Chapter 7
Alkenes and Alkynes I:
Properties and Synthesis.
Elimination Reactions
of Alkyl Halides
Created by
Professor William Tam & Dr. Phillis Chang
Copyright © 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.
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Table of Contents
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
(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
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Table of Contents
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
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
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Table of Contents
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
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
© 2014 by John Wiley & Sons, Inc. All rights reserved.
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
© 2014 by John Wiley & Sons, Inc. All rights reserved.
1. Introduction

Alkenes
● Hydrocarbons containing C=C
● Old name: olefins
CH2OH
H3C
Vitamin A
H3C
H
HO
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H
Cholesterol

Alkynes
● Hydrocarbons containing C≡C
● Common name: acetylenes
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2. The (E) - (Z) System for Designating Alkene Diastereomers

Cis-Trans System
● Useful for 1,2-disubstituted alkenes
● Examples:
H
(1)
Cl
Br
Br
vs
Cl
H
trans -1-Bromo2-chloroethene
H
H
cis -1-Bromo-
2-chloroethene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Examples
H
(2)
vs
H
H
H
cis -3-Hexene
trans -3-Hexene
(3) Br
Br
trans -1,3-
Dibromopropene
vs
Br
Br
cis -1,3-
Dibromopropene
© 2014 by John Wiley & Sons, Inc. All rights reserved.

(E) - (Z) System
● Difficulties encountered for
trisubstituted and tetrasubstituted
alkenes
e.g.
CH3
Cl
Br
cis or trans?
H
Cl is cis to CH3
and trans to Br
© 2014 by John Wiley & Sons, Inc. All rights reserved.

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
© 2014 by John Wiley & Sons, Inc. All rights reserved.

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
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Examples
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● Examples
Cl
CH3
Br
H
 (E )-2-Bromo-1-chloropropene
Br
Cl
CH3
H
 (Z )-2-Bromo-1-chloropropene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Other examples
(1) Cl
H
2
1
Cl
H
(2)
Cl
C1: Cl > H
C2: Cl > H
2
1
Br
(E )-1,2-Dichloroethene
[or trans-1,2-Dichloroethene]
Cl
(Z )-1-Bromo-1,2-dichloroethene
C1: Br > Cl
C2: Cl > H
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Other examples
Br
4
(3)
1
3
2
8
7
5
6
(Z )-3-Bromo-4-tert-butyl-3-octene
C3: Br > C
C4: tBu > nBu
© 2014 by John Wiley & Sons, Inc. All rights reserved.
3. Relative Stabilities of Alkenes

Cis and trans alkenes do not have the
same stability
crowding
R
R
C
H
H
R
C
C
H
Less stable
R
C
H
More stable
© 2014 by John Wiley & Sons, Inc. All rights reserved.
3A. Heat of Reaction
C

C
+
H
H
Pt
Heat of hydrogenation
● ∆H° ≃ -120 kJ/mol
© 2014 by John Wiley & Sons, Inc. All rights reserved.
C
C
H
H
+ H2
Enthalpy
7 kJ/mol
+ H2
DH° = -127 kJ/mol
5 kJ/mol
+ H2
DH° = -120 kJ/mol
DH° = -115 kJ/mol
≈
≈
© 2014 by John Wiley & Sons, Inc. All rights reserved.
≈
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.
© 2014 by John Wiley & Sons, Inc. All rights reserved.

R
Relative Stabilities of Alkenes
R
R
R
>
R
R
R
H
tetratrisubstituted substituted
R
H
>
R
H
>
R
H
R
R
>
H
R
R
H
>
H
H
disubstituted
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H
H
H
H
>
H
H
monounsubstituted substituted

Examples of stabilities of alkenes
(1)
(2)
>
>
© 2014 by John Wiley & Sons, Inc. All rights reserved.
4. Cycloalkenes

Cycloalkenes containing 5 carbon
atoms or fewer exist only in the cis
form
cyclopropene
cyclobutene
cyclopentene
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Trans – cyclohexene and trans –
cycloheptene have a very short lifetime
and have not been isolated
cyclohexene
Hypothetical
trans - cyclohexene
(too strained to exist at r.t.)
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Trans – cyclooctene has been isolated.
The molecule is chiral and exists as a
pair of enantiomers.
cis - cyclooctene
trans - cyclooctenes
© 2014 by John Wiley & Sons, Inc. All rights reserved.
5. Synthesis of Alkenes via
Elimination Reactions

Dehydrohalogenation of Alkyl Halides
H
H
C
H
C
X
H

H
base
-HX
H
H
H
H
Dehydration of Alcohols
H
H
C
H
H
H
C
OH
H+, heat
H
H
-HOH
H
H
© 2014 by John Wiley & Sons, Inc. All rights reserved.
6. Dehydrohalogenation of Alkyl
Halides

The best reaction conditions to use
when synthesizing an alkene by
dehydrohalogenation are those that
promote an E2 mechanism
H
B:
C
E2
C
C
C
X
© 2014 by John Wiley & Sons, Inc. All rights reserved.
+ 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)

© 2014 by John Wiley & Sons, Inc. All rights reserved.

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.]
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Sodium ethoxide in ethanol
(EtONa/EtOH) and potassium tertbutoxide 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.)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
6B. Zaitsev’s Rule

(1)
Examples of dehydrohalogenations
where only a single elimination product
is possible
EtONa
EtOH, 55 C
Br
EtONa
(2)
(3)
o
Br
( )
n
o
EtOH, 55 C
t -BuOK
Br
t -BuOH, 40oC
(79%)
(91%)
( )
© 2014 by John Wiley & Sons, Inc. All rights reserved.
n
(85%)

Rate = k
H3C
H
C
CH3
EtO
(2nd order overall)
 bimolecular
Br
̶̶̶ H
B
Ha
a
Hb
2-methyl-2-butene
Br
̶̶̶ H
b
2-methyl-1-butene
© 2014 by John Wiley & Sons, Inc. All rights reserved.

⊖
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:
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Zaitsev’s Rule
● In elimination reactions, the more
highly substituted alkene product
predominates ‫يسود‬
 Stability of alkenes

Me
Me
C
Me
Me
>
C
Me
Me
C
Me
Me
>
H
>
C
H
Me
>
C
H
H
C
H
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H
C
H
Me
C
Me
C
H
C
Me
Mechanism for an E2 Reaction
α
β
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
© 2014 by John Wiley & Sons, Inc. All rights reserved.
C=C is fully
formed and
the other
products are
EtOH and Br⊖

O Et
H3C
Et O
H
Free Energy
H3C
CH3CH2


C
CH3
C CH3
H
Br
C
Br

DG2‡
H
C
H
H
DG1‡
CH3
CH3
EtO- + CH3CH2
C
CH3CH2
C
CH2 + EtOH + Br
CH3
Br
CH3
CH3CH
Reaction Coordinate
© 2014 by John Wiley & Sons, Inc. All rights reserved.
C
CH3 + EtOH + Br
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
C
C
H
OH
HA
heat
C
C
© 2014 by John Wiley & Sons, Inc. All rights reserved.
+
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
H
H
C
C
H
OH
H
conc. H2SO4
H
180oC
H
H
C
Ethanol (a 1o alcohol)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
+ H2O
C
H
● Secondary alcohols usually dehydrate
under milder conditions. Cyclohexanol,
for example, dehydrates in 85%
phosphoric acid at 165–170°C
OH
85% H3PO4
+
165-170oC
Cyclohexanol
Cyclohexene
(80%)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H2O
● 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
CH3
H3C
C
CH2
20% H2SO4
OH
85oC
CH3
tert-Butyl alcohol
H3C
CH3
+ H2O
2-Methylpropene
(84%)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● The relative ease with which
alcohols will undergo dehydration is
in the following order:
R
R
C
R
OH
R
3o alcohol
>
R
C
H
OH
>
H
2o alcohol
© 2014 by John Wiley & Sons, Inc. All rights reserved.
R
C
OH
H
1o alcohol

Some primary and secondary alcohols
also undergo rearrangements of their
carbon skeletons during dehydration
CH3
H3C
C
CH CH3
85% H3PO4
80oC
CH3OH
3,3-Dimethyl-2-butanol
H3C
CH3
C
C
H3C
CH3
2,3-Dimethyl-2-butene
(80%)
CH3
H3C
+
C
CHCH 3
H2C
2,3-Dimethyl-1-butene
(20%)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Notice that the carbon skeleton of
the reactant is
C
C
C
C
C
C
while that of the product is
C
C
C
C
C
C
(For mechanism of this
rearrangement, see: slides 60-64)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
7A. Mechanism for Dehydration of 2
o
& 3 Alcohols: An E1 Reaction

o
Consider the dehydration of tert-butyl
alcohol
+ H O
● Step 1
H
CH3
H3C
C
CH3
H
O
H + H
O
H
H3C
H3C
H
C
O
CH3
protonated
alcohol
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H
● Step 2
H3C
H3C
H
C
O
CH3
H
H3C
CH3
C
CH3
+ H
O
H
a carbocation
● Step 3
H
H
H3C
C
C
H
CH3
+ H
CH2
O
H
H3C
C
H
+ H
CH3
2-Methylpropene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
O
H
7B. Carbocation Stability & the
Transition State

Recall
R
R
C
H
> R
R
3o
C
H
> H
R
>
2o
C
H
> H
R
>
1o
most
stable
C
H
>
methyl
least
stable
© 2014 by John Wiley & Sons, Inc. All rights reserved.
© 2014 by John Wiley & Sons, Inc. All rights reserved.
7C. A Mechanism for Dehydration of
Primary Alcohols: An E2 Reaction
protonated
alcohol
o
1 alcohol
H
C
C
H
H
O
H + H
A
acid
catalyst
H
H
O
fast
slow
r.d.s
H
+ HA +
C
C
H alkene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H
H
C
C
O
H
H
H
+A
conjugate
base
8. Carbocation Stability & Occurrence
of Molecular Rearrangements
8A. Rearrangements during Dehydration of Secondary Alcohols
CH3
H3C
C
CH CH3
85% H3PO4
heat
CH3OH
3,3-Dimethyl-2-butanol
H3C
CH3
C
C
CH3
H3C
+
H3C
CH3
2,3-Dimethyl-2-butenol
(major product)
C
CHCH 3
H2C
2,3-Dimethyl-1-butene
(minor product)
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Step 1
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
Step 2
CH3
H3C
C
CH
CH3
CH3
a 2o carbocation
+ H
O
H
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Step 3
CH3
H3C
C
CH3


C
CH

CH
CH3
H3C
CH3
CH3
CH3
2o carbocation
transition state
(less stable)
3o carbocation
o
The less stable 2
carbocation rearranges
o
to a more stable 3
carbocation.
CH3
H3C
C
CH
CH3
© 2014 by John Wiley & Sons, Inc. All rights reserved.
(more stable)
CH3

Step 4
A
(a)
(b)
H
H
(a) or (b)
CH2
C
C
CH3
CH3 CH3
(a)
(b)
(major)
(minor)
H3C
HA +
CH3
C
H3C
H
H2C
C
C
CH3
more stable alkene
H3C
C
CH3 + HA
CH3
less stable alkene
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Other common examples of
carbocation rearrangements
● Migration of a methyl group
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● Migration of a hydride
H
H3C
C
CH
CH3
CH3
hydride
migration
H
H3C
o
a 2 carbocation
© 2014 by John Wiley & Sons, Inc. All rights reserved.
C
C
CH3
CH3
3o carbocation
8B. Rearrangement after Dehydration
of a Primary Alcohol
R
H
H
C
C
C
H
R
H
H +H
O
A
R
C
H
H
C
E2
R
H
R
H
H
R
+H
C
H
A
protonation
H
+
A
H
H
C
C
C
R
H
C
R
R
H
C
H + A
H
R
A
O +H
R
C
C
+ H
C
C
H
H
C
H
deprotonation
© 2014 by John Wiley & Sons, Inc. All rights reserved.
C
R
C
H
H +H
A
9. The Acidity of Terminal Alkynes
Acetylenic hydrogen
sp
sp2
H
H
C
C
H
H
C
C
H
pKa = 25

sp3
H
H
pKa = 44
H
H
C
C
H
H
H
pKa = 50
Relative basicity of the conjugate base
CH3CH2
> CH2
CH
> CH
© 2014 by John Wiley & Sons, Inc. All rights reserved.
CH
Comparison of acidity and basicity of
1st row elements of the Periodic Table
● Relative acidity

H
OH > H
pKa 15.7
OR > H
CR > H
C
16-17
NH2 > H
25
CH
38
CH2 > H
44
CH2CH3
50
● Relative basicity
OH <
OR <
C
CR <
NH2 <
CH
© 2014 by John Wiley & Sons, Inc. All rights reserved.
CH2 <
CH2CH3
10. Synthesis of Alkynes by
Elimination Reactions

Synthesis of Alkynes by
Dehydrohalogenation of Vicinal
Dihalides
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Mechanism
R
H
H
C
C
R
NH2
Br Br
NH2
R
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R

Examples
Br
(1)
H
Br
H
NaNH2
heat
© 2014 by John Wiley & Sons, Inc. All rights reserved.
(78%)

Synthesis of Alkynes by
Dehydrohalogenation of Geminal
Dihalides
© 2014 by John Wiley & Sons, Inc. All rights reserved.
11. Terminal Alkynes Can Be
Converted to Nucleophiles for
Carbon-Carbon Bond Formation

R
The acetylide anion can be prepared by
H
NaNH2
liq. NH3
R
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Na
+ NH3

R
Acetylide anions are useful
intermediates for the synthesis of other
alkynes
R' X
R
R' + X
∵ 2nd step is an SN2 reaction, usually
o
only good for 1 R’ or methyl halides
o
o
 2 and 3 R’ usually undergo E2
elimination

© 2014 by John Wiley & Sons, Inc. All rights reserved.

Ph
Examples
NaNH2
liq. NH3
Ph
CH3
H
I
Na
I
H
SN2
Ph
E2
CH3
+
NaI
© 2014 by John Wiley & Sons, Inc. All rights reserved.
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
© 2014 by John Wiley & Sons, Inc. All rights reserved.

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
© 2014 by John Wiley & Sons, Inc. All rights reserved.
12. Hydrogenation of Alkenes

Hydrogenation is an example of
addition reaction
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Examples
H2
Pd/C
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H
H
13. Hydrogenation: The Function
of the Catalyst

Hydrogenation of an alkene is an
exothermic reaction
● ∆H° ≃ -120 kJ/mol
R
CH
CH
R
hydrogenation
R
CH2
CH2
+ heat
+ H2
© 2014 by John Wiley & Sons, Inc. All rights reserved.
R
© 2014 by John Wiley & Sons, Inc. All rights reserved.
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
C
Y
C
X
C
C
+
H
H
syn
addition
Y
Pt
C
H
C
H
Catalytic hydrogenation is a syn addition.
© 2014 by John Wiley & Sons, Inc. All rights reserved.

An anti addition places parts of the
adding reagent on opposite faces of
the reactant
Y
C
C
+
X
C
Y
X
© 2014 by John Wiley & Sons, Inc. All rights reserved.
C
anti
addition
14. Hydrogenation of Alkynes
H2
H
H
Pt or Pd
H2
H
H

H
H
Using the reaction conditions, alkynes
are usually converted to alkanes and
are difficult to stop at the alkene stage
© 2014 by John Wiley & Sons, Inc. All rights reserved.
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)

Ni
O
O
NaBH4
CH3
EtOH
2
Ni2B
(P-2)
● Lindlar’s catalyst
 Pd/CaCO3, quinoline
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Semi-hydrogenation of alkynes using
Ni2B (P-2) or Lindlar’s catalyst causes
syn addition of hydrogen
● Examples
H2
H
Ni2B (P-2)
Ph
CH3
H
(97%)
(cis)
H2
H
Pd/CaCO3
quinoline
Ph
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H
CH3
(86%)
14B. Anti Addition of Hydrogen:
Synthesis of trans-Alkenes
 Alkynes can be converted to transalkenes by dissolving metal reduction
 Anti addition of dihydrogen to the
alkyne
H
o
R
R'
1. Li, liq. NH3, -78 C
2. aqueous work up
© 2014 by John Wiley & Sons, Inc. All rights reserved.
R'
R
H

Example
o
1. Li, liq. EtNH2, -78 C
2. NH4Cl
H
H
anti addition
© 2014 by John Wiley & Sons, Inc. All rights reserved.

Mechanism
radical anion
R
R
C
C
R
C
vinyl radical
H
C
NHEt
R
H
C
C
R
R
Li
Li
R
H
C
H
C
EtHN
R
trans alkene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H
R
H
C
C
R
vinyl anion
Summary of Methods for the Preparation of
Alkenes
C
H
C
X
(Dehydrohalogenation
of alkyl halides)
H+
base, heat
C
C
H
OH
heat (Dehydration
H2, Ni2B (P-2)
or Lindlar's catalyst
(give (Z)-alkenes)
C
C
C
(Semihydrogenation
of alkynes)
of alcohols)
C
Li, liq. NH 3
(give (E)-alkenes)
C
(Dissolving
metal reduction
of alkynes)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
C
Summary of Methods for the Preparation of
Alkynes
X
R'
H
R
R'
R
H
X
Cl
(Dehydrohalogenation
Cl
of geminal dihalide)
NaNH2
heat
NaNH2
heat
(Dehydrohalogenation
of vicinal dihalide)
R
C
C
R'
(Deprotonation of terminal
alkynes and SN2 reaction of
the acetylide anion)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
H
H
 END OF CHAPTER 7 
© 2014 by John Wiley & Sons, Inc. All rights reserved.