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Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Click to edit Master title style,5-,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Alkenes:Structure andNomenclature,Chapter 5,Unsaturated Hydrocarbons,Unsaturated hydrocarbon:,contains one or more carbon-carbon double or triple bonds,Alkene:,contains a carbon-carbon double bond and has the general formula C,n,H,2n,Alkyne:,contains a carbon-carbon triple bond and has the general formula C,n,H,2n-2,Unsaturated Hydrocarbons,Arenes:,benzene and its derivatives(Ch 21-22),we do not study arenes until Chapters 21&22,however,we show structural formulas of compounds containing the phenyl group before that time,the phenyl group is not reactive under any of the conditions we describe in Ch 6-20,Structure of Alkenes,A double bond consists of,one sigma bond formed by the overlap of,sp,2,hybrid orbitals and one pi bond formed by the overlap of parallel 2,p,orbitals,the two carbon atoms of a double bond and the four atoms bonded to them lie in a plane,with bond angles of approximately 120,Structure of Alkenes,it takes approximately 264 kJ(63 kcal)/mol to break the pi bond in ethylene;that is,to rotate one carbon by 90 with respect to the other so that there is no overlap between 2,p,orbitals on adjacent carbons,Cis,Trans,Isomerism in Alkenes,Cis,trans,isomers:,isomers that have the same connectivity but a different arrangement of their atoms in space due to the presence of either a ring or a carbon-carbon double bond,Index of Hydrogen Deficiency,Index of hydrogen deficiency(IHD):,the sum of the number of rings and pi bonds in a molecule,To determine IHD,compare the number of hydrogens in an unknown compound with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds,the molecular formula of the reference hydrocarbon is C,n,H,2n+2,Index of Hydrogen Deficiency,for each atom of a Group 7 element(F,Cl,Br,I),add one H,no correction is necessary for the addition of atoms of Group 6 elements(O,S)to the reference hydrocarbon,for each atom of a Group 5 element(N,P),subtract one hydrogen,IDH=,2,(H,reference,-H,molecule,),Index of Hydrogen Deficiency,Problem:,isopentyl acetate has a molecular formula of C,7,H,14,O,2,.Calculate its IHD,reference hydrocarbon C,7,H,16,IHD=(16-14)/2=1,Problem:,calculate the IHD for niacin,molecular formula C,6,H,6,N,2,O,reference hydrocarbon C,6,H,16,IHD=(16-6)/2=5,O,O,Isopentyl acetate,IUPAC Nomenclature,1.Number the longest chain of carbon atoms that contains the double bond in the direction that gives the carbons of the double bond the lowest numbers,2.Locate the double bond by the number of its first carbon,3.Name substituents,4.Number the carbon,locate and name substituents,locate the double bond,and name the main chain,Common Names,Despite the precision and universal acceptance of IUPAC nomenclature,some alkenes,particularly low-molecular-weight ones,are known almost exclusively by their common names,Common Names,the common names methylene,vinyl,and allyl are often used to show the presence of the following alkenyl groups,The,Cis,Trans,System,Configuration is determined by the orientation of atoms of the main chain,The,E,Z,System,uses priority rules(Chapter 3),if groups of higher priority are on the same side,the configuration is,Z,(German,zusammen,),if groups of higher priority are on opposite sides,the configuration is,E,(German,entgegen,),The,E,Z,System,Example:,name each alkene and specify its configuration by the,E,Z,system,Cis,Trans,Isomerism,Cycloalkenes,in small-ring cycloalkenes,the configuration of the double bond is,cis,these rings are not large enough to accommodate a,trans,double bond,Cis,Trans,Isomerism,trans,-cyclooctene is the smallest,trans,cyclooctene that has been prepared in pure form and is stable at room temperature,the,cis,isomer is 38 kJ(9.1 kcal)/mol more stable than the,trans,isomer,the,trans,isomer is chiral even though it has no chiral center,Dienes,Trienes,and Polyenes,For alkenes containing two or more double bonds,change the infix-,en,-to-,adien,-,-,atrien,-,etc.,those containing several double bonds are often referred more generally as polyenes,following are three dienes,Dienes,Trienes,and Polyenes,for alkenes with,n,double bonds,each of which can show,cis,trans,isomerism,2,n,stereoisomers are possible,example:2,2,=4,cis,trans,isomers are possible for 2,4-heptadiene,Dienes,Trienes,and Polyenes,vitamin A,a biologically important compound for which a number of,cis,trans,isomers is possible,there are four double bonds about which,cis,trans,isomerism is possible,for 2,4,=16 stereoisomers,Physical Properties,Alkenes are nonpolar compounds,The only attractive forces between their molecules are dispersion forces,The physical properties of alkenes are similar to those of alkanes,Terpenes,Terpene:,a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene,Terpenes,Myrcene,C,10,H,16,a component
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