有机化学结构与功能第三章.pdf

1 Bond Dissociation Energies Radical Halogenation and Relative Reactivity 2 Gottlieb Daimler 1834 1900 3 3 1 Strength of Alkane Bonds Radicals 3 2 Structure of Alkyl Radicals Hyperconjugation 3 3 Conversion of Petroleum Pyrolysis 3 4 Chlorination of Methane The Radical Chain Mechanism 3 5 Other Radical Halogenations of Methane 3 6 Chlorination of Higher Alkanes Relative Reactivity and Selectivity 3 7 Selectivity in Radical Halogenation with Fluorine and Bromine 3 8 Synthetic Radical Halogenation 3 9 Synthetic Chlorine Compounds and The Stratospheric Ozone Layer 3 10 Combustion and the relative Stabilities of Alkanes 4 3 1 Strength of Alkane Bonds Radicals Radicals are formed by homolytic cleavage Homolytic Cleavage A B A B Radicals Heterolytic Cleavage A BA B Ions 5 Bond Dissociation Energies of Various A B DHoin kcal mol 1 B in A B A in A B H F Cl Br I OH NH2 H 1041351038771119107 CH3 1051108571579380 CH3CH2 981078068539277 CH3CH2CH2 981078168539178 CH3 2CH 94 51068168539293 CH3 3C 931108167529393 The stability of radicals determines the C H bond strengths Compare the bond dissociation energies of CH3 F CH3 OH and CH3 NH2 Why do the bonds get weaker along this series even though the orbitals participating in overlap become better matched in size and energy 6 Bond Dissociation Energies for Some Alkanes CompoundDHo kcal mol 1 CompoundDHo kcal mol 1 CH3 H105CH3 CH390 C2H5 H98C2H5 CH386 C3H7 H98C3H7 CH387 CH3 2CHCH2 H98C2H5 C2H582 CH3 2CH H94 5 CH3 2C CH386 CH3 3C H93 CH3 3 CH384 CH3 3 CH3 372 7 Figure 3 1 The different energies needed to form radicals from an alkane CH3CH2CHR2 Radical stability increases from primary to secondary to tertiary 8 Which C C bond would break first the bond in ethane or that in 2 2 dimethylpropane Stability of Alkyl Radicals CH3 pirmary secondary tertiary 9 3 2 Structure of Alkyl Radicals Hyperconjugation Figure 3 2 The hybridization change upon formation of a methyl radical from methane The nearly planar arrangement is reminiscent of the hybridization in BH3 10 Figure 3 3 Hyperconjugation green dashed lines resulting from the donation of electrons in filled sp3 hybrids to the partly filled p orbital in A ethyl and B 1 methylethyl and 1 1 dimethylethyl radicals The resulting delocalization of electron density has a net stabilizing 11 3 3 Conversion of Petroleum Pyrolysis Pyrolysis of Hexane Cleavage of C C bonds into radicals CH3CH2CH2CH2CH2CH3 1 2 3 4 C1 C2 cleavage CH3 CH2CH2CH2CH2CH3 C2 C3 cleavage CH3CH2 CH2CH2CH2CH3 C3 C4 cleavage CH3CH3CH3 CH2CH2CH2CH3 Examples of Radical Combination Reactions CH3 CH2CH3CH3CH2CH3 CH2CH2CH2CH2CH3CH3CH2CH2 CH3CH2CH2CH2CH2CH2CH2CH3 Examples of hydrogen abstraction reactions CH3CH2CH2CHCH3 H CH3CH3CH2 CHCH3 H CH2CH2CH2CH2CH3 H CH2 CH2CH2CH2CH3 H 12 E Reaction coordinate E a E ca t The Function of a Catalyst Dodecane Zeolite 482 oC 2 min C3 C4 C5 C6 other products 13 Product Distribution in Typical Distillation of Crude Petroleum Amount of volume Boiling Point oC Carbon AtomsProducts 1 2 30C1 C4 Natural Gas methane propane butane liquefied petroleum gas 15 30400 C25 Residual oil paraffin waxes asphalt tar 14 3 4 Chlorination of Methane The Radical Chain Mechanism Ho energy input energy output DHo bonds broken DHo bonds formed 105 58 85 103 25 kcal mol 1 CH3HClCl 10558 DHo kcal mol 1 CH3HClCl 85103 or hv 15 Experimental observation Only a relatively small number of initiation events are necessary to enable a great many methane and chlorine molecules to undergo conversion into products ClCl Chlorine atom Cl or hv 2 Ho DHo Cl2 58 kcal mol 1 INITIATION Cl Cl Ho DHo CH3 H DHo H Cl 2 kcal mol 1 PROPAGATION STEP 1 C H H HH H C H H H DHo kcal mol 1 105 103 16 Figure 3 4 Approximate molecular orbital description of the abstraction of a hydrogen atom by a chlorine atom to give a methyl radical and hydrogen chloride Notice the rehybridization at carbon in the planar methyl radical The additional three nonbonded electron pairs on chlorine have been omitted The orbitals are not drawn to scale The symbol identifies the transition state 17 Figure 3 5 Potential energy diagram of the reaction of methane with a chlorine atom Partial bonds in the transition state are depicted by dotted lines This process propagation step 1 in the radical chain chlorination of methane is slightly endothermic 18 Ho DHo Cl Cl DHo CH3 Cl 27 kcal mol 1 PROPAGATION STEP 2 DHo kcal mol 1 58 Cl Cl C H H H C H H HCl Cl 85 CH4 Cl Slightly unfavorable CH3 HClCH3Cl Cl HCl Very favorable Cl2 CH4 ClCH3 HCl CH3 Cl2CH3Cl Cl Ho kcal mol 1 2 27 CH4 Cl2CH3Cl HCl 25 19 Figure 3 6 Complete potential energy diagram for the formation of CH3Cl from methane and chlorine Propagation step1 has the higher transition state energy and is therefore slower The Hoof the overall reaction CH4 Cl2 CH3Cl HCl amounts to 25 kcal mol 1 20 Experimental observation Small amounts of ethane are identified among the products of chlorination of methane CHAIN TERMINATION Cl Cl Cl2 Cl CH3CH3Cl CH3 CH3 CH3CH3 A Radical Chain Mechanism InitiationPropagation steps XX X or hv 2 X RHR HX R X2 RX X Termination 21 Chlorination of ethane furnishes chloroethane Write a mechanism for this transformation 22 3 5 Other Radical Halogenations of Methane DHovalues for the elemental halogens HalogenDHo kcal mol 1 F237 Cl258 Br246 I236 F2is easier to dissociate than Cl2 So is I2 However 23 Enthalpies of the Propagation Steps in the Halogenation of Methane kcal mol 1 Reaction X CH4 CH3 HX 30 2 18 34 F Cl Br I CH3 X2 CH3X X 73 27 25 21 CH4 X2 CH3X HX 103 25 7 13 24 Figure 3 7 Potential energy diagrams 25 Hammond Postulate Early transition states are frequently characteristic of fast exothermic processes Late transition states are frequently typical of relatively slow endothermic transformations Prof George S Hammond 1921 2005 Georgetown Univ Washington D C JACS 1955 77 334 26 Predict the product distribution of the reaction of methane with an equimolar mixture of chlorine and bromine CH4 Cl2 Br2 CH3Cl CH4 Cl2 Br2 HCl 27 3 6 Chlorination of Higher Alkanes Relative Reactivity and Selectivity Chlorination of Ethane CH3CH3 Cl2 or hv CH3CH2Cl HClHo 27 kcal mol 1 Chloroethane Propagation Steps CH3CH3 ClCH3CH2 HClHo 5 kcal mol 1 CH3CH2 Cl2CH3CH2Cl ClHo 22 kcal mol 1 28 Secondary C H bonds are more reactive than primary ones 29 Statistical ratio of products Relative reactivity of a secondary hydrogen Relative reactivity of a primary hydrogen Yield of product from secondary hydrogen abstraction Yield of product from primary hydrogen abstraction number of secondary hydrogens number of primary hydrogens 57 2 43 6 4 Expected statistical ratio Expected C H bond reactivity ratio Experimental ratio 25 oC 3 1 Less more 43 57 Chlorination of Propane CH3CH2CH3 Cl2 or hv CH3CH2CH2Cl CH3CHCH3 HCl Cl 30 Tertiary C H bonds are more reactive than secondary ones hv Expected statistical ratio Expected C H bond reactivity ratio Experimental ratio 25 oC 9 1 Less more 64 36 Relative reactivity of a tertiary hydrogen Relative reactivity of a primary hydrogen Yield of product from tertiary hydrogen abstraction Yield of product from primary hydrogen abstraction number of tertiary hydrogens number of primary hydrogens 36 1 64 9 5 Cl2C CH3 CH3 H3CH C CH3 CH3 HClCH2 C CH3 CH3 H3CCl 31 Calculate the relative reactivity ratio of the reaction below Primary secondary tertiary halide 41 36 23 Relative reactivity primary secondary tertiary 41 9 36 2 23 1 Chrorination of 2 Methylbutane hv Cl2C CH3 H H3CCH2CH3 HCl C CH3 H ClH2CCH2CH3C CH3 H H3CCH2CH2ClC CH3 H H3CCHCH3 Cl C CH3 Cl H3CCH2CH3 27 14 36 23 32 3 7 Selectivity in Radical Halogenation with Fluorine and Bromine X2 C CH3 H3C CH3 HC CH3 H3C CH3 XC CH3 XH2C CH3 H Fluorination of tert butane X F 14 86 X Br 99 1 Statistically Expected 1 9 33 34 35 Cl2 HCl Cl Cyclopentane Large excess 92 7 Chlorocyclopentane hv Chloroination of a Molecule with Only One Type of Hydrogen 3 8 Synthetic Radical Haloge