Alkenes, Alkynes

1
Alkenes, Alkynes
2
Required
background Thermodynamics from general
chemistry Hybridization Molecular geometry Curved
arrow notation Acidity and basicity
Essential for 1.
Reactions of elimination 2. Selective
reactions 3. Multistep reaction mechanisms 4.
Role of petroleum in the economy 5.
Stereochemistry of reactions 6. Role of polymers
in the economy 7. Chemistry of aromatic compounds
3

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
4
s (sigma)-bonds are symmetrical with respect to
rotation around the bond. Rotation of fragments
around the bond does not break the bond.
p (pi)-bonds are non-symmetrical with respect to
rotation around the bond. Rotation of fragments
around the bond breaks the bond. The p-bonds are
normally weaker, than the s-bonds, due to a
weaker orbital overlap.
5
(No Transcript)
6
Together, s- and p- bonds form a double bond. Now
we need to choose a hybridization to describe
systems, containing a double bond. The valence
shell of the atom of carbon has one s-orbital and
three p-orbitals. When the carbon is not bonded
by any p-bonds, all s- and p-orbitals are
involved in the formation of s-bonds, through
the sp3-hybrid orbitals. When the carbon is
bonded by one p-bond, one p-orbital participates
in the formation of this p- bond, and the
remaining one s-orbital and two p-orbitals are
involved in the formation of s-bonds through the
sp2-hybrid orbitals. The presence of
sp2-hybridized carbons (hosts of sp2-orbitals) is
characteristic for alkenes.
7
(No Transcript)
8
The presence of a weaker p-bond in alkenes
accounts for their higher chemical reactivity,
comparing with alkanes. The simplest alkene is
ethylene (C2H4). It is the simplest signaling
agent in biology, responsible for ripening apples
and other fruits.
9

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
10
Due to the planar geometry of the double bond,
two substituents can be located at either the
same side of the double bond, or at the opposite
sides of the double bond.
Note cis-trans-isomerism is impossible if at
least one carbon at the double bond has two
identical substituents.
11
(No Transcript)
12
Nomenclature of alkenes
Same rules as for alkanes, except 1. Replace
-ane with -ene 2. The principal carbon chain
must contain the double bond 3. Numbering of the
principal chain The double bond must have the
lowest number 4. In the chemical name, indicate
position of the double bond
5. If the compound contains more, than one
double bond, replace -ene with -diene,
-triene etc. 6. Indicate stereochemistry (cis-
or trans-)
13
E-, Z- Nomenclature
Step 1. For each double bond, assign relative
priorities of attached fragments Step 2. Find
highest priority fragments at each carbon. If
they are cis-, the isomer is Z. If they are
trans, the isomer is E.
14
(No Transcript)
15

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
16
Dipole moments are a little higher, than for
alkanes due to polarization of p-bonds
Dipole moments for cis-isomers are normally
higher, than for trans-isomers
17
More substituted double bonds are more stable,
than less substituted double bonds.
Trans-isomers are more stable, than cis-isomers.
18
To compare relative stability of isomeric alkenes
with higher accuracy, heats of hydrogenation can
be used instead of heats of combustion.
19

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
20
Dehydration of alcohols
Zaitsevs Rule Hydrogen comes off the carbon
with the least number of hydrogens attached
21
Dehydrohalogenation
Dehydrogenation
22
(No Transcript)
23

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
24
(No Transcript)
25
(No Transcript)
26

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
27
Stability of alkyl substituted carbocations
tertiary gt secondary gt primary
28
Delocalization of positive charge increases
stability of carbocations. So far, we compared
stabilities of primary, secondary and
tertiary alkyl carbocations, based on the s to p
interaction.
Participation of higher in energy p-orbitals
strongly increases stability of carbocations at
double bonds due to the p to p interaction.
Benzylic gt allylic gt alkyl gt vinylic
29
The first carbocation (triphenylmethyl cation)
was synthesized in 1901 by Noris and Kehrmann.
Existence of carbocations was proved by George
Olah (NMR, X-Ray) and brought him the Nobel Prize
in 1994.
Carbocations are considered alongside with
carboanions, carbenes and radicals among the most
reactive intermediates in organic chemistry
30

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
31
(No Transcript)
32
Markovnikovs rule (1896)
The halogen of a hydrogen halide attaches to the
carbon of the alkene bearing the fewer number of
hydrogens and greater number of carbons
33
Examples of hydrohalogenation
34

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
35
For the reaction of hydration the Markovnikovs
rule works the best
600,000,000 lb of ethanol is produced annually in
the US by this reaction. Hydration is the
reaction of dehydration, going backwards.
36

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
37
There are several mechanisms of halogenation of
alkenes. We will consider electrophilic
halogenation.
38
Consequence of the bromonium cation formation
anti-stereoselectivity of addition
39

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
40
This reaction was introduced by Herbert Brown in
1955 and brought him the Nobel Prize in 1979.
Hydroboration, followed by oxidation is used when
we need to perform hydration of a double bond
against the Markovnikovs rule.
41
The intermediate R-BH2 normally reacts further
with another molecule of alkene until a
trialkylborane R3B is formed. It does not change
the reaction product, but enhances regioselectivit
y due to the steric hindrance around the
fragment R. The mechanism of hydroboration
determines its syn-stereoselectivity
42

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
43
(No Transcript)
44
(No Transcript)
45

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
46
In alkynes, one s- and two p- bonds form a triple
bond. Now we need to choose a hybridization to
describe systems, containing a triple bond. The
valence shell of the atom of carbon has one
s-orbital and three p-orbitals. When the carbon
is bonded by two p-bonds, each of two p-orbitals
participates in the formation of this p- bond,
and the remaining one s-orbital and one p-orbital
are equally involved in the formation of s-bonds,
giving rise to the sp-hybridization state. The
presence of sp-hybridized carbons is
characteristic for alkynes.
47
s- and p-bonds in alkynes
48
(No Transcript)
49

Outline 1. Bonding in Alkenes 2. Nomenclature of
Alkenes. Cis-trans-isomerism 3. Physical
properties of Alkenes 4. Preparation of
Alkenes 5. Catalytic Hydrogenation of Alkenes 6.
Carbocations 7. Hydrohalogenation of Alkenes 8.
Acid-catalyzed Hydration of Alkenes 9.
Halogenation of Alkenes 10. Hydroboration-Oxidatio
n of Alkenes 11. Polymers 12. Bonding in
Alkynes 13. Acidity of Alkynes
50
(No Transcript)
51
The relatively high acidity of alkynes
significantly affects their chemical properties.