Substituents with Unshared Electrons

Substituents that carry nonbonding electrons (n electrons) can cause shifts in the primary and secondary absorption bands. The nonbonding electrons can increase the length of the 7t system through resonance. 

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The more available these n electrons are for interaction with the r systan of the aromatic ring, the greater the shifts will be. Examples of groups with n electrons are the amino, hydroxyl, and methoxy groups, as well as the halogens. 

Such an excited state is often called a charge-transfer or an electron-transfer excited state. 

In compounds that are acids or bases, pH changes can have very significant effects on the positions of the primary and secondary bands. Table 10.10 illustrates the effects of changing the pH of the solution on the absorption bands of various substituted benzenes. In going from benzene to phenol, notice the shift from 203.5 to 210.5 nm—a 7-nm shift—in the primary band. The secondary band shifts from 254 to 270 nm—a 16-nm shift. However, in phenoxide ion, the conjugate base of phenol, the primary band shifts from 203.5 to 235 nm (a 31.5-nm shift), and the secondary band shifts from 254 to 287 nm (a 33-nm shift). The intensity of the secondary band also increases. In phenoxide ion, there are more n electrons, and they are more available for interaction with the aromatic k system than in phenol. 

The comparison of aniline and anUitiium ion illustrates a reverse case. Aniline exhibits shifts similar to those of phenol. From benzene to anihne, the primary band shifts from 203.5 to 230 nm (a 26.5-nm shift), and the secondary band shifts from 254 to 280 nm (a 26-nm shift). However, these large shifts are not observed in the case of anilinium ion, the conjugate acid of aniline. For anilin-ium ion, the primary and secondary bands do not shift at all. The quaternary nitrogen of anilinium ion has no unshared pairs of electrons to interact with the benzene n system. Consequently, the spectrum of anihnium ion is almost identical to that of benzene. 

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In spite of their efficiency, in most cases hydrogen fluoride/pyridine mixtures are not suitable for the thermal fluorodediazoniation of ort/io-substituted anilines, especially those in which the substituent bears a heteroatom with unshared electron pairs (halogen, methoxy, etc.). 

Similar effects are observed for benzene derivatives in which the substituent has unshared electron pairs that can be conjugated with the benzene ring (e.g., —NH2, —OH, —Cl ). An unshared electron pair is to some extent delocalized to become a part of the aromatic 77-electron system in both the ground and excited states, but more importantly in the excited state. This is... 

Heteroatoms with unshared electron pairs can also interact with adjacent ct bonds. For example, oxygen and nitrogen substituents substantially weaken an adjacent (geminal) C—H bond. 

The absolute configuration at sulfur is specified by the Cahn-Ingold-Prelog method with the provision that the unshared electron pah is considered to be the lowest ranking substituent. 

The parameters of Table V show many of the trends previously recognized (2i, 4, 10, 19). The substituents (-/ ) having a first row element with an unshared electron pair as first atom (F, OCsH , OM2, NHCOMe, NH2 and NMe2) show enhanced pi delocalization across the scales i.e., -a < °R increment of the scales is greater between... 

With substituents such as OH and OMe that have unshared electron pairs, an electron-donating, i.e. base-strengthening, mesomeric effect can be exerted from the o- and p-, but not from the m-position, with the result that the p-substituted aniline is a stronger base than the corresponding w-compound. The m-compound is a weaker base than aniline itself, due to the electron-withdrawing inductive effect exerted by the oxygen atom in each case. As so often, the effect of the o-substituent remains somewhat anomalous, due to direct interaction with the NH2 group by both steric and polar effects. The substituted anilines are found to have related pAa values as follows ... 

Involvement of AModo species in electrophilic C-iodinations needs to be considered since a number of imidazoles are known to form such compounds in basic medium. Charge-transfer complexes, too, are quite well known. They seem to be of the n -type through the unshared electron pair at N-3. Equilibrium constants for their formation are known to increase regularly in line with electron-donating powers of substituents (or vice versa). Some KCT values at 20°C (L M are imidazole (200), 1-methylimidazole (333), 1,2-dimethylimidazole (1165), 4-phenylimidazole (152), and 4,5-diphenylimidazole (141) (83BSB923). The charge-transfer complexes formed between iodine and imidazole-2-thiones appear to involve the sulfur atoms (88JA2586). 

The polarity alternation rule (PAR) considers two kinds of substituents. The donors are. those having unshared electronic pairs or -electrons, and +1 groups. These include OH, OR, OCOR, NH2, NRR, N(R)COR, SH, SR, halogens and alkyl groups. The donor properties of the alkyl groups may reflect the existence of hyperconjugation. On the other hand, the acceptors are electron sinks, i.e. polarizable it-bonds, atoms with empty orbitals, and —I groups. Examples of acceptors are C=0 (aldehydes, ketones, carboxylic acid derivatives), CN, S02, N02, SiRj. 

Such substituents are relatively uncommon, and most of the reported H—X additions have been carried out with Y groups having unshared electron pairs on an atom connected directly to a carbon of the double bond ... 

Azo dyes with ionizable groups, or substituents with an unshared pair of electrons, next to the azo linkage form complexes with transition metals.103 Many such complexes have been described, and many have been used as fabric dyes because of their superior light stability.104... 

On the other hand, for RCXJ and R2CF+ the cation stability increases along with the increase of resonance (crR) effect of a halogen F > Cl > Br > I [59]. The significant stabilizing effect of fluorine substituent was explained as a result of back-donation of an unshared electron pair of F on the vacant orbital of carbon. Stability of substituted fluoromethyl cations in gas phase increases going from CF3 to 12 [15] ... 

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