Substitution in Naphthalene

Polycyclic aromatic hydrocarbons undergo electrophilic aromatic substitution when treated with the same reagents that react with benzene. In general, polycyclic aromatic hydrocarbons are more reactive than benzene. Most lack the symmetry of benzene, however, and mixtures of products may be formed even on monosubstitution. Among polycyclic aromatic hydrocarbons, we will discuss only naphthalene, and that only briefly. 

Two sites are available for substitution in naphthalene C-1 and C-2. The more reactive site of electrophilic attack is normally C-1. 

C-1 is more reactive because the intermediate formed when the electrophile bonds there is a relatively stable carbocation. A benzene-type pattern of bonds is retained in one ring, and the positive charge is delocalized by allylic resonance. 

To involve allylic resonance in stabilizing the carbocation intermediate formed when the electrophile bonds to C-2, the benzene-like character of the other ring is sacrificed. 

Sulfonation of naphthalene is reversible at elevated temperature. A different isomer of naphthalenesulfonic acid is the major product at 160°C than is the case at OX. Which isomer is the product of kinetic control Which one is formed under conditions of thermodynamic control Can you think of a reason why one isomer is more stable than the other  

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In Chapter 2, electrophilic substitution in naphthalene was discussed, when consideration of the stability of the cationic intermediates arising... 

The FE-NBMO and HMO-NBMO coefficients of it radicals corresponding to a- and 0-substitutions in naphthalene are derived in Chart I to serve as examples. In the case of a-substitution, the FEMO model offers two possibilities for treating the effective potential at a bridge head atom adjacent to a substitution site, that is, either as a joint or a nonjoint atom. In the first case, an appendage of the free-electron pathway is considered to be pointing toward the a-position. In this chapter, these types of atoms in PAH fragments are taken to be joint positions. 

FIGURE 5.33. Distributions of formal charge in transition states for electrophilic substitution in naphthalene and anthracene and corresponding reactivity numbers. 

Rate and Regioselectivity in the Nitration of (Trifluoromethyl)benzene 474 Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents 476 Substituent Effects in Electrophilic Aromatic Substitution Strongly Deactivating Substituents 480 Substituent Effects in Electrophilic Aromatic Substitution Halogens 482 Multiple Substituent Effects 484 Retrosynthetic Analysis and the Synthesis of Substituted Benzenes 486 Substitution in Naphthalene 488 Substitution in Heterocyclic Aromatic Compounds 489... 

Toluene sulfonates more readily than benzene, while o- and n-xylcnc and mesitylene may be slowly sulfonated with 95 per cent II2SO4, even without heating. Para derivatives, such as p-xylcnc, dissolve less readily (separation of the xylenes), while p-dihalogcn benzenes require 20 per cent fuming sulfuric acid and heating to 100° to 120° for sulfonation. Substitution in naphthalene takes place more readily than in benzene and therefore concentrated H2SO4 may be used. 

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