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Organic Chemistry II - Reactions of Aromatic Compounds

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    Study Guide Organic Chemistry II – Reactions of Aromatic Compounds 1. Friedel - Crafts Alkylation Reaction The Friedel - Crafts alkylation reaction is a key process for adding alkyl groups to a benzene ring, forming an alkylated benzene . Here’s how this reaction works, step by step. Step 1: Generating the Electrophile • The reaction begins by generating an electrophile . • Methyl chloride (CH ₃ Cl) reacts with aluminum chloride (AlCl ₃ ) . o AlCl ₃ is a catalyst that helps break the bond between carbon and chlorine in CH ₃ Cl . o This generates a methyl cation (CH ₃⁺ ) , which is highly reactive. Step 2: The Electrophile Attacks the Benzene Ring • The methyl cation (CH ₃⁺ ) is the electrophile.

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    Study Guide • This electrophile attacks the π electron system of the benzene ring. • The attack forms a nonaromatic carbocation (a positively charged carbon atom) attached to the ring. Step 3: Delocalization of the Positive Charge • The positive charge on the carbocation is delocalized throughout the benzene ring. • The charge shifts across the molecule as the ring maintains its overall stability. Step 4: Restoring Aromaticity • To restore the aromaticity of the benzene ring: o A proton (H ⁺ ) is lost from the carbocation formed. o The loss of the proton allows the benzene ring to return to its stable, aromatic state.

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    Study Guide Step 5: Regenerating the Catalyst • The proton (H ⁺ ) then reacts with AlCl ₄⁻ to regenerate the AlCl ₃ catalyst . • The product HCl (hydrochloric acid) is formed as a by - product. Example: Methylation of Benzene When methyl chloride (CH ₃ Cl) reacts with benzene in the presence of AlCl ₃ , toluene (methylbenzene) is formed. Rearrangements in Friedel - Crafts Alkylation In some cases, carbocations can rearrange during the reaction, leading to unpredicted products . Here's how it works: Example: Propylation of Benzene • Propyl chloride (C ₃ H ₇ Cl) reacts with benzene and AlCl ₃ .

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    Study Guide o The initial carbocation formed is a primary carbocation . o However, this carbocation can rearrange to a more stable secondary carbocation . This leads to the formation of isopropyl benzene (from the rearranged carbocation), which is the major product . The minor product results from the primary carbocation. 1,2 - Hydride Shift • The carbocation undergoes a 1,2 - hydride shift . o A hydride ion (H ⁻ ) moves from one carbon to another. o This rearrangement stabilizes the carbocation by shifting the positive charge to a more stable location. Key Takeaway • The Friedel - Crafts alkylation is an electrophilic substitution reaction where an alkyl group is added to a benzene ring. • The reaction involves generating a carbocation , which attacks the benzene ring, followed by the loss of a proton to restore aromaticity. • AlCl ₃ is a key catalyst in generating the reactive electrophile. • Carbocation rearrangements can occur, leading to different products. • This reaction is widely used for introducing alkyl groups into aromatic compounds.

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    Study Guide 2. Friedel - Crafts Acylation Reaction Overview The Friedel ‐ Crafts acylation reaction , another example of an electrophilic aromatic substitution reaction, is similar to the Friedel ‐ Crafts alkylation reaction except that the substance that reacts with benzene is an acyl halide, instead of an alkyl halide, R &bond; X. An acetyl chloride reaction appears as: The mechanism for the generation of the acylium ion, Is The remainder of the mechanism is identical to that of the alkylation of benzene. Because the acylium ion is resonance stabilized, no rearrangements occur.

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    Study Guide How Does It Work? Here’s the basic process: 1. The Reactants : The benzene ring reacts with the acyl halide in the presence of a special catalyst, AlCl ₃ (aluminum chloride). In a common example, an acyl halide like acetyl chloride is used. 2. Formation of the Acylium Ion : The acyl halide reacts with AlCl ₃ , breaking the bond between the acyl group and the halide (Cl). This forms the acylium ion (R - C ⁺ ). The acylium ion is a very strong electrophile, meaning it is highly reactive and ready to attack the benzene ring. The Mechanism Here’s the step - by - step breakdown of how the reaction proceeds: 1. Step 1 : The acyl chloride reacts with AlCl ₃ , which helps break the bond between the acyl group (R - C) and the chlorine (Cl), forming the acylium ion (R - C ⁺ ). 2. Step 2 : The benzene ring acts as a nucleophile, meaning it has electrons that can attack the positively charged acylium ion. This attack creates a nonaromatic carbocation in the benzene ring.

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    Study Guide 3. Step 3 : The positive charge in the carbocation spreads over the entire benzene ring, stabilizing it. 4. Step 4 : To restore the aromaticity of the benzene ring, a proton (H ⁺ ) is lost from the position where the acyl group has attached. 5. Step 5 : The proton reacts with the AlCl ₄⁻ (from the catalyst) to regenerate the AlCl ₃ catalyst and form HCl.

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    Study Guide Why Is This Important? This reaction is a great way to add an acyl group (like R - C=O ) to a benzene ring. It’s useful in organic chemistry to create aromatic compounds with an acyl group attached, which are key intermediates in making pharmaceuticals, plastics, and more. Key Takeaway • Electrophilic aromatic substitution is the general reaction type. • The key reactants are benzene and an acyl halide . • The reaction is catalyzed by AlCl ₃ . • The product is an aryl ketone , with the acyl group attached to the benzene ring. This reaction is one of the many fundamental reactions in organic chemistry, and understanding it will help you with many other reactions involving aromatic compounds. 3. Directing Group Influence on Benzene Rings When we add groups to a benzene ring, they can affect where other groups will attach to the ring. This happens through electrophilic aromatic substitution , where a group already on the ring influences the placement of new groups. There are two main categories of groups that affect this: 1. Activating Groups : These groups make the benzene ring more reactive. They push electron density towards the ring, especially to the ortho and para positions, making these spots more attractive for incoming groups. Examples of activating groups include: o NH ₂ (amine group) o OH (hydroxyl group) o OCH ₃ (methoxy group) 2. Deactivating Groups : These groups pull electrons away from the benzene ring, making the ring less reactive. They direct new groups to the meta position. Examples of deactivating groups include:

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