Alkenes

Properties

Nomenclature

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

What’s My Name?

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

Hi

Hi

Lo

Lo

Establish Priority of Substituents on Each sp2

Carbon

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

Preparation of Alkenes

• E2 - Elimination reactions of alkyl halides and tosylates (or mesylates)

• E1 - Acid catalyzed dehydration of alcohols

BrNaOEt, EtOH

heat+ NaBr

OH

H2SO4 (aq) heat+ H2O

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.

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

3o Carbocation forms Preferentially

Carbocation Stabilitymore highly substituted, lower energy

Asymmetric Center is Generated Racemic Mixture Formed

H-Br

HH

Br

Br

a)

b)

Br

H

a) Br

H

b)(R) (S)

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

HBr Addition with RO-ORAnti-Markovnikov

HBr

RO-OR, h

H Br

Free-Radical Mechanism

Initiation:

Propagation: i)

ii)

RO-OR 2 RO

RO + HBr Br + ROH

+ BrBr

Br+ H-Br

H Br+ Br

.

. .

. .

. .

Addition of Br2

Br

Br

Br2

More Definitions

• Stereospecific – only one stereoisomer is formed at the expense of the other (e.g. trans vs. cis)

• Stereoselective – one stereoisomer is formed preferentially over the other.

Anti-Addition Stereospecific Reaction

Anti-addition

Br

Br Br

Br

+Br2

Bromonium Ion Intermediate

Bromonium ion

- Br

Br

Br

BrBr

+ BrBr

Trans is formed exclusivelyNo Meso is formed (cis)

racemic mixture

S SR RBr BrBr Br

Br2

Bromonium Ion is Opened Equally from Both Sides

Br2

Br Br Br BrR R S S

racemic mixture

Br

Br

Br

Br

Br Br + -

Brominations Often Generate Asymmetric Centers

Br2

R RS S

racemic mixture

CH3CH3

H H

CH3CH3

Br BrH H

CH3CH3

Br BrH H

CH3

CH3 H

H CH3

CH3 H

H

Br

Br

CH3CH3

Br BrH H

S Rmeso

Br2

trans alkene + anti addition = MESO

CH2CH3

CH2CH3 H

H

Br2

CH2CH3

CH2CH3

HH

Br

Br

Br Br

CH2CH3CH2CH3HH

meso

Provide a Mechanism

BrHCl

Br

Cl

Cl

Br

+

racemic

Halohydrin FormationAddition of Br – OH

Stereospecific & Regiospecific

bromonium ion

BrBrBr2

+ BrBr

H2O

in H2O

Br

OH

HBr

OH

Br

-

Unsymmetrical Bromonium ionH2O opens ring at more hindered site

Br

CH3 H

+

+

H2O

BrCH3

HO H

Br and OH are trans in anti addition

+ enantiomer

CH3

OH

BrBr2 in H2O

CH3

Catalytic Hydrogenationsyn addition

Mechanism

Syn Addition of H2

CH3CH3

H2, 1% Pt/C

75 psi

CH3 CH3H H

cis only

Reactions that Generate Chirality Centers

Hydrogenation, syn

CH3CH3

CH2CH3CH2CH3

H2, Pt/C

CH2CH3 CH2CH3

CH3CH3

HH

CH2CH3

CH2CH3

H CH3

H CH3

product is meso

Hydrogenationall alkene bonds are reduced

H2, Pt/C

Three mol equivalents of hydrogen gas are consumed.

Asymmetric Induction

PPh2

PPh2

RuCl2

CH3

OHH2

Ru(BINAP)Cl2

CH3H

OH

96% e.e.

Noyori and Knowles shared Nobel Prize in Chemistry, 2001

Preparation of (L)-Dopafor Treatment of Parkinson’s

HO

HO

CH2C

CO2H

NH2

H

l-(-) Dopa

HO

HO

CH2CH2NH2

Dopaminecannot cross blood-brain

barrier

C=CNH2

CO2HH

HO

HO

H2Rh(DIOP)Cl2

enz.

10 pt. problem7.(10) Consider the compound 3(S),5,5-

trimethylcyclohexene. Upon reduction with H2 on a 1% Pt/C catalyst, the resulting product has an absolute configuration of (R). Draw the equation for this reaction, clearly drawing the starting alkene and the product alkane and explain why the absolute configuration is completely inverted in this reaction.

HydrationAddition of H2O

C CH

H

CH3

CH3

H2SO4 (aq)

CH3

CH3C

H

H

H

C

OH

H

C CH

H

HCH3

CH3

H2O

OH

H

C CH

H

HCH3

CH3

H2O

-H

catalytic

Oxymercuration HydrationMarkovnikov additionRegiospecific Reaction

1) Hg(OAc)2 inTHF/H2O

2) NaBH4

OH

H

Oxymercuration Mechanism

OH H

(H )NaBH4

organomercurial alcohol

HgOAc

OH

OAc

HgOAc

OH

H

OAcHgOAc

in H2O

OH2

+

+

Hg

OAc

Hg(OAc)2

Hydroboration Hydration Anti-Markovnikov

Syn addition

31) BH3-THF

2) H2O2, NaOH

H OH3

Hydroboration

BH3H BH2 H B

HH2

B

H

H H

Trialkylborane

H2O2

OH-

H OH3

3

Alkylborane

Draw the Major Products

CH3

1) BH3, THF

2) H2O2, NaOH

1) Hg(OAc)2, H2O-THF

2) NaBH4

2 Complementary Hydration Reactions

CH3

1) BH3, THF

2) H2O2, NaOH

OH

CH3

H

1) Hg(OAc)2, H2O-THF

2) NaBH4

OH

CH3

+ enant.

Cyclopropanation

Simmons-Smith Reaction

H Hin ether

CH2I2, Zn(Cu)

In situ Preparation of Carbene

C

II

H H+ Zn

HH

I I

C

Zn

C

H Hcarbene

CHH

sp2 hybridized

+ ZnI2

H H

Two MCAD Inhibitors

CO2H CO2H

MCPA SPA

MCPA and SPA

C CHCH2CH2OHC

H

H

Al(CH3)3

1 eq. CH2I2

in CH2Cl2, r.t.OH

CO2H

oxidize

MCPAinhibitors of MCAD enzymes

2 eq. CH2I2AlCl3in CH2Cl2, r.t.

CO2H

SPA

then oxidize

2) H2O2 , OH-

1) BH3-THF

f)

H2 1%Pt/C

e)

d)

c)

HBr with peroxidesb)

2) NaBH4

in H2O, THF

1) Hg(OAc)2a)

CH3

h

Br2

HCl

OzonolysisAlkene Cleavage

C=C

CH3

CH3

CH3

H

O3 in CH2Cl2C O

H

CH3

C

CH3

CH3

O+

H

CH3

CH3

CH3

C=C

OO

O

1)

2) CH3SCH3or Zn/HOAc

OO

O

H O

OO

H

ozonide

DMS

+ DMSO

Mechanism of Ozonolysis

Addition polymers

nn

isoprene polyisoprene (a trimer)latex, rubber (tires)

Air Pollution Reacts with Tires

n

O3

O

OO

O

O

O

O

OO

rubber

?

O3 reacts with all alkene Bonds

CH2

1) O3

2) CH3SCH3

O OO

H H+ HCH

O

O

O O

O

CH2

Problem

An unknown compound A, C8H16, reacts with

H2 on a 1% Pt/C catalyst to form B (C8H18).

B has two chiral centers but is optically inactive due to the presence of an internal plane of symmetry. Treatment of A with O3

followed by Zn/HOAc affords butanone only. Identify A and B and draw B in a Fischer projection.

O

butanone

O

HH

H2

O3

A

B

What is the Structure of Limonene?Solomons 8.39

O O

O

H

Limonene1) O3

2) Zn, HOAc+ HCH

O

H2, 1% Pt/C1-isopropyl-4-methylcyclohexane

C=C bonds become carbonyl groups

Limonene

1) O3

2) Zn, HOAc

H2, 1% Pt/C

1-isopropyl-4-methylcyclohexane

O

OO

O

C

HH

Acid Catalyzed DimerizationSolomons 8.58

+

major minor

H3O+

2

Hoffmann Product via E1

+

major minor

H3O+

2

H

H

OH2