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1、Chapter 8 Arenes Aromatic hydrocarbons. Often abbreviated ArH. Ar- : an aryl group. Aliphatic hydrocarbons: compounds that do not contain benzene or benzene-like rings as structural units. (Historically, aliphatic was used to describe compounds derived from fats and oils.) Benzene Kekul and the stru
2、cture of benzene 8.1 Benzene and its homologs In 1865, August Kekul proposed structure of benzene based on three premises: 1. Benzene is C6H6. 2. All the hydrogens of benzene are equivalent. 3. The structural theory requires that there be four bonds to each carbon. Different!? Benzene is not cyclohe
3、xatriene, nor is it a pair of rapidly equilibrating cyclohexatriene isomers. What is it about benzene that makes it behave so much differently from other unsaturated compounds? The 140-pm bond distances in benzene are exactly midway between the typical sp2sp2 single-bond distance of 146 pm and the s
4、p2sp2 double-bond distance of 134 pm. British chemist Sir Robert Robinson: benzene is regarded as a hybrid of the two Kekul structures. Aromaticity: Special stability associated with aromatic compounds. (KJ/mol) resonance energy (23 atm pressure) (100%) (a) The framework of bonds shown in the tube m
5、odel of benzene are bonds. (b) Each carbon is sp2-hybridized and has a 2p orbital perpendicular to the framework. Overlap of the 2p orbitals generates a system encompassing the entire ring. (c) Electrostatic potential plot of benzene. The red area in the center corresponds to the region above and be
6、low the plane of the ring where the electrons are concentrated. An orbital hybridization view of bonding in benzene (a) (b) (c) The molecular orbitals of benzene Benzene is said to have a closed-shell electron configuration. 1 2 3 4 5 6 Substituted derivatives of benzene and their nomenclature Physi
7、cal properties of arenes Arenes resemble other hydrocarbons in their physical properties. Insoluble in water, but quite soluble in non-polar solvents. Less dense than water. Boiling point (b.p.) Melting point (m.p.) Benzene and toluene are widely used as solvents. 1. One mode of chemical reactivity
8、involves the ring itself as a functional group and includes (a) Electrophilic aromatic substitution (b) Reduction 2. The second family of reactions are those in which the aryl group acts as a substituent and affects the reactivity of a functional unit to which it is attached. Reactions of arenes Ele
9、ctrophilic aromatic substitution 1. Halogenation 2. Nitration 3. Sulfonation 5. Friedel-Crafts acylation 4. Friedel-Crafts alkylation Charles Friedel 1832-1899 French chemist James Mason Crafts 1839-1917 American chemist Mechanism Step 2: Step 1: Nitration of benzene Step 2: Step 1: Mechanism Sulfon
10、ation of benzene Removing the water formed in the reaction allows benzenesulfonic acid to be obtained in virtually quantitative yield. NaOHH+SO3NaONaOHStep 3: Step 1: Step 2: The mechanism of sulfonation of benzene Halogenation of benzene Step 1: Step 2: (6575%) Mechanism Friedel-Crafts alkylation o
11、f benzene (60%) Step 1: Step 2: (66%) One drawback to FriedelCrafts alkylation is that rearrangements can occur, especially when primary alkyl halides are used. (20%) (40%) Industrial preparation of styrene Laboratory preparation of styrene (1) NBS / CCl4, 80oC(2) NaOC2H5 / C2H5OHCH2=CHCl, AlCl3 54%
12、 (6568%) Other carbocation precursors can be used in place of alkyl halides. Friedel-Crafts acylation of benzene (88%) Step 1: Mechanism Step 2: Acyl chlorides are readily available: (7683%) Acyl cations do not rearrange: Synthesis of alkylbenzenes by acylationreduction FriedelCrafts acylation, foll
13、owed by Clemmensen or WolffKishner reduction is a standard sequence used to introduce a primary alkyl group onto an aromatic ring. (86%) (73%) Clemmensen reduction (82%) Wolff-Kishner-Huang Minglong reduction (84%) (80%) Haworth reaction Haworth reaction Bogert-Cook Synthesis Limitations on FriedelC
14、rafts Reactions 1.The organic halide that reacts with the arene must be an alkyl halide or an acyl halide. 2. Rearrangement of alkyl groups can occur, especially with primary alkyl halides (RCH2CH2X and R2CHCH2X). X X 3. Strongly deactivated aromatic rings do not undergo Friedel-Crafts alkylation or
15、 acylation. GG = 4. It is sometimes difficult to limit Friedel-Crafts alkylation to monoalkylation. FriedelCrafts reactions are possible with any carbocationic intermediate. H(+)-pinene, hexaneRRHAlCl3 / HCl or CF3SO3H(+)-pinene: 81% 5. FriedelCrafts alkylation is a reversible reactiondealkylation.
16、Rate and regioselectivity in electrophilic aromatic substitution 1. What is the effect of a substituent on the rate of electrophilic aromatic substitution? 2. What is the effect of a substituent on the regioselectivity of electrophilic aromatic substitution? Nitration rate: 20-25 1 1/40000 A methyl substituent is an ortho, para dire
