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1、5.1 Alkene Nomenclature,characterized by molecular formula CnH2n have a C=C double bond said to be “unsaturated“ or “olefins”,Chapter 5 Structure and Preparation of Alkenes: Elimination Reactions,Chapter 5; Alkanes and Elimination Reactions,Nomenclature Geometric Isomers Cis/trans and E/Z Priority R
2、ules Physical Properties Alkene Stability Elimination Reactions E1 and E2 Mechanisms :H and :R shifts Dehydrohalogenation Reactions,1) Find the longest continuous chain that includes the double bond. 2) Replace the -ane ending of the unbranched alkane having the same number of carbons by -ene. 3) Nu
3、mber the chain in the direction that gives the lowest number to the doubly bonded carbon.,1-Butene,Alkene Nomenclature,4) If a substituent is present, identify its position by number. The double bond takes precedence over alkyl groups and halogens when the chain is numbered. The compound shown above
4、 is 4-bromo-3-methyl-1-butene.,Alkene Nomenclature,4) If a substituent is present, identify its position by number. Hydroxyl groups take precedence over the double bond when the chain is numbered. The compound shown above is 2-methyl-3-buten-1-ol.,Alkene Nomenclature,Alkene Nomenclature,If more than
5、 one double bond is present; name as a diene or triene. Use numbers to indicate multiple bond positions.,Cycloalkene Nomenclature,1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene.,Cyclohexene,1) Replace the -ane ending of the cycloalkane having the same number
6、of carbons by -ene. 2) Number through the double bond in the direction that gives the lower number to the first-appearing substituent.,6-Ethyl-1-methylcyclohexene,Cycloalkene Nomenclature,Planar Restricted C-C Bond Rotation,Structure of Ethylene,Isomers,Constitutional isomers,Stereoisomers,Requireme
7、nts for Geometric Isomers (Type of Stereoisomer),Restricted Bond Rotation Cycloalkane Multiple Bond Two Different Carbon Atoms that Have Two Different Substituents Attached to Them,2-Methylpropene,1-Butene,cis-2-Butene,Constitutional Isomers of C4H8,cis-2-Butene,trans-2-Butene,Stereoisomers of C4H8,
8、Also known as Geometric isomers, cis/trans isomers, or E/Z isomers,E, Z nomenclature is used when cis/trans ranking is ambiguous,What is needed: 1) systematic body of rules for ranking substituents 2) new set of stereochemical symbols other than cis and trans,E : higher ranked substituents on opposi
9、te sides Z : higher ranked substituents on same side,Entgegen,higher,higher,lower,lower,The E-Z Notational System,(1) Higher atomic number outranks lower atomic number,Br F Cl H,(Z )-1-Bromo-2-chloro-1-fluoroethene,Table 5.1 CIP Rules,(2) When two atoms are identical, compare the atoms attached to t
10、hem on the basis of their atomic numbers. Precedence is established at the first point of difference.,CH2CH3 outranks CH3,Table 5.1 CIP Rules,(3) Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain.,CH(CH3)2 outra
11、nks CH2CH2OH,C(C,C,H),C(C,H,H),Table 5.1 CIP Rules,(4) Evaluate substituents one by one. Dont add atomic numbers within groups.,CH2OH outranks C(CH3)3,C(O,H,H),C(C,C,C),Table 5.1 CIP Rules,(5) An atom that is multiply bonded to another atom is considered to be replicated as a substituent on that ato
12、m.,CH=O outranks CH2OH,C(O,O,H),C(O,H,H),Table 5.1 CIP Rules,Alkene Polarity and Boiling Point,Dipoles cancel; Trans isomer nonpolar Lower boiling pt,Dipoles reinforce; Cis isomer polar Higher boiling pt,Double bonds are classified according to the number of carbons attached to them.,monosubstituted
13、,H,R,H,R,disubstituted,H,R,H,R,disubstituted,Double bonds are classified according to the number of carbons attached to them.,In general; the more substituted the alkene is the more stable it is.,Electronic (larger effect) disubstituted alkenes are more stable than monosubstituted alkenes Steric (sm
14、aller effect) For disubstituted alkenes; trans alkenes are more stable than cis alkenes,Substituent Effects on Alkene Stability,+ 6O2,4CO2 + 8H2O,2707 kJ/mol,2717 kJ/mol,2710 kJ/mol,Figure 5.4 Heats of combustion of C4H8 isomers.,cis-2-butene,trans-2-butene,Figure 5.5 cis and trans-2-Butene,Give the structure or make a molecular model of the most stable C6H12 alkene.,Problem 5.8,
