Download Alkenes and Alkynes I

Chapter 7
Alkenes and Alkynes I:
Properties and Synthesis.
Elimination Reactions of Alkyl Halides
Ch. 7 - 1
1. Introduction

Alkenes
● Hydrocarbons containing C=C
● Old name: olefins
CH2OH
Vitamin A
H3C
H3C
H
HO
H
Cholesterol
Ch. 7 - 2

Alkynes
● Hydrocarbons containing C≡C
● Common name: acetylenes
H
N
Cl
O
O
F3C C
C
Efavirenz
(antiviral, AIDS therapeutic)
I
C
Cl
C
O
Cl
Cl
Haloprogin
(antifungal, antiseptic)
Ch. 7 - 3
2.

The (E ) - (Z ) System for
Designating Alkene Diastereomers
Cis-Trans System
● Useful for 1,2-disubstituted alkenes
● Examples:
H
(1)
Cl
Br
Br
H
trans -1-Bromo2-chloroethene
vs
Cl
H
H
cis -1-Bromo-
2-chloroethene
Ch. 7 - 4
● 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
Ch. 7 - 5

(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
Ch. 7 - 6

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
Ch. 7 - 7

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 “zusammer”; it
means “together” in German
Ch. 7 - 8
● Examples
Cl
CH3
1
2
Br
H
On carbon 2: Priority of Br > C
On carbon 1: Priority of Cl > H
⇒
highest priority groups
are Br (on carbon 2)
and Cl (on carbon 1)
Ch. 7 - 9
● Examples
Cl
CH3
Br
H
⇒ (E )-2-Bromo-1-chloropropene
Br
Cl
CH3
H
⇒ (Z )-2-Bromo-1-chloropropene
Ch. 7 - 10
● 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
Ch. 7 - 11
● Other examples
Br
4
(3)
8
7
1
3
2
5
6
(Z )-3-Bromo-4-tert-butyl-3-octene
C3: Br > C
n
t
C4: Bu > Bu
Ch. 7 - 12
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
Ch. 7 - 13
3A. Heat of Reaction
C

C
+
H
H
Pt
C
C
H
H
Heat of hydrogenation
● ∆H° ≃ -120 kJ/mol
Ch. 7 - 14
+ H2
Enthalpy
7 kJ/mol
+ H2
∆H° = -127 kJ/mol
5 kJ/mol
+ H2
∆H° = -120 kJ/mol
∆H° = -115 kJ/mol
≈
≈
≈
Ch. 7 - 15
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.
Ch. 7 - 16

R
Relative Stabilities of Alkenes
R
R
R
>
R
R
R
H
tetratrisubstituted substituted
R
H
>
R
H
>
R
H
R
R
>
H
R
disubstituted
R
H
>
H
H
H
H
H
H
>
H
H
monounsubstituted substituted
Ch. 7 - 17

Examples of stabilities of alkenes
(1)
(2)
>
>
Ch. 7 - 18
4. Cycloalkenes

Cycloalkenes containing 5 carbon
atoms or fewer exist only in the cis
form
cyclopropene
cyclobutene
cyclopentene
Ch. 7 - 19

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.)
Ch. 7 - 20

Trans – cyclooctene has been isolated
and is chiral and exists as a pair of
enantiomers
cis - cyclooctene
trans - cyclooctenes
Ch. 7 - 21
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
Ch. 7 - 22
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
+ B:H + X
X
Ch. 7 - 23
6A. How to Favor an E2 Mechanism
Use a secondary or tertiary alkyl halide
if possible. (Because steric hinderance
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)

Ch. 7 - 24

Use a high concentration of a strong
and nonpolarizable 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.
Ch. 7 - 25

Sodium ethoxide in ethanol
(EtONa/EtOH) and potassium tertbutoxide in tertbutyl alcohol (t-BuOK/tBuOH) 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)
Ch. 7 - 26
6B. Zaitsev’s Rule

(1)
Examples of dehydrohalogenations
where only a single elimination product
is possible
EtONa
EtOH, 55 C
Br
EtONa
(2)
(3)
(79%)
o
Br
( )
n
(91%)
o
EtOH, 55 C
Br
t -BuOK
o
t -BuOH, 40 C
( )
n
(85%)
Ch. 7 - 27

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
Ch. 7 - 28
⊖
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:

(1)
Ha
Hb
+
EtOH
70oC
Br
(2)
NaOEt
Br
69%
31%
(eliminate Ha) (eliminate Hb)
KOEt
+
EtOH
51%
+
18%
69%
31%
Ch. 7 - 29
Zaitsev’s Rule
● In elimination reactions, the more
highly substituted alkene product
predominates
 Stability of alkenes

Me
Me
C
Me
C
>
Me
>
Me
Me
C
Me
Me
Me
C
H
C
H
C
>
Me
C
H
>
H
Me
C
Me
H
C
H
H
C
H
Ch. 7 - 30
Mechanism for an E2 Reaction
Et O
CH3
α
C C CH3
H 3C β
Br
H
Et O
H
EtO⊖ removes
a β proton;
C−H breaks;
new π bond
forms and Br
begins to
depart
H
H 3C
C
CH3
C CH3
H
Br
δ−
δ−
H3C
H
C C
+
Et OH
Partial bonds in
the transition
state: C−H and
C−Br bonds
break, new π
C−C bond forms
CH3
CH3
+ Br
C=C is fully
formed and
the other
products are
EtOH and Br⊖
Ch. 7 - 31
δ−
O Et
H 3C
Et O
H
Free Energy
H 3C
CH3CH2
δ−
C
CH3
C CH3
H
Br
δ−
C
Br
δ−
∆G2‡
H
C
H
H
∆G1‡
CH3
CH3
EtO- + CH3CH2
C
CH3CH2
C
CH2 + EtOH + Br
CH3
Br
CH3
CH3CH
C
CH3 + EtOH + Br
Reaction Coordinate
Ch. 7 - 32
6C. Formation of the Less Substituted
Alkene Using a Bulky Base

Hofmann’s Rule
● Most elimination reactions follow
Zaitsev’s rule in which the most
stable alkenes are the major
products. However, under some
circumstances, the major
elimination product is the less
substituted, less stable alkene
Ch. 7 - 33
● Case 1: using a bulky base
EtO
(small)
CH3CH CHCH3
+
CH3CH2CH CH2
(80%)
(20%)
CH3CH2CHCH3
Br
t
BuO
(bulky)
⊖
EtO
(small base)
H
CH3CH CHCH3
(30%)
+
CH3CH2CH CH2 (70%)
H
H
H
H
C
C
C
C
H
H
Br H
tBuO⊖
H
(bulky base)
Ch. 7 - 34
● Case 2: with a bulky group next
to the leaving halide
less crowded β-H
H3C
Me H
Br H
C
C
C
Me H
C
H
Me H
more crowded β-H
EtO
Me H
H3C
C
C
Me H
C
CH2
Me
(mainly)
Ch. 7 - 35

Zaitsev Rule vs. Hofmann Rule
● Examples
(1)
Ha
Hb
Br
+
a
(eliminate H )
(eliminate Hb)
NaOEt, EtOH, 70oC
69%
31%
t
t
o
KO Bu, BuOH, 75 C
28%
72%
Ch. 7 - 36
● Examples
Hb
(2)
Br
Ha
+
a
(eliminate H )
b
(eliminate H )
o
NaOEt, EtOH, 70 C
91%
9%
t
o
t
KO Bu, BuOH, 75 C
7%
93%
Ch. 7 - 37
6D. The Stereochemistry of E2
Reactions
The 5 atoms involved in the transition
state of an E2 reaction (including the
base) must lie in the same plane
 The anti coplanar conformation is the
preferred transition state geometry
● The anti coplanar transition state is
staggered (and therefore of lower
energy), while the syn coplanar
transition state is eclipsed

Ch. 7 - 38
B
B
H
C
LG
H
C
C
C
LG
Anti coplanar
transition state
(preferred)
Syn coplanar
transition state
(only with certain
rigid molecules)
Ch. 7 - 39

Orientation Requirement
● H and Br have to be anti periplanar
(trans-coplanar)
● Examples
+ EtO
CH3CH2
Br
CH3CH2
CH3
CH3
Br
since:
CH3CH2
EtO
H
H
H
CH3
Only H is
anti periplanar
to Br
Ch. 7 - 40

E2 Elimination where there are two
axial β hydrogens
(a)
EtO
EtO
H3C
H
b
H
4
H
H
Ha
3
a
1
2
H3C
1
4
3
2
1-Menthene (78%)
(more stablealkene)
CH(CH3)2
H
Cl
Hb
Both
and
hydrogens
are anti to the chlorine in
this, the more stable
conformation
CH(CH3)2
(b)
H3C
1
4
3
CH(CH3)2
2
2-Menthene (22%)
(less stable alkene)
Ch. 7 - 41

E2 elimination where the only axial β
hydrogen is from a less stable
Conformer
H
H3C
1
4
H
H
3
CH3
H
2
CH(CH3)2
Cl
H
Menthyl chloride
(more stable conformer)
Elimination is not possible
for this conformation
because no hydrogen is anti
to the leaving group
Cl
H
H
H
H
CH(CH3)2
Menthyl chloride
(less stable conformer)
Elimination is possible for
this conformation because
the green hydrogen is anti
to the chlorine
Ch. 7 - 42
The transition state for
the E2 elimination is
anti coplanar
CH3
CH3
Cl
H
H
H
H
CH(CH3)2
Cl
H
H
H
H
CH(CH3)2
OEt
2-Menthene (100%)
H3C
CH(CH3)2
Ch. 7 - 43
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
+
H2O
Ch. 7 - 44

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
Ethanol (a 1o alcohol)
H
C
+ H2O
C
H
Ch. 7 - 45
● 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
+
H2O
Cyclohexene
(80%)
Ch. 7 - 46
● 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
OH
CH3
tert-Butyl alcohol
CH2
20% H2SO4
85oC
H3C
CH3
+ H2O
2-Methylpropene
(84%)
Ch. 7 - 47
● 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
>
R
C
OH
H
1o alcohol
Ch. 7 - 48

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
CH3
H3C
2,3-Dimethyl-2-butene
(80%)
CH3
H3C
+
C
CHCH3
H2C
2,3-Dimethyl-1-butene
(20%) Ch. 7 - 49
● 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
Ch. 7 - 50
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
H
CH3
protonated
alcohol
Ch. 7 - 51
● 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
CH3
H
+ H
O
H
2-Methylpropene
Ch. 7 - 52
7B. Carbocation Stability & the
Transition State

Recall
R
R
C
H
> R
R
3o
most
stable
C
H
> H
R
>
2o
C
H
> H
R
>
1o
C
H
>
methyl
least
stable
Ch. 7 - 53
Ch. 7 - 54
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
H
H
C
C
O
H
H
H
+A
conjugate
base
Ch. 7 - 55
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
C
C
CH3
H3C
CH3
+
CH3
H3C
2,3-Dimethyl-2-butenol
(major product)
C
CHCH3
H2C
2,3-Dimethyl-1-butene
(minor product)
Ch. 7 - 56

Step 1
CH3
H3C
C
CH3
CH
CH3 O
CH3
H3C
H
CH3
CH
CH3 OH2
H
+ H
C
O
H
protonated
alcohol
+ H
O
H
Ch. 7 - 57

Step 2
CH3
CH3
H3C
C
H3C
CH
OH2
CH3
H3C
C
CH
CH3
CH3
o
a 2 carbocation
+ H
O
H
Ch. 7 - 58

Step 3
CH3
H3C
C
CH3
δ+
δ
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
(more stable)
CH3
CH3
Ch. 7 - 59

Step 4
A
(a)
(b)
H
H
(a) or (b)
CH2
C
C
CH3
CH3 CH3
(a)
(b)
(major)
(minor)
CH3
H3C
HA +
C
H3C
H
H2C
C
C
CH3
less stable alkene
H3C
C
CH3 + HA
CH3
more stable alkene
Ch. 7 - 60

Other common examples of
carbocation rearrangements
● Migration of an allyl group
CH3
H3C
C
CH
CH3
CH3
o
a 2 carbocation
methanide
migration
CH3
H3C
C
C
CH3
CH3
3o carbocation
Ch. 7 - 61
● Migration of a hydride
H
H3C
C
CH
CH3
CH3
o
a 2 carbocation
hydride
migration
H
H3C
C
C
CH3
CH3
3o carbocation
Ch. 7 - 62
8B. Rearrangement after Dehydration
of a Primary Alcohol
R
R
H
H
C
C
C
H
R
H
H +H
O
A
C
H
E2
H
C
R
H
R
H
H
R
+H
C
H
A
protonation
H
+
A
H
H
C
C
C
R
R
H
H
C
C
R
H + A
H
R
A
O +H
R
C
C
+ H
C
C
H
H
C
H
deprotonation
C
R
C
H
H +H
A
Ch. 7 - 63
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
CH
Ch. 7 - 64

H
Comparison of acidity and basicity of
1st row elements of the Periodic Table
● Relative acidity
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
CH2 <
CH2CH3
Ch. 7 - 65
10. Synthesis of Alkynes by
Elimination Reactions

Synthesis of Alkynes by
Dehydrohalogenation of Vicinal
Dihalides
H
H
C
C
Br Br
NaNH2
heat
C
C
Ch. 7 - 66

Mechanism
R
H
H
C
C
Br Br
R
NH2
H
R
E2
R
Br
NH2
R
R
Ch. 7 - 67

Examples
Br
H
NaNH2
(1)
H
(2)
Br
Ph
Ph
heat
Br2
CCl4
Br
Ph
(78%)
H
Ph
H
NaNH2
heat
Br
Ph
Ph
Ch. 7 - 68

Synthesis of Alkynes by
Dehydrohalogenation of Geminal
Dihalides
O
R
Cl
PCl5
CH3
0oC
R
Cl
CH3
gem-dichloride
1. NaNH2 (3 equiv.), heat
2. HA
Ph
H
Ch. 7 - 69
11. Replacement of the Acetylenic
Hydrogen Atom of Terminal
Alkynes

R
The acetylide anion can be prepared by
H
NaNH2
liq. NH3
R
Na
+ NH3
Ch. 7 - 70

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’
o
o
 2 and 3 R’ usually undergo E2
elimination

Ch. 7 - 71

Ph
Examples
NaNH2
liq. NH3
Ph
CH3
H
I
Na
I
H
SN2
Ph
CH3
+
E2
Ph
H
+
NaI
+
I
Ch. 7 - 72
13. Hydrogenation of Alkenes
C
C

C
C
H2
Pt, Pd or Ni
solvent
heat and pressure
H
H
C
C
H2
Pt, Pd or Ni
H
H
solvent
heat and pressure
C
C
H
H
Hydrogenation is an example of
addition reaction
Ch. 7 - 73

Examples
H
H2
H
Rh(PPh3)3Cl
H2
Pd/C
H
H
Ch. 7 - 74
14. Hydrogenation: The Function
of the Catalyst

Hydrogenation of an alkene is an
exothermic reaction
● ∆H° ≃ -120 kJ/mol
R
CH
CH
+ H2
R
hydrogenation
R
CH2
CH2
R
+ heat
Ch. 7 - 75
Ch. 7 - 76
14A. 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.
Ch. 7 - 77

An anti addition places parts of the
adding reagent on opposite faces of
the reactant
Y
C
C
+
X
C
Y
X
C
anti
addition
Ch. 7 - 78
15. 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
Ch. 7 - 79
15A. 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
● Lindlar’s catalyst
 Pd/CaCO3, quinoline
Ni2B
(P-2)
Ch. 7 - 80

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
H
CH3
(86%)
Ch. 7 - 81
15B. 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
R'
R
H
Ch. 7 - 82

Example
o
1. Li, liq. EtNH2, -78 C
2. NH4Cl
H
H
anti addition
Ch. 7 - 83

Mechanism
radical anion
R
R
C
C
R
C
vinyl radical
H
C
NHEt
R
H
C
C
R
R
Li
Li
H
R
C
H
C
R
trans alkene
EtHN
H
R
H
C
C
R
vinyl anion
Ch. 7 - 84
16. An Introduction to Organic Synthesis
16A. Why Do Organic Synthesis?

To make naturally occurring compounds
which are biologically active but difficult (or
impossible) to obtain
AcO
Ph
O
O
OH
BzN
H
OH
TAXOL
HO
H
OH
O
OAc
Anti-tumor,
anti-cancer
agent
Ch. 7 - 85
TAXOL

Isolated from Pacific Yew tree
Leaves
Cones and Fruit
seed
pollen cones
usually appear on separate
male and female trees
Ch. 7 - 86
TAXOL



Approved by the U.S. Food & Drug
Administration in 1992 for treatment of
several types of cancer, including breast
cancer, lung cancer, and melanoma
An estimation: a 100-year old yew tree
must be sacrificed in order to obtain 300 mg
of Taxol, just enough for one single dose for
a cancer patient
Obviously, synthetic organic chemistry
methods that would lead to the synthesis of
Taxol would be extremely useful
Ch. 7 - 87
16B. Retrosynthetic Analysis
target
molecule
1st
precursor
2nd
precursor
starting
compound
Ch. 7 - 88

When doing retrosynthetic analysis, it is
necessary to generate as many possible
precursors, hence different synthetic routes,
as possible
1st precursor A
target
molecule
1st precursor B
1st precursor C
2nd precursor a
2nd precursor b
2nd precursor c
2nd precursor d
2nd precursor e
2nd precursor f
Ch. 7 - 89
16C. Identifying Precursors

Synthesis of
C
C
(target molecule)
Ch. 7 - 90

Retrosynthetic Analysis
o
SN2 on 1 alkyl halide: good
C
disconnection 1
C
C
+
δ−
X
δ+
C
disconnection 2
δ+
X
δ−
+
o
SN2 on 2 alkyl halide: poor
⇒ will get E2 as major pathway
Ch. 7 - 91

Synthesis
C
C
H
NaNH2
C
liq. NH3
(SN2)
NaI +
C Na
I
C
C
Ch. 7 - 92
16D. Raison d’Etre
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)
C
of alcohols)
Li, liq. NH3
(give (E)-alkenes)
C
(Dissolving
metal reduction
of alkynes)
C
Ch. 7 - 93
Summary of Methods for the Preparation of
Alkynes
X
(Dehydrohalogenation
Cl
of geminal dihalide)
R'
H
R
H
H
NaNH2
heat
NaNH2
heat
(Dehydrohalogenation
of vicinal dihalide)
R'
R
H
X
Cl
R
C
(Deprotonation of terminal
alkynes and SN2 reaction of
the acetylide anion)
R
C
R'
1. NaNH2, liq. NH3
2. R'-X (R' = 1o alkyl group)
C
C
H
Ch. 7 - 94
 END OF CHAPTER 7 
Ch. 7 - 95