5.1 Alkene Nomenclature : 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
Slide2 : 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) Number the chain in the direction that gives the lowest number to the doubly bonded carbon. 1-Butene Alkene Nomenclature
Slide3 : 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 is 4-bromo-3-methyl-1-butene. Alkene Nomenclature
Slide4 : 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 : Alkene Nomenclature If more than one double bond is present; name as a –diene or –triene. Use numbers to indicate multiple bond positions.
Cycloalkene Nomenclature : Cycloalkene Nomenclature 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. Cyclohexene
Cycloalkene Nomenclature : 1) Replace the -ane ending of the cycloalkane having the same number 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
Structure of Ethylene : Planar
Restricted C-C Bond Rotation Structure of Ethylene
Slide10 : 2-Methylpropene 1-Butene cis-2-Butene Constitutional Isomers of C4H8
Slide11 : cis-2-Butene trans-2-Butene Stereoisomers Also known as Geometric isomers, cis/trans isomers, or
E/Z isomers
E, Z nomenclature is used when cis/trans ranking is ambiguous : 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
The E-Z Notational System : E : higher ranked substituents on opposite sides
Z : higher ranked substituents on same side Entgegen higher higher lower lower The E-Z Notational System
Table 5.1 CIP Rules : (1) Higher atomic number outranks lower atomic number Br > F Cl > H (Z )-1-Bromo-2-chloro-1-fluoroethene Table 5.1 CIP Rules
Table 5.1 CIP Rules : (2) When two atoms are identical, compare the atoms attached to them on the basis of their atomic numbers. Precedence is established at the first point of difference. —CH2CH3 outranks —CH3 Table 5.1 CIP Rules
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 outranks —CH2CH2OH —C(C,C,H) —C(C,H,H) Table 5.1 CIP Rules
Table 5.1 CIP Rules : (4) Evaluate substituents one by one. Don't add atomic numbers within groups. —CH2OH outranks —C(CH3)3 —C(O,H,H) —C(C,C,C) Table 5.1 CIP Rules
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 atom. —CH=O outranks —CH2OH —C(O,O,H) —C(O,H,H) Table 5.1 CIP Rules
Alkene Polarity and Boiling Point : 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. : Double bonds are classified according to the number of carbons attached to them. monosubstituted H R H R' disubstituted H R H R' disubstituted
Double bonds are classified according to the number of carbons attached to them. : 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.
Substituent Effects on Alkene Stability : Electronic (larger effect)
disubstituted alkenes are more stable than monosubstituted alkenes
Steric (smaller effect)
For disubstituted alkenes;
trans alkenes are more stable than cis alkenes Substituent Effects on Alkene Stability
Figure 5.4 Heats of combustion of C4H8 isomers. : + 6O2 4CO2 + 8H2O 2707 kJ/mol 2717 kJ/mol 2710 kJ/mol Figure 5.4 Heats of combustion of C4H8 isomers.
Figure 5.5 cis and trans-2-Butene : cis-2-butene trans-2-butene Figure 5.5 cis and trans-2-Butene
Problem 5.8 : Give the structure or make a molecular model of the most stable C6H12 alkene. Problem 5.8