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