高等有机化学1取代基效应.pptx

,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,#,Lewis Structures,Resonance,Hybrid orbitals,Aromaticity,Functional Groups,Lewis structure:,the electron-pair bond,The Lewis structural formula can serve as a convenient starting point to understanding the,valence-bond theory,of molecular structure.,The ionic bonding force,arises from the electrostatic attraction between ions of opposite charge.,The covalent bonding force,arises from sharing of electron pairs between atoms.,I.Lewis Structures,Ionic,covalent,and polar bonds,ionic bonds,:,transfer,of electrons,covalent bonds,:,sharing,of electrons,electro negativity,:,relative attraction for electrons in a bond,-increases going up and to the right in the periodic table,The valence-shell occupancy,must not exceed 2 for hydrogen and must not exceed 8 for atoms of the first row of the periodic table.,For elements of the second and later rows,the valence-shell occupancy may exceed 8.,SF,6,H,2,SO,4,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.,Resonance structures,The,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 ion,actual species is an average of the two(,resonance hybrid,),delocalized,electrons,I.Lewis Structures,Resonance structures,more stable,major contributor,less stable,minor contributor,Draw resonance structures for the following species.If the structures are not equivalent,indicate which would be the major contributor.,CH,3,NO,2,CH,2,CHO,III.Valence Bond Model,A.Atomic and molecular orbitals,H+H,HH,Two electrons in,s,1,s,are lower energy than in the separate atoms,covalent bond,Shape and Orientation,The electron distribution must have the,characteristics of both s(spherical)and p,(node)orbitals.,The four new electron distributions,(orbitals)will orient as far away from each,other as possible.Toward the corners of a,tetrahedron.,III.Valence Bond Model,B.Hybrid atomic orbitals,1.,sp,3,hybridization,CH,4,facts:,tetrahedral,4 equivalent bonds,III.Valence Bond Model,Hybrid atomic orbitals,1.,sp,3,hybridization,III.Valence Bond Model,B.Hybrid atomic orbitals,2.,sp,2,hybridization,C,2,H,4,facts:,all six atoms lie,in same plane,trigonal planar=,sp,2,III.Valence Bond Model,B.Hybrid atomic orbitals,2.,sp,2,hybridization,III.Valence Bond Model,B.Hybrid atomic orbitals,3.,sp,hybridization,C,2,H,2,facts:,linear=,sp,III.Valence Bond Model,B.Hybrid atomic orbitals,3.,sp,hybridization,III.Valence Bond Model,B.Hybrid atomic orbitals,What is the hybridization of each indicated atom in the following compound?,17-ethynylestradiol,(“The Pill”),IV.Functional Groups,Atoms or groups of atoms that behave similarly,regardless of the structure to which they are attached.,methanol,ethanol,b,-phenethyl alcohol(lilacs),geraniol,retinol(vit A),IV.Functional Groups,Hydrocarbons(C&H only),Heteroatomic compounds,aliphatic,aromatic,alkanes,alkenes,alkynes,cyclic compounds,alcohols,ethers,aldehydes,ketones,carboxylic acids,esters,amines,amides,In the allyl system each carbon is trigonal,and each uses sp,2,hybrids to make bonds to its neighbors.,Delocalized,bonding,Resonance and Aromaticity,1.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 effective 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,.,AROMATICITY,Aromatic compounds,are,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.,Antiaromatic compounds,are cyclic unsaturated compounds that are thermodynamically less stable 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 ring,The simple resonance theory fails to explain the contrast between the properties of,benzene,on the one 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 ring,If the number of pairs is even(4n),the last pair will be the only occupants of a doubly degenerate level and so one electron will go into each orbital with spins parallel.,It is found that the 4n+2 rings are stabilized compared with the openchain analog,whereas the 4n systems are destabilized.,Theory thus explains the classification of cyclic conjugated systems as aromatic or antiaromatic according to the energy criterion,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.,Theory suggests 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,.,分子高度不饱和（,C/H,比值很高）,.,高度稳定：难加成 难氧化 易亲电取代,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）,NMR,：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）,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,）,Homoaromaticity,They are systems in which the,systems is interrupted at one or more points by a saturated center but in which geometry still permits significant overlap of the,p,orbitals across the insulating gap.,同芳体系是指被一个或两个,Csp,3,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,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,。,实验判断：,NMR,显示出该体系具有顺磁环电流。,环外质子向高磁场移动。,。,反芳体系很少存在，因一切分子总趋于使本身能量更低。,Interactions between molecules,HOMO-LUMO Interactions,The 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,system,Figure 1.48,p80,