高等有机化学教案1取代基效应.ppt

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1、Mechanism and Theory in Organic ChemistryT.H.Lowry Harper&Row 1987高等有机化学诨魁宏、高宏宾、任责忠编 高等教育出版社19888Part 1:THE COVALENT BONDI.Lewis StructuresII.ResonanceIII.Hybrid orbitalsIV.AromaticityV.Functional GroupsLewis structure:the electron-pair bondThe Lewis structural formula can serve as a convenient star

2、ting point to understanding the valence-bond theory of molecular structure.nThe ionic bonding force arises from the electrostatic attraction between ions of opposite charge.nThe covalent bonding force arises from sharing of electron pairs between atoms.I.Lewis Structures Ionic,covalent,and polar bon

3、dsionic bonds:transfer of electronscovalent bonds:sharing of electronselectro negativity,:relative attraction for electrons in a bond-increases going up and to the right in the periodic tablenThe valence-shell occupancy must not exceed 2 for hydrogen and must not exceed 8 for atoms of the first row

4、of the periodic table.nFor elements of the second and later rows,the valence-shell occupancy may exceed 8.SF6,H2SO4The heat of hydrogenation of benzene is less exothermic by about 21 kcal mol-1(88kj mol-1)than one would have expected from Lewis structure 1 on the basis of known properties of single

5、and double bonds.Resonance structuresThe superposition of two or more Lewis structures with different electron distributions but identical nuclear positions into a composite picture is called resonance.neither of these accurately describes the formate ionactual species is an average of the two(reson

6、ance hybrid)delocalizedelectronsI.Lewis Structures Resonance structuresmore stablemajor contributorless stableminor contributorDraw resonance structures for the following species.If the structures are not equivalent,indicate which would be the major contributor.CH3NO2CH2CHOIII.Valence Bond ModelA.At

7、omic and molecular orbitalsH+H HHTwo electrons in s1s are lower energy than in the separate atoms covalent bondShape and Orientation n The electron distribution must have thecharacteristics of both s(spherical)and p(node)orbitals.n The four new electron distributions(orbitals)will orient as far away

8、 from eachother as possible.Toward the corners of atetrahedron.III.Valence Bond ModelB.Hybrid atomic orbitals1.sp3 hybridizationCH4 facts:tetrahedral,4 equivalent bondsIII.Valence Bond ModelHybrid atomic orbitals1.sp3 hybridizationIII.Valence Bond ModelB.Hybrid atomic orbitals2.sp2 hybridizationC2H4

9、 facts:all six atoms liein same planetrigonal planar=sp2III.Valence Bond ModelB.Hybrid atomic orbitals2.sp2 hybridizationIII.Valence Bond ModelB.Hybrid atomic orbitals3.sp hybridizationC2H2 facts:linear=spIII.Valence Bond ModelB.Hybrid atomic orbitals3.sp hybridizationIII.Valence Bond ModelB.Hybrid

10、atomic orbitalsWhat is the hybridization of each indicated atom in the following compound?17-ethynylestradiol(“The Pill”)IV.Functional GroupsAtoms or groups of atoms that behave similarly,regardless of the structure to which they are attached.methanolethanolb-phenethyl alcohol(lilacs)geraniolretinol

11、(vit A)IV.Functional GroupsHydrocarbons(C&H only)Heteroatomic compoundsaliphaticaromaticalkanesalkenesalkynescyclic compoundsalcoholsethersaldehydesketonescarboxylic acidsestersaminesamidesnIn the allyl system each carbon is trigonal,and each uses sp2 hybrids to make bonds to its neighbors.Delocaliz

12、ed bonding Resonance and Aromaticity1.Benzene is much less reactive toward electrophiles,such as halogens,than are simple alkenes,and when it reacts it gives substitution rather than addition.2.NMR of benzene and its derivatives shows the protons bonded to the ring to be experiencing a stronger effe

13、ctive magnetic field than do protons attached to simple alkenes.3.The heat of hydrogenation of benzene is less exothermic by about 21 kcal mol-1(88kj mol-1)than one would have expected from Lewis structure 1 on the basis of known properties of single and double bonds.AROMATICITYnAromatic compounds a

14、re cyclic unsaturated molecules characterized by certain magnetic effects and by substantially different chemical reactivity and greater thermodynamic stability than would be expected from a noncyclic model.nAntiaromatic compounds are cyclic unsaturated compounds that are thermodynamically less stab

15、le than would be expected from a noncyclic model.Capable of existence only when immobilized by freezing in an inert matrix at very low temperature.Though a stable compd.,does not have a planar ringnThe simple resonance theory fails to explain the contrast between the properties of benzene on the one

16、 hand and of cyclobutadiene and cyclooctatetraene on the other,but we may turn to molecular orbitals for the solution.Huckel theory:any regular plane polygon conjugated ringnIf the number of pairs is even(4n),the last pair will be the only occupants of a doubly degenerate level and so one electron w

17、ill go into each orbital with spins parallel.nIt is found that the 4n+2 rings are stabilized compared with the openchain analog,whereas the 4n systems are destabilized.nTheory thus explains the classification of cyclic conjugated systems as aromatic or antiaromatic according to the energy criterion,

18、which we may define as follows:a molecule is aromatic if it is thermodynamically more stable than expected for the open-chain analog,and it is antiaromatic if it is thermodynamically less stable.A compound showing neither stabilization nor destabilization would be classed as nonaromatic.nTheory sugg

19、ests that an applied magnetic field perpendicular to a conjugated ring that has 4n+2 electrons will cause the electrons to behave as though they were circulating around the ring and generating their own small magnetic field.This circulation is called the ring current.(p.40,Fig.1.27)芳香性概念n.分子高度不饱和（C/

20、H比值很高）n.高度稳定：难加成 难氧化 易亲电取代 n芳香化合物均具有相当大的共振能（离域能）（芳香性分子能量低）nNMR：芳香性：在外界磁场的作用下，产生一个与外界磁场方向相反、并与环平面垂直的磁场，使环外质子的化学位移向低场 （值较大）n 芳香性的表现归结为：独特的化学反应性能；独特的热力学稳定性和独特的光谱性质（主要是NMR）HomoaromaticitynThey are systems in which the systems is interrupted at one or more points by a saturated center but in which geomet

21、ry still permits significant overlap of the p orbitals across the insulating gap.n同芳体系是指被一个或两个Csp3隔开的环状体系，其分子的几何 形状允许P轨道超越间隔间隙进行有效重叠，且电子数符合4n+2规则而具有芳香性。因此产生芳香性的结构，叫同芳结构。此结构可由NMR证明。（一）反芳结构 n 分子因电子离域而更不稳定。由于电子离域而相当大地提高了能量的分子体系，叫反芳体系。离域的电子数为4n。n 实验判断：NMR显示出该体系具有顺磁环电流。n 环外质子向高磁场移动。n 反芳体系很少存在，因一切分子总趋于使本身

22、能量更低。Interactions between moleculesHOMO-LUMO InteractionsThe strongest interactions will be between those orbitals that are close to each other in energy.The significant interactions are between filled orbitals of one molecule and empty orbitals of the other.The most important interactions are between HOMO of one molecule and the LUMO of the other.(p70.Fig.1.38 and Fig.1.39)Application to systemnFigure 1.48,p80