# Organic Chemistry: Aromaticity, Electrophilic & Nucleophilic Aromatic Substitution > Hückel's rule for aromaticity and anti-aromaticity, heteroaromatic rings, Electrophilic Aromatic Substitution (EAS) mechanisms and directing effects, Nucleophilic Aromatic Substitution (SNAr and benzyne), benzylic reactions, arenediazonium chemistry, and Birch reductions. - **Canonical URL:** https://flashcards.world/flashcards/sets/1dbc26cb-a077-4d1b-a292-b83ddaae53e1/ - **Markdown URL:** https://flashcards.world/flashcards/sets/1dbc26cb-a077-4d1b-a292-b83ddaae53e1.md - **Level:** advanced - **Languages:** en → en - **Total Cards:** 32 ## Flashcards (32 Cards) | # | Front (Term / Prompt) | Back (Definition / Answer) | Example | Multiple-Choice Options | | :--- | :--- | :--- | :--- | :--- | | 1 | What isotopic labeling experiment proved the existence of the benzyne intermediate? | In John D. Roberts's classic 1953 experiment, reacting [1-¹⁴C]-chlorobenzene with KNH₂ in liquid NH₃ yielded an approximately 1:1 mixture (~47:53, differing slightly from 50:50 only by the ¹²C/¹⁴C kinetic isotope effect) of [1-¹⁴C]-aniline and [2-¹⁴C]-aniline, proving a symmetric benzyne intermediate was formed. | | | | 2 | What reagents are used in the Clemmensen reduction to reduce aryl ketones to alkyl chains? | Zinc amalgam (Zn(Hg)) and concentrated aqueous hydrochloric acid (HCl) under reflux conditions reduce benzylic carbonyls into methylene groups (-CH2-). | | | | 3 | What reaction conditions selectively reduce an aromatic nitro group (-NO2) to an amine (-NH2)? | Catalytic hydrogenation (H2 over Pd/C) or dissolving active metals in aqueous acid (Fe/HCl or Sn/HCl) followed by basic neutralization cleanly reduces nitrobenzene to aniline. | | | | 4 | What is diazo coupling and what synthetic products does it generate? | Arenediazonium cations act as weak electrophiles attacking highly activated aromatic rings (phenols or anilines) via EAS to generate brightly colored azo compounds (-N=N-) used as dyes. | | | | 5 | What product is formed when an alkylbenzene side chain is oxidized with hot KMnO4 or Na2Cr2O7? | Any benzylic carbon possessing at least one benzylic C-H bond is cleaved and oxidized completely into benzoic acid; quaternary benzylic carbons (e.g., tert-butylbenzene) are inert. | | | | 6 | How can an arenediazonium salt be converted directly into a phenol? | By heating the aqueous arenediazonium salt solution in dilute sulfuric acid (H2O, H2SO4, heat), which displaces molecular N2 with water to synthesize phenols in high yield. | | | | 7 | What are Sandmeyer reactions and what copper salts are used? | Arenediazonium salts react with copper(I) chloride (CuCl), copper(I) bromide (CuBr), or copper(I) cyanide (CuCN) to replace the diazonium group with -Cl, -Br, or -CN via radical mechanisms. | | | | 8 | How can an iodine atom be attached to an aromatic ring using diazonium chemistry? | Arenediazonium salts react cleanly with potassium iodide (KI) in aqueous solution at room temperature to produce aryl iodides without requiring any copper catalyst. | | | | 9 | Why is 1,3,5,7-cyclooctatetraene (COT) non-aromatic rather than anti-aromatic at room temperature? | With 8 pi electrons, planarity would force anti-aromaticity; COT relieves destabilization by puckering into a non-planar 'tub' conformation, breaking p-orbital overlap. | | | | 10 | Why is a Lewis acid catalyst (FeBr3 or AlCl3) required for the halogenation of benzene? | Benzene's aromatic stabilization requires a powerful electrophile; FeBr3 polarizes the Br-Br bond to generate a highly electrophilic complex (Br+...FeBr4-) capable of disrupting aromaticity. | | | | 11 | Why does Friedel-Crafts acylation avoid the polyalkylation and rearrangement issues of alkylation? | The resonance-stabilized acylium ion (R-C≡O+) resists rearrangement, and the resulting acyl group is strongly electron-withdrawing, deactivating the product ring against further substitution. | | | | 12 | Under what conditions does unactivated chlorobenzene react with NaNH2 via an elimination-addition mechanism? | Extremely strong base (NaNH2 in liquid NH3) abstracts an ortho proton, expelling chloride to form a highly strained benzyne intermediate featuring a formal carbon-carbon triple bond in the ring. | | | | 13 | What is the general two-step mechanism of Electrophilic Aromatic Substitution (EAS)? | Benzene attacks an electrophile to form a non-aromatic, resonance-stabilized arenium ion (sigma complex / Wheland intermediate), followed by base abstraction of the proton to restore aromaticity. | | | | 14 | When two directing groups conflict on a disubstituted benzene, which group directs incoming EAS substitution? | The more powerful activating group dominates directing orientation; activating groups supersede deactivating groups (e.g., -OH > -CH3 > -Cl > -NO2). | | | | 15 | What reagents are used in the Wolff-Kishner reduction to reduce aryl ketones? | Hydrazine (H2NNH2) and potassium hydroxide (KOH) heated in a high-boiling solvent like diethylene glycol reduce carbonyl groups to methylene groups via hydrazone intermediates. | | | | 16 | Why are halogen substituents on benzene unique in their EAS directing and activating behavior? | Halogens are net deactivating due to strong inductive electron withdrawal (-I), but ortho/para directing because their lone pairs can donate electron density by resonance (+R) to stabilize ortho/para arenium ions. | | | | 17 | Why is the cyclopentadienyl anion exceptionally stable and aromatic? | Deprotonating cyclopentadiene leaves a carbanion with two electrons in an unhybridized p-orbital, yielding a fully conjugated cyclic planar ring with 6 pi electrons (satisfying 4n + 2 where n = 1). | | | | 18 | What are the regiochemical outcomes of Birch reductions on substituted benzenes? | Using Na or Li in liquid NH₃ with an alcohol proton source converts benzenes into unconjugated 1,4-cyclohexadienes: electron-donating groups (-OCH₃, -R) leave the ipso and para carbons sp²-hybridized, whereas electron-withdrawing groups (-COOH) reduce and protonate the ipso and para carbons to sp³ centers. | | | | 19 | How can aromatic sulfonation be reversed synthetically? | Sulfonation with fuming H2SO4 yields benzenesulfonic acid; heating benzenesulfonic acid with dilute aqueous acid or steam reverses the reaction, cleanly removing the -SO3H blocking group. | | | | 20 | What defines an anti-aromatic molecule and why are such systems unusually unstable? | A cyclic, planar, fully conjugated system possessing 4n pi electrons; according to Frost circle analysis, anti-aromatic systems possess unpaired electrons in non-bonding molecular orbitals, destabilizing the system. | | | | 21 | What four criteria must a molecule satisfy to be classified as aromatic by Hückel's rule? | A monocyclic system must be cyclic, planar, fully conjugated around the ring (a continuous ring of overlapping p-orbitals), and contain (4n + 2) pi electrons, where n is a non-negative integer (2, 6, 10, 14, …). | | | | 22 | Why are activating groups in EAS invariably ortho/para directors? | They donate electron density into the ring through resonance (lone pairs) or hyperconjugation, placing additional electron density at ortho/para positions and stabilizing the resulting arenium ion intermediates. | | | | 23 | What is the reactive electrophile in aromatic nitration and how is it generated? | The nitronium ion (NO2+), generated when sulfuric acid (H2SO4) protonates nitric acid (HNO3), which loses water to form the linear sp-hybridized electrophile. | | | | 24 | How does N-bromosuccinimide (NBS) selectively brominate alkylbenzenes? | Irradiation with light (hν) or radical initiator generates bromine radicals that selectively abstract the weakly bonded benzylic hydrogen to form an exceptionally stable resonance-delocalized benzylic radical. | | | | 25 | What three major synthetic limitations restrict Friedel-Crafts alkylation of benzene? | Carbocation rearrangements to more stable isomers, polyalkylation because alkyl groups activate the ring, and failure on aromatic rings containing moderate or strong deactivating substituents. | | | | 26 | How do the electronic contributions of heteroatom lone pairs differ between pyridine and pyrrole? | Pyridine's nitrogen is already double-bonded and its lone pair resides in an sp2 hybrid orbital outside the aromatic sextet; pyrrole's nitrogen contributes its lone pair from a p-orbital to complete the 6 pi-electron aromatic ring. | | | | 27 | What is the Schiemann reaction and what reagent does it utilize? | Treating arenediazonium salts with fluoroboric acid (HBF4) precipitates an arenediazonium tetrafluoroborate salt, which undergoes thermal decomposition to synthesize aryl fluorides. | | | | 28 | What reagent replaces an arenediazonium group with hydrogen (-H) in synthetic sequences? | Hypophosphorous acid (H3PO2); this reductive deamination allows amino or nitro groups to serve as temporary directing or activating handles before being excised. | | | | 29 | Why do standard electron-withdrawing groups (-NO2, -CF3, -CN) direct EAS meta? | Electrophilic attack at ortho or para positions places the positive arenium charge directly on the ring carbon bearing the electron-withdrawing group, an extremely destabilized high-energy configuration avoided by meta attack. | | | | 30 | How are arenediazonium salts (Ar-N2+ Cl-) prepared from primary aromatic amines? | Aniline is treated with sodium nitrite (NaNO2) and hydrochloric acid at 0-5°C; maintaining ice temperatures is required to prevent explosive decomposition and spontaneous nitrogen loss. | | | | 31 | What structural prerequisites are required for Nucleophilic Aromatic Substitution via addition-elimination (SNAr)? | The aromatic ring must possess a strong electron-withdrawing group (such as -NO2) positioned ortho or para to a good leaving group (such as halide). | | | | 32 | What is the Meisenheimer complex formed during SNAr reactions? | A resonance-stabilized cyclohexadienyl carbanionic intermediate formed when the nucleophile attacks the halide-bearing carbon, with negative charge delocalized into the ortho/para nitro groups. | | | --- Study this deck interactively with FSRS-6 spaced repetition, audio review, and multiple-choice mode at [Flashcards World](https://flashcards.world/flashcards/sets/1dbc26cb-a077-4d1b-a292-b83ddaae53e1/).