有机Chapt-10自由基.ppt

1、Chapt 10 Radiacl Reactions,烷烃与卤素的自由基反应 烯烃与HBr的自由基反应 自由基聚合,The Chemistry of Radicals in Biology, Medicine, and Industry,Nitric oxide ( ) plays a remarkable number of important roles in living systems.,Radical are all around us because molecular oxygen ( ) is itself a diradical.,Radical are capable of

2、 randomly damaging all components of the body because they are highly reactive. Radical reactions are important in many industrial processes. Polymer,Introduction:,Homolytic bond dissociation (homolysis): electronically symmetrical bond breaking produces radicals (free radicals).,Single-barbed arrow

3、s are used for the movement of a single electron.,Production of Radicals,Reactions of Radicals:,Almost all small radicals are short-lived, highly reactive species. Abstraction of an atom from another molecule:,Hydrogen abstraction:,Addition to a compound containing a multiple bond to produce a new,

4、larger radical:,The Reactions of Alkanes with Halogens,1)With methane the reaction produces a mixture of halomethanes and a HX. 2)Alkanes do not react appreciably with iodine.,Multiple Substitution Reactionsversus Selectivity,Multiple substitutions almost always occur in the halogenation of alkanes.

5、 Chlorination of most of higher alkanes gives a mixture of isomeric monochloro products as well as more highly halogenated compounds.,Chlorine is relatively unselective,Chlorination of Methane: Mechanism of Reaction,Step 1 (chain-initiating step radicals are created),Chlorination of methane:,CH4 + C

6、l2 CH3Cl + HCl,Step 2 (chain-propagating step one radical generates another),With repetition of steps 2 and 3, molecules of chloromethane and HCl are procuded chain reaction.,Step 3 (chain-propagating step one radical generates another),Chain-terminating steps: used up one or both radical intermedia

7、tes.,Mechanism for the formation of CH2Cl2:,Step 3a,Step 2a,Reaction of Methane with Other Halogens: F2 Cl2 Br2,Relative Stabilities of Radicals:,The order of stability of alkyl radicals is the same as for carbocations.,Although alkyl radicals are uncharged, the carbon that bears the odd electrons i

8、s electron deficient.,Selectivity of Bromine,Bromine is less reactive toward alkanes than chlorine, but is more selective.,The Geometry of Alkyl Radicals,The geometrical structure of most alkyl radicals is trigonal planar at the carbon having the unpaired electron.,Generation of a Second Stereocente

9、r in a Radical Halogenation:,When a chiral molecule reacts to yield a product with a second stereocenter, The products of the reactions are diastereomers with different amounts.,Redical Addition to Alkenes: the Anti-Markovnikov Addition of HBr,When alkenes that contained peroxides or hydroperoxides

10、reacted with HBr anti-Markovnikov addition of HBr occurred.,CH3CH=CH2 + HBr CH3CH2CH2Br,HF, HCl, and HI do not give anti-Markovnikov addition even in the presence of peroxides.,In the absence of peroxides, or in the presence of compounds that would “trap” radicals, normal Markovnikov addition occurs

11、.,Mechanism for Anti-Markovnikov Addition,Step 3,Step 2,Step 1,Step 4,Summary of Markovnikov versus Anti-Markovnikov Addition of HBr to Alkenes,In the absence of peroxides, the reagent that attacks the double bond first is a proton. Proton is small steric effects are unimportant. Proton attaches its

12、elf to a carbon atom by an ionic mechanism to form the more stable carbocation Markovnikov addition.,Markovnikov product,More stable carbocation,In the presence of peroxides, the reagent that attacks the double bond first is the larger bromine atom. Bromine attaches itself to the less hindered carbo

13、n atom by a radical mechanism to form the more stable radical intermediate,anti-Markovnikov product,More stable radical,ROOR,Radical Polymerization of Alkenes: Chain-Growth Polymers,Polymers, called macromolecules, are made up of many repeating subunits (monomers) by polymerization reactions.,The polyethylene is useful only when it has a molecular weight of nearly 1,000,