Aromaticity hydrocarbons aliphatic aromatic alkanes alkenes alkynes

1. Aromaticity hydrocarbons aliphaticaromatic alkanesalkenesalkynes

2. Aliphatichydrocarbons 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). Aromatichydrocarbons are unsaturated ring compounds that resist the typical addition reactions of aliphatic unsaturated compounds, instead undergoing substitution reactions. They are also much more stable. Benzene and related compounds.

3. 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 • Benzene resists addition reaction, undergoes substitution reactions.

4. d) Heats of hydrogenation and combustion are far lower than they should be 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

5. Experimental evidence suggest that all six carbon-carbon bonds in benzene are identical. • The properties, including the above one, of benzene can only be explained by assuming that the actual structure of benzene is an average of the above two possible equivalent structures-known as resonance. • Simple aromatic compounds like benzene are non-polar, insoluble in water, volatile, and flammable. • Unlike alkenes, several aromatic hydrocarbons are toxic. Benzene itself is implicated as a cancer causing chemical.

6. 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

7. 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 CH2=C=CHCH=C=CH2 CH3CH=C=CHCCH

8. CH 2 =C=C=CH2 CH2 H2C CH2 CH2 =C=CH2 CH2 H2C= =CH2

9. =CH2 Which of these structures is benzene?

10. 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

11. 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 + CH2=C=CHCH=C=CH2 four + CH3CH=C=CHCCH four +

12. CH2 one possible =C=C=CH2 CH2 H2C two CH2 CH2 =C=CH2 two + CH2 three + HC2= =CH2 two

13. =CH2 one three two

14. There are three disubstituted benzenes of any type: three dibromobenzenes C6H4Br2, etc. CH2 CH3CC-CCCH3 two possible CH2 CH2 four four

15. 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:

16. 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. But benzene doesn’t undergo the reactions typical of unsaturated hydrocarbons!

17. Reagent Cyclohexene Benzene

18. Unusual Reactions • Alkene + KMnO4 diol (addition)Benzene + KMnO4 no reaction. • Alkene + Br2/CCl4  dibromide (addition)Benzene + Br2/CCl4  no reaction. • With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain. => Chapter 16

19. 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.

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

21. 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) • benzene + 3 H2, Ni, heat, pressure  cyclohexane + 49.8 Kcal/mol (predicted value = 3 X 28.6 = 85.8 Kcal/mole) • Heat of hydrogenation for benzene is 36 Kcal/mole

22. 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!

23. Resonance! We can draw more than one classic structure that differ only in where the electrons are. The two structures are of the same energy, so resonance is important. The molecule cannot be adequately represented by classic structures but must be thought of as a hybrid of the contributing structures. Additionally, the hybrid is more stable than any of the contributing structures (resonance stabilization energy).

24. Facts about benzene: Formula = C6H6 • Isomer number: • one monosubstituted isomer C6H5Y known • three disubstituted isomers C6H4Y2 known • 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.

25. Can we predict which compounds will be aliphatic and which ones will be aromatic like benzene? Yes. • In order to be aromatic, the compound must be: • cyclic with p-atomic orbitals on all members of the ring. • and • have 4i + 2electrons in the p-orbitals of the ring (where i = 0, 1, 2, 3, …). • [ eg. = 2 or 6 or 10 or 14 or 18 or 22 or 26… ]

27. annulenes: monocyclic compounds with the formula: -[-CH=CH-]n-

28. • : 5 e- 6 e- 4 e- 4 e- : • 6 e- does not have p-orbitals on all atoms 8 e- 7 e-

29. Nomenclature for benzene: monosubstituted benzenes: Special names:

30. Mercedes Benzene

31. Disubsituted benzenes: ortho- meta- 1,2- 1,3- 1,4- para-

33. Phenyl and Benzyl Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon. => Chapter 16

34. New Attached Groups • Phenyl Benzyl Nitro -NO2 2,4,6-trinitrotoluene 4-phenyl-1-butene Benzyl alcohol