Alkenes. Chapter 4 Chapter 4 1

Contents of Chapter 3  General Formulae and Nomenclature of Alkenes  Cis-Trans Isomerism  Reactivity Considerations  Thermodynamics and Kinetics Chapter 4 2

General Molecular Formula for Alkenes  General molecular formula for acyclic alkanes is C n H 2n+2

CH 3 CH 2 CH 2 CH 2 CH 3 an alkane C 5 H 12 C n H 2n+2

Chapter 4 3

General Molecular Formula for Alkenes  Each  bond introduced, reduces the H content by 2

CH 3 CH 2 CH 2 CH=CH 2 an alkene C 5 H 10 C n H 2n

Chapter 4 4

General Molecular Formula for Alkenes  Each ring also reduces the H content by 2

a cyclic alkane C 5 H 10 C n H 2n

Chapter 4 5

General Molecular Formula for Alkenes   Generalization: The molecular formula for a hydrocarbon is C n H 2n+2 every  minus 2 hydrogens for bond and/or ring present in the molecule Each  bond or ring is considered a

unit of unsaturation

.

C 5 H 8 C n H 2n-2 a cyclic alkene with 2 units of unsaturation

Chapter 4 6

Saturated and Unsaturated Hydrocarbons  Alkanes or

saturated hydrocarbons

contain the maximum number of carbon-hydrogen bonds

CH 3 CH 2 CH 2 CH 2 CH 3 a saturated hydrocarbon

Chapter 4 7

Saturated and Unsaturated Hydrocarbons  Alkenes contain fewer than the maximum number of carbon-hydrogen bonds and are therefore referred to as

unsaturated hydrocarbons CH 3 CH 2 CH 2 CH=CH 2 an unsaturated hydrocarbon

Chapter 4 8

Nomenclature of Alkenes IUPAC names of alkenes are based on the corresponding alkane with “ane” replaced by “ene” Chapter 4 9

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 10

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 11

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 12

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 13

IUPAC Rules for Alkene Nomenclature 5. 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 14

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 15

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 16

IUPAC Rules for Alkene Nomenclature The

sp 2

carbons of an alkene are called vinylic An

sp 3

adjacent carbon is called allylic Chapter 4 17

IUPAC Nomenclature of Dienes • Find the longest chain containing both double bonds CH 2 CH 2 CH 2 CH 2 CH 3 CHCHCH CH 2 1 2 3 4 5

3-butyl-1,4-pentadiene

Chapter 4 18

IUPAC Nomenclature of Dienes • Use corresponding alkane name but replace the “

ne”

ending with “

diene”

CH 2 CH 2 CH 2 CH 2 CH 3 CHCHCH CH 2

3-butyl-1,4-pentadiene

“pentane” changed to “pentadiene” Chapter 4 19

IUPAC Nomenclature of Dienes • Number in the direction that gives the lowest number to a double bond CH 2 CHCH 2 CH 2 CH

1,5-heptadiene not 2,6-heptadiene

CHCH 3 Chapter 4 20

• IUPAC Nomenclature of Dienes List substituents in alphabetical order CH 3 CH 3 C CHCH CH 2 CH 3 CCH 2 CH 3

5-ethyl-2-methyl-2,4-heptadiene

Chapter 4 21

• 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 CH 3 CH 3 C CHCH CH 2 CH 3 CCH 2 CH 3

5-ethyl-2-methyl-2,4-heptadiene or 5-ethyl-2-methyl-hepta-2,4-diene

Chapter 4 22

The

E, Z

System of Nomenclature Br Cl Br CH 3 C C C C H CH 3 H Cl Which isomer is cis and which is trans?

A more definitive nomenclature is needed!

Chapter 4 23

The

E, Z

System of Nomenclature    First prioritize the groups bonded to the two

sp 2

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 24

• • 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

sp 2

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 25

• • 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 26

Relative Stabilities of Alkenes Chapter 4 27

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 28

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 29

Reaction Mechanisms  We use curved arrows to indicate the movement of pairs of electrons as two molecules, ions or atoms interact Chapter 4 30

Reaction Mechanisms  Curved arrows are drawn only from the electron-rich site to the electron deficient site Chapter 4 31

Thermodynamics  When  G ° is negative the reaction is

exergonic

Chapter 4 32

Thermodynamics  When  G ° is positive the reaction is

endergonic

Chapter 4 33

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 34

Free Energy of Activation Chapter 4 35

Rate-Determining Step  Formation of the carbocation intermediate is the slower of the two steps  It is the rate-determining step Chapter 4 36

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 37

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 38

Transition States and Intermediates  An intermediate  is at a local minimum energy in the reaction coordinate diagram  may be isolated in some cases Chapter 4 39