Naphthalene, alternant hydrocarbon

An important distinction for conjugated hydrocarbons is the classification into alternant and non-alternant hydrocarbons. Alternant hydrocarbons are those like ethene, hexatriene, benzene and naphthalene where we can divide the carbon atoms into two sets called starred and unstarred , such that no member of one set is directly bonded to another member (Figure 7.4). 

Methylisoquinolinium 2-carboxylate (230), originally prepared by Quast (70LA64), was recently identified as a defensive betaine from Photuris versicolor fireflies (99JNP378). It is a pseudo-cross-conjugated mesomeric betaine isoconjugate to the odd alternant hydrocarbon 2-isopropenyl-naphthalene anion which is an odd alternant hydrocarbon anion. This compound therefore is a member of class 13, which is very rare. The UV absorption maxima Imax (methanol) were found at 235 (4.35), 320 (shoulder, 3.97), and 326 (3.99) nm. This compound undergoes similar reactions as Homarine 19 (Scheme 75). The NMR data are presented in Table VIII. 

Aromatic hydrocarbons can be divided into two types alternant and nonalternant.In alternant hydrocarbons, the conjugated carbon atoms can be divided into two sets such that no two atoms of the same set are directly linked. For convenience, one set may be starred. Naphthalene is an alternant and azulene a nonaltemant hydrocarbon ... 

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. 

To illustrate the effect of solvation on temporary anions we will consider the naphthalene molecule. This molecule is particularly interesting because it is an alternant hydrocarbon (14), and for such molecules, the pairing theorem (15) predicts that the anion and cation spectra should be identical. This theorem is valid for both Huckel and PPP model Hamiltonians, but is not valid for ab initio or CNDO calculations. It has been found (1 ) to be true to a good approximation ( /0.1 eV) in organic glasses (16). The ETS spectra allows an examination of the validity of this... 

For alternant hydrocarbons cj, = cj,. for all p, and there is no first-order energy change for the HOMO LUMO transition, as is apparent from Figure 2.26. This result is remarkably well confirmed by the absorption spectra of naphthalene, quinoline, and isoquinoline shown in Figure 2.27. 

Let us now specialise the argument to the case of an even, alternant hydrocarbon with nj2 doubly-occupied bonding-orbitals and n/2 unoccupied anti-bonding orbitals. This is the situation, for example, in the ground state of butadiene (Fig. 2-8) and of naphthalene (Fig. 6-7). From equation (4-4), the charge on the rth carbon atom, qr, is... 

The reader may, however, object that there are a number of molecules, the alternant hydrocarbons which we discussed in great detail in Chapter Six, in which the charge densities, q at all carbon atoms, r, (r = 1,2,..., ) are identically unity, by part 3 of the Coulson-Rushbrooke theorem. Examining the 7r-electron charge at the various sites in such a molecule is, therefore, no longer a way of distinguishing one position from another. For example, is the a-position in naphthalene more reactive than the -position, and, if so, why Such a distinction cannot depend upon the qr, for, as we have just observed, they are all equal. In that case we shall just have to make appeal to the next-highest-order differential—a procedure which introduces a new set of properties, called polarisabilities, which have proved quite important in the study of this kind of system. The word polarisability is rather an unfortunate one, but we shall use it and deal here with so-called atom-atom polarisabilities . 

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-... 

Alternant Hydrocarbons. Note that the HMOs of naphthalene, like those of butadiene and benzene, are paired, meaning that for each HMO with the energy a - x)8 there is an HMO with energy a -I- x)8. This can be proved to be true for every alternant hydrocarbon (for the proo see Problem 16.24). An alternant hydrocarbon is a planar conjugated hydrocarbon in which the carbon atoms can be divided into a starred set and an unstarred set, with starred carbons bonded only to unstarred carbons, and vice versa (Rg. 16.7). All planar conjugated hydrocarbons are alternants except those containing a ring with an odd number of carbons. 

FIGURE 16.7 Naphthalene is an alternant hydrocarbon, and azulene is a nonalternant hydrocarbon. 

The ground state approach would clearly fail to account for the greater ease of substitution of naphthalene in the a position compared to the p position by electrophilic reagents. This is because naphthalene is an alternant hydrocarbon and is predicted to have equal charges at each ir-electron center. 

The use of to evaluate changes in E.jj with changes produced in by polar reagents does not solve the problem of predicting the relative position of attack on alternate hydrocarbons such as naphthalene or butadiene. One ratllle iw w s-... 

Other perturbation approaches to the reactivity of alternant hydrocarbons, such as naphthalene, have been discussed... 

Figure 8-18 ESR splitting constants in gauss versus HMO unpaired spin densities. The systems are fused ring alternant hydrocarbon radical anions (naphthalene, anthracene, tetracene, pyrene). The underlined point is thought to result from negative spin density. (Data from Streitwieser [7].)...

Perhaps the most obvious reaction index to use for the earliest stages of electrophilic or nucleophilic reactions is the n-electron density. If Cl is attracted to tt charge, it should be attracted most to those sites where tt density is greatest. (Such an ion should be attracted to sites having excessive a charge density also, but our basic HMO assumptions ignore any variations in a density.) For an alternant hydrocarbon like naphthalene, all tt densities are unity, so this index is of no use. For nonaltemant molecules, however, it can be quite helpful. Azulene has varying HMO n densities (XXI). (More sophisticated calculations described in future chapters are in qualitative... 

One of the smallest non-alternant hydrocarbons is acenaphthylene (Figure 2.2). It can be considered a C2H2 unit fused peri to naphthalene, and Its first synthesis was just that in 1867, Berthelot heated naphthalene with acetylene in a tube furnace to provide acenaphthylene. The reactivity of acenaphthylene is dominated by the... 

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