Key Concepts of Aromatic Substitution Reactions

Aromatic substitution reactions are key in Organic Chemistry II, allowing for the modification of aromatic compounds. These reactions, like electrophilic aromatic substitution and Friedel-Crafts reactions, enable the introduction of various functional groups, shaping the properties of the resulting molecules.

1. Electrophilic Aromatic Substitution (EAS)

   • A reaction where an electrophile replaces a hydrogen atom on an aromatic ring.
   • The aromatic system acts as a nucleophile, attacking the electrophile to form a sigma complex.
   • Regeneration of aromaticity occurs through deprotonation, restoring the aromatic character.

2. Friedel-Crafts Alkylation

   • Involves the introduction of an alkyl group onto an aromatic ring using an alkyl halide and a Lewis acid catalyst.
   • Can lead to carbocation rearrangements, resulting in unexpected products.
   • Generally, it is less effective on deactivated aromatic rings.

3. Friedel-Crafts Acylation

   • Introduces an acyl group (RCO-) onto an aromatic ring using an acyl chloride and a Lewis acid.
   • Produces ketones and avoids carbocation rearrangements, providing more control over the product.
   • The acyl group is a deactivating group, which can limit further substitutions.

4. Halogenation

   • The substitution of a hydrogen atom with a halogen (Cl, Br) using a halogen and a Lewis acid catalyst.
   • Requires the presence of a catalyst to generate a more reactive electrophile.
   • Typically occurs at the ortho and para positions relative to existing substituents.

5. Nitration

   • The introduction of a nitro group (NO2) onto an aromatic ring using a mixture of nitric acid and sulfuric acid.
   • The nitronium ion (NO2+) acts as the electrophile in this reaction.
   • Nitration is regioselective, favoring the formation of ortho and para products.

6. Sulfonation

   • The addition of a sulfonyl group (SO3H) to an aromatic ring using sulfur trioxide (SO3) or oleum.
   • The electrophile is the sulfonium ion (SO3H+), which reacts with the aromatic system.
   • Sulfonic acid groups are strong deactivators but can be removed by hydrolysis.

7. Nucleophilic Aromatic Substitution (NAS)

   • A reaction where a nucleophile replaces a leaving group on an aromatic ring.
   • Typically occurs in aromatic compounds with strong electron-withdrawing groups (EWGs) that stabilize the negative charge.
   • Mechanisms include the addition-elimination pathway or the elimination-addition pathway.

8. Directing effects of substituents

   • Substituents on an aromatic ring influence the position of new substituents during EAS.
   • Electron-donating groups (EDGs) direct substitution to the ortho and para positions.
   • Electron-withdrawing groups (EWGs) direct substitution to the meta position.

9. Activating and deactivating groups

   • Activating groups increase the reactivity of the aromatic ring towards electrophilic substitution.
   • Deactivating groups decrease the reactivity and can influence the regioselectivity of substitution.
   • The strength of the activating or deactivating effect is determined by the electronic nature of the substituent.

10. Ortho, para, and meta directors

    • Ortho and para directors are typically electron-donating groups that stabilize the carbocation intermediate.
    • Meta directors are usually electron-withdrawing groups that destabilize the carbocation at the ortho and para positions.
    • The position of substitution is crucial for predicting the outcome of reactions involving substituted aromatic compounds.