created by professor william tam & dr. phillis chang chapter 7 alkenes and alkynes i: properties...

Created byProfessor William Tam & Dr. Phillis

Chapter 7

Alkenes and Alkynes I:Properties and

Synthesis.Elimination Reactions

of Alkyl Halides

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

Vitamin A

H HCholesterol

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

Efavirenz(antiviral, AIDS therapeutic)

Haloprogin(antifungal, antiseptic)

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

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

alkenes●Examples:

(1) vs

trans -1-Bromo-2-chloroethene

cis -1-Bromo-2-chloroethene

●ExamplesH

(2) vs

trans -3-Hexene cis -3-Hexene

Br(3) vs

trans -1,3-Dibromopropene

cis -1,3-Dibromopropene

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)

●Examples

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

●Examples

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

●Other examples

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

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

C1: Br > ClC2: Cl > H

C1: Cl > HC2: Cl > H

●Other examples

(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

crowding

Less stable More stable

3A.Heat of Reaction

C C + H HPt

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

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

tetra-substituted

> > > > > >

tri-substituted

di-substituted

mono-substituted

un-substituted

Examples of stabilities of alkenes

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

Dehydration of Alcohols

HH+, heat

6. Dehydrohalogenation of AlkylHalides

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

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)

EtOH, 55oC

EtOH, 55oC(91%)

(3) Br t -BuOK

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

Rate = H3CHC

CH3 EtOk

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:

HbHaEtONa

EtOH70oC

69% 31%

(eliminate Ha) (eliminate Hb)

(2)Br EtOK

51% 18% 31%

Zaitsev’s Rule●In elimination reactions, the

more highly substituted alkene product predominates يسود

Stability of alkenes

Mechanism for an E2 Reaction

CH3CH3

Et OH + Br

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

Reaction Coordinate

DG2‡

EtO- +

CCH3CH2

+ EtOH + Br-CH3

CCH3CH2 CH2

+ EtOH + Br-CH3

CCH3CH CH3

CH3CH3

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

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

Hconc. H2SO4

180oCC C

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

OH20% H2SO4

85oC+ H2O

H3C CH3

tert-Butyl alcohol 2-Methylpropene(84%)

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

3o alcohol 2o alcohol 1o alcohol

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

CH3H3C

3,3-Dimethyl-2-butanol

85% H3PO4

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

+ C CHCH3

H3C CH3

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

●Notice that the carbon skeleton of the reactant is

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

H3C O H

protonatedalcohol

●Step 2H3C

H3C O H

H3C CH3

Ha carbocation

●Step 3

H3C CH3

CH3C CH3

2-Methylpropene

7B.Carbocation Stability & theTransition State

Recall

3o 2o 1o methyl> >>

moststable

leaststable

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

O H + H Afast

slowr.d.s

alkene

1o alcohol

acidcatalyst

protonatedalcohol

conjugatebase

8. Carbocation Stability & Occurrenceof Molecular Rearrangements

8A.Rearrangements during Dehy-dration of Secondary Alcohols

CH3H3C

3,3-Dimethyl-2-butanol

85% H3PO4heat

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

+ C CHCH3

H3C CH3

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

Step 1

H3CHC CH3

protonatedalcohol

H3CHC CH3

Step 2

H3CHC CH3

a 2o carbocation

H3C CH CH3

Step 3

H3C CH CH3

2o carbocation

(less stable)

transition state

H3C CH CH3

3o carbocation

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

Step 4

C CH3CH2

(a) or (b)

(a) (b)

H2C+ HAC

H3CHA +

(major) (minor)

less stable alkenemore stable alkene

a 2o carbocation

H3C CH CH31 2

Other common examples of carbocation rearrangements

●Migration of a methyl group

3o carbocation

H3C C CH3

methanide

migration

(1,2-methyl shift)

●Migration of a hydride

a 2o carbocation

H3C CH CH3

3o carbocation

H3C C CH3

hydride

migration

8B.Rearrangement after Dehydrationof a Primary Alcohol

C C C O H

H A+E2

H A+protonation

HA+C C

H A+deprotonation

A + C C

sp sp2 sp3

9. The Acidity of Terminal Alkynes

pKa = 25 pKa = 44 pKa = 50

Acetylenic hydrogen

C CH H C C

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

C C2 NaNH2

Mechanism

Examples

H (78%)

heat(1)

NaNH2(2 eq.)heat

CCl4 Ph

Synthesis of Alkynes by Dehydrohalogenation of Geminal DihalidesO

0oC R CH3

gem-dichloride

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

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

NaNH2liq. NH3

SN2 E2

Ph CH3

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

Pt, Pd, or Ni

solventheat and pressure

2 equiv. H2Pt, Pd, or Ni

solventheat and pressure

Examples

Rh(PPh3)3Cl

(Wilkinson's catalyst)

13. Hydrogenation: The Functionof the Catalyst

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

R CH CH R

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

synaddition

C C + H H C C

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

antiaddition

14. Hydrogenation of Alkynes

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

Pt or Pd

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

EtOHNi2B

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

Ni2B(P-2)

H2 H H

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

Example

2. NH4Cl

1. Li, liq. EtNH2, -78oC

anti addition

Mechanism

radical anion

C C RR

H NHEt

H HEtHN

vinyl radical

vinyl aniontrans alkene

Summary of Methods for the Preparation of Alkenes

H OHH+

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

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

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)

NaNH2heat

C C HR

1. NaNH2, liq. NH3

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

NaNH2heat

END OF CHAPTER 7

created by professor william tam & dr. phillis chang chapter 7 alkenes and alkynes i: properties...

john wiley sons

university of guelph

professor tam

university of guelph

professor william tam

university of toronto

organic synthesis

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