有机化学课件及复习题答案chapter

1、1TriptyceneFullereneCHAPTER 5, Aromatic Compounds25.1 Preface The word aromatic was used to describe some fragrant compounds in the early days of organic chemistry( in early 19th century). Today, we use the word aromatic to refer to the class of compounds that contain benzene-like, six-membered ring

2、s with three double bonds. Many important compounds are aromatic in part, including the steroidal hormone estrone and the analgestic ibuprofen. BenzeneIbuprofenEstroneCOOHOH3CHHHHOCOOHOOCCH3Aspirin3*5.2 Structure of Benzene: The Kekule and Resonance Proposal All carbon-carbon bonds in benzene have t

3、he same length, intermediate between a typical single bond and a typical double bond. Most C-C single bonds have length near 0.154nm, and most C=C bonds are about 0.134nm, But the carbon-carbon bonds in benzene are 0.139nm long. Electronic density in all six carbon-carbon bonds is identical. Benzene

4、 is a flat, symmetrical molecule with the shape of a regular hexagon. AllC-C-C bond angles are 1200, each carbon atom is sp2-hybridized, and each carbonhas a p orbitals perpendicular to the plane of six-membered ring. Each p orbital overlaps equally well with both neighboring p orbitals, leading to

5、a structure for benzene in which the electrons are delocalized around the ring in two doughnut-shaped clouds.4567HHHHHH An orbital picture of benzene. Each of six carbon atoms has a p orbital that can overlap equally well with neighboring p orbitals on both sides. The electrons are thus delocalized

6、around the ring in two doughnut-shaped clouds.85.3 Naming Aromatic CompounddsMany have common names: Toulene = Methylbenzene, Aniline =aminobenzene etc)Monosubstituted benzenes systematic names as hydrocarbons with-Benzenes: C6H5Br: Bromobenzene, C6H5NO2: NitrobenzenePhenyl, BenzylDisubstituted benz

7、enes: 1,2: ortho(o); 1,3: meta(m); 1,4: para(p)ortho-DichlorobenzeneClClClClClClmeta-Dichlorobenzenepara-Dichlorobenzene9*5.4 Chemistry of Benzene: Electrophilic Aromatic Substitution Reactions The most important reaction of aromatic compounds is electrophilic aromaticsubstitution. That is, an elect

8、ron-poor reagent (an electrophile, E+) reacts with an aromatic ring and substitutes for one of the ring hydrogens:HHHHHH+E+HHHEHH+H+ Many different substitute can be introduced onto the aromatic ring by electrophilic substitution reactions.10*Electrophilic Aromatic Substitution Reactions E+XX=F, Cl,

9、 Br, INO2SO3HRCORHalogenationNitrationSulfonationAlkylationAcylation11*Mechanism of Bromination of BenzeneBenzene electrons participate as Lewis base in reactionThe product is formed by loss a proton, which is replaced by bromineFeBr3 used as a catalyst to polarize the bromine reagent.The cationic i

10、ntermediate was first proposed by G W Wheland of University of Chicago and often called wheland intermediate BrBrBrHBrHBrHHBrBrGeorge Willard Wheland(1907-1974)1213BrBrslowBrHFeBr4Nonaromatic carbocationfastBr+ HBr+ FeBr3The cationic addition intermediate transfers a proton to FeBr4 (from Br- and Fe

11、Br3)This restores the aromaticity ( in contrast with addition in alkenes-14Aromatic Chlorination and IodinationChlorine and Iodine (but not fluorine, which is too reactive) can produce aromatic substitution with the addition of other reagents to promote the reactionChlorination requires FeCl3Iodine

12、must be oxidized to form a more powerful I species (with Cu or Peroxide)I2CuCl2II2HNO3I15Aromatic Nination and Sulfonation+HNO3H2SO4(Con)NO2NOOOHHNOOOHHNO2fuming sulfuric acidSO3HSO3+ H2SO4SO3H+ HSO4Nitration of aromatic rings is a key step in the synthesis of fine chemicalsSulfonation is also a key

13、 step in the synthesis of fine chemicals. Sulfonation is reversible reaction16*The Friedel-Crafts Alkylation and Acylation+Cl+OClOAromatic substitution of a R+ for HAluminum chloride (Lewis acid) promotes the formation of the carbocationWheland intermediate17The limitations of Friedel-Crafts Alkylat

14、ionAryl halides and vinylic halides do not reactAminobenzene and a strongly electron-withdrawing group on the benzenes do not workMultiple alkylations can occur because the first alkylation is activatingRearrangement of R+ often take place18CH2CH=CH2+AlCl3HClCHCH3CH3OSnCl4CH2Cl2OHHONHCOCF3SMeSO3HCH2

15、Cl2NHCOCF3S19CH3COCH3 + 2OHOHH2SO4HOCCH3CH3+ HCHO + HClCH2ClH3PO4CH3COOH77%20*5.4 Substituent Effects in Electrophilic Aromatic Substitution Reactions OHNO2ClHRelative rate of Nitration6X10-80.0331100021H2SO4OH250CHNO3OHNO2OHNO2OHNO2+50%0%50%H2SO4CN250CHNO3CNNO2CNNO2CNNO2+17%81%2%H2SO4Cl250CHNO3ClNO

16、2ClNO2ClNO2+30%1%69%22CH3(alkyl)NHCOCH3OCH3OHNH2Ortho and para-directing activatorsOrtho and para-directing deactivatorsMeta-directing deactivatorsNR3NO2C NSO3HC CH3OC OHOC OCH3OC HOIBrClF23*5.5 An Explanation of Substituent Effects Activating groups donate electrons to the ring, stabilizing the Whe

17、land intermediate Deactivating groups withdraw electron from the ring, destabilized the Wheland intermediateGHGE+E+E+GHGEHEHEHG donate electrons; carbocation is more stable and the ring is more reactiveG withdraw electrons; carbocation is less stable and the ring is less reactive24Origins of Substit

18、uent EffectsInductive effect: withdrawal or donation of electrons through a bondResonance effect: withdrawal or donation of electrons through a bond due to the overlap of a orbital on the substituent with a orbital on the aromatic ring25Inductive effectsControlled by electronegativity and the polari

19、ty of bonds in functional groupsHalogens, C=O, CN and NO2 withdraw electron through bond connected to the ringAlkyl groups donate electronsXCOC NNOO-(X=F, Cl, Br, I)-+-+CH326Resonance effects 1: Electron WithdrawalC=O, CN and NO2 substituents withdraw electrons from aromatic ring by resonance electr

20、ons flow from the ring to substituentsYZCOC NNOO27Resonance effects 2: Electron DonatingHalogens, OH, OR and amino substituents donate electrons electrons flow from the substituents to the ringThe effect is greatest at ortho and paraYORNH2OHX28Contrasting EffectsHalogens, OH and OR withdraw electron

21、s inductively so that deactivate the ringResonance interaction affect orientationThe strongest effects dominate29CH3OrthoAttackAttackAttackMetapara63%3%34%CH3NO2HCH3NO2HCH3NO2HMost stableHNO2CH3HNO2CH3HNO2CH3CH3HNO2CH3HNO2CH3HNO2Most stable30OHOrthoAttackAttackAttackMetapara50%0%50%OHNO2HOHNO2HOHNO2HMost stableHNO2OHHNO2OHHNO2OHOHHNO2OHHNO2OHHNO2Most stableOHNO2HOHHNO231ClOrthoAttackAttackAttackMetapara35%1%64%ClNO2HClNO2HClNO2HMost stable