CHAPTER 16 Electrophilic Attack on Derivatives of Benzene: Substituents Control Regioselectivity

16-1 Activation or Deactivation by Substituents on a Benzene Ring

Substituent on an aromatic ring affect the stability of the intermediate pentadienyl cation. As the formation of this cation is endothermic, the transitition state and therefore the rate of its formation are significantly affected the substitutent. Those substituents that stabilize carbocations will accelerate the rate of substitution compared with a hydrogen.

16-2 Directing Inductive Effects of Alkyl Groups

The effect of the substituent on the intermediate pentadienyl cation will vary with its position on this cation: the two end and the middle carbons bear positive charge whereas the other two do not. Substituents with lone-pairs of electrons can stabilize the cation by lone pair donation but only when they are on the end or middle carbons. All such substituents destabilize the cation when on the other two carbons. Groups that are electron withdrawing direct substitution so that they are on a carbon of the pentadienyl cation that does NOT bear positive charge.

 

16-3 Directing Effects of Substituents in Conjugation with the Benzene Ring

Substituents with lone-pairs of electrons can stabilize the cation by lone pair donation but only when they are on the end or middle carbons. All such substituents destabilize the cation when on the other two carbons.

16-4 Electrophilic Attack on Disubstituted Benzenes

When two substituents are present, it is possible that they direct to the same positions. When this is not the case, the one that is higher on the list of substituent rates (fastest at the top) directs the third group.

16-5 Synthetic Strategies Toward Substituted Benzenes

Key here is that there are only five groups that can be directly introduced by electrophilic, aromatic substitution: alkyl, acyl, halogen, nitro, sulfonic acid. Other groups, such as NH2 and OH are the result of modification of one of the five groups introduced by EAS

 

16-6 Reactivity of Polycyclic Benzenoid Hydrocarbons

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Naphthalene undergoes electrophilic attach in the same way that benzene does. Substitution at the alpha carbon is favored.

16-7 Polycylic Aromatic Hydrocarbons and Cancer

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In the right concentration, everything will kill you. All peanut butter contains trace amounts of aflatoxins produced by a fungus that love peanuts as much as people do. Antropologists were studying a tribe in africa that had an unusually high level of liver cancer. Yep, peanuts were a major part of their diet. Aflotoxin B1 is shown below. How many functional groups do you see?