Naphthalene reactivity toward electrophiles

Condensed aromatic hydrocarbons are more reactive towards electrophilic reagents, and naphthalene, for example, may be brominated quite readily in solution in carbon tetrachloride without the need for a catalyst electrophilic attack takes place at the more reactive a-position to yield 1-bromonaphthalene (Expt 6.25). 

It is known that whereas benzene and naphthalene are relatively unreactive, the remaining compounds have much higher reactivity toward electrophiles and 1,2- or 1,4-additions. In the case of benzene, addition across a double bond will cause a 34 kcal/mol loss of stabilization (36 less 2 kcal/mol for the butadiene fragment) and addition to naphthalene will cause a 22 kcal/mol loss of stabilization (i.e. 60 kcal/mol less 38 kcal/mol for the remaining styrene unit). On the other hand, addition across the 9,10 positions in anthracene will only lead to an 8 kcal/mol loss... 

In contrast to benzene, the bond lengths in naphthalene are not all equal, as illustrated in 4. The resonance energy of naphthalene is 255 kJ mol", which is higher than, though not twice that of, benzene (151 kJ mol" ). In the canonical forms 5 and 7 that contribute to the valence bond structure for naphthalene, only one of the two rings is fully benzenoid. Naphthalene is less aromatic than benzene, which accounts for its higher reactivity towards electrophilic attack compared with benzene. 

The relative ease of sulfonation of 235 in comparison with chalcone 231 is due to the greater reactivity of the naphthalene nucleus towards electrophilic substitution and the orientation of sulfonation occurs in the predicted 4-position of the naphthalene ring in view of the electron-donating character of the alkenic double bond. 2-, 3-, and 4-Methoxy, 3,4-dimethoxy- and 4-phenyl-chalcone, together with the 2-thienyl and cinnamylidene analogues reacted with chlorosulfonic acid (six equivalents or three equivalents in thionyl chloride) at room temperature to give the corresponding sulfonyl chlorides. The orientation of sulfonation was discussed in terms of stereoelectronic factors and was confirmed by NMR spectral analysis. 

The benzanthrone system is susceptible to both electrophilic and nucleophilic attack. The most reactive sites towards electrophiles are the 3- and 9-positions, which can be compared with the 4,4 -positions in biphenyl. The 9-position is somewhat deactivated by the carbonyl group, however. Thus, for example, monobromination takes place at the 3-position and further substitution gives 3,9-dibromobenzanthrone. Nitration and benzoylation similarly give rise to the 3-substituted product. The 3-position is in fact peri-hindered (compare naphthalene) so that sulphonation yields the 9-sulphonic acid. Electron withdrawal by the carbonyl group activates the 4- and 6-positions towards nucleophilic attack for example, hydroxylation occurs at these sites. 

The text points out that C-l of naphthalene is more reactive than C-2 toward electrophilic aromatic substitution. Thus, of the two possible products of sulfonation, naphthalene-1-sulfonic acid should be formed faster and should be the major product under conditions of kinetic control. Since the problem states that the product under conditions of thermodynamic control is the other isomer, naphthalene-2-sulfonic acid is the major product at elevated temperature. 

Naphthalene is more reactive than benzene toward electrophilic substitution because the carbocation intermediate is more stable and, therefore, easier to form than the analogous carbocation intermediate formed from benzene. Because of naphthalene s increased reactivity, a Lewis acid is not needed for bromination or chlorination. 

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