Aromatic compounds must be: cyclic, planar, fully conjugated, and have 4n+2 pi electrons (n=0,1,2...). 6 electrons (benzene), 10, 14, 18... Antiaromatic = 4n pi electrons. Nonaromatic = not cyclic/conjugated.
Electron Donating Groups (OH, NH2, alkyl, halogens*): direct ortho/para, activate ring. Electron Withdrawing Groups (NO2, CN, COOH, CHO): direct meta, deactivate ring. *Halogens are o/p directors but deactivate.
⚗️ Aromatic Compounds
EAS mechanism: electrophile attacks → arenium ion → deprotonation restores aromaticity
EAS Mechanism
Step 1: Electrophile (E+) attacks pi system → forms arenium ion (carbocation, loses aromaticity). Step 2: Base removes proton → restores aromaticity. Aromaticity drives the reaction to completion.
Dissolving metal reduction (Na or Li in liquid NH3 + alcohol). Reduces benzene ring to 1,4-cyclohexadiene. EDG groups: double bonds remain on substituted carbons. EWG groups: double bonds away from substituent.
⚗️ Aromatic Compounds
Resonance structures of benzene: 3 double bonds delocalized over 6 carbons
Benzene Structure & Resonance
Benzene (C₆H₆) has 6 equivalent C-C bonds — intermediate between single and double (1.40 Å). Delocalization of 6 pi electrons over the ring gives extra stability (resonance energy ~36 kcal/mol). This is why benzene undergoes substitution (preserves aromaticity) rather than addition (would destroy aromaticity). Each carbon is sp² hybridized, all atoms coplanar.
Multiple fused benzene rings: Naphthalene (2 rings, 10 pi e⁻, aromatic). Anthracene (3 rings linear). Phenanthrene (3 rings angular — more stable than anthracene). PAHs are important industrially and biologically — some are carcinogens (benzo[a]pyrene). More rings = less soluble, higher melting point. Each ring still satisfies Hückel's rule as part of the fused system.
Aromatic rings containing atoms other than carbon. Pyridine: N contributes 1 lone pair to sigma, 1 electron to pi system (sp² N). Weakly basic. Furan/Thiophene: O/S contribute 2 electrons to pi system. Pyrrole: N-H nitrogen contributes 2 electrons to pi system (lone pair in ring). More basic? Pyridine is basic (lone pair not in ring). Pyrrole is NOT basic (lone pair in ring — deprotonation destroys aromaticity).
Pyridine
N lone pair NOT in pi system — basic (pKa ~5)
Pyrrole
N lone pair IN pi system — NOT basic
Furan
O lone pair in pi system — 6 pi electrons
Thiophene
S lone pair in pi system — 6 pi electrons
Imidazole
Two N: one pyridine-like (basic), one pyrrole-like
⚗️ Aromatic Compounds
Nucleophilic aromatic substitution (SNAr): EWG activate ring toward Nu attack
Nucleophilic Aromatic Substitution
Unlike normal aromatics (electrophilic EAS), rings with strong EWG undergo nucleophilic substitution (SNAr). Requirements: strong EWG (NO₂, CN) ortho/para to the leaving group, good nucleophile, good leaving group (F, Cl, Br). Mechanism: Meisenheimer complex (anionic sigma complex) → expulsion of leaving group. F is the best leaving group in SNAr (despite being worst in SN2) because it stabilizes the Meisenheimer complex.
Requirements
EWG ortho/para + leaving group on ring
Mechanism
Addition of Nu → Meisenheimer complex → loss of LG
Best LG in SNAr
F (worst in SN2, best in SNAr!)
Why F?
C-F bond polarization stabilizes Meisenheimer complex
⚗️ Aromatic Compounds
Phenol acidity: pKa ~10 — more acidic than alcohols due to ring resonance
Phenol Properties & Reactivity
Phenol (C₆H₅OH, pKa ~10) is much more acidic than typical alcohols (pKa ~16) because the phenoxide anion is resonance-stabilized — the negative charge delocalizes into the ring. EWG on ring increase acidity (lower pKa); EDG decrease it. Phenols are activated toward EAS (OH is a powerful o/p director). React with NaOH → sodium phenoxide (soluble). Do NOT react with NaHCO₃ (unlike carboxylic acids) — useful for separation.
Acidity
pKa ~10 — between alcohol (~16) and carboxylic acid (~5)
Why acidic
Phenoxide anion resonance-stabilized by ring
EAS activation
OH strongly activates ring — o/p director
Separations
Dissolves in NaOH but not NaHCO₃
⚗️ Aromatic Compounds
Ortho/para ratio: steric effects favor para; electronic effects favor both
Predicting Ortho vs Para Ratios
Both ortho and para positions are activated by EDG. Para product is usually major because: (1) there are 2 ortho positions but they are sterically hindered near the substituent, (2) large groups strongly favor para. Small substituents (F, OH) give more ortho. Large substituents (t-Bu, NO₂) give predominantly para. Steric bulk of incoming electrophile also matters. On exams, when predicting EAS — state o/p director and name both products.
⚗️ Aromatic Compounds
Benzyne intermediate: forms in elimination-addition mechanism under strong base
Benzyne & Elimination-Addition
When aryl halides react with very strong bases (NaNH₂, n-BuLi), an elimination occurs to form benzyne (a highly strained triple bond in the ring). Benzyne is an extremely reactive intermediate. Addition of nucleophile can occur at either carbon of the 'triple bond' → mixture of ortho products. Evidence: labeling studies show scrambling. This explains why certain substitution products don't follow normal SNAr patterns.
Diazonium salts (ArN₂⁺) react with electron-rich aromatic compounds (phenols, arylamines) via electrophilic aromatic substitution → azo compounds (Ar-N=N-Ar'). The diazonium acts as a weak electrophile — only activated (electron-rich) rings react. Coupling occurs at para position preferentially. Azo dyes are extensively colored (extended conjugation) and used industrially. Reaction at 0–5°C to prevent decomposition of diazonium.
🎓 Common Exam Questions
Q: State Hückel's rule for aromaticity.
A: A compound is aromatic if it is: (1) cyclic, (2) planar, (3) fully conjugated (continuous p orbital overlap), and (4) has 4n+2 pi electrons (n = 0, 1, 2...). Examples: benzene (6 e⁻, n=1), naphthalene (10 e⁻, n=2). Antiaromatic = 4n pi electrons (cyclobutadiene, 4 e⁻). Nonaromatic = not cyclic or not fully conjugated.
Q: What is the difference between ortho/para directors and meta directors in EAS?
A: Electron-donating groups (EDG: OH, NH₂, alkyl, OR): activate the ring and direct to ortho and para positions. Electron-withdrawing groups (EWG: NO₂, CN, COOH, CHO): deactivate the ring and direct to meta. Exception: halogens (F, Cl, Br, I) are ortho/para directors but deactivate the ring (withdraw electrons inductively but donate via resonance).
Q: Describe the mechanism of electrophilic aromatic substitution (EAS).
A: Step 1: Electrophile (E⁺) attacks the pi system → arenium ion (carbocation intermediate) — aromaticity is lost. Step 2: Base (often the conjugate base of the electrophile) removes a proton → aromaticity is restored. The driving force is restoration of aromaticity. Unlike electrophilic addition, EAS gives substitution product because re-aromatization is thermodynamically favorable.
Q: What are the reagents and electrophiles for common EAS reactions?
A: Nitration: HNO₃ + H₂SO₄ → NO₂⁺ (nitronium ion). Sulfonation: H₂SO₄/SO₃ → SO₃. Halogenation: X₂ + Lewis acid (FeBr₃, AlCl₃) → X⁺ equivalent. Friedel-Crafts alkylation: R-X + AlCl₃ → carbocation. Friedel-Crafts acylation: RCOCl + AlCl₃ → acylium ion (RCO⁺). Acylation is preferred over alkylation — no rearrangement, product deactivated so mono-acylation occurs.
Q: What is Birch reduction and what product does it give?
A: Birch reduction uses dissolved metal (Na or Li) in liquid ammonia with an alcohol proton source. Reduces benzene ring to 1,4-cyclohexadiene (not fully reduced). EDG substituents: double bonds remain ON the substituted carbons. EWG substituents: double bonds form AWAY from the substituent. The reaction gives a partially reduced, non-aromatic ring useful in synthesis.
0
Correct
0
Missed
0
Remaining
What does this mean / stand for?
0
Correct
0
Wrong
0
Remaining
No saved cards yet. Click ☆ Save on any memory trick to save it here.