Created by Professor William Tam & Dr. Phillis Chang Chapter 7 Alkenes and Alkynes I: Properties and...

Created byProfessor William Tam & Dr. Phillis

ChangCopyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Chapter 7

Alkenes and Alkynes I:Properties and

Synthesis.Elimination Reactions

of Alkyl Halides

© 2014 by John Wiley & Sons, Inc. All rights reserved.

About The Authors

These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang.

Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem.

Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.


Table of Contents (hyperlinked)1. Introduction2. The (E) - (Z) System for Designating Alkene

Diastereomers3. Relative Stabilities of Alkenes4. Cycloalkenes5. Synthesis of Alkenes via Elimination Reactions6. Dehydrohalogenation of Alkyl Halides7. Acid-Catalyzed Dehydration of Alcohols8. Carbocation Stability & Occurrence of

Molecular Rearrangements9. The Acidity of Terminal Alkynes10. Synthesis of Alkynes by Elimination Reactions11. Terminal Alkynes Can Be Converted

to Nucleophiles for Carbon-Carbon Bond Formation12. Hydrogenation of Alkenes13. Hydrogenation: The Function of the Catalyst14. Hydrogenation of Alkynes15. An Introduction to Organic Synthesis


Table of Contents 1. Introduction2. The (E) - (Z) System for Designating Alkene

Diastereomers3. Relative Stabilities of Alkenes4. Cycloalkenes5. Synthesis of Alkenes via Elimination Reactions6. Dehydrohalogenation of Alkyl Halides7. Acid-Catalyzed Dehydration of Alcohols8. Carbocation Stability & Occurrence of Molecular

Rearrangements9. The Acidity of Terminal Alkynes10. Synthesis of Alkynes by Elimination Reactions11. Terminal Alkynes Can Be Converted to Nucleophiles

for Carbon-Carbon Bond Formation12. Hydrogenation of Alkenes13. Hydrogenation: The Function of the Catalyst14. Hydrogenation of Alkynes15. An Introduction to Organic Synthesis


Table of Contents1. Introduction2. The (E) - (Z) System for Designating Alkene

Diastereomers3. Relative Stabilities of Alkenes4. Cycloalkenes5. Synthesis of Alkenes via Elimination Reactions6. Dehydrohalogenation of Alkyl Halides7. Acid-Catalyzed Dehydration of Alcohols8. Carbocation Stability & Occurrence of Molecular

Rearrangements9. The Acidity of Terminal Alkynes10. Synthesis of Alkynes by Elimination Reactions11. Terminal Alkynes Can Be Converted to Nucleophiles

for Carbon-Carbon Bond Formation12. Hydrogenation of Alkenes13. Hydrogenation: The Function of the Catalyst14. Hydrogenation of Alkynes15. An Introduction to Organic Synthesis


In this chapter we will consider:

The properties of alkenes and alkynes and how they are name

How alkenes and alkynes can be transformed into alkanes

How to plan the synthesis of any organic molecule


1. Introduction

Alkenes●Hydrocarbons containing C=C●Old name: olefins

CH2OH

Vitamin A

HO

H3C

H3C

H HCholesterol


Alkynes●Hydrocarbons containing C≡C●Common name: acetylenes

O

NH

O

Cl

Efavirenz(antiviral, AIDS therapeutic)

CF3CC

Cl

ClCl

OCC

I

Haloprogin(antifungal, antiseptic)


2. The (E) - (Z) System for Design-ating Alkene Diastereomers

Cis-Trans System●Useful for 1,2-disubstituted

alkenes●Examples:

ClBr

H

H

ClH

Br

H

(1) vs

trans -1-Bromo-2-chloroethene

cis -1-Bromo-2-chloroethene


●ExamplesH

H

(2) vs

trans -3-Hexene cis -3-Hexene

HH

Br(3) vs

trans -1,3-Dibromopropene

cis -1,3-Dibromopropene

BrBr

Br


ClBr

CH3

H

e.g. cis or trans?

Cl is cis to CH3

and trans to Br

(E) - (Z) System●Difficulties encountered for

trisubstituted and tetrasubstituted alkenes


The Cahn-Ingold-Prelog (E) - (Z) Convention

●The system is based on the atomic number of the attached atom

●The higher the atomic number, the higher the priority


The Cahn-Ingold-Prelog (E) - (Z) Convention● (E) configuration – the highest

priority groups are on the opposite side of the double bond “E ” stands for “entgegen”; it

means “opposite” in German● (Z) configuration – the highest

priority groups are on the same side of the double bond “Z ” stands for “zusammen”; it

means “together” in German


On carbon 2: Priority of Br > COn carbon 1: Priority of Cl > H

highest priority groups are Br (on carbon 2) and Cl (on carbon 1)

ClBr

CH3

H

21

●Examples


ClBr

CH3

H (E )-2-Bromo-1-chloropropene

●Examples

ClCH3

Br

H (Z )-2-Bromo-1-chloropropene


●Other examples

(1)

(2)

ClCl

H

H

12

(E )-1,2-Dichloroethene[or trans-1,2-Dichloroethene]

ClCl

Br1

2 (Z )-1-Bromo-1,2-dichloroethene

C1: Br > ClC2: Cl > H

C1: Cl > HC2: Cl > H


●Other examples

(3)

Br

87

65

43

2

1

(Z )-3-Bromo-4-tert-butyl-3-octene

C3: Br > CC4: tBu > nBu


3. Relative Stabilities of Alkenes

Cis and trans alkenes do not have the same stability

C C

H

R

H

R

C C

R

H

H

R

crowding

Less stable More stable


3A.Heat of Reaction

C C + H HPt

C C

H H

Heat of hydrogenation●∆H° ≃ -120 kJ/mol


En

thalp

y+ H2

≈

+ H2

≈

+ H2

≈

7 kJ/mol

5 kJ/mol

DH° = -115 kJ/mol

DH° = -127 kJ/mol

DH° = -120 kJ/mol


3B.Overall Relative Stabilities ofAlkenes

The greater the number of attached alkyl groups (i.e., the more highly substituted the carbon atoms of the double bond), the greater the alkene’s stability.


Relative Stabilities of Alkenes

R

R

R

R

tetra-substituted

R

R

H

R

R

R

H

H

H

R

R

H

H

R

H

R

H

R

H

H

H

H

H

H

> > > > > >

tri-substituted

di-substituted

mono-substituted

un-substituted


Examples of stabilities of alkenes

(1) >

(2) >


4. Cycloalkenes

Cycloalkenes containing 5 carbon atoms or fewer exist only in the cis form

cyclopropene cyclobutene cyclopentene


Trans – cyclohexene and trans – cycloheptene have a very short lifetime and have not been isolated

cyclohexene Hypotheticaltrans - cyclohexene

(too strained to exist at r.t.)


Trans – cyclooctene has been isolated. The molecule is chiral and exists as a pair of enantiomers.

cis - cyclooctene trans - cyclooctenes


5. Synthesis of Alkenes viaElimination Reactions

Dehydrohalogenation of Alkyl Halides

C CH

XH

H

H

H

H

H

H

Hbase

-HX

Dehydration of Alcohols

C CH

OHH

H

H

H

H

H

H

HH+, heat

-HOH


6. Dehydrohalogenation of AlkylHalides

The best reaction conditions to use when synthesizing an alkene by dehydrohalogenation are those that promote an E2 mechanism

C C

H

X

B: C CE2

B:H + X+


6A.How to Favor an E2 Mechanism

Use a secondary or tertiary alkyl halide if possible. (Because steric hindrance in the substrate will inhibit substitution)

When a synthesis must begin with a primary alkyl halide, use a bulky base. (Because the steric bulk of the base will inhibit substitution)


Use a high concentration of a strong and non-polarizable base, such as an alkoxide. [Because a weak and polarizable base would not drive the reaction toward a bimolecular reaction, thereby allowing unimolecular processes (such as SN1 or E1 reactions) to compete.]


Sodium ethoxide in ethanol (EtONa/EtOH) and potassium tert-butoxide in tert-butyl alcohol (t-BuOK/t-BuOH) are bases typically used to promote E2 reactions

Use elevated temperature because heat generally favors elimination over substitution. (Because elimination reactions are entropically favored over substitution reactions.)


Examples of dehydrohalogenations where only a single elimination product is possible

6B.Zaitsev’s Rule

Br(79%)(1)

EtONa

EtOH, 55oC

(2)Br

EtONa

EtOH, 55oC(91%)

(3) Br t -BuOK

t -BuOH, 40oC(85%)( )n

( )n


Rate = H3CHC

Br

CH3 EtOk

Br

HbHaB2-methyl-2-butene

2-methyl-1-butene

YYY Ha

YYY Hb

(2nd order overall) bimolecular


When a small base is used (e.g. EtO⊖ or HO⊖) the major product will be the more highly substituted alkene (the more stable alkene). Examples:

Br

HbHaEtONa

EtOH70oC

+

69% 31%

(eliminate Ha) (eliminate Hb)

(1)

(2)Br EtOK

EtOH

51% 18% 31%

+ +

69%


Zaitsev’s Rule●In elimination reactions, the

more highly substituted alkene product predominates يسود

Stability of alkenes

C C

Me

Me

Me

Me

> C C

Me

Me

H

Me

> C C

H

Me

Me

H

> C C

H

Me

H

Me

> C C

H

Me

H

H


Mechanism for an E2 Reaction

Et O

C CBr

H

HH3C

CH3CH3

Et O

C CBr

H

HH3C

CH3CH3

C C

H

H3C

CH3

CH3

Et OH + Br

+β

α

EtO⊖ removes a b hydrogen; C−H breaks; new p bond forms and Br begins to depart

Partial bonds in the transition state: C−H and C−Br bonds break, new p C−C bond forms

C=C is fully formed and the other products are EtOH and

Br⊖


Free

Ene

rgy

Reaction Coordinate

DG2‡

EtO- +

CH3

CCH3CH2

Br

CH3

+ EtOH + Br-CH3

CCH3CH2 CH2

+ EtOH + Br-CH3

CCH3CH CH3

Et O

C CBr

H

HH3C

CH3CH3

EtO

CCBr

H

HH

H3CCH3CH2

DG1‡


7. Acid-Catalyzed Dehydration ofAlcohols

Most alcohols undergo dehydration (lose a molecule of water) to form an alkene when heated with a strong acid

C

H

C

OH

HA

heatC C + H2O


The temperature and concentration of acid required to dehydrate an alcohol depend on the structure of the alcohol substrate● Primary alcohols are the most

difficult to dehydrate. Dehydration of ethanol, for example, requires concentrated sulfuric acid and a temperature of 180°C

H

CH

H

C

H

OH

Hconc. H2SO4

180oCC C

H

H

H

H

+ H2O

Ethanol (a 1o alcohol)


● Secondary alcohols usually dehydrate under milder conditions. Cyclohexanol, for example, dehydrates in 85% phosphoric acid at 165–170°C

OH85% H3PO4

165-170oC+ H2O

Cyclohexanol Cyclohexene(80%)


● Tertiary alcohols are usually so easily dehydrated that extremely mild conditions can be used. tert-Butyl alcohol, for example, dehydrates in 20% aqueous sulfuric acid at a temperature of 85°C

H3C C

CH3

CH3

OH20% H2SO4

85oC+ H2O

CH2

H3C CH3

tert-Butyl alcohol 2-Methylpropene(84%)


●The relative ease with which alcohols will undergo dehydration is in the following order:

C

R

R OH

R

> C

R

R OH

H

> C

H

R OH

H

3o alcohol 2o alcohol 1o alcohol


Some primary and secondary alcohols also undergo rearrangements of their carbon skeletons during dehydration

C CH

OH

CH3H3C

3,3-Dimethyl-2-butanol

85% H3PO4

80oC

C C

H3C

H3C

CH3

CH3

2,3-Dimethyl-2-butene(80%)

+ C CHCH3

H2C

H3C CH3

2,3-Dimethyl-1-butene(20%)

CH3

CH3


●Notice that the carbon skeleton of the reactant is

C CC

C

C

C

C C

C

C

C

C

while that of the product is

(For mechanism of this rearrangement, see: slides 60-64)


7A.Mechanism for Dehydration of 2o & 3o Alcohols: An E1 Reaction

Consider the dehydration of tert-butyl alcohol●Step 1

CH3

C

CH3

H3C O H

H

O

H

H+

H3C

C

CH3

H3C O H

H

+ H O

H

protonatedalcohol


●Step 2H3C

C

CH3

H3C O H

H

C

CH3

H3C CH3

+ H O

Ha carbocation

●Step 3

C

C

H3C CH3

+ H O

H

H

H

HH

O

H

H+CH2

CH3C CH3

2-Methylpropene


7B.Carbocation Stability & theTransition State

Recall

R

CR

R

H

CR

R

H

CH

R

H

CH

H

> >>

3o 2o 1o methyl> >>

moststable

leaststable



7C. A Mechanism for Dehydration of Primary Alcohols: An E2 Reaction

C C

H

H

H

O H + H Afast

C C

H

H

H

O H

H

+ A

C C

H

H

+HA+

H

OH

slowr.d.s

alkene

1o alcohol

acidcatalyst

protonatedalcohol

conjugatebase


8. Carbocation Stability & Occurrenceof Molecular Rearrangements

8A.Rearrangements during Dehy-dration of Secondary Alcohols

C CH

OH

CH3H3C

3,3-Dimethyl-2-butanol

85% H3PO4heat

C C

H3C

H3C

CH3

CH3

2,3-Dimethyl-2-butenol(major product)

+ C CHCH3

H2C

H3C CH3

2,3-Dimethyl-1-butene(minor product)

CH3

CH3


Step 1

CH3

C

CH3

H3CHC CH3

HOH

H

+

O H

+ H O

H

protonatedalcohol

CH3

C

CH3

H3CHC CH3

OH2


Step 2

CH3

C

H3C

H3CHC CH3

OH2

+ H O

H

a 2o carbocation

CH3

C

CH3

H3C CH CH3


Step 3

CH3

C

CH3

H3C CH CH3

2o carbocation

(less stable)

transition state

CH3

C

CH3

H3C CH CH3

C

CH3

H3C CH CH3

CH3

3o carbocation

(more stable)The less stable 2o carbocation rearranges to a more stable 3o carbocation.


Step 4

H

C

CH3

C CH3CH2

CH3

H

A

(a) or (b)

(a)

(b)

(a) (b)

H

C

CH3

C CH3

H3C

H2C+ HAC

CH3

C

CH3

H3C

H3CHA +

(major) (minor)

less stable alkenemore stable alkene


a 2o carbocation

CH3

C

CH3

H3C CH CH31 2

Other common examples of carbocation rearrangements

●Migration of a methyl group

3o carbocation

C

CH3

H3C C CH3

methanide

migration

CH3

(1,2-methyl shift)


●Migration of a hydride

a 2o carbocation

H

C

CH3

H3C CH CH3

3o carbocation

C

CH3

H3C C CH3

hydride

migration

H


8B.Rearrangement after Dehydrationof a Primary Alcohol

+ H O

H

C C C O H

R

H

H

R

H

H

H A+E2

C C

R

C

H

H

R

HH A+

H A+protonation

C C

R

C

H

H

R

HA+C C

R

C

H

H

R

HH

H A+deprotonation

C C

R

C

H

HR

A + C C

R

C

H

H

R

HH

H


sp sp2 sp3

9. The Acidity of Terminal Alkynes

pKa = 25 pKa = 44 pKa = 50

Acetylenic hydrogen

C CH H C C

H

H

H

H

C C

H

H

H H

H

H

Relative basicity of the conjugate base

CH3CH2 >CH2 CH >CH CH


Comparison of acidity and basicity of 1st row elements of the Periodic Table●Relative acidity

●Relative basicity

H OH H OR C CRH H NH2 H CH CH2 H CH2CH3> > > > >

pKa 15.7 16-17 25 38 44 50

OH OR C CR NH2 CH CH2 CH2CH3< < < < <


10. Synthesis of Alkynes by Elimination Reactions

Synthesis of Alkynes by Dehydrohalogenation of Vicinal Dihalides

Br

C

H

C

Br

H

C C2 NaNH2

heat


Mechanism

Br

R C

H

C R

Br

H NH2

R R

NH2

R

BrH

RE2


Examples

Br

Br H

H (78%)

NaNH2

heat(1)

NaNH2(2 eq.)heat

Ph

Ph

Br2

CCl4 Ph

Br H

BrH

PhPh

Ph(2)


Synthesis of Alkynes by Dehydrohalogenation of Geminal DihalidesO

R CH3

PCl5

0oC R CH3

Cl Cl

gem-dichloride

1. NaNH2 (3 equiv.), heat2. HA

R H


11. Terminal Alkynes Can Be Converted to Nucleophiles for Carbon-Carbon Bond Formation

The acetylide anion can be prepared by

R HNaNH2

liq. NH3R Na + NH3


Acetylide anions are useful intermediates for the synthesis of other alkynes

R R' X R R' X+

∵ 2nd step is an SN2 reaction, usually only good for 1o R’ or methyl halides

2o and 3o R’ usually undergo E2 elimination


Examples Ph H

Ph Na

NaNH2liq. NH3

CH3 I

I

H

SN2 E2

Ph CH3

+

NaI

Ph H

+

+ I


11A. General Principles of Structure and Reactivity Illustrated by the Alkylation of Alkynide Anions

Preparation of the alkynide anion involves simple Brønsted–Lowry acid–base chemistry


Once formed, the alkynide anion is a Lewis base with which the alkyl halide reacts as an electron pair acceptor (a Lewis acid). The alkynide anion can thus be called a nucleophile because of the negative charge concentrated at its terminal carbon—it is a reagent that seeks positive charge


The alkyl halide can be called an electrophile because of the partial positive charge at the carbon bearing the halogen—it is a reagent that seeks negative charge


12. Hydrogenation of Alkenes

Hydrogenation is an example of addition reaction

C C

H

C C

HH2

Pt, Pd, or Ni

solventheat and pressure

C C

H

C C

H

H

H

2 equiv. H2Pt, Pd, or Ni

solventheat and pressure


Examples

H2

Rh(PPh3)3Cl

H

H

(Wilkinson's catalyst)

H

H

H2

Pd/C


13. Hydrogenation: The Functionof the Catalyst

Hydrogenation of an alkene is an exothermic reaction●∆H° ≃ -120 kJ/mol

R CH CH R

+ H2

hydrogenationR CH2 CH2 R

+ heat



13A. Syn and Anti Additions An addition that places the parts

of the reagent on the same side (or face) of the reactant is called syn addition

C C + X Y C C

X Y

synaddition

C C + H H C C

H H

Pt

Catalytic hydrogenation is a syn addition.


An anti addition places parts of the adding reagent on opposite faces of the reactant

C C + X Y C C

Y

X

antiaddition


14. Hydrogenation of Alkynes

Using the reaction conditions, alkynes are usually converted to alkanes and are difficult to stop at the alkene stage

H2

Pt or Pd

H H

H2

H H

HH


14A. Syn Addition of Hydrogen: Synthesis of cis-Alkenes

Semi-hydrogenation of alkynes to alkenes can be achieved using either the Ni2B (P-2) catalyst or the Lindlar’s catalyst● Nickel boride compound (P-2

catalyst)

● Lindlar’s catalyst Pd/CaCO3, quinoline

NiO CH3

O

2

NaBH4

EtOHNi2B

(P-2)


Semi-hydrogenation of alkynes using Ni2B (P-2) or Lindlar’s catalyst causes syn addition of hydrogen●Examples

Ni2B(P-2)

H2 H H

(cis)

(97%)

Pd/CaCO3quinoline

H2Ph CH3

Ph CH3

H H(86%)


14B. Anti Addition of Hydrogen: Synthesis of trans-Alkenes

Alkynes can be converted to trans-alkenes by dissolving metal reduction

Anti addition of dihydrogen to the alkyne

R'R2. aqueous work up

1. Li, liq. NH3, -78oCH

RR'

H


Example

2. NH4Cl

1. Li, liq. EtNH2, -78oC

H

H

anti addition


C C

R

R

H

Mechanism

C C

R

R

radical anion

C C RR

Li

H NHEt

C C

R

R

H

Li

C C

H

R

R

H HEtHN

vinyl radical

vinyl aniontrans alkene


Summary of Methods for the Preparation of Alkenes

C C

C C

H OHH+

heat

C C

Li, liq. NH3(give (E)-alkenes)

C C

H2, Ni2B (P-2)or Lindlar's catalyst(give (Z)-alkenes)

C C

H X base, heat

(Dehydration of alcohols)

(Dissolvingmetal reduction of alkynes)

(Semi-hydrogenation of alkynes)

(Dehydrohalogenationof alkyl halides)


Summary of Methods for the Preparation of Alkynes

C C R'R

(Dehydrohalogenation of geminal dihalide)

(Deprotonation of terminal alkynes and SN2 reaction of the acetylide anion)

(Dehydrohalogenation of vicinal dihalide)

R H

ClCl

H

R'

NaNH2heat

C C HR

1. NaNH2, liq. NH3

2. R'-X (R' = 1o alkyl group)

R H

HX

X

R'

NaNH2heat


END OF CHAPTER 7

Date post:	28-Dec-2015
Category:	Documents
Upload:	ethan-bryan
View:	295 times
Download:	23 times

Created by Professor William Tam & Dr. Phillis Chang Chapter 7 Alkenes and Alkynes I: Properties and...

Documents