Naphthalene, alternant substitution

With naphthalene, electrophilic substitution (e.g. nitration) is found to take place preferentially at the 1- (a-), rather than the alternative 2- (/ -), position. This can be accounted for by the more effective delocalisation, and hence stabilisation, that can take place in the Wheland intermediate for 1 - attack (60a - 606) compared with that for 2-attack (61) ... 

In an alternative method, the senior component is regarded as substituted by RN=N—, this group R being named as a radical. Thus 2-(7-phenylazo-2-naphthylazo)anthracene is the name by this alternative method for the compound named anthracene-2-azo-2 -naphthalene-7 -azobenzene. 

In alternant hydrocarbons (p. 55), the reactivity at a given position is similar for electrophilic, nucleophilic, and free-radical substitution, because the same kind of resonance can be shown in all three types of intermediate (cf. 20,22, and 23). Attack at the position that will best delocalize a positive charge will also best delocalize a negative charge or an unpaired electron. Most results are in accord with these predictions. For example, naphthalene is attacked primarily at the 1 position by NOj, NHJ, and Ph, and always more readily than benzene. 

In contrast to 1, the related pure host 7 may be obtained in crystalline form 68). The crystal structure of 7 is built via helical chains of alternating intra- and inter-molecular H-bonding through the carboxyl functions. This structure supplies the information that the carboxyl groups are therefore already positioned in an appropriate way to facilitate analogous H-bonding in the known inclusions of 7. As discussed later (Sect. 4.2.2), these are exclusively salt-type associates and as such, intimately interact with the carboxyl groups. Hence one may infer that displacement of the carboxyl functions from position 2 in 1 to position 8 in 7 reduces the ability of inclusion formation. Similar reasons such as the solid-solubility differences observed in the classical naphthalene/chloronaphthalene systems (alpha- vs. beta-substituted derivatives, cf. Ref. 28 may also be applied here. 

The sulphonation of naphthalene with concentrated H2S04 at 80° is found to lead to almost complete 1 -substitution, the rate of formation of the alternative 2-sulphonic acid being very slow at this temperature, i.e. kinetic control. Sulphonation at 160°, however, leads to the formation of no less than 80 % of the 2-sulphonic acid, the remainder being the 1-isomer. That we are now seeing thermodynamic control is confirmed by the observation that heating pure naphthalene 1- or... 

There are many other aromatic hydrocarbons, i.e. compounds like benzene, which contain rings of six carbon atoms stabilised by electron delocalisation. For example, if one of the hydrogen atoms in benzene is replaced by a methyl group, then a hydrocarbon called methylbenzene (or toluene) is formed. It has the structural formulae shown. Methylbenzene can be regarded as a substituted alkane. One of the hydrogen atoms in methane has been substituted by a or —group, which is known as a phenyl group. So an alternative name for methylbenzene is phenylmethane. Other examples of aromatic hydrocarbons include naphthalene and anthracene. 

The quantum-chemical calculation of charge-transfer states as possible intermediates in electrophilic aromatic substitution reactions, making allowance for solvation effects, has been reviewed.6 It has been shown that a simple scaled Hartree-Fock ab initio model describes the ring proton affinity of some polysubstituted benzenes, naphthalenes, biphenylenes, and large alternant aromatics, in agreement with experimental values. The simple additivity rule observed previously in smaller... 

Ansamycins are a class of macrocyclic compounds in which non-adjacent positions on an aromatic ring system are spanned by the long aliphatic bridge (Latin ansa = handle). The aromatic portion may be a substituted naphthalene or naphthaquinone, or alternatively a substituted benzene ring. The macrocycle in the ansamycins is closed by an amide rather than an ester linkage, i.e. ansamycins are lactams. The only ansamycins currently used therapeutically are semi-synthetic naphthalene-based macrocycles produced from rifamycin B. 

As mentioned above, one major drawback of the Trost methodology is its restriction to the parent compound. It was the Cohen group who found an alternative approach to phenylthiocyclopropyl lithium chemistry by using a reductive lithiation of readily accessible cyclopropanone dithioketals which also works for alkyl-substituted cyclopropanes. The anions obtained by reduction with two equivalents of lithium naphthalene or preferably lithium l-(dimethylamino)naphthalene (LDMAN) can effectively be trapped by apt electrophiles (equation 112). 

A one-pot procedure was developed for the preparation of aromatic amines from phenols via a one-pot Smiles rearrangement by N.P. Peet et al.° This new approach can be considered as an alternative of the Bucherer reaction which only works well for naphthalene derivatives and gives very poor yields for substituted benzene derivatives. In the current procedure, the phenol was reacted with 2-bromo-2-methylpropionamide to give 2-aryloxy-2-methylpropionamide which upon treatment with base underwent the Smiles rearrangement. The hydrolysis of the resulting A/-aryl-2-hydroxypropionamide afforded the aromatic amine. 

Naphthalene undergoes electrophilic aromatic substitution at the position next to the second ring. The sigma complex leading to the obtained product has more resonance structures, indicating a more delocalized charge than that of the alternative. 

Dodd 81) has found that Eq. (46) combined with the model accounting for the hyperconjugation effect reproduces reasonably well the magnitude and direction of the shifts of characteristic bands of the naphthalene and anthracene radical ions upon methyl substitution. In general, with alternant hydrocarbons the shifts of the first bands upon substitution by groups with the inductive effect (e.g. CHs or NH3) can be expected to be more considerable for radical ions than for the parent hydrocarbons, inasmuch as the first transition energy in closed-... 

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