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Chapter 4 1
Alkenes. Alkenes. Chapter 4Chapter 4
Chapter 4 2
Contents of Chapter 3 General Formulae and Nomenclature of General Formulae and Nomenclature of
AlkenesAlkenes Cis-Trans IsomerismCis-Trans Isomerism Reactivity ConsiderationsReactivity Considerations Thermodynamics and KineticsThermodynamics and Kinetics
Chapter 4 3
General Molecular Formula for Alkenes
General molecular formula for acyclic alkanes is CnH2n+2
CH3CH2CH2CH2CH3
an alkane
C5H12
CnH2n+2
Chapter 4 4
General Molecular Formula for Alkenes
Each bond introduced, reduces the H content by 2
CH3CH2CH2CH=CH
2 an alkene
C5H10
CnH2n
Chapter 4 5
General Molecular Formula for Alkenes
Each ring also reduces the H content by 2
a cyclic alkane
C5H10
CnH2n
Chapter 4 6
General Molecular Formula for Alkenes
Generalization: The molecular formula for a hydrocarbon is CnH2n+2 minus 2 hydrogens for every bond and/or ring present in the molecule
Each bond or ring is considered a unit of unsaturation.
a cyclic alkene with 2 units of unsaturation
C5H8
CnH2n-2
Chapter 4 7
Saturated and Unsaturated Hydrocarbons Alkanes or saturated hydrocarbons
contain the maximum number of carbon-hydrogen bonds
CH3CH2CH2CH2CH3
a saturated hydrocarbon
Chapter 4 8
Saturated and Unsaturated Hydrocarbons Alkenes contain fewer than the
maximum number of carbon-hydrogen bonds and are therefore referred to as unsaturated hydrocarbons
CH3CH2CH2CH=CH2
an unsaturated hydrocarbon
Chapter 4 9
Nomenclature of Alkenes
IUPAC names of alkenes are based on the corresponding alkane with “ane” replaced by “ene”
Chapter 4 10
IUPAC Rules for Alkene Nomenclature
1. The longest chain containing the functional group (the double bond) is numbered such that the double bond is the lowest possible number
Chapter 4 11
IUPAC Rules for Alkene Nomenclature
2. If there are substituents, the chain is still numbered in a direction that gives the double bond the lowest number
Chapter 4 12
IUPAC Rules for Alkene Nomenclature
3. If chain has more than one substituent, they are cited in alphabetical (not numerical) order.
Rules for alphabetizing are the same as for alkanes
Chapter 4 13
IUPAC Rules for Alkene Nomenclature
4. If the same number for the double bond is obtained in both directions, number in the direction that gives lowest number to a substituent.
Chapter 4 14
IUPAC Rules for Alkene Nomenclature5. In cyclic compounds, a number is not
needed to denote the position of the functional group
The double bond is assumed to be between carbons 1 and 2
Chapter 4 15
IUPAC Rules for Alkene Nomenclature
6. If both directions yield same low number for a functional group and for one substituent, number in the direction which yields the lower number for one of the remaining substituents
Chapter 4 16
IUPAC Rules for Alkene Nomenclature Two groups containing double bonds
that are used as names for substituents are the vinyl group and the allyl group
Chapter 4 17
IUPAC Rules for Alkene Nomenclature
The sp2 carbons of an alkene are called vinylic
An sp3 adjacent carbon is called allylic
Chapter 4 18
IUPAC Nomenclature of Dienes
• Find the longest chain containing both double bonds
1 2 3 4 5
CHCHCH
CH2CH2CH2CH3
CH2CH2
3-butyl-1,4-pentadiene
Chapter 4 19
IUPAC Nomenclature of Dienes
• Use corresponding alkane name but replace the “ne” ending with “diene”
CHCHCH
CH2CH2CH2CH3
CH2CH2
“pentane” changed to “pentadiene”
3-butyl-1,4-pentadiene
Chapter 4 20
IUPAC Nomenclature of Dienes
• Number in the direction that gives the lowest number to a double bond
CH2 CHCH2CH2CH CHCH3
1,5-heptadiene
not 2,6-heptadiene
Chapter 4 21
IUPAC Nomenclature of Dienes
• List substituents in alphabetical order
CH3C
CH3
CHCH CCH2CH3
CH2CH3
5-ethyl-2-methyl-2,4-heptadiene
Chapter 4 22
IUPAC Nomenclature of Dienes
• Place numbers indicating the double bond positions either in front of the parent compound or in the middle of the name immediately before the diene suffix
CH3C
CH3
CHCH CCH2CH3
CH2CH3
5-ethyl-2-methyl-2,4-heptadiene
or 5-ethyl-2-methyl-hepta-2,4-diene
Chapter 4 23
The E, Z System of Nomenclature
Br
C
H
C
Cl
CH3
Br
C
H
C
CH3
Cl
Which isomer is cis and which is trans?
A more definitive nomenclature is needed!
Chapter 4 24
The E, Z System of Nomenclature
First prioritize the groups bonded to the two sp2 carbons
If the higher priority group for each carbon is on the same side of the double bond, it is the Z isomer (for Zusammen, German for “together”)
If the higher priority group for each carbon is on the opposite side of the double bond, it is the E isomer (for Entgegen, German for “opposite”)
Chapter 4 25
The E, Z Prioritization Rules
• Relative priorities depend first on the atomic number of the atom (not the formula weight of the group) bonded to the sp2 carbon
• In the case of a tie, the atomic numbers of the atoms bonded to the tied atoms are considered next (e.g. C, C, & H beats C, H, & H)
Chapter 4 26
The E, Z Prioritization Rules
• If an atom is doubly bonded to another atom, the system treats it as if it were bonded to two such atoms
• In the case of isotopes, the isotope with the greater mass number has the higher priority
Chapter 4 27
Relative Stabilities of Alkenes
Chapter 4 28
Relative Stabilities of Alkenes The more alkyl substituents attached to
a double bond the more stable the double bond.
Trans alkenes more stable than cis alkenes
Not difficult concepts but should be learned now in order to understand Chapter 9 later.
Chapter 4 29
Reactivity Considerations
Electrophiles react with nucleophiles An alkene has electron density above
and below the bond making it electron-rich and therefore a nucleophile
Therefore alkenes react with electrophiles
Chapter 4 30
Reaction Mechanisms We use curved arrows to indicate the
movement of pairs of electrons as two molecules, ions or atoms interact
Chapter 4 31
Reaction Mechanisms Curved arrows are drawn only from
the electron-rich site to the electron deficient site
Chapter 4 32
Thermodynamics When G° is negative the reaction is
exergonic
Chapter 4 33
Thermodynamics When G° is positive the reaction is
endergonic
Chapter 4 34
Kinetics Knowing the G° of a reaction will not
tell us how fast it will occur or if it will occur at all
We need to know the rate of reaction The rate of a reaction is related to the
height of the energy barrier for the reaction, G‡, the free energy of activation
Chapter 4 35
Free Energy of Activation
Chapter 4 36
Rate-Determining Step Formation of the carbocation intermediate
is the slower of the two steps It is the rate-determining step
Chapter 4 37
Rate-Determining Step Carbocation intermediates are consumed
by bromide ions as fast as they are formed
The rate of the overall reaction is determined by the slow first step
Chapter 4 38
Transition States and Intermediates
It is important to distinguish between a transition state and a reaction intermediate
A transition state is a local maximum in the reaction coordinate
diagram has partially formed and partially broken bonds has only fleeting existence
Chapter 4 39
Transition States and Intermediates
An intermediate is at a local minimum energy in the reaction
coordinate diagram may be isolated in some cases
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