42. 4.2 Orientation of Alkene
Addition Reaction
(1) Regiospecific Rxn.
(2) Markovnikov observed in the 19th century that in
the addition of HX to alkene, the H attaches to
the carbon with the most H’s and X attaches
to the other end (to the one with the most
alkyl substituents)
• This is Markovnikov’s rule
43. Example of Markovnikov’s Rule
Addition of HCl to 2-methylpropene
Regiospecific
If both ends have similar substitution, then not
regiospecific
47. Energy of Carbocations and Markovnikov’s
Rule
More stable carbocation forms faster
Tertiary cations and associated transition states are
more stable than primary cations
48. Mechanistic Source of Regiospecificity in
Addition Reactions
If addition involves a
carbocation
intermediate
and th
d there are two
t
possible ways to add
the route producing
the more alkyl
substituted cationic
center is lower in
energy
alkyl groups stabilize
y g p
carbocation
53. 4.4 Addition of H2O
Hydration of an alkene is the addition of H-OH to
H OH
to give an alcohol
Acid catalysts are used in high temperature industrial
processes: ethylene is converted to ethanol
60. 4.7 Oxidation of Alkenes: Hydroxylation and
Cleavage
Cl
Hydroxylation: in basic solution addition
solution,
of one or more –OH group to a molecule.
61. Cleavage: in acidic solution
No H on C: C=O; O
N C C O; One H on C: COOH; t
C=O C COOH; two Hs
H
on C: CO2
62. Alkene Cleavage: Ozone
Ozone, O3, adds to alkenes to form molozonide
Reduce molozonide t obtain k t
R d l id to bt i ketones and/or
d/
aldehydes
64. Examples of Ozonolysis of Alkenes
Cleavage products reveal an alkene’s
alkene s
structure
65. 4.8 Alkene polymers
A polymer is a very large molecule consisting
of repeating units of simpler molecules,
f ti it f i l l l
formed by polymerization
Alkenes react with radical catalysts to
undergo radical polymerization
Ethylene is polymerized to poyethylene, for
poyethylene
example
67. Polymerization: Propagation
Radical from intiation adds to alkene to
generate alkene derived radical
This radical adds to another alkene, and so
s ad ca a ot e a e e, a d
on many times
68. Polymerization: Termination
Chain propagation ends when two radical
chains combine
Not controlled specifically but affected by
ot co t o ed spec ca y a ected
reactivity and concentration
70. 4.9 Conjugate Dienes
Conjugated dienes: they are more than one double
& separated by only one single bond and their
t db l i l b d d th i
orbitals interact
H2C=CH—CH=CH2 H2C=CH—CH2—CH=CH2
1,3-Butadiene 1,4-Pentadien
（conjugated;alternating （nonconjugated; nonalterinating
double and single bonds） double and single bonds）
Conjugated dienes are somewhat more stable
than nonconjugated dienes
77. The Diels-Alder Cycloaddition Reaction
Conjugate dienes can combine with alkenes to form
six-membered
six membered cyclic compounds
The formation of the ring involves no intermediate
(
(concerted formation of two bonds) )
Discovered by Otto Paul Hermann Diels and Kurt
Alder in Germany in the 1930’s
81. Resonance forms are imaginary
g y
Resonance forms differ only in the placement
of their π or non-bonding electrons.
82. ■
Different resonance forms of a substance don’t have
to be equivalent
equivalent.
83. ■ Resonance forms must be valid Lewis structures
and obey normal rules of valency
valency.
■
Resonance leads to stability
stability.
84. 4.12 Alkynes’ Reactions
Naming Alkynes:
General hydrocarbon rules apply wuith “-
yne
yne” as a suffix indicating an alkyne
85. 1. Reduction of Alkynes
Addition of H2 using chemically deactivated
palladium on calcium carbonate as a catalyst (the
Lindlar catalyst) produces a cis alkene
The two hydrogens add syn (from the same side of
y g y (
the triple bond)
86. 2. Reactions of Alkynes:
Addition of HX and X2
Addition reactions of alkynes are similar to
those of alkenes
th f lk
Intermediate alkene reacts further with
excess eagent
e cess reagent
Regiospecificity according to Markovnikov
87. Addition of Bromine and Chlorine
Initial addition gives trans intermediate
Product with excess reagent is tetrahalide
88. 3. Hydration of Alkynes
Alkynes do not react with aqueous protic acids
Mercuric ion (as the sulfate) is a Lewis acid catalyst
that promotes addition of water in Markovnikov
orientation
The immediate product is a vinylic alcohol, or enol,
which spontaneously transforms to a ketone
89. Keto-enol
Enols rearrange to the isomeric ketone by
the rapid transfer of a proton from the
hydroxyl to the alkene carbon
The keto form is usually so stable
compared to the enol that only the keto
form can be observed
90. Hydration of Unsymmetrical Alkynes
If the alkyl groups at either end of the C-C triple bond
are not the same, both products can form and this is
same
not normally useful
If the triple bond is at the first carbon of the chain
(then H is what is attached to one side) this is called a
terminal alkyne
Hydration of a terminal always gives the methyl
ketone, which is useful
91. 4. Oxidative Cleavage of
Alkynes
Strong oxidizing reagents (O3 or KMnO4) cleave
internal alkynes, producing t
i t l lk d i two carboxylic acids
b li id
Terminal alkynes are oxidized to a carboxylic acid
and carbon dioxide
Neither process is useful in modern synthesis – were
used to elucidate structures because the products
indicate the structure of the alkyne precursor
i di h f h lk
92. 5. Alkyne Acidity: Formation of
Acetylide Anions
Terminal alkynes are weak Brønsted acids
y
(alkenes and alkanes are much less acidic
(pKa ~ 25)
Reaction of strong anhydrous bases with
a terminal acetylene produces an
acetylide ion
93. Alkylation of Acetylide Anions
Acetylide ions can react as nucleophiles
as well as bases
Reaction with a primary alkyl halide
p y y
produces a hydrocarbon that contains
carbons from both partners, providing a
general route to larger alkynes
