Organic Chemistry II - Reactions of Aromatic Compounds - Quick Guides

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Study GuideOrganic Chemistry II–Reactions of AromaticCompounds1.Friedel-Crafts Alkylation ReactionTheFriedel-Crafts alkylation reactionis a key process for adding alkyl groups to a benzene ring,forming analkylated benzene. Here’s how this reaction works, step by step.Step 1: Generating the Electrophile•The reaction begins by generating anelectrophile.•Methyl chloride (CH₃Cl)reacts withaluminum chloride (AlCl₃).oAlCl₃is a catalyst that helps break the bond between carbon and chlorine inCH₃Cl.oThis generates amethyl cation (CH₃⁺), which is highly reactive.Step 2: The Electrophile Attacks the Benzene Ring•Themethyl cation (CH₃⁺)is the electrophile.

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Study Guide•This electrophile attacks theπelectron systemof the benzene ring.•The attack forms anonaromatic carbocation(a positively charged carbon atom) attached tothe ring.Step 3: Delocalization of the Positive Charge•The positive charge on the carbocation isdelocalizedthroughout 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:oAproton (H⁺)is lost from the carbocation formed.oThe loss of the proton allows the benzene ring to return to its stable, aromatic state.

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Study GuideStep 5: Regenerating the Catalyst•The proton (H⁺) then reacts withAlCl₄⁻to regenerate theAlCl₃catalyst.•The productHCl(hydrochloric acid) is formed as a by-product.Example: Methylation of BenzeneWhen methyl chloride (CH₃Cl) reacts with benzene in the presence of AlCl₃,toluene(methylbenzene)is formed.Rearrangements in Friedel-Crafts AlkylationIn some cases,carbocationscan rearrange during the reaction, leading tounpredicted products.Here's how it works:Example: Propylation of Benzene•Propyl chloride(C₃H₇Cl) reacts with benzene and AlCl₃.

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Study GuideoTheinitial carbocationformed is aprimary carbocation.oHowever, this carbocation can rearrange to amore stable secondary carbocation.This leads to the formation ofisopropyl benzene(from the rearranged carbocation), which is themajor product. Theminor productresults from the primary carbocation.1,2-Hydride Shift•The carbocation undergoes a1,2-hydride shift.oAhydride ion (H⁻)moves from one carbon to another.oThis rearrangement stabilizes the carbocation by shifting the positive charge to amore stable location.Key Takeaway•TheFriedel-Crafts alkylationis an electrophilic substitution reaction where an alkyl group isadded to a benzene ring.•The reaction involvesgenerating a carbocation, which attacks the benzene ring, followed bythe loss of a proton to restore aromaticity.•AlCl₃is a key catalyst in generating the reactive electrophile.•Carbocation rearrangementscan occur, leading to different products.•This reaction is widely used for introducing alkyl groups into aromatic compounds.

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Study Guide2.Friedel-Crafts Acylation Reaction OverviewTheFriedel‐Crafts acylation reaction,another example of an electrophilic aromatic substitutionreaction, is similar to the Friedel‐Crafts alkylation reaction except that the substance that reacts withbenzene 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,IsThe remainder of the mechanism is identical to that of the alkylation of benzene. Because the acyliumion is resonance stabilized, no rearrangements occur.

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Study GuideHow Does It Work?Here’s the basic process:1.The Reactants:Thebenzene ringreacts with theacyl halidein the presence of a special catalyst,AlCl₃(aluminum chloride).In a common example, an acyl halide likeacetyl chlorideis used.2.Formation of the Acylium Ion:The acyl halide reacts with AlCl₃, breaking the bond between the acyl group and the halide (Cl).This forms theacylium ion(R-C⁺). The acylium ion is a very strong electrophile, meaning it ishighly reactive and ready to attack the benzene ring.The MechanismHere’s the step-by-step breakdown of how the reaction proceeds:1.Step 1: The acyl chloride reacts with AlCl₃, which helps break the bond between the acylgroup (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 thepositively charged acylium ion. This attack creates anonaromatic carbocationin thebenzene ring.

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Study Guide3.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 positionwhere the acyl group has attached.5.Step 5: The proton reacts with the AlCl₄⁻(from the catalyst) to regenerate the AlCl₃catalystand form HCl.

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Study GuideWhy Is This Important?This reaction is a great way to add anacyl group(likeR-C=O) to a benzene ring. It’s useful inorganic chemistry to create aromatic compounds with an acyl group attached, which are keyintermediates in making pharmaceuticals, plastics, and more.Key Takeaway•Electrophilic aromatic substitutionis the general reaction type.•The key reactants arebenzeneand anacyl halide.•The reaction iscatalyzedbyAlCl₃.•The product is anaryl 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 willhelp you with many other reactions involving aromatic compounds.3.Directing Group Influence on Benzene RingsWhen we add groups to a benzene ring, they can affect where other groups will attach to the ring.This happens throughelectrophilic aromatic substitution, where a group already on the ringinfluences 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 electrondensity towards the ring, especially to theorthoandparapositions, making these spots moreattractive for incoming groups. Examples ofactivating groupsinclude:oNH₂(amine group)oOH (hydroxyl group)oOCH₃(methoxy group)2.Deactivating Groups: These groups pull electrons away from the benzene ring, making thering less reactive. They direct new groups to themetaposition. Examples ofdeactivatinggroupsinclude:

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