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AROMATIC

AROMATIC. BENZENE. Benzene . This aromatic hydrocarbon was first discovered in 1825 but its structure was not generally agreed upon until 1946. Facts about benzene: Formula = C 6 H 6 Isomer number: one monosubstituted isomer C 6 H 5 Y known

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AROMATIC

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  1. AROMATIC BENZENE Dr Seemal Jelani Chem-160

  2. Benzene. • This aromatic hydrocarbon was first discovered in 1825 but its structure was not generally agreed upon until 1946. • Facts about benzene: • Formula = C6H6 • Isomer number: • one monosubstituted isomer C6H5Y known • three disubstituted isomers C6H4Y2 known Dr Seemal Jelani Chem-160

  3. Benzene resists addition reaction, undergoes substitution reactions. • Heats of hydrogenation and combustion are far lower than they should be. • From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds. Dr Seemal Jelani Chem-160

  4. Formula = C6H6 • Max. number of H’s for 6-carbons = 14 • Benzene only has 6 hydrogens. Given one degree of unsaturation (double bond or ring) for every two missing hydrogens less than the maximum, benzene has 4 degrees of unsaturation; that is four combinations of pi-bonds and rings. • CH3CC-CCCH3 HCC-CC-CH2CH3 • HCCCH2CCCH3 HCCCH2CH2CCH Dr Seemal Jelani Chem-160

  5. CH3 HCCCHCCH CH2=CHCCCH=CH2 CH2=CHCH=CHCCH CH2=C=CHCH2CCH CH3 CH2=C=CHCCCH3 CH2=C=CCCH CH=CH2 CH2=C=CHCH=C=CH2 CH2=C CCH CH3CH=C=CHCCH Dr Seemal Jelani Chem-160

  6. CH2 =C=C=CH2 CH2 H2C CH2 CH2 =C=CH2 CH2 HC2= =CH2 Dr Seemal Jelani Chem-160

  7. =CH2 Which of these structures is benzene? Dr Seemal Jelani Chem-160

  8. Isomer number. There is only one monosubstituted benzene of any type: only one bromobenzene C6H5Br, only one nitrobenzene C6H5NO2, etc. • CH3CC-CCCH3HCC-CC-CH2CH3 • one possiblethree possibles • HCCCH2CCCH3HCCCH2CH2CCH • three possible two possible Dr Seemal Jelani Chem-160

  9. CH3 HCCCHCCH CH2=CHCCCH=CH2 three two + CH2=CHCH=CHCCH CH2=C=CHCH2CCH five + four + CH3 CH2=C=CHCCCH3 CH2=C=CCCH three + three + CH=CH2 CH2=C=CHCH=C=CH2 CH2=C four + CCH four + CH3CH=C=CHCCH four + Dr Seemal Jelani Chem-160

  10. CH2 one possible =C=C=CH2 CH2 H2C two CH2 CH2 =C=CH2 two + CH2 three + HC2= =CH2 two Dr Seemal Jelani Chem-160

  11. =CH2 three one two Dr Seemal Jelani Chem-160

  12. There are three disubstituted benzenes of any type: three dibromobenzenes C6H4Br2, etc. CH2 CH3CC-CCCH3 two possible CH2 CH2 four four Dr Seemal Jelani Chem-160

  13. No classical valence bond structure for C6H6 correctly explains the existence of only one monosubstituted benzene and three disubstituted benzenes. Kekulé (1890) proposed that the following were in rapid equilibrium: 1,3,5-cyclohexatriene Dr Seemal Jelani Chem-160

  14. But benzene doesn’t undergo the reactions typical of unsaturated hydrocarbons! Interesting features of benzene Dr Seemal Jelani Chem-160

  15. If benzene is 1,3,5-cyclohexatriene as Kekulé proposed, what should its chemistry be? Alkenes, dienes, cyclcoalkenes, etc. typically give addition reactions with electrophiles. Dr Seemal Jelani Chem-160

  16. Reagent Cyclohexene Benzene Dr Seemal Jelani Chem-160

  17. Benzene + 3 H2, Ni, room temp.  NR Benzene + 3 H2, Ni, 200oC, 1500 psi cyclohexane Although highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead. Dr Seemal Jelani Chem-160

  18. Reactions of benzene: • Nitration • C6H6 + HNO3, H2SO4 C6H5NO2 + H2O • Sulfonation • C6H6 + H2SO4, SO3 C6H5SO3H + H2O • Halogenation • C6H6 + X2, Fe  C6H5X + HX • Freidel-Crafts alkylation • C6H6 + RX, AlCl3 C6H5R + HX • substitutions Dr Seemal Jelani Chem-160

  19. Heats of hydrogenation and combustion are far lower than they should be. cyclohexene + H2, Ni  cyclohexane + 28.6 Kcal/mole 1,3-cyclohexadiene + 2 H2, Ni  cyclohexane + 55.4 Kcal/mole (predicted value = 2 X 28.6 = 57.2 Kcal/mole) Dr Seemal Jelani Chem-160

  20. benzene + 3 H2, Ni, heat, pressure  cyclohexane + 49.8 Kcal/mole • (predicted value = 3 X 28.6 = 85.8 Kcal/mole) • Heat of hydrogenation for benzene is 36 Kcal/mole lower than predicted! Dr Seemal Jelani Chem-160

  21. e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds. C—C single bonds  1.50 Å C = C double bonds  1.34 Å The bonds in benzene are all equal and 1.39 Å but 1,3,5-cyclohexatriene has three double bonds and three single bonds! Dr Seemal Jelani Chem-160

  22. Facts about benzene: • Formula = C6H6 • Isomer number: • one monosubstituted isomer C6H5Y known • three disubstituted isomers C6H4Y2 known Dr Seemal Jelani Chem-160

  23. Benzene resists addition reaction, undergoes substitution reactions. • Heats of hydrogenation and combustion are far lower than they should be. • From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds. Dr Seemal Jelani Chem-160

  24. Aliphatic • Hydrocarbons are open-chain and ring compounds that react like open chain compounds: • Saturated: alkanes and cycloalkanes (typical reaction = substitution) • Unsaturated: alkenes, alkynes, dienes, cycloalkenes (typical reaction = addition). Dr Seemal Jelani Chem-160

  25. Aromatic • Hydrocarbons are unsaturated ring compounds that resist the typical addition reactions of aliphatic unsaturated compounds, instead undergoing substitution reactions • They are also much more stable than they should be. Dr Seemal Jelani Chem-160

  26. Nomenclature for benzene: Monosubstituted benzenes: Special names: Dr Seemal Jelani Chem-160

  27. Mercedes Benzene Dr Seemal Jelani Chem-160

  28. Disubsituted benzenes: ortho- meta- para- 1,2- 1,3- 1,4- Dr Seemal Jelani Chem-160

  29. Dr Seemal Jelani Chem-160

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  31. Dr Seemal Jelani Chem-160

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