有机化学Chapter 2. Alkanes 烷烃

1、2.1 Classes of hydrocarbon2.2 Structures of Alkanes 2.2.1 Structures of Alkanes sp3 Hybridized orbital Structure of methane Structure of ethane Shapes of alkanes2.2.2 Isomerization of Alkanes2.3 Nomenclature of Alkanes2.3.1 Unbranched Alkanes:Chapter 2. Alkanes (烷烃）烷烃）2.3.2 Branched Alkanes2.3.3 Bra

2、nched alkyl groups2.3.4 Cycloalkane Nomenclature2.4 Properties of Alkanes and Cycloalkanes 2.5 Conformation (构象构象) of Alkanes and Cycloalkanes2.5.1 Conformation of Alkanes1. Conformational Analysis of Ethane2. Conformational Analysis of Butane2.5.2 Conformation of Cyclohexane1. Cis-trans isomerism i

3、n Cyclohexane2. Conformation of Cyclohexane 3. Conformational Analysis of Mono-substituted Cyclohenaxes 4. Conformational Analysis of Disubstituted CyclohexanesHydrocarbons (烃烃)C. H elementsAliphatic(脂肪的脂肪的)Aromatic(芳香的芳香的)AlkenesAlkynesAlicyclic CHCHHHHHCCHHHHC CHH2.1 Classes of HydrocarbonsAlkanes

4、SaturatedUnsaturatedHydrocarbonsAlkanesAlkenesAlkynesArene2.2 Structures of Alkanes 2.2.1 Structures of Alkanes C :1s2 2s2 2px1 2py1 2pz0Electronic configurationin ground state of a carbon atomHybrid orbitals(杂化轨道杂化轨道):Ground state2 2p p2 2s s1 1s sPromotionof electronExcitated state2 2p p2 2s s1 1s

5、 ssp3-Hybridizedstate1 1s sspsp3 3Hybri-dized2s2s2p2py y2p2pz z2p2px xAn sp3 orbital s orbital p orbital4 tetrahedralsp3 orbitalsGeometric structure:tetrahedron109.5109.5109.5109.5Four tetrahedral Four tetrahedral spsp3 3 orbital orbital The Structure of Methane4 equivalent C键键Bond angle:109.5C: sp3

6、 orbitalH: 1s orbitalGeometric structure: tetrahedronA ball-and-stick model of methaneA space and filling model of methaneThe Structure of ethaneC2H6CH3CH3CHHHCHHHThe carbon-carbon bond is formed by overlap of 2 carbon sp3 hybrid orbitals.C CC CH HH HH HC CC CH HH HH H bondbondThe character of bond:

7、 Higher bond energy; Lower polarizability； Rotation about bond axes. An ball-and-stick model of ethaneAn ball-and-stick model of ethane( (球棒模型球棒模型) )Shapes of alkanesZigzag not straight（曲折性）（曲折性）2.2.2 Isomerization of AlkanesBeginning with C4H10CH3CH2CH2CH2CH3n-Pentanen-PentaneStraight-chain ,unbran

8、ched alkanesor normal alkanes IsopentaneNeopentaneBranched-chain alkaneGeneral formula:CnH2n+2 CH3CHCH3CH2CH3CH3CCH3CH3CH3ConstitutionalIsomers(构造异构体构造异构体)CH3CH2CH2CHCH3CH32.3 Nomenclature of AlkanesCommon nomenclatureIsohexaneIUPAC Nomenclature:2.3.1 Unbranched Alkanes:AlkanesAlkanesAlkyl groupAlky

9、l groupAbbreviationAbbreviationCH4MethMethaneaneCH3MethMethyl ylMeMeCHCH3 3CHCH3 3CHCH3 3CHCH2 2EtEthEthaneaneEthEthyl ylCHCH3 3CHCH2 2CHCH3 3CHCH3 3CHCH2 2CHCH2 2PrPropPropaneanePropPropyl ylaneaneyl ylPrefixPrefixParentParentSuffixSuffixWhere are the substituents?How manyCarbons?What Family?CHCH3

10、3CHCH2 2CHCH2 2CHCH3 3CHCH3 3CHCH2 2CHCH2 2CHCH2 2ButButaneaneButButyl ylBuBu2.3.2 Branched Alkanes2.3.2 Branched AlkanesStep 1. Find the parent hydrocarbon Find the longest continuous chain of carbon atomsCHCH CH CH2CH2CH3CH3CH3CH3CH CH3CH2CH3CHCH CH CH2CH2CH3CH3CH3CH3CH CH3CH2CH3 To equal long cha

11、ins, choose the chain containing the most branchedchain.Heptane(庚烷庚烷)Step 2 Number the atoms in the main chain. Beginning at the end nearer the first branch point, number each C atom in the parent chain.CHCH2CH2CH CHCH2CH3CH3CH3CH2CH3CH2CH3987654321CH CH2CH2CH CHCH2CH3CH3CH3CH2CH3CH2CH3219876543( I

12、)( II )3. Designate the location of the substituents5-丙基丙基-4-异丙基壬烷异丙基壬烷CH3CH2CH2CHCH CH2CH2CHCH3CH3CH2CH2CH3CH2CH31234756894-Isopropyl-5-propylnonane List the substituents in alphabetical order. When the same substituents appears more than once, use the multiplying prefixes: di-, tri-, tetra- and so

13、 on (一，二，三，四一，二，三，四).Sequence Rule for atom and group (see Prof. Gao,Pg 73)Sequence Rule for atom and group (see Prof. Gao,Pg 73)3) For Atoms: I Br Cl S P F O N C D H2) For groups: CH2Br CH2Cl CH2H1) For C-C double bond or triple bond:C CHC(C)(C)C(C)(C)HCCH3CH3CH3CHCH2C(C)C(C)HHHCHCH3CH3H2C CH3 -CH3

14、H3CH2CCH2CCH2CHCH3CH3CH3CH3CH32,2,4-Trimethylpentane2,2,4-2,2,4-三甲基戊烷三甲基戊烷CH3CH2CHCHCHCH2CH2CH3CH3CH3CH2CH34-Ethyl-3,5-dimethyloctane3,5-3,5-二甲基二甲基-4-4-乙基辛烷乙基辛烷Replicating prefixes areignored when alphabetizing.2.3.3 Branched alkyl groupsCCHHPrimary carbon(伯碳伯碳)CCHCSecondary carbon(仲碳仲碳)Alkyl groups

15、 NameCHCH3CH3CH2CH3CH3CHCHCH3CH3CH2CH3CCH3CH3Isopropyl(异丙基异丙基)sec-butyl (or sec-Bu)(仲丁基仲丁基)Isobutyl(异丁基异丁基)tert-butylor t-Butyl, t-Bu)(叔丁基叔丁基)CCCCTertiary carbon(叔碳叔碳)CH3CH2CH2CH2CH2CH3C(CH3)33-tert-Butylhexane(3-叔丁基己烷）叔丁基己烷）2.3.4 Cycloalkane Nomenclature CH3CH3CH(CH3)21 12 24 45 53 31,1-二甲基二甲基-3-异丙

16、基异丙基环环戊烷戊烷3-Isopropyl-1,1-dimethylcyclopentane2.3.5 Bridged Hydrocarbon Nomenclature 12341234567812345671234576ABCDBicyclo1.1.0butane6-Methyl-bicyclo-3.2.1octane6-Ethyl-2, 7,7-trimethyl-bicyclo2.2.1heptane2, 7, 7-Trimethyl-tricyclo-2.2.1.03,5heptane2.3.6 Spiro Hydrocarbon Nomenclature AB123456789101

17、234567Spiro4.5decane4-methyl-Spiro2.4heptane2.4 Properties of Alkanes and CycloalkanesPhysic propertiesBoiling pointMelting pointSolubility (in water)Density(g/100 mL-1)Refractive index（折射率）（折射率）Source of Alkanes and Cycloalkanes:Nature gas: Methane, ethane and propane. . Petroleum: 150 kinds of hyd

18、rocarbons. Kerosene(煤油）煤油）, gas oil（柴油）（柴油）. .Lubricating oil, paraffin wax, asphalt.-300-200-1000100200300温度（）12345678910 11 12 13 14C原子数烷烃熔沸点图熔点沸点Number of carbonNumber of carbonTemperature (Temperature () )A plot of melting and boiling points versusA plot of melting and boiling points versus numb

19、er of carbons number of carbons Alkanes show regular increases in both boiling point and melting point as mole- cular weight increases.Van der Waals Forces (范德华力）范德华力）Molecular size , Branched alkanes have lower boiling points than their unbranched isomers.Van der waals forces .molecular surface are

20、as , ,CH3CH2CH2CH2CH3CH3CHCH2CH3CH3CH3CCH3CH3CH3Pentane 2-Methylbutane 2,2-Dimethyl- propane bp: 36 28 9 bp: 36 28 9 2.5 Conformation (构象构象) of Alkanes and Cycloalkanes2.5.1 Conformation of AlkanesFor alkanes:Ex.Conformation:Different spatial arrangements of atoms or atomic groups in a molecule that

21、 are generated by rotation about C C single bond.1. Conformational Analysis of EthaneNewmanProjection(纽曼投影式纽曼投影式)Sawhorse(锯架式锯架式)Eclipsed conformation (重叠式重叠式)HHHHHHHHHHHHStaggered conformation (交叉式交叉式)HHHHHHHHHHHHNewman Projection(纽曼投影式纽曼投影式):HHHHHHHHHHHHHHHHHH能量能量 (kJ.mol(kJ.mol-1-1) )0 0606012012

22、0旋转角度旋转角度 / ( / ( ) )12.612.6120120120120Energy: Eclipsed Staggered 重叠式重叠式 交错式交错式Torsional strain(扭转张力扭转张力: 处于重叠式构象中两相处于重叠式构象中两相邻的邻的C-X (X= H, Cl, Br, OH, CHC-X (X= H, Cl, Br, OH, CH3 3, C, C2 2H H5 5 etc) etc)键，键，存在着一种要旋转成交叉式的张力，称为扭转张力存在着一种要旋转成交叉式的张力，称为扭转张力 ）HHHHHHHHHHHHInterconversionRelation betw

23、een conformations:Conformers(构象异构体构象异构体) or RotamersTorisonal Strain( (扭转张力扭转张力 or 排排斥力斥力) )Van der Waals Radius ( (范德华半径与范德华张力范德华半径与范德华张力) ) In a molecular crystal, One-half the distance between two atoms of an element that are as close to each other as possible without being formally bonded to eac

24、h other except for van der Waals forces.HaHaHHHH154pm110.7pm109.3o229pmrepulsionVan der Waals Radius for H: 120 pmDistance for Ha-Ha: 229 pmNonbonded interaction of ethanewhen two uncharged atoms approach each other, the interaction between them is very small at large distances, becomes increasingly

25、 attractive as the separation approaches the sum of their van der Waals radii, but then becomes strongly repulsives as the separation becomes less than the sum of their van der Waals radii.2. Conformational Analysis of ButaneRotation about C2C3 bond in butaneHHCH3HCH3HCH3HHCH3HHHHCH3HCH3HCH3HHH3CHH对

26、位交叉对位交叉部分重叠部分重叠 部分交叉部分交叉全重叠全重叠 全重叠全重叠对位交叉对位交叉 Stability:部分交叉部分交叉 部分重叠部分重叠 CH3CH3HHCH3CH3HH2.5.2 Conformation of Cyclohexane 1. Cis-trans isomerism in CyclohexaneEx. 1,2-Dimethylcyclopentane:The rotation is limited by “rigid” ringStereoisomers:trans-trans-cis-cis-The same constitution,the different s

27、pacial arrangement of atomsor atomic groups. Rotation freedom around CC bond in open-chainalkanes. trans-1,2-Dimethylcyclopentanecis-1,2-DimethylcyclopentaneCH3CH3HHCH3CH3HHThe structure of cyclohexane is nonplanar:Chair conformation（椅式构象）（椅式构象）Boat conformation（船式构象）（船式构象）2. Conformation of Cyclohe

28、xaneBoth conformations are interconverted:0.25 nm1234560.18 nm123456But the chair conformation is more stablethan boat (1nm=1000pm).3HHHHHHHHHHHH12456CH2CH2HHHHHHHH145362123456HHHHHHHHHHHHCH2CH2HHHHHHHH415632 In chair conformation, all CH bondsare staggered;but in boat conformation,all CH bonds are

29、eclipsed. In chair conformation, C1,C3,C5 are co-planar; C2,C4,C6 are co-planar. Two kinds of CH bonds: 6 CH bonds are axials; 6 CH bonds are equatorials. .a aa aa aa aa aa ae e124563a ae ea ae ea ae ee ea aa aa ae ee ee ee ee ee ee eOne chair conformation is converted to another chair :R Ri in ng g

30、- -f fl li ip pAs the result of ring inversion, any axialbond becomes equatorial.Conformational mobility of cyclohexaneChair-chair interconversion :Chair Half-chair Twist boat Boat Twist boat Half-chair Chair Energy:Chair Twist boat Boat Half-chair 3. Conformational Analysis of mono-substituted cycl

31、ohexanesIn methylcyclohexane:Van der waals strain1,3-diaxial repulsion(排斥排斥)Methyl group: From axial to equatorial5%5%95%95%HHCCH3HH162345123456fastHHHCHHHC(CH3)3HCH3CH3H3C0.01%0.01%99.99%99.99%As the group is larger, the conformationof cyclohexane with the group equatorialis more stable than the ax

32、ial form.4 4. Conformational Analysis of Disubstituted cyclohexanesH3CCH3HHH3CCH3HHWedge-and-dash description(伞式伞式): cis-1,4-Dimethylcyclohexane顺顺-1,4-1,4-二甲基环己烷二甲基环己烷trans-反反- -Heats of Combustion (kj / mol)cis-1,4-Dimethylcyclohexane 5219trans-1,4-Dimethylcyclohexane 5212The trans-form is more sta

33、bleCH3H3CHHCH3H3CHHcis-Form:(a,e)(a,e)(e,a)(e,a)trans-Form:CH3H3CHHCH3H3CHH(a,a)(a,a)(e,e)(e,e)The general rule:a. The conformation with equatorial substituent is more stable than axial form. .b. The conformation with the larger substituent in equatorial position is more stable than the axial form.H

34、3CHHHHH3Cc.To a polysubstituted cyclohexane, the more substituents are in equatorialbonds, the stable is its conformation.H3CHHcis-1-Methyl-4-tert-butyl-Cyclohexane:(e.a)(e.a)(a,e)(a,e)2.6 Properties of Alkanes and Cycloalkanes2.6.1 Stability of carbocation and radical a) Stability of carbocationH C

35、+HHMethylR C+HHR C+RHR C+RRCCC+PrimarySecondaryTertiaryAllylic(1o)(2o)(3o) B(2) The Effect of Hyperconjugation for CarboncationCHHCHHHCHCHHHCHHHCHCHHCHH bond+(1) The induction effect substituted groupb)The Stability of Carbon RadicalCRRR120 osp2 hybrid, 120 o, vacant p orbitalCGCGABStability: A BThe

36、 Effect of Hyperconjugation for Carbon RadicalCHHCHHHCHCHHHCHHHCHCHHCHH bondCH3CHCH3CH3CH3CCH3CH3CH3CH2 仲氢仲氢 伯氢伯氢反应活性反应活性比比 5.0 3.5 1.0(4) (4) 反应活性与选择性反应活性与选择性(Pg54)(Pg54)卤素的反应活性次序：卤素的反应活性次序：F2 Cl2 Br2 I2CH3CHCH2CH2CH3 + Br2h60CH3CH3CCH2CH2CH3 + HBrCH3Br2 2甲基戊烷甲基戊烷 2 2甲基甲基2 2溴戊烷溴戊烷(76%)(76%)溴的反应性溴的反应性低，选择性低，选择性高。高。2.6.2 2.6.2 氧化反应氧化反应(oxidation)(oxidation)烷烃和环烷烃在空气中可以燃烧烷烃和环烷烃在空气中可以燃烧: :CH4 + 2O2燃燃烧烧CO2 + 2H2O + 891 kJm