Alkenes

Properties

Nomenclature

Stability

Addition Reactions

Ethylene

Electron Rich -Bond

Cis and Transno rotation about -bond

Elements or Degrees of Unsaturation: -Bond or Ring

Halogens replace Hydrogens

C5H10Cl2 is saturated

ClCl

C6H9Br has 2 degrees of unsaturation

Br

Br

Br

Br

Saturated compounds with Oxygen and Nitrogen

C C

HH

H

H

HH

C2H6

HH

H

C O C

H

HH

C2H6O

N C

H

HHH

C

H

HH

C2H7N

CnH2n+2CnH2n+2O CnH2n+3N

Determine the # elements of Unsaturation

C8H10Br2OC8H18 is saturatedcompound is deficient by 6 "H's"3 degrees of unsaturation

O

BrBrCH2OH

CH3

Br

Br

e.g.

OH Br

Br

Alkene Nomenclature

Cyclic alkenes

Alkyl Groups with -Bonds

CH=CH2

Br

cis 1-bromo-3-vinylcyclohexane

a vinyl group CH2CH=CH2

4-allylcyclopentene

an allyl group(or ethenyl)(or 2-propenyl)

a phenyl group

3-phenyl-1-nonene

Alkylidene GroupsDouble Bonds Fused to Rings

CH2

methylenecyclohexane

CHCH3

ethylidenecyclopentane

Polyenes

Name these Alkenes

cis-trans Isomers

E/Z System

Establish Priority of Substituents on Each sp2

Carbon

What’s My Name?

(Z)-4-ethyl-5-isopropyl-4-nonene

Hi

Hi

Lo

Lo

Priorities with Multiple Bonds

E or Z?

E or Z?

Name These

3,3-dimethyl-1,4-cyclohexadiene 2-butyl-1,5-hexadiene

(E) 1-ethylidene-2-methylcyclopentane3-allylcyclohexene

Hydrogenation Data Helps to Determine Stability

Hhydrogenation of Alkenes

Enthalpy Change Shows Relative Energy of Alkene

Both cis and trans 2-Butene are Hydrogenated to Butane

“E” is More Stable than “Z” by 2.3 KJ/mol

Relative Stabilities of Alkenes

Hyperconjugation bond associates with adjacent C-H bond

C

C

C

mono-substituted disubstituted

C

1-butene trans 2-butene

The Addition Reaction

HBr Addition

Markovnikov’s Rule

The addition of H-X across a double bond results in the more

highly substituted alkyl halide as the major product.

Depicting a Reaction

Addition of HBr or HClMarkovnikov Addition

Markovnikov

Br

C CH

H

HCH3

CH3

HBr

CH3

CH3

C

H

H

C

CH3

CH3C

H

H

Br

C

H

not formed

Regiochemistry Determined by Stability of Intermediate

H-BrBr

H

HBr Br H

3o carbocation

Hyperconjugation

Carbocation Stabilitymore highly substituted, lower energy

3o Carbocation forms Preferentially

What Alkenes are Needed to form Theses Alkyl Halides?

Definitions

• Regioisomers – two constitutional isomers that could result from an addition reaction.

• Regiospecific – only one regiosisomer forms at the expense of the other.

• Regioselective – both regioisomers are formed, but one is formed in preference.

Determine the major product:

a) HCl

b)HBr

c) 2 mol HCl

a) HCl

b)HBr

c) 2 mol HCl

Cl

H

Br

H

Cl ClHH

cis and trans

Rearrangements

CH3

CH3

HCl CH3

CH3

Cl

HH Cl

CH3

CH3

H

methide shiftCH3

CH3

Cl

32o o

Propose a Mechanism

2 Possible pathways to the Carbocation Intermediates

Endo- transition state looks like productExo- transition state looks like Reactant

Hammond’s Postulate

• Related species that are close in energy are close in structure.

• In an endothermic reaction, the transition state is similar to the product in structure and stability.

• In an exothermic reaction, the transition state is similar to the reactant in structure and stability.

• i.e. the structure of the transition state resembles the structure of the most stable species.