Aromatic Electrophilic Substitution

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EAS Halogenation

• To create a strong electrophile either AlCl₃ or FeBr₃ is needed. 

• Carbocation intermediate is formed. 

• Formulates a benzene ring with a halogen attached.

EAS Nitration

• Acid (i.e. H₂SO₄) reacts with HNO₃ to form a unstable electrophile.

• Benzene reacts with the electrophile.

• Adds NO₂ to the benzene.

EAS Sulfonation

• Forms its conjugate base and acid as byproducts.

• Benzene reacts with unstable electrophile.

• Forms a benzene with a HSO₃ group attached.

• Needs acid and heat to react.

EAS Alkylation

• Reacts with AlCl₃/FeCl₃.

• Alkyl group is attached.

• Carbocation rearrangement can occur.

EAS Acylation

• Reacts with AlCl₃/FeCl₃.

• Adds carbonyl group to benzene; forms a ketone.

• No carbocation rearrangement. 

Clemmensen Reduction

• Reacts using Zn(Hg) and HCl.

• Needs heat.

• Uses acidic conditions.

• Takes off the =O.

Wolff-Kishner Reduction

• Reacts using H₂N-NH₂, KOH, and heat.

• Uses basic conditions.

• Takes off the =O.

EAS with Group on Ring

• Electron-donating groups lower ΔG^‡ (faster reaction) and electron-withdrawing groups raise ΔG^‡ (slower reaction). 

Weakly Activating Groups

• CH₃.

• Alkyl.

• Aryl (C₆H₅).

Moderately Activating Groups

• -OCH₃, -OAlkyl.

• OC(=O)R. 

• NHC(=O)R. 

Strongly Activating Groups

• -OH, -O^-. 

• NH₂, -NHR, -NR₂.

Weak Deactivating Groups

• Cl, Br, F.

• However, acts like a weak activator.

Moderately Deactivating Groups

• C≡N. 

• C(=O)R, -C(=O)H. 

• C(=O)OH, -C(=O)OR. 

• SO₃H.

Strongly Deactivating Groups

• NO₂.

• NR₃⁺, CX₃ (X = F, Cl). 

EDG with a Group on the Ring

• When there is already a group on the ring, the specific substituent will determine the location of the second group. 

• Activating substituent = ortho/para (para is favoured). 

• One exception are the halogens. 

EWG with a Group on the Ring

• When there is already a group on the ring, the specific substituent will determine the location of the second group. 

• Deactivating substituent = meta.

Ortho/Para Direction

• Weak activators favour o/p substitution though inductive stabilization. 

• Para is usually major (even though there are two ortho positions) unless it is blocked.

Ortho/Para Activation

• Strong activators favour o/p substitution though extra resonance stabilization.

• Resonance stabilization by strong activators wins out over inductive stabilization. 

• Can sometimes overactivate.

Decreasing Activation

• In times of overactivation, the activating group can be made less activating. 

• Allows the other reactant to reach the proper position. 

Meta Direction

• Strong deactivators disfavour o/p subsitution though charge repulsion.

Benzylic Halogenation Radicalization

• Generic free radical halogenation reaction.

• Reacts with a halogen (X₂) along heat or light.

• Proceeds via hydrogen abstraction to form a free radical.

Benzylic Halogenation Radicalization Favour

• Abstraction of benzylic H favoured over ring hydrogens.

• Also favoured over other non-benzylic side-chain hydrogens.

Radical Chain Process

Oxidative Degradation

• Hot KMnO₄ with KOH for first reaction, then H₃O⁺ for second reaction.

• Leads to benzoic acid formation. 

Catalytic Hydrogenation | Fe/HCl Reduction

• Conversion of NO₂ to NH₂ (nitro to aniline).

• However, other substituents on the benzene can react.

• Fe, HCl can ensure no other reactions occur other than NO₂ → NH₂.