Aromaticityhydrocarbons aliphaticaromaticalkanes alkenes .ppt

1、Aromaticityhydrocarbonsaliphatic aromaticalkanes alkenes alkynes,Aliphatic compounds: open-chain compounds and ring compounds that are chemically similar to open-chain compounds. Alkanes, alkenes, alkynes, dienes, alicyclics, etc.Aromatic compounds: unsaturated ring compounds that are far more stabl

2、e than they should be and resist the addition reactions typical of unsaturated aliphatic compounds. Benzene and related compounds.,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 num

3、ber:one monosubstituted isomer C6H5Y knownthree 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 CC bonds in benzene are the same length and intermed

4、iate in length between single and double bonds.,a) Formula = C6H6 Max. number of Hs 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 comb

5、inations of pi-bonds and rings.CH3CC-CCCH3 HCC-CC-CH2CH3HCCCH2CCCH3 HCCCH2CH2CCH,CH3 HCCCHCCH CH2=CHCCCH=CH2CH2=CHCH=CHCCH CH2=C=CHCH2CCHCH3 CH2=C=CHCCCH3 CH2=C=CCCHCH=CH2 CH2=C=CHCH=C=CH2 CH2=CCCH CH3CH=C=CHCCH,CH2,H2C,CH2,=C=C=CH2,=C=CH2,CH2,CH2,CH2,=CH2,HC2=,=CH2,Which of these structures is benz

6、ene?,Isomer number. There is only one monosubstituted benzene of any type: only one bromobenzene C6H5Br, only one nitrobenzene C6H5NO2, etc.CH3CC-CCCH3 HCC-CC-CH2CH3one possible three possiblesHCCCH2CCCH3 HCCCH2CH2CCHthree possible two possible,CH3 HCCCHCCH CH2=CHCCCH=CH2three two +CH2=CHCH=CHCCH CH

7、2=C=CHCH2CCHfive + four +CH3 CH2=C=CHCCCH3 CH2=C=CCCHthree + three +CH=CH2 CH2=C=CHCH=C=CH2 CH2=Cfour + CCH four +CH3CH=C=CHCCH four +,CH2,H2C,CH2,=C=C=CH2,=C=CH2,CH2,CH2,CH2,=CH2,HC2=,one possible,two,three +,two +,two,=CH2,three,one,two,CH3CC-CCCH3,There are three disubstituted benzenes of any typ

8、e: three dibromobenzenes C6H4Br2, etc.,CH2,CH2,CH2,two possible,four,four,No classical valence bond structure for C6H6 correctly explains the existence of only one monosubstituted benzene and three disubstituted benzenes. Kekul (1890) proposed that the following were in rapid equilibrium:,If benzene

9、 is 1,3,5-cyclohexatriene as Kekul proposed, what should its chemistry be? Alkenes, dienes, cyclcoalkenes, etc. typically give addition reactions with electrophiles. But benzene doesnt undergo the reactions typical of unsaturated hydrocarbons!,Reagent Cyclohexene Benzene,Benzene + 3 H2, Ni, room tem

10、p. NRBenzene + 3 H2, Ni, 200oC, 1500 psi cyclohexaneAlthough highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead.,Reactions of benzene: NitrationC6H6 + HNO3, H2SO4 C6H5NO2 + H2O SulfonationC

11、6H6 + H2SO4, SO3 C6H5SO3H + H2O HalogenationC6H6 + X2, Fe C6H5X + HX Freidel-Crafts alkylationC6H6 + RX, AlCl3 C6H5R + HX substitutions,d) Heats of hydrogenation and combustion are far lower than they should be.cyclohexene + H2, Ni cyclohexane + 28.6 Kcal/mole1,3-cyclohexadiene + 2 H2, Ni cyclohexan

12、e + 55.4 Kcal/mole(predicted value = 2 X 28.6 = 57.2 Kcal/mole)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!,e) From X-ray, all of the CC bonds in benzene are the sam

13、e length and intermediate in length between single and double bonds.CC single bonds 1.50 C = C double bonds 1.34 The bonds in benzene are all equal and 1.39 ,but 1,3,5-cyclohexatriene has three double bonds and three single bonds!,Resonance!,We can draw more than one classic structure that differ on

14、ly 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 contribu

15、ting structures (resonance stabilization energy).,Facts about benzene: Formula = C6H6 Isomer number:one monosubstituted isomer C6H5Y knownthree disubstituted isomers C6H4Y2 known Benzene resists addition reaction, undergoes substitution reactions. Heats of hydrogenation and combustion are far lower

16、than they should be. From X-ray, all of the CC bonds in benzene are the same length and intermediate in length between single and double bonds.,Aliphatic hydrocarbons are open-chain and ring compounds that react like open chain compounds: saturated: alkanes and cycloalkanes (typical reaction = subst

17、itution) unsaturated: alkenes, alkynes, dienes, cycloalkenes (typical reaction = addition). 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 th

18、an they should be.,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 + 2 electrons in the p-orbitals of the ring (where i = 0, 1, 2,

19、3, ). eg. = 2 or 6 or 10 or 14 or 18 or 22 or 26 ,annulenes: monocyclic compounds with the formula:-CH=CH-n-,:,:,4 e-,4 e-,5 e-,does not have p-orbitals on all atoms,8 e-,7 e-,6 e-,6 e-,Nomenclature for benzene: monosubstituted benzenes: Special names:,Mercedes Benzene,Disubsituted benzenes:ortho- meta- para-1,2- 1,3- 1,4-,