Unit IV Model Questions & Answers: Polynuclear Hydrocarbons
Practice two, five and ten mark questions on naphthalene, anthracene, phenanthrene, diphenylmethane and triphenylmethane with structures, reaction schemes and concise model answers.
Part A Two Mark Questions
1. Define polynuclear hydrocarbons.
Definition: Hydrocarbons containing two or more aromatic ring nuclei.
Classes: Fused: naphthalene, anthracene, phenanthrene. Non-fused: diphenylmethane and triphenylmethane.
2. Differentiate fused and non-fused polynuclear hydrocarbons.
Fused: Adjacent rings share a pair of carbon atoms and their bond.
Non-fused: Separate rings connect through a bond or a bridge carbon.
3. Write the molecular formulae of naphthalene, anthracene and phenanthrene.
Naphthalene: C10H8
Anthracene: C14H10
Phenanthrene: C14H10
4. What are α- and β-positions in naphthalene?
Structure: Naphthalene, C10H8. Alpha positions: 1, 4, 5, 8; beta: 2, 3, 6, 7; fusion: 4a, 8a.
Reason: C-1 attack forms a sigma complex with contributors preserving an aromatic outer ring; alpha substitution is usually kinetically favored.
5. Why does electrophilic substitution of naphthalene generally occur at the α position?
Structure: Naphthalene, C10H8. Alpha positions: 1, 4, 5, 8; beta: 2, 3, 6, 7; fusion: 4a, 8a.
Reason: C-1 attack forms a sigma complex with contributors preserving an aromatic outer ring; alpha substitution is usually kinetically favored.
6. What is Haworth synthesis of naphthalene?
Definition: Stepwise ring building from benzene and succinic anhydride.
Sequence: Beta-benzoylpropionic acid → gamma-phenylbutyric acid → 1-tetralone → tetralin → naphthalene.
7. Differentiate industrial isolation and Haworth synthesis of naphthalene.
Coal tar isolation: Cooling, crystallization and purification recover existing naphthalene.
Haworth synthesis: Chemical construction of the fused ring from benzene and succinic anhydride.
8. Write the nitration reaction of naphthalene.
Nitration of naphthalene
Electrophile: NO2+ from the mixed acid.
Reason: C-1 attack gives the more stabilized sigma complex.
9. State the effect of temperature on sulfonation of naphthalene.
Temperature dependent sulfonation of naphthalene
Kinetic alpha product
Reason: C-1 substitution forms faster.
Thermodynamic beta product
Reason: Reversible sulfonation allows equilibration to the more stable C-2 isomer.
10. Differentiate kinetic and thermodynamic sulfonation products of naphthalene.
Temperature dependent sulfonation of naphthalene
Kinetic alpha product
Reason: C-1 substitution forms faster.
Thermodynamic beta product
Reason: Reversible sulfonation allows equilibration to the more stable C-2 isomer.
11. What are the oxidation products of naphthalene?
Oxidation products
12. Differentiate tetralin and decalin.
Comparison: Tetralin C10H12 retains one aromatic ring; decalin C10H18 has two saturated rings.
13. Define anthracene and state its molecular formula.
Structure: Anthracene, C14H10. Three linearly fused benzene rings; 14 pi electrons.
14. Why are C-9 and C-10 important in anthracene?
Structure: Anthracene, C14H10. C-9 and C-10 lie on the central ring.
Reason: Reactions at C-9/C-10 preserve benzene-like aromaticity in the two outer rings.
15. What is anthraquinone?
Definition: Anthraquinone is anthracene-9,10-dione, with two central C=O groups.
16. Differentiate anthrone and anthraquinone.
Anthrone: One C=O at C-9 and CH2 at C-10.
Anthraquinone: Two C=O groups at C-9 and C-10.
17. Differentiate anthracene and phenanthrene.
Comparison: Both C14H10; anthracene is linear and phenanthrene angular.
18. Write the condensed structures of diphenylmethane and triphenylmethane.
Structure: Diphenylmethane, C13H12. Two benzene rings linked by CH2.
Structure: Triphenylmethane, C19H16. Three benzene rings attached to one CH.
Classification: The phenyl rings share no atoms. The central bridge carbon is sp3 and breaks continuous pi conjugation.
Nomenclature: Benzyl: C6H5CH2–; benzhydryl: (C6H5)2CH–; trityl: (C6H5)3C–. Primed locants distinguish positions on separate rings.
Part B Five Mark Questions
1. Explain the classification and structural features of polynuclear hydrocarbons.
Definition: Two or more aromatic nuclei in one hydrocarbon.
Fused rings: Naphthalene, anthracene and phenanthrene share carbon pairs.
Non-fused rings: Biphenyl has a direct interring bond; diphenylmethane and triphenylmethane have sp3 carbon bridges.
2. Describe the Haworth synthesis of naphthalene with reagents and intermediates.
Haworth synthesis of naphthalene
Step 1 Friedel Crafts acylation
Key point: The succinyl fragment forms a phenyl ketone bearing a carboxyl group.
Step 2 Carbonyl reduction
Key point: Clemmensen reduction converts the aryl ketone to CH2.
Step 3 Intramolecular cyclization
Key point: Acylation closes the second six-membered ring.
Step 4 Ketone reduction
Key point: Reduction removes the ring carbonyl oxygen.
Step 5 Dehydrogenation
Key point: Loss of hydrogen restores the second aromatic ring.
3. Discuss the important electrophilic substitution reactions of naphthalene.
Nitration of naphthalene
Electrophile: NO2+ from the mixed acid.
Reason: C-1 attack gives the more stabilized sigma complex.
Bromination
Chlorination
Friedel Crafts alkylation: RCl / anhydrous AlCl3 gives mainly a 1-alkylnaphthalene under suitable conditions; the identity of R determines the complete product.
Friedel Crafts acylation: RCOCl / anhydrous AlCl3 gives mainly a 1-acylnaphthalene under suitable conditions; specify R before drawing a definite product.
Temperature dependent sulfonation of naphthalene
Kinetic alpha product
Reason: C-1 substitution forms faster.
Thermodynamic beta product
Reason: Reversible sulfonation allows equilibration to the more stable C-2 isomer.
4. Explain oxidation and reduction reactions of naphthalene.
Oxidation of naphthalene
Reason: Oxidative cleavage removes part of one fused ring.
Hydrogenation of naphthalene
Structures: Tetralin retains one aromatic ring; decalin is fully saturated and can be cis or trans.
5. Explain the medicinal importance of selected naphthalene derivatives.
Medicinal naphthalene derivatives
Naproxen
Class: Arylpropionic acid NSAID
Use: Analgesic and anti-inflammatory
Propranolol
Class: Aryloxypropanolamine beta blocker
Use: Nonselective beta-adrenergic blockade
Naftifine
Class: Allylamine antifungal
Use: Topical treatment of dermatophyte skin infections
Terbinafine
Class: Allylamine antifungal
Use: Topical or oral treatment of dermatophyte infections
Interpretation: The substituents in each complete molecule determine its medicinal action.
6. Describe the structure, aromaticity and special reactivity of anthracene.
Structure: Anthracene, C14H10. Planar, conjugated linear tricyclic aromatic compound with 14 pi electrons (4n + 2; n = 3).
Key positions: C-9 and C-10 react readily because two outer benzene rings can retain aromatic stabilization.
7. Describe the Haworth-type synthesis of anthracene.
Haworth type synthesis of anthracene
Step 1 Acylation
Step 2 Clemmensen reduction
Step 3 Ring closure
Step 4 Deoxygenation
8. Discuss the important reactions of anthracene.
Oxidation to anthraquinone
Product: Anthracene-9,10-dione has carbonyl groups at both central carbons.
Controlled hydrogenation
Reason: Hydrogen adds at the reactive central 9,10 region.
Bromine addition at 9 and 10
Reagent: Br2 in CCl4 or another inert solvent.
Product: Br adds at C-9 and C-10.
Electrophilic bromination at 9
Diels Alder cycloaddition
Reactants: Anthracene (9,10-diene region) + maleic anhydride (dienophile).
Reagent and condition: Heat in a suitable solvent; no Lewis acid is required.
Product: 9,10-ethanoanthracene-11,12-dicarboxylic anhydride.
Reason: The two outer benzene rings remain aromatic in the bridged adduct.
Photodimerization
Reaction: 2 anthracene → 9,10:9′,10′-dianthracene (UV light).
Product: Two units joined through C-9/C-9′ and C-10/C-10′.
9. Compare anthracene and phenanthrene.
Structure: Anthracene, C14H10. Linear fusion.
Structure: Phenanthrene, C14H10. Angular fusion.
Reason: Phenanthrene is more resonance-stabilized; their fusion pattern changes characteristic 9,10 reactivity.
10. Describe the structures and distinguishing features of diphenylmethane and triphenylmethane.
Structure: Diphenylmethane, C13H12. Two benzene rings linked by CH2.
Structure: Triphenylmethane, C19H16. Three benzene rings attached to one CH.
Classification: The phenyl rings share no atoms. The central bridge carbon is sp3 and breaks continuous pi conjugation.
Nomenclature: Benzyl: C6H5CH2–; benzhydryl: (C6H5)2CH–; trityl: (C6H5)3C–. Primed locants distinguish positions on separate rings.
Part C Ten Mark Questions
1. Discuss naphthalene under the headings: structure, preparation, electrophilic substitution, oxidation, reduction and medicinally important derivatives.
Structure: Naphthalene, C10H8. Two fused rings; 10 pi electrons; alpha 1,4,5,8 and beta 2,3,6,7.
Haworth synthesis of naphthalene
Step 1 Friedel Crafts acylation
Key point: The succinyl fragment forms a phenyl ketone bearing a carboxyl group.
Step 2 Carbonyl reduction
Key point: Clemmensen reduction converts the aryl ketone to CH2.
Step 3 Intramolecular cyclization
Key point: Acylation closes the second six-membered ring.
Step 4 Ketone reduction
Key point: Reduction removes the ring carbonyl oxygen.
Step 5 Dehydrogenation
Key point: Loss of hydrogen restores the second aromatic ring.
Nitration of naphthalene
Electrophile: NO2+ from the mixed acid.
Reason: C-1 attack gives the more stabilized sigma complex.
Bromination
Chlorination
Friedel Crafts alkylation: RCl / anhydrous AlCl3 gives mainly a 1-alkylnaphthalene under suitable conditions; the identity of R determines the complete product.
Friedel Crafts acylation: RCOCl / anhydrous AlCl3 gives mainly a 1-acylnaphthalene under suitable conditions; specify R before drawing a definite product.
Temperature dependent sulfonation of naphthalene
Kinetic alpha product
Reason: C-1 substitution forms faster.
Thermodynamic beta product
Reason: Reversible sulfonation allows equilibration to the more stable C-2 isomer.
Oxidation of naphthalene
Reason: Oxidative cleavage removes part of one fused ring.
Hydrogenation of naphthalene
Structures: Tetralin retains one aromatic ring; decalin is fully saturated and can be cis or trans.
Medicinal naphthalene derivatives
Naproxen
Class: Arylpropionic acid NSAID
Use: Analgesic and anti-inflammatory
Propranolol
Class: Aryloxypropanolamine beta blocker
Use: Nonselective beta-adrenergic blockade
Naftifine
Class: Allylamine antifungal
Use: Topical treatment of dermatophyte skin infections
Terbinafine
Class: Allylamine antifungal
Use: Topical or oral treatment of dermatophyte infections
Interpretation: The substituents in each complete molecule determine its medicinal action.
2. Describe anthracene in detail with structure, aromaticity, Haworth-type synthesis, important reactions and medicinally relevant derivatives.
Structure: Anthracene, C14H10. Three linear fused rings, planar conjugation and 14 pi electrons.
9 and 10 positions: Central-ring attack retains the two outer aromatic rings.
Haworth type synthesis of anthracene
Step 1 Acylation
Step 2 Clemmensen reduction
Step 3 Ring closure
Step 4 Deoxygenation
Important reactions
Oxidation to anthraquinone
Product: Anthracene-9,10-dione has carbonyl groups at both central carbons.
Controlled hydrogenation
Reason: Hydrogen adds at the reactive central 9,10 region.
Bromine addition at 9 and 10
Reagent: Br2 in CCl4 or another inert solvent.
Product: Br adds at C-9 and C-10.
Electrophilic bromination at 9
Diels Alder cycloaddition
Reactants: Anthracene (9,10-diene region) + maleic anhydride (dienophile).
Reagent and condition: Heat in a suitable solvent; no Lewis acid is required.
Product: 9,10-ethanoanthracene-11,12-dicarboxylic anhydride.
Reason: The two outer benzene rings remain aromatic in the bridged adduct.
Photodimerization
Reaction: 2 anthracene → 9,10:9′,10′-dianthracene (UV light).
Product: Two units joined through C-9/C-9′ and C-10/C-10′.
Medicinally relevant anthracene related derivatives
Dithranol: 1,8-dihydroxy-9-anthrone; anthrone derivative used topically for psoriasis.
Anthraquinone: 9,10-dione scaffold; individual glycosides and substituted derivatives have distinct uses.
Sennosides: Dianthrone glycosides; stimulant laxatives. Full glycoside stereostructures must be identified by the particular sennoside (A, B, etc.).
Doxorubicin: Anthracycline glycoside; anticancer agent. Its tetracyclic aglycone and daunosamine sugar distinguish it from anthracene.
Mitoxantrone: Substituted anthracenedione; antineoplastic agent.
3. Compare naphthalene, anthracene and phenanthrene with respect to structure, formula, ring arrangement, reactive positions and characteristic reactions.
Structure: Naphthalene, C10H8. Two fused rings; alpha C-1 is favored in electrophilic substitution.
Structure: Anthracene, C14H10. Three linear rings; the 9,10 region favors additions and oxidation.
Structure: Phenanthrene, C14H10. Three angular rings; oxidation can form phenanthrene-9,10-quinone.
Characteristic naphthalene reaction
Characteristic anthracene reaction
Characteristic phenanthrene reaction
Comparison: Naphthalene has 10 pi electrons; anthracene and phenanthrene each have 14 pi electrons but differ in ring fusion and stabilization.
4. Discuss the important reactions of anthracene and explain why the 9,10 positions are especially reactive.
Structure: Anthracene, C14H10. Three linear fused rings.
Reason: 9,10 reaction can preserve aromatic character in both outer rings.
Oxidation to anthraquinone
Product: Anthracene-9,10-dione has carbonyl groups at both central carbons.
Controlled hydrogenation
Reason: Hydrogen adds at the reactive central 9,10 region.
Bromine addition at 9 and 10
Reagent: Br2 in CCl4 or another inert solvent.
Product: Br adds at C-9 and C-10.
Electrophilic bromination at 9
Diels Alder cycloaddition
Reactants: Anthracene (9,10-diene region) + maleic anhydride (dienophile).
Reagent and condition: Heat in a suitable solvent; no Lewis acid is required.
Product: 9,10-ethanoanthracene-11,12-dicarboxylic anhydride.
Reason: The two outer benzene rings remain aromatic in the bridged adduct.
Photodimerization
Reaction: 2 anthracene → 9,10:9′,10′-dianthracene (UV light).
Product: Two units joined through C-9/C-9′ and C-10/C-10′.
5. Explain the medicinal importance of naproxen, propranolol, naftifine and terbinafine as naphthalene-containing derivatives.
Medicinal naphthalene derivatives
Naproxen
Class: Arylpropionic acid NSAID
Use: Analgesic and anti-inflammatory
Propranolol
Class: Aryloxypropanolamine beta blocker
Use: Nonselective beta-adrenergic blockade
Naftifine
Class: Allylamine antifungal
Use: Topical treatment of dermatophyte skin infections
Terbinafine
Class: Allylamine antifungal
Use: Topical or oral treatment of dermatophyte infections
Interpretation: The substituents in each complete molecule determine its medicinal action.
6. Discuss diphenylmethane and triphenylmethane under structure, classification, bridge carbon, nomenclature and relevance.
Structure: Diphenylmethane, C13H12. Two benzene rings linked by CH2.
Structure: Triphenylmethane, C19H16. Three benzene rings attached to one CH.
Classification: The phenyl rings share no atoms. The central bridge carbon is sp3 and breaks continuous pi conjugation.
Nomenclature: Benzyl: C6H5CH2–; benzhydryl: (C6H5)2CH–; trityl: (C6H5)3C–. Primed locants distinguish positions on separate rings.
Relevance: Substituted diphenylmethanes and triphenylmethanes are useful chemical scaffolds; a medicinal use belongs to a specified complete derivative.
Unit IV Reaction Checklist
Check: Naphthalene nitration: mixed acid, 50–60 °C → 1-nitronaphthalene.
Check: Naphthalene sulfonation: ~80 °C → 1-sulfonic acid; ~160 °C → 2-sulfonic acid.
Check: Haworth sequence: benzoylpropionic acid → phenylbutyric acid → tetralone → tetralin → naphthalene.
Check: Oxidation: naphthalene → phthalic anhydride or phthalic acid; anthracene → anthraquinone.
Check: Hydrogenation: naphthalene → tetralin → decalin; anthracene → 9,10-dihydroanthracene.
Check: Anthracene Br2 in inert solvent gives 9,10 addition; Br2/FeBr3 gives aromatic substitution.
Check: For medicinal compounds, include the entire derivative structure and its class and use.