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8-8-1 1HaloalkanesChapter 8Chapter 88-8-2 2Structureu uHaloalkane(alkyl halide):Haloalkane(alkyl halide):a compound containing a halogen covalently bonded to an sp3 hybridized carbon;given the symbol RXu uHaloalkene(vinylic halide):Haloalkene(vinylic halide):a compound containing a halogen bonded to an sp2 hybridized carbonu uHaloarene(aryl halide):Haloarene(aryl halide):a compound containing a halogen bonded to a benzene ring;given the symbol ArX(we do not study vinylic or aryl halides in this chapter)8-8-3 3Nomenclaturenumber the parent chain to give the substituent encountered first the lowest number,whether it is halogen or an alkyl groupindicate halogen substituents by the prefixes fluoro-,chloro-,bromo-,and iodo-,and list them in alphabetical order with other substituentslocate each halogen on the parent chain by giving it a number preceding the name of the halogenin haloalkenes,number the parent chain to give carbon atoms of the double bond the lower set of numbers8-8-4 4Nomenclatureexamplesu uCommon names:Common names:name the alkyl group followed by the name of the halide8-8-5 5Nomenclatureseveral polyhaloalkanes are common solvents and are generally referred to by their common or trivial nameshydrocarbons in which all hydrogens are replaced by halogens are commonly named as perhaloalkanes or perhaloalkenes8-8-6 6Dipole MomentsuDipole moment of RX depends on:the sizes of the partial charges the distance between them the polarizability of the unshared electrons on halogen8-8-7 7van der Waals ForcesuHaloalkanes are associated in the liquid state by van der Waals forcesu uvan der Waals forces:van der Waals forces:a group intermolecular attractive forces including dipole-dipole forcesdipole-induced dipole forcesinduced dipole-induced dipole(dispersion)forcesuvan der Waals forces pull molecules togetheras molecules are brought closer and closer,van der Waals attractive forces are overcome by repulsive forces between electron clouds of adjacent atoms or molecules8-8-8 8van der Waals Forcesthe energy minimum is where the attractive forces are the strongestnonbonded interatomic and intermolecular distances at these minima can be measured by x-ray crystallography and each atom and group of atoms can be assigned a van der Waals radiusvan der Waals radiusnonbonded atoms in a molecule cannot approach each other closer than the sum of their van der Waals radii without causing nonbonded interaction strain 8-8-9 9Boiling PointsuFor an alkane and a haloalkane of comparable size and shape,the haloalkane has the higher boiling pointthe difference is due almost entirely to the greater polarizability of the three unshared pairs of electrons on halogen compared with the low polarizability of shared electron pairs of covalent bonds polarizability:polarizability:a measure of the ease of distortion of the distribution of electron density about an atom in response to interaction with other molecules and ions;fluorine has a very low polarizability,iodine has a very high polarizability8-8-1010Boiling Pointsamong constitutional isomers,branched isomers have a more compact shape,decreased area of contact,decreased van der Waals attractive forces between neighbors,and lower boiling points8-8-1111Boiling Pointsboiling points of fluoroalkanes are comparable to those of hydrocarbons of similar molecular weight and shapethe low boiling points of fluoroalkanes are the result of the small size of fluorine,the tightness with which its electrons are held,and their particularly low polarizability8-8-1212DensityuThe densities of liquid haloalkanes are greater than those of hydrocarbons of comparable molecular weighta halogen has a greater mass per volume than a methyl or methylene groupuAll liquid bromoalkanes and iodoalkanes are more dense than wateruDi-and polyhalogenated alkanes are more dense than water8-8-1313Bond Lengths,StrengthsuC-F bonds are stronger than C-H bonds;C-Cl,C-Br,and C-I bonds are weaker8-8-1414Halogenation of AlkanesuIf a mixture of methane and chlorine is kept in the dark at room temperature,no change occursuIf the mixture is heated or exposed to visible or ultraviolet light,reaction begins at once with the evolution of heatu uSubstitution:Substitution:a reaction in which an atom or group of atoms is replaced by another atom or group of atoms8-8-1515Halogenation of Alkanesif chloromethane is allowed to react with more chlorine,a mixture of chloromethanes is formed8-8-1616RegioselectivityuRegioselectivity is high for bromination,but not as high for chlorination8-8-1717RegioselectivityuRegioselectivity is 3 2 1for bromination,approximately 1600:80:1for chlorination,approximately 5:4:1Example:Example:draw all monobromination products and predict the percentage of each for this reaction 8-8-1818EnergeticsuBond Dissociation Enthalpies(BDE)8-8-1919EnergeticsuUsing BDE,we can calculate the heat of reaction,H0,for the halogenation of an alkane8-8-2020Mechanismu uRadical:Radical:any chemical species that contains one or more unpaired electronsradicals are formed by homolytic bond cleavagehomolytic bond cleavagethe order of stability of alkyl radicals is 3 2 1 methyl8-8-2121Radical Chain Mechanismu uChain initiation:Chain initiation:a step in a chain reaction characterized by formation of reactive intermediates(radicals,anions,or cations)from nonradical or noncharged molecules8-8-2222Radical Chain Mechanismu uChain propagation:Chain propagation:a step in a chain reaction characterized by the reaction of a reactive intermediate and a molecule to form a new reactive intermediate and a new moleculeu uChain length:Chain length:the number of times the cycle of chain propagation steps repeats in a chain reaction8-8-2323Radical Chain Mechanismu uChain termination:Chain termination:a step in a chain reaction that involves destruction of reactive intermediates8-8-2424Chain Propagation StepsuFor any set of chain propagation steps,theirequations add to the observed stoichiometryenthalpies add to the observed H0 8-8-2525Regioselectivity?uThe regioselectivity of chlorination and bromination can be accounted for in terms of the relative stabilities of alkyl radicals(3 2 1 methyl)uBut how do we account for the greater regioselectivity of bromination(1600:80:1)compared with chlorination(5:4:1)8-8-2626Hammonds Postulateu uHammonds Postulate:Hammonds Postulate:the structure of the transition state for an exothermic step looks more like the reactants of that step than the productsfor an endothermic step looks more like the products of that step than the reactantsuThis postulate applies equally well to the transition state for a one-step reaction and to each transition state in a multi-step reaction8-8-2727Hammonds Hammonds PostulatePostulate8-8-2828Hammonds Postulatein halogenation of an alkane,hydrogen abstraction(the rate-determining step)is exothermic for chlorination but endothermic for bromination8-8-2929Hammonds PostulateuBecause hydrogen abstraction for chlorination is exothermic:the transition state resembles the alkane and a chlorine atomthere is little radical character on carbon in the transition stateregioselectivity is only slightly influenced by radical stability8-8-3030Hammonds PostulateuBecause hydrogen abstraction for bromination is endothermic:the transition state resembles an alkyl radical and HBrthere is significant radical character on carbon in the transition stateregioselectivity is greatly influenced by radical stabilityradical stability is 3 2 1 methyl,and regioselectivity is in the same order8-8-3131Hammonds Postulate8-8-3232StereochemistryuWhen radical halogenation produces a chiral center or takes place at a hydrogen on a chiral center,the product is a mixture of R and S enantiomers as a racemic mixturefor simple alkyl radicals,the carbon bearing the radical is sp2 hybridized and the unpaired electron occupies the unhybridized 2p orbital(see next screen)8-8-3333Stereochemistry8-8-3434Allylic Halogenationu uAllylic carbon:Allylic carbon:a C adjacent to a C-C double bondu uAllylic hydrogen:Allylic hydrogen:an H on an allylic carbonan allylic C-H bond is weaker than a vinylic C-H bond8-8-3535Allylic BrominationuAllylic bromination using NBS8-8-3636Allylic BrominationuA radical chain mechanismChain initiationChain propagation8-8-3737Allylic Brominationchain terminationuBr2 is provided by the reaction of NBS with HBr 8-8-3838The Allyl RadicaluA hybrid of two equivalent contributing structures(Equivalent contributing structures)CH2CHCH2CHCH2CH28-8-3939The Allyl RadicaluMolecular orbital model of the allyl radical8-8-4040The Allyl RadicaluUnpaired electron spin density map of the allyl radicalthe unpaired electron density (green lobes)appears only on carbons 1 and 38-8-4141Allylic Halogenation Example 8.5Example 8.5 Account for the fact that allylic bromination of 1-octene by NBS gives these isomeric products8-8-4242Radical Autoxidationu uAutoxidation:Autoxidation:oxidation requiring oxygen,O2,and no other oxidizing agentoccurs by a radical chain mechanism similar to that for allylic halogenationin this section,we concentrate on autoxidation of the hydrocarbon chains of polyunsaturated triglyceridesthe characteristic feature of the fatty acid chains in polyunsaturated triglycerides is the presence of 1,4-dienesradical abstraction of a doubly allylic hydrogen of a 1,4-diene forms a particularly stable radical8-8-4343Radical Autoxidationautoxidation begins when a radical initiator,X,abstracts a doubly allylic hydrogenthis radical is stabilized by resonance with both double bonds8-8-4444Radical Autoxidationthe doubly allylic radical reacts with oxygen,itself a diradical,to form a peroxy radicalthe peroxy radical then reacts with another 1,4-diene to give a new radical,R,and a hydroperoxidevitamin A,a naturally occurring antioxidant,reacts preferentially with the initial peroxy radical to give a resonance-stabilized phenoxy radical,which is very unreactive,and scavenges another peroxide radical8-8-4545Radical Autoxidationvitamin E as an antioxidant8-8-4646Radical Addition of HBr to AlkenesuAddition of HBr to alkenes gives either Markovnikov addition or non-Markovnikov addition depending on reaction conditionsMarkovnikov addition occurs when radicals are absentnon-Markovnikov addition occurs when peroxides or other sources of radicals are present8-8-4747Radical Addition of HBr to Alkenesaddition of HCl and HI gives only Markovnikov productsto account for the the non-Markovnikov addition of HBr in the presence of peroxides,chemists proposed a radical chain mechanismuChain initiation8-8-4848Radical Addition of HBr to AlkenesuChain propagation8-8-4949Radical Addition of HBr to AlkenesuChain terminationuThis pair of addition reactions illustrates how the products of a reaction can often be changed by a change in experimental conditionspolar addition of HBr is regioselective,with Br adding to the more substituted carbonradical addition of HBr is also regioselective,with Br adding to the less substituted carbon8-8-5050HaloalkanesEnd Chapter 8End Chapter 8 更多精品资请访问更多精品资请访问 更多品资源请访问更多品资源请访问
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