高等有机化学 教案 1 取代基效应.ppt

Mechanism and Theory in Organic Chemistry T.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 19888 Part 1: THE COVALENT BOND,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. SF6, H2SO4,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. CH3NO2 CH2CHO,III. Valence Bond Model,A. Atomic and molecular orbitals,H + H HH,Two electrons in s1s 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. sp3 hybridization,CH4 facts:,tetrahedral, 4 equivalent bonds,III. Valence Bond Model,Hybrid atomic orbitals,1. sp3 hybridization,III. Valence Bond Model,B. Hybrid atomic orbitals,2. sp2 hybridization,C2H4 facts:,all six atoms lie in same plane,trigonal planar = sp2,III. Valence Bond Model,B. Hybrid atomic orbitals,2. sp2 hybridization,III. Valence Bond Model,B. Hybrid atomic orbitals,3. sp hybridization,C2H2 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 sp2 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 over