Resonance structures pyridine

Acridine is a heterocyclic aromatic compound obtained from coal tar that is used in the synthesis of dyes. The molecular formula of acridine is C13H9N, and its ring system is analogous to that of anthracene except that one CH group has been replaced by N. The two most stable resonance structures of acridine are equivalent to each other, and both contain a pyridine-like structural unit. Write a structural formula for acridine. 

It is interesting to speculate why the basicities of the substituted pyridines are well correlated by normal a-values, rather than by a+-values. Writing the resonance structures for the conjugate acid (5<->6)... 

This improves reactivity towards electrophiles. Consideration of resonance structures shows positions 2, 4, and 6 are now electron rich. Nitration of pyridine A-oxide occurs at C-4 very little 2-nitration... 

The pyridine nucleotides NAD and NADP always function in unbound form. The oxidized forms contain an aromatic nicotinamide ring in which the positive charge is delocalized. The right-hand example of the two resonance structures shown contains an electron-poor, positively charged C atom at the para position to nitrogen. If a hydride ion is added at this point (see above), the reduced forms NADH or NADPH arise. No radical intermediate steps occur. Because a proton is released at the same time, the reduced pyridine nucleotide coenzymes are correctly expressed as NAD(P)H+HT... 

Electron spin resonance. Structure of pyridine derivatives with unpaired electrons NMR proton signals in pyridine... 

The behavior of pyridine in substitution reactions can be understood on the basis of its resonance structures (la-d) and on the basis of the electron-density distribution at the various ring positions as derived from molecular-orbital-theoretical calculations, An example of the published pi-electron density distribution is shown in II, The resonance energy of pyridine is 35 kcal/mole (versus 39 kcal/mole for benzene). 

When substitution occurs at C-6 or C-7, only one resonance structure that retains an aromatic pyridine ring is possible. 

Electrophilic attack on pyridine at the 2-position gives an unstable intermediate, with one of the resonance structures showing a positive charge and only six electrons on nitrogen. In contrast, electrophilic attack at the 3-position gives a more stable intermediate with the positive charge spread over three carbon atoms and not on nitrogen. 

When the imidazole ring is considered to be something resembling a pyrrole-pyridine combination (1) it would appear that any electrophilic attack should take place preferably at C-5 (pyrrole-or, pyridine-j8). Such a model, though, fails to take account of the tautomeric equivalence of C-4 and C-5 (Section 4.06.5.1). The overall reactivities of imidazole and benzimidazole can be inferred from sets of resonance structures in which the dipolar contributors have finite importance (Section 4.06.2) or by mesomeric structures such as (2). These predict electrophilic attack in imidazole at N-3 or any ring carbon atom, nucleophilic attack at C-2 or C-1, and also the amphoteric nature of the molecule. In benzimidazole the acidic and basic properties, the preference for nucleophilic attack at C-2 and the tendency for electrophiles to react at the fused benzene ring can be readily rationalized. 

Attempts to correlate reaction mechanisms, electron density calculations and experimental results have met with only limited success. As mentioned in the previous chapter (Section 4.06.2), the predicted orders of electrophilic substitution for imidazole (C-5 > -2 > -4) and benzimidazole (C-7>-6>-5>-4 -2) do not take into account the tautomeric equivalence of the 4- and 5-positions of imidazole and the 4- and 7-, 5- and 6-positions of benzimidazole. When this is taken into account the predictions are in accord with the observed orientations of attack in imidazole. Much the same predictions can be made by considering the imidazole molecule to be a combination of pyrrole and pyridine (74) — the most likely site for electrophilic attack is C-5. Furthermore, while sets of resonance structures for the imidazole and benzimidazole neutral molecules (Schemes 1 and 2, Section 4.06.2) suggest that all ring carbons have some susceptibility to electrophilic attack, consideration of the stabilities of the expected tr-intermediates (Scheme 29) supports the commonly observed preference for 5- (or 4-) substitution. In benzimidazole attack usually occurs first at C-5 and a second substituent enters at C-6 unless other substituent effects intervene. 

Pyridine is a heterocycle containing a six-membered ring with three n bonds and one nitrogen atom. Like benzene, two resonance structures can be drawn. 

Pyridine, like benzene, has six Jl-electrons. The electron-withdrawing nitrogen atom deactivates the ring, and electrophilic substitution is slower than that for benzene. Substitution occurs principally at the 3-position of the ring, as attack at the 2-/4-position produces less stable cation intermediates (i.e. with one resonance structure having a positive charge on the divalent nitrogen). 

Structure of pyridine resonance contributors (mesomeric structures)... 

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