Amino group electronic effects

To test the effectiveness of the new CS INDO-based procedure we searched for compounds exhibiting large solvent effects on their structural and optical properties. From this point of view merocyanine dyes are most suitable in view of the dramatic dependence of their UV/vis spectra on the solvent polarity [25,26]. The current interpretation of this behaviour is based on the fact that merocyanines are formed by an amino group (electron donor) and a carbonyl... 

Although nitrenes are isoelectronic with carbenes there are important differences in their electronic structures, which accounts for their different reactivity. Chemically nitrenes behave similarly to carbenes, that is, they add to carbon-carbon double bonds and insert into C-H single bonds. In addition they isomerize to imines, abstract a hydrogen atom to form primary amino groups, and effect ring closures. 

The resonance effect of the amino group is far more important than its inductive effect in electrophilic aromatic substitution, and this resonance effect makes the amino group electron releasing. 

Ultraviolet. Benzene has a series of relatively low intensity absorption bands in the region of 230 to 270 nm. When there is a substituent on the ring with nonbonding electrons, such as an amino group, there is a pronounced increase in the intensity of these bands and a shift to longer wavelength. Aniline shows an absorption band at 230 nm (e = 8600) and a secondary band at 280 nm (e = 1430). Protonation of the amino groups reduces these effects and the spectmm resembles that of the unsubstituted benzene. 

As might be expected from a consideration of electronic effects, an amino substituent activates pyrazines, quinoxalines and phenazines to electrophilic attack, usually at positions ortho and para to the amino group thus, bromination of 2-aminopyrazine with bromine in acetic acid yields 2-amino-3,5-dibromopyrazine (Scheme 29). 

A mechanism has been proposed to rationalize the results shown in Figure 23. The relative proportion of the A -pyrazolines obtained by the reduction of pyrazolium salts depends on steric and electronic effects. When all the substituents are alkyl groups, the hydride ion attacks the less hindered carbon atom for example when = Bu only C-5 is attacked. The smaller deuterohydride ion is less sensitive to steric effects and consequently the reaction is less selective (73BSF288). Phenyl substituents, both on the nitrogen atom and on the carbon atoms, direct the hydride attack selectively to one carbon atom and the isolated A -pyrazoline has the C—C double bond conjugated with the phenyl (328 R or R = Ph). Open-chain compounds are always formed during the reduction of pyrazolium salts, becoming predominant in the reduction of amino substituted pyrazoliums. 

Notice that the MO picture gives the same qualitative picture of the substituent effects as described by resonance structures. The amino group is pictured by resonance as an electron donor which causes a buildup of electron density at the /3 carbon, whereas the formyl group is an electron acceptor which diminishes electron density at the /3 carbon. 

Fig. 4.7 shows the titration with perchloric acid of a mixture of piperidine, ethylenediamine and p-toluidine. Fig. 4.8 illustrates the effect of different chain lengths on the titration (with perchloric acid) of diamines in nitrobenzene containing 2.5% (v/v) of methanol once the first amino group of EDA has been protonated, the resulting proton bridge with the lone pair of electrons of the second amino group lowers the basicity of the latter considerably the effect decreases on the introduction of more intermediate CH2 groups until complete disappearance when six are present. 

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