Reactivity of Polycyclic Benzenoid Hydrocarbons16-6

Naphthalene is activated toward electrophilic substitution.Naphthalene undergoes electrophilic substitution rather than addition.

Naphthalene is activated with respect to electrophilic aromatic substitution. Bromination at C1 occurs without a catalyst under mild conditions.

Nitration, as well as other electrophilic substitutions, occur readily and are highly selective for reaction at C1.

The ease of reaction and preference for reaction at C1 for naphthalene can be explained on the basis of the resonance structures for the carbocation transition state:

Note that there are 5 resonance structures, two of which contain the particularly stable benzene ring.

Attack at C2 also leads to a carbocation having 5 resonance structures, however, not one of these has the structure of an intact benzene.

Attack at C1 leads to a more stable transition state than does attack at C2.

Electrophiles attack substituted naphthalenes regioselectively.The naphthalene ring carrying a substituent is the ring most affected toward electrophilic attack.

An activating group directs the incoming electrophile to the same ring; a deactivating group directs the incoming electrophile to the other ring.

Deactivating groups direct electrophilic substitutions to the other ring, preferentially to position C5 and C8.

Resonance structures aid in predicting the regioselectivity of larger polycyclic aromatic hydrocarbons.

Resonance, steric considerations and the directing power of substituents apply to larger derivatives of naphthalene, such as phenanthrene.

Electrophilic attack in this system is at C9 and C10 because substitution at these positions leads to a carbocation resonance structure having two intact benzene rings.

Polycyclic Aromatic Hydrocarbons and Cancer16-7

Many polycyclic benzenoid hydrocarbons are carcinogenic.

A particularly well-studied environmental carcinogenic pollutant is benzo[a]pyrene.

This molecule is generated by gasoline and oil combustion, incineration of refuse, forest fires, cigarettes, cigars and in roasting meats.

Benzo[a]pyrene is biologically converted into the ultimate carcinogen, an oxacyclopropane, diol derivative:

The carcinogenic activity of the epoxide derivative of benzo[a]pyrene is believe to be due to its interaction with a guanine base in DNA.