1. Properties and Synthesis. Elimination Reactions
Chapter 7
Alkenes and Alkynes I:
Properties and Synthesis.
Elimination Reactions
of Alkyl Halides
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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 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.
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3. Table of Contents (hyperlinked)
Introduction
The (E) - (Z) System for Designating Alkene Diastereomers
Relative Stabilities of Alkenes
Cycloalkenes
Synthesis of Alkenes via Elimination Reactions
Dehydrohalogenation of Alkyl Halides
Acid-Catalyzed Dehydration of Alcohols
Carbocation Stability & Occurrence of Molecular Rearrangements
The Acidity of Terminal Alkynes
Synthesis of Alkynes by Elimination Reactions
Terminal Alkynes Can Be Converted to Nucleophiles for Carbon-Carbon Bond Formation
Hydrogenation of Alkenes
Hydrogenation: The Function of the Catalyst
Hydrogenation of Alkynes
An Introduction to Organic Synthesis
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Table of Contents
Introduction
The (E) - (Z) System for Designating Alkene Diastereomers
Relative Stabilities of Alkenes
Cycloalkenes
Synthesis of Alkenes via Elimination Reactions
Dehydrohalogenation of Alkyl Halides
Acid-Catalyzed Dehydration of Alcohols
Carbocation Stability & Occurrence of Molecular Rearrangements
The Acidity of Terminal Alkynes
Synthesis of Alkynes by Elimination Reactions
Terminal Alkynes Can Be Converted to Nucleophiles for Carbon-Carbon Bond Formation
Hydrogenation of Alkenes
Hydrogenation: The Function of the Catalyst
Hydrogenation of Alkynes
An Introduction to Organic Synthesis
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5. © 2014 by John Wiley & Sons, Inc. All rights reserved.
Table of Contents
Introduction
The (E) - (Z) System for Designating Alkene Diastereomers
Relative Stabilities of Alkenes
Cycloalkenes
Synthesis of Alkenes via Elimination Reactions
Dehydrohalogenation of Alkyl Halides
Acid-Catalyzed Dehydration of Alcohols
Carbocation Stability & Occurrence of Molecular Rearrangements
The Acidity of Terminal Alkynes
Synthesis of Alkynes by Elimination Reactions
Terminal Alkynes Can Be Converted to Nucleophiles for Carbon-Carbon Bond Formation
Hydrogenation of Alkenes
Hydrogenation: The Function of the Catalyst
Hydrogenation of Alkynes
An Introduction to Organic Synthesis
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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
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1. Introduction
Alkenes
Hydrocarbons containing C=C
Old name: olefins
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Alkynes
Hydrocarbons containing C≡C
Common name: acetylenes
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2. The (E) - (Z) System for Design- ating Alkene Diastereomers
Cis-Trans System
Useful for 1,2-disubstituted alkenes
Examples:
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Examples
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(E) - (Z) System
Difficulties encountered for trisubstituted and tetrasubstituted alkenes
Cl is cis to CH3
and trans to Br
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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
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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
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Examples
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Examples
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Other examples
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Other examples
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3. Relative Stabilities of Alkenes
Cis and trans alkenes do not have the same stability
crowding
Less stable
More stable
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3A. Heat of Reaction
Heat of hydrogenation
∆H° ≃ -120 kJ/mol
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7 kJ/mol ≈
DH° = -127 kJ/mol
5 kJ/mol
Enthalpy
DH° = -120 kJ/mol
DH° = -115 kJ/mol
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3B. Overall Relative Stabilities of Alkenes
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.
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Relative Stabilities of Alkenes
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Examples of stabilities of alkenes
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4. Cycloalkenes
Cycloalkenes containing 5 carbon atoms or fewer exist only in the cis form
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Trans – cyclohexene and trans – cycloheptene have a very short lifetime and have not been isolated
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Trans – cyclooctene has been isolated. The molecule is chiral and exists as a pair of enantiomers.
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5. Synthesis of Alkenes via Elimination Reactions
Dehydrohalogenation of Alkyl Halides
Dehydration of Alcohols
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6. Dehydrohalogenation of Alkyl Halides
The best reaction conditions to use when synthesizing an alkene by dehydrohalogenation are those that promote an E2 mechanism
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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)
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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.]
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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.)
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6B. Zaitsev’s Rule
Examples of dehydrohalogenations where only a single elimination product is possible
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Rate =
(2nd order overall)
 bimolecular
̶̶̶ Ha
̶̶̶ Hb
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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:
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Zaitsev’s Rule
In elimination reactions, the more highly substituted alkene product predominates يسود
Stability of alkenes
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Mechanism for an E2 Reaction 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⊖
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DG1‡
DG2‡
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7. Acid-Catalyzed Dehydration of Alcohols
Most alcohols undergo dehydration (lose a molecule of water) to form an alkene when heated with a strong acid
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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
Ethanol (a 1o alcohol)
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Secondary alcohols usually dehydrate under milder conditions. Cyclohexanol, for example, dehydrates in 85% phosphoric acid at 165–170°C
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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
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The relative ease with which alcohols will undergo dehydration is in the following order:
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Some primary and secondary alcohols also undergo rearrangements of their carbon skeletons during dehydration
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Notice that the carbon skeleton of the reactant is
while that of the product is
(For mechanism of this rearrangement, see: slides 60-64)
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7A. Mechanism for Dehydration of 2o & 3o Alcohols: An E1 Reaction
Consider the dehydration of tert-butyl alcohol
Step 1
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Step 2
Step 3
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7B. Carbocation Stability & the Transition State
Recall
most
stable
least
stable
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7C. A Mechanism for Dehydration of Primary Alcohols: An E2 Reaction
protonated
alcohol
1o alcohol
acid
catalyst
conjugate
base
alkene
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8. Carbocation Stability & Occurrence of Molecular Rearrangements
8A. Rearrangements during Dehy- dration of Secondary Alcohols
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Step 1
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Step 2
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Step 3
The less stable 2o carbocation rearranges to a more stable 3o carbocation.
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Step 4
(a)
(b)
(a) or (b)
(a)
(b)
(major)
(minor)
more stable alkene
less stable alkene
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Other common examples of carbocation rearrangements
Migration of a methyl group
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Migration of a hydride
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8B. Rearrangement after Dehydration of a Primary Alcohol
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9. The Acidity of Terminal Alkynes
Acetylenic hydrogen sp
sp2
sp3
pKa = 25
pKa = 44
pKa = 50
Relative basicity of the conjugate base
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Comparison of acidity and basicity of 1st row elements of the Periodic Table
Relative acidity
Relative basicity
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10. Synthesis of Alkynes by Elimination Reactions
Synthesis of Alkynes by Dehydrohalogenation of Vicinal Dihalides
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Mechanism
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Examples
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Synthesis of Alkynes by Dehydrohalogenation of Geminal Dihalides
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11. Terminal Alkynes Can Be Converted to Nucleophiles for Carbon-Carbon Bond Formation
The acetylide anion can be prepared by
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Acetylide anions are useful intermediates for the synthesis of other alkynes
∵ 2nd step is an SN2 reaction, usually only good for 1o R’ or methyl halides
2o and 3o R’ usually undergo E2 elimination
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Examples
SN2 E2
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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
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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
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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
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12. Hydrogenation of Alkenes
Hydrogenation is an example of addition reaction
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Examples
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13. Hydrogenation: The Function of the Catalyst
Hydrogenation of an alkene is an exothermic reaction
∆H° ≃ -120 kJ/mol
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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
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An anti addition places parts of the adding reagent on opposite faces of the reactant
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14. Hydrogenation of Alkynes
Using the reaction conditions, alkynes are usually converted to alkanes and are difficult to stop at the alkene stage
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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
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Semi-hydrogenation of alkynes using Ni2B (P-2) or Lindlar’s catalyst causes syn addition of hydrogen
Examples
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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
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Example
anti addition
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Mechanism
radical anion
vinyl radical
trans alkene
vinyl anion
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82. Summary of Methods for the Preparation of Alkenes
(Dehydrohalogenation
of alkyl halides)
(Dehydration of alcohols)
(Semi-hydrogenation
of alkynes)
(Dissolving
metal reduction of alkynes)
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83. Summary of Methods for the Preparation of Alkynes
(Dehydrohalogenation of geminal dihalide)
(Dehydrohalogenation of vicinal dihalide)
(Deprotonation of terminal alkynes and SN2 reaction of the acetylide anion)
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 END OF CHAPTER 7 
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