiles and matches functions, this is an extension to standard SQL. Some form of this transformation function is... 

Figure 6.3 A SMIRKS notation specifying the conversion of a charge-separated nitro group to a representation with two double bonds and a 5-valent nitrogen atom. Corresponding atoms on each side have the same number (shown after the colon in the SMIRKS), and the atom attached to the nitro group is shown in the SMIRKS as indicating any atom .

Typical rules include converting nitro groups to the uncharged form (6.8), expanding shortcuts (6.9), replacing dative bonds in coordination compounds with separated charges (6.10), and neutralising zwitterions (6.11). 

Figure 14 represents very logical relationships between impact sensitivity and the net charges of the primarily reacting nitro group here linear condensed polynitro (poly)arenes are sharply separated from spatially con-... 

One notable exception to this rule involves compounds containing the nitro group (—NO2), which have resonance structures with more than two charges. Why We saw earlier that the structure of the nitro group must be drawn with charge separation in order to avoid violating the octet rule ... 

NO2 substitution in meta (compound VI) resulted in lower NC values than the para analog (compound VII) (6.1 vs. 8.1, respectively Table I), resonance processes that would involve ring stabilization of a 4-substituted benzenesulphenyl ion may be implicated separation of inductive and resonance effects is complex and has not been attempted in our analysis. Nitro substitution in the ortho position (compound V) resulted in lesser activation (NC value 2.4 Table I) than expected or seen at the other positions. This effect could result from a stabilizing intramolecular H bond of the 2-nitro group that renders the thiophenol species less acidic, or from an inhibition of the approach to a negatively charged thiolate sulfur due to the... 

We see that the structure of the azido group cannot be drawn without charge separation (much like a nitro group, which also cannot be drawn without charge separation). Nevertheless, the azido group is stiU stabilized by resonance, as seen here ... 

Resonance stabilization of meta substituted toluene. Note that positive charge does not reside directly under the methyl group. This is, therefore, not as low in energy as the ortho and para substituted toluenes. As a result, the main products of nitration of toluene are o-nitro-toluene and p-nitrotoluene. These can be separated to give p-nitrotoluene. The synthesis of p-nitrotoluene from toluene can be summarized as ... 

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