Alkenes: Structure, Bonding, and Naming
1435-1436
2014-2015
Learning Objectives
Learning Objectives
Chapter two discusses the following topics and the student by the end of this
Chapter two discusses the following topics and the student by the end of this
chapter will:
chapter will:
Know the structure, hybridization and bonding of alkenes
Know the structure, hybridization and bonding of alkenes
Know the c
Know the c
ommon and IUPAC naming of alkenes
ommon and IUPAC naming of alkenes
Know the geometry of the double bond i.e. cis/trans isomerization
Know the geometry of the double bond i.e. cis/trans isomerization
Know the physical properties of alkenes
Know the physical properties of alkenes
Know the different methods used for preparation of alkenes (elimination
Know the different methods used for preparation of alkenes (elimination
reactions ; dehydrogenation, dehydration and alkenes stability (Zaitsev’s rule)
reactions ; dehydrogenation, dehydration and alkenes stability (Zaitsev’s rule)
play an important role in understanding these reactions
play an important role in understanding these reactions
Know the addition reactions of alkenes and the effect of Markovnikov’s rule
Know the addition reactions of alkenes and the effect of Markovnikov’s rule
in determining the regioselectivity of this reaction.
in determining the regioselectivity of this reaction.
Alkenes
They are unsaturated hydrocarbons
They are unsaturated hydrocarbons
– made up of C and H atoms
– made up of C and H atoms
and contain
and contain
one or more C=C
one or more C=C
double bond somewhere in their
double bond somewhere in their
structures.
structures.
Their general formula is
Their general formula is
C
C
n
n
H
H
2n
2n
- for non-cyclic alkenes
- for non-cyclic alkenes
Their general formula is
Their general formula is
C
C
n
n
H
H
2n-2
2n-2
- for cyclic alkenes
- for cyclic alkenes
S
S
t
t
r
r
u
u
c
c
t
t
u
u
r
r
e
e
O
O
f
f
A
A
l
l
k
k
e
e
n
n
e
e
s
s
3
Alkenes
T
r
i
g
o
n
a
l
p
l
a
n
a
r
2
s
2
2
p
3
3
x
s
p
2
2
p
s
s
p
p
2
2
H
H
y
y
b
b
r
r
i
i
d
d
i
i
z
z
a
a
t
t
i
i
o
o
n
n
O
O
f
f
O
O
r
r
b
b
i
i
t
t
a
a
l
l
s
s
I
I
n
n
A
A
l
l
k
k
e
e
n
n
e
e
s
s
T
T
h
h
e
e
e
e
l
l
e
e
c
c
t
t
r
r
o
o
n
n
i
i
c
c
c
c
o
o
n
n
f
f
i
i
g
g
u
u
r
r
a
a
t
t
i
i
o
o
n
n
o
o
f
f
a
a
c
c
a
a
r
r
b
b
o
o
n
n
a
a
t
t
o
o
m
m
i
i
s
s
1
1
s
s
2
2
2
2
s
s
2
2
2
2
p
p
2
2
T
T
h
h
u
u
s
s
promotion
hybridization
2
p
2
2
s
1
Alkenes
In ethylene
In ethylene
(ethene),
(ethene),
each carbon atom use an
each carbon atom use an
sp
sp
2
2
orbital
orbital
to form a
to form a
single
single
C-C bond.
C-C bond.
Because of the two
Because of the two
sp
sp
2
2
orbitals overlap
orbitals overlap
by end-
by end-
to- end
to- end
the resulting bond is called
the resulting bond is called
σ
σ
bond.
bond.
The
The
pi (
pi (
π
π
)
)
bond
bond
between the two carbon atoms is formed by
between the two carbon atoms is formed by
side- by-side
side- by-side
overlap
overlap
of the two unhybridized p- orbitals (2
of the two unhybridized p- orbitals (2
p
p
–2
–2
p )
p )
for
for
maximum overlap
maximum overlap
and hence
and hence
the
the
strongest bond
strongest bond
, the 2p
, the 2p
orbitals are in line
orbitals are in line
and perpendicular to the molecular plane
and perpendicular to the molecular plane
.
.
This gives rise to the planar arrangement around C=C
This gives rise to the planar arrangement around C=C
bonds
bonds
. Also s orbitals of hydrogen atoms overlap
. Also s orbitals of hydrogen atoms overlap
with the
with the
sp
sp
2
2
orbitals in carbon atoms to form two
orbitals in carbon atoms to form two
C-H bonds with each carbon atom.
C-H bonds with each carbon atom.
The resulting shape of ethene molecule is
The resulting shape of ethene molecule is
planar
planar
with
with
bond angles of
bond angles of
120º
120º
and
and
C=C bond length is
C=C bond length is
1.34 Å
1.34 Å
5
O
O
r
r
b
b
i
i
t
t
a
a
l
l
O
O
v
v
e
e
r
r
l
l
a
a
p
p
I
I
n
n
E
E
t
t
h
h
e
e
n
n
e
e
Alkenes
two sp
2
orbitals overlap to form a sigma
bond between the two carbon atoms
O
O
r
r
b
b
i
i
t
t
a
a
l
l
O
O
v
v
e
e
r
r
l
l
a
a
p
p
I
I
n
n
E
E
t
t
h
h
e
e
n
n
e
e
two 2p orbitals overlap to form a pi
bond between the two carbon atoms
s orbitals in hydrogen atoms overlap with the
sp
2
orbitals in carbon atoms to form C-H
bonds
the resulting shape is planar
with bond
angles of 120º and
C=C (1.34 Å)
s
p
2
h
y
b
r
i
d
i
z
e
d
c
a
r
b
o
n
a
t
o
m
s
6
Alkenes
Nomencalture Of Alkenes And Cycloalkenes
1.
Alkene common names:
Substituent groups containing double bonds are:
Substituent groups containing double bonds are:
H
H
2
2
C=CH– Vinyl group
C=CH– Vinyl group
H
H
2
2
C=CH–CH
C=CH–CH
2
2
– Allyl group
– Allyl group
7
Alkenes
2.
IUPAC Nomenclature Of Alkenes
IUPAC Nomenclature Of Alkenes
Find the longest continuous Carbon chain containing the double bond this
Find the longest continuous Carbon chain containing the double bond this
determines the root name then add the suffix
determines the root name then add the suffix
-ene.
-ene.
Number the C- chain from the end that is nearer to the double bond. Indicate
Number the C- chain from the end that is nearer to the double bond. Indicate
the location of the double bond by using the number of the first atom of the
the location of the double bond by using the number of the first atom of the
double bond just before the suffix ene or as a prefix.
double bond just before the suffix ene or as a prefix.
Indicate the positions of the substituents using numbers of carbon atoms to
Indicate the positions of the substituents using numbers of carbon atoms to
which they are bonded and write their names in alphabetical order (
which they are bonded and write their names in alphabetical order (
N.B.
N.B.
discard the suffixes
discard the suffixes
tert
tert
-, di, tri,---when alphabetize the substituents
-, di, tri,---when alphabetize the substituents
) and if
) and if
more than one substituent of the same type are present use the prefixes di- or
more than one substituent of the same type are present use the prefixes di- or
tri or tetra or penta,--- to indicate their numbers.
tri or tetra or penta,--- to indicate their numbers.
8
Alkenes
Alkenes
In cycloalkenes the double bond carbons are assigned ring locations #1 and #2.
In cycloalkenes the double bond carbons are assigned ring locations #1 and #2.
Which of the two is #1 may be determined by the nearest substituent rule.
Which of the two is #1 may be determined by the nearest substituent rule.
If the substituents on both sides of the = bond are at the same distance, the
If the substituents on both sides of the = bond are at the same distance, the
numbering should start from the side that gives the substituents with lower alphabet
numbering should start from the side that gives the substituents with lower alphabet
the lower number.
the lower number.
10
10
Alkenes
When the longer chain cannot include the C=C, a substituent name
When the longer chain cannot include the C=C, a substituent name
is used.
is used.
Alkenes
Geometrical Isomerism In Alkenes
Geometrical Isomerism In Alkenes
G
G
. I. found in some, but not all, alkenes
. I. found in some, but not all, alkenes
It occurs in alkenes having two different groups / atoms attached to each carbon
It occurs in alkenes having two different groups / atoms attached to each carbon
atom of the = bond
atom of the = bond
G. I.
x
G. I.
X
G. I.
G. I.
12
12
A=C or B=D
No
Cis or tran
s
e
(G. I.
X
)
A≠C B≠D,
A =B or C=D Cis / A =D or C= B transe
G. I.
Alkenes
13
It occurs due to the
It occurs due to the
Restricted Rotation
Restricted Rotation
of C=C bonds so the groups on either
of C=C bonds so the groups on either
end of the bond are ‘fixed’ in one position in space; to flip between the two groups
end of the bond are ‘fixed’ in one position in space; to flip between the two groups
a bond must be broken.
a bond must be broken.
Geometrical isomers can not convert to each at room temperature.
Geometrical isomers can not convert to each at room temperature.
Alkenes
Geometrical Isomerism In Alkenes
Geometrical Isomerism In Alkenes
A) Cis / trans isomerism in alkenes
A) Cis / trans isomerism in alkenes
Exhibited by alkenes having two H’s and two other similar groups or
Exhibited by alkenes having two H’s and two other similar groups or
atoms attached to each carbon atom of the = bond (or generally the
atoms attached to each carbon atom of the = bond (or generally the
alkene have only two types of atom or groups i.e. ABC=CAB)
alkene have only two types of atom or groups i.e. ABC=CAB)
Cis prefix
Cis prefix
used when hydrogen atoms on both carbon atoms are on
used when hydrogen atoms on both carbon atoms are on
the
the
SAME
SAME
side of C=C bond
side of C=C bond
Trans prefix
Trans prefix
used when non-hydrogen groups / atoms are on the
used when non-hydrogen groups / atoms are on the
opposite
opposite
sides of C=C bond
sides of C=C bond
14
14
Types Of Geometric Isomerism
Types Of Geometric Isomerism
Alkenes
C
i
s
G
r
o
u
p
s
/
a
t
o
m
s
a
r
e
o
n
t
h
e
S
a
m
e
S
i
d
e
o
f
t
h
e
d
o
u
b
l
e
b
o
n
d
T
r
a
n
s
G
r
o
u
p
s
/
a
t
o
m
s
a
r
e
o
n
O
p
p
o
s
i
t
e
S
i
d
e
s
a
c
r
o
s
s
t
h
e
d
o
u
b
l
e
b
o
n
d
15
15
Alkenes
16
If the groups attached to the C=C are different, we distinguish
If the groups attached to the C=C are different, we distinguish
the two isomers by adding the prefix Z (from German word
the two isomers by adding the prefix Z (from German word
Zusammen) if the higher-priority groups are together in the
Zusammen) if the higher-priority groups are together in the
same side or E (from German word Entgegen) if the higher-
same side or E (from German word Entgegen) if the higher-
priority groups are opposite sides depending on the atomic
priority groups are opposite sides depending on the atomic
number of the atoms attached to each end of the C=C.
number of the atoms attached to each end of the C=C.
Atoms with higher atomic numbers receive higher priority
Atoms with higher atomic numbers receive higher priority
I> Br > Cl > F > O > N > C > H
I> Br > Cl > F > O > N > C > H
B) Z/ E isomerism in alkenes
B) Z/ E isomerism in alkenes
Alkenes
17
Alkenes
18
Alkenes
Exercise
Exercise
Q1-Which of the following compounds can exhibit cis / trans isomerism
Q1-Which of the following compounds can exhibit cis / trans isomerism
a)
2-Methylpropene
2-Methylpropene
b)
1-Butene
1-Butene
c)
2-Methyl-2-pentene
2-Methyl-2-pentene
d)
2-Butene
2-Butene
e)
3-Methyl-2-hexene
3-Methyl-2-hexene
Q2- Name the following compounds according to IUPAC system
Q2- Name the following compounds according to IUPAC system
19
19
Alkenes
20
Alkenes
E-
3-Methyl-2-pentene
Physical Properties of Alkenes
Physical Properties of Alkenes
Alkenes are nonpolar compounds thus:
Alkenes are nonpolar compounds thus:
Insoluble in water
Insoluble in water
Soluble in nonpolar solvents ( hexane, benzene,…)
Soluble in nonpolar solvents ( hexane, benzene,…)
The boiling point of alkenes increase as the number of carbons
The boiling point of alkenes increase as the number of carbons
increase.
increase.
21
21
Alkenes
Preparation Of Aalkenes
Preparation Of Aalkenes
1-
1-
Dehydration of alcohols
( removal of OH group and a proton from
two adjacent carbon atoms ) using mineral acids such as H
2
SO
4
or
H
3
PO
4
22
22
Alkenes
23
23
Zaitsev’sRule
Zaitsev’sRule
If there are different protons can be eliminated with the hydroxyl
If there are different protons can be eliminated with the hydroxyl
group or with halogen atom, in this case more than one alkene can be
group or with halogen atom, in this case more than one alkene can be
formed, the major product will be the alkene with the most alkyl
formed, the major product will be the alkene with the most alkyl
substituents attached to the double bonded carbon.
substituents attached to the double bonded carbon.
Zaitsev rule:
an elimination occurs to give the most stable, more highly
substituted alkene
Alkenes
2-
2-
Dehydrohalogenation of alkyl halides using a base
Dehydrohalogenation of alkyl halides using a base
24
24
Alkenes
or NaOH
25
For example:
Dehalogenation of 1,2-Dibromobutane leads to the formation of 1-
Butene. In the presence of catalyst ( AcOH acetic acid).
Alkenes
Reactions Of Alkenes
Reactions Of Alkenes
26
26
Alkenes
27
Alkenes
A
n
e
l
e
c
t
r
o
p
h
i
l
e
,
a
n
e
l
e
c
t
r
o
n
-
p
o
o
r
s
p
e
c
i
e
s
,
(
f
r
o
m
t
h
e
G
r
e
e
k
w
o
r
d
s
m
e
a
n
i
n
g
e
l
e
c
t
r
o
n
l
o
v
i
n
g
)
.
I
t
i
s
a
s
p
e
c
i
e
s
(
a
n
y
m
o
l
e
c
u
l
e
,
i
o
n
o
r
a
t
o
m
)
t
h
a
t
a
c
c
e
p
t
a
p
a
i
r
o
f
e
l
e
c
t
r
o
n
s
t
o
f
o
r
m
a
n
e
w
c
o
v
a
l
e
n
t
b
o
n
d
.
A
n
u
c
l
e
o
p
h
i
l
e
,
a
n
e
l
e
c
t
r
o
n
-
r
i
c
h
s
p
e
c
i
e
s
,
,
(
f
r
o
m
t
h
e
G
r
e
e
k
w
o
r
d
s
m
e
a
n
i
n
g
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u
c
l
e
u
s
l
o
v
i
n
g
)
.
I
t
i
s
a
s
p
e
c
i
e
s
(
a
n
y
m
o
l
e
c
u
l
e
,
i
o
n
o
r
a
t
o
m
)
t
h
a
t
d
o
n
a
t
e
a
n
e
l
e
c
t
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p
a
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a
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w
c
o
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a
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t
b
o
n
d
.
28
Electrophilic Addition Reaction
Electrophilic Addition Reaction
1- Additions To The Carbon-Carbon Double Bond
1- Additions To The Carbon-Carbon Double Bond
1.1 Addition Of Hydrogen: Hydrogenation (Reduction)
1.1 Addition Of Hydrogen: Hydrogenation (Reduction)
Alkenes
Cis-
1,2-Dimethyl cyclohexane
1.2.Addition of Halogens( Halogenation)
1.2.Addition of Halogens( Halogenation)
29
29
Alkenes
Trans-
1,2- Dibromo-1,2-Dimethyl cyclohexane
1.3. Addition of Hydrogen Halides
1.3. Addition of Hydrogen Halides
30
30
However, if the double bond carbon atoms are not structurally equivalent, i.e.
However, if the double bond carbon atoms are not structurally equivalent, i.e.
unsymmetrical alkenes
unsymmetrical alkenes
as in molecules of 1- propene, 1-butene, 2-methyl-2-butene and 1-
as in molecules of 1- propene, 1-butene, 2-methyl-2-butene and 1-
methylcyclohexene, the
methylcyclohexene, the
reagent may add in two different ways
reagent may add in two different ways
to give two isomeric
to give two isomeric
products. This is shown for 1-propene in the following equation.
products. This is shown for 1-propene in the following equation.
Only
Only
one product
one product
is possible from the addition of these strong acids to
is possible from the addition of these strong acids to
symmetrical alkenes
symmetrical alkenes
such as ethene, 2-butene and cyclohexene.
such as ethene, 2-butene and cyclohexene.
Alkenes
31
31
Alkenes
Stability of carbocation
Stability of carbocation
32
Alkenes
Markovnikov’s rule stats that
Markovnikov’s rule stats that
: In addition of
: In addition of
unsymmetrical reagent
unsymmetrical reagent
to
to
unsymmetrical alkenes
unsymmetrical alkenes
the
the
positive ion
positive ion
adds
adds
to the
to the
carbon of the alkene
carbon of the alkene
that bears the greater number of hydrogen atoms
that bears the greater number of hydrogen atoms
and
and
the negative ion
the negative ion
adds to the other carbon of the alkene
adds to the other carbon of the alkene
.
.
However w
hen the addition reactions to such unsymmetrical alkenes are
hen the addition reactions to such unsymmetrical alkenes are
carried out, it was found that
carried out, it was found that
2-bromopropane is nearly the exclusive
2-bromopropane is nearly the exclusive
product
product
. Thus it said the reaction proceeded according to
. Thus it said the reaction proceeded according to
Markovnikov’s
Markovnikov’s
rule
rule
1.4. Addition of HOX halogen in aqueous solution (
1.4. Addition of HOX halogen in aqueous solution (
-
-
OH, X
OH, X
+
+
):
):
Halohydrin formation
Halohydrin formation
33
33
Only one product is possible from the addition of HOX acids (formed from
Only one product is possible from the addition of HOX acids (formed from
mixture of H2O and X2) to symmetrical alkenes such as ethene and cyclohexene.
mixture of H2O and X2) to symmetrical alkenes such as ethene and cyclohexene.
Alkenes
34
Alkenes
However, addition reactions to unsymmetrical alkenes will result in the formation of
However, addition reactions to unsymmetrical alkenes will result in the formation of
Markovonikov’s product
Markovonikov’s product
preferentially.
preferentially.
1.5. Addition of H
1.5. Addition of H
2
2
O: Hydration
O: Hydration
35
35
Only
Only
one product
one product
is possible from the addition of H
is possible from the addition of H
2
2
O in presence of acids as
O in presence of acids as
catalysts to
catalysts to
symmetrical alkenes
symmetrical alkenes
such as ethene and cyclohexene.
such as ethene and cyclohexene.
However, addition reactions to
However, addition reactions to
unsymmetrical alkenes
unsymmetrical alkenes
will result in the formation of
will result in the formation of
Markovonikov’s product preferentially
Markovonikov’s product preferentially
.
.
Alkenes
2.1- Ozonolysis:
2.1- Ozonolysis:
Oxidation with ozone
Oxidation with ozone
(Oxidative cleavage):
(Oxidative cleavage):
This reaction involves rupture of the C=C to give aldehydes or ketones according to the
This reaction involves rupture of the C=C to give aldehydes or ketones according to the
structure of the original alkene.
structure of the original alkene.
36
36
Alkenes
2-Oxidation Reaction:
2-Oxidation Reaction:
37
37
2- Oxidation with KMnO
2- Oxidation with KMnO
4
4
(Oxidative addition):
(Oxidative addition):
Alkenes
Cis- diol
Thank You for your kind attention !
Thank You for your kind attention !
Questions?
Comments
38
Alkenes
Alkenes are unsaturated hydrocarbons containing a C=C double bond. This chapter covers topics such as alkene structure, hybridization, naming, isomerization, physical properties, preparation methods, stability rules, and addition reactions. Learn about the sp2 hybridization of orbitals in alkenes, orbital overlap in ethene, and nomenclature of alkenes and cycloalkenes.
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Presentation Transcript
Unsaturated Hydrocarbons Alkenes 1435 1435- -1436 2014 2014- -2015 1436 2015
Alkenes Learning Objectives Chapter two discusses the following topics and the student by the end of this chapter will: Know the structure, hybridization and bonding of alkenes Know the common and IUPAC naming of alkenes Know the geometry of the double bond i.e. cis/trans isomerization Know the physical properties of alkenes Know the different methods used for preparation of alkenes (elimination reactions ; dehydrogenation, dehydration and alkenes stability (Zaitsev s rule) play an important role in understanding these reactions Know the addition reactions of alkenes and the effect of Markovnikov s rule in determining the regioselectivity of this reaction.
Alkenes Structure Of Alkenes They are unsaturated hydrocarbons made up of C and H atoms and contain one or more C=C double bond somewhere in their structures. Their general formula is CnH2n - for non-cyclic alkenes Their general formula is CnH2n-2 - for cyclic alkenes 3
Alkenes sp2 Hybridization Of Orbitals In Alkenes The electronic configuration of a carbon atom is 1s22s22p2 Thus 2p3 2s2 2s1 3 x sp2 2p 2p2 promotion hybridization Trigonal planar
Alkenes Orbital Overlap In Ethene In ethylene (ethene), each carbon atom use an sp2 orbital to form a single C-C bond. Because of the two sp2 orbitals overlap by end- to- end the resulting bond is called bond. The pi ( ) bond between the two carbon atoms is formed by side- by-side overlap of the two unhybridized p- orbitals (2p 2p ) for maximum overlap and hencethe strongest bond, the 2p orbitals are in line and perpendicular to the molecular plane. This gives rise to the planar arrangement around C=C bonds. Also s orbitals of hydrogen atoms overlap with the sp2 orbitals in carbon atoms to form two C-H bonds with each carbon atom. The resulting shape of ethene molecule is planar with bond angles of 120 and C=C bond length is 1.34 http://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/Ethene-2D-flat.png/220px-Ethene-2D-flat.png 5
Alkenes Orbital Overlap In Ethene two sp2 orbitals overlap to form a sigma bond between the two carbon atoms sp2 hybridized carbon atoms s orbitals in hydrogen atoms overlap with the sp2 orbitals in carbon atoms to form C-H bonds two 2p orbitals overlap to form a pi bond between the two carbon atoms the resulting shape is planar with bond angles of 120 and C=C (1.34 ) 6
Alkenes Nomencalture Of Alkenes And Cycloalkenes 1. Alkene common names: CH3 CH2 H2C CH2 CH3-CH CH2 H3C C Common: Ethylene Propylene Isobutene Substituent groups containing double bonds are: H2C=CH Vinyl group H2C=CH CH2 Allyl group Br Cl Common: Allyl bromide Vinyl chlorride 7
Alkenes 2. IUPAC Nomenclature Of Alkenes Find the longest continuous Carbon chain containing the double bond this determines the root name then add the suffix -ene. Number the C- chain from the end that is nearer to the double bond. Indicate the location of the double bond by using the number of the first atom of the double bond just before the suffix ene or as a prefix. 1 2 3 4 5 6 1 2 3 4 H2C CH CH2CH3 CH3CH Hex-2-ene or 2-Hexene (not 4-Hexene) CHCH2CH2CH3 But-1-ene or 1-Butene (not 3-Butene) Indicate the positions of the substituents using numbers of carbon atoms to which they are bonded and write their names in alphabetical order (N.B. discard the suffixes tert-, di, tri,---when alphabetize the substituents) and if more than one substituent of the same type are present use the prefixes di- or tri or tetra or penta,--- to indicate their numbers. 8
Alkenes 8 Cl CH3 7 1 5 3 H3C C 2 CH 3 CH3 4 4 6 6 1 2 4 2 2-Methyl-but-2-ene or 2-Methyl-2-butene (not 3-Methyl-2-butene) 3 5 1 CH3 6-Methyl-2-octene 3-Chloro-2-hexene (not 2-Chloro-1-methyl-1-pentene) Br 1 3 CH3-CH2-CH2-CH=CH-C-CH3 5 6 7 4 (CH3CHCHCH2OCH3) = OCH3 2 1 3 4 2 Br CH3 1-Methoxy-but-2-ene (not 4-Methoxy-but-2-ene) 2,2-Dibromo-3-heptene (not 6,6-Dibromo-4-heptene) 1 3 5 2 1 2 4 2 3 9 3 5 7 1 CN 6 4 8 5-Methylcyclopenta-1,3-diene An ''a'' is added due to inclusion of di put two consonants consecutive 4-Cyano-2-ethyl-1-pentene (not 2-Ethyl-4-cyano-1-pentene) 2,3,7-Trimethyl-non-3-ene (not 2-Isopropyl-6-methyl--2-octene)
Alkenes In cycloalkenes the double bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule. 1 CH3 2 6 1 3 5 H3C CH3 4 3,5-Dimethyl-cyclohexene (not 4,6-Dimethylcyclohexen) (not 1,5-Dimethyl-2-cyclohexen) 1-Methyl cyclopentene (not 2-Methylcyclopeneten) If the substituents on both sides of the = bond are at the same distance, the numbering should start from the side that gives the substituents with lower alphabet the lower number. 1 2 3 4 7 5 6 3-tert-Butyl-7-isopropyl-cycloheptene (not 3-Isopropyl-7-tert-butylcycloheptene) 10
Alkenes When the longer chain cannot include the C=C, a substituent name is used. 1 2 1 3 CH CH2 CH CH2 6 5 4 Vinyl-cyclohexane 3-Vinyl-cyclohexene
Alkenes Geometrical Isomerism In Alkenes A B C G. I. found in some, but not all, alkenes It occurs in alkenes having two different groups / atoms attached to each carbon atom of the = bond D A=C or B=D No Cis or transe (G. I. X) A C B D, A =B or C=D Cis / A =D or C= B transe G. I. G. I. x 12 G. I. X G. I. G. I.
Alkenes Geometrical Isomerism In Alkenes It occurs due to the Restricted Rotation of C=C bonds so the groups on either end of the bond are fixed in one position in space; to flip between the two groups a bond must be broken. X Geometrical isomers can not convert to each at room temperature. 13
Alkenes Types Of Geometric Isomerism A) Cis / trans isomerism in alkenes Exhibited by alkenes having two H s and two other similar groups or atoms attached to each carbon atom of the = bond (or generally the alkene have only two types of atom or groups i.e. ABC=CAB) Cis prefix used when hydrogen atoms on both carbon atoms are on the SAME side of C=C bond Trans prefix used when non-hydrogen groups / atoms are on the opposite sides of C=C bond 14 Cis-But-2-ene Trans-But-2-ene
Alkenes H Cl H H H Cl Cl Cl cis-1,2-Dichloro-ethene trans-1,2-Dichloro-ethene Cis Trans Groups / atoms are on the Same Side of the double bond Groups / atoms are on Opposite Sides across the double bond = Cis-Oct-4-ene Trans-Oct-4-ene H H H H H CH3 H3C H CH3 H H H CH3 cis-cis-2,4-hexadiene H trans-trans-2,4-hexadiene trans-4-ethyl-3-heptene 15
Alkenes B) Z/ E isomerism in alkenes If the groups attached to the C=C are different, we distinguish the two isomers by adding the prefix Z (from German word Zusammen) if the higher-priority groups are together in the same side or E (from German word Entgegen) if the higher- priority groups are opposite sides depending on the atomic number of the atoms attached to each end of the C=C. Atoms with higher atomic numbers receive higher priority I> Br > Cl > F > O > N > C > H 16
Alkenes 17
Alkenes Br I CH3 I CH3 Cl Br Cl Z-2-bromo-1-chloro-1-iodopropene E-2-bromo-1-chloro-1-iodopropene O I CH3 CH2 Cl H C CH3 H H O Z Z 18
Alkenes Exercise Q1-Which of the following compounds can exhibit cis / trans isomerism a) 2-Methylpropene b) 1-Butene c) 2-Methyl-2-pentene d) 2-Butene e) 3-Methyl-2-hexene Q2- Name the following compounds according to IUPAC system c) b) a) 19
Alkenes 3 5 4 2 2 4 Br 1 3 1 3-Bromo propene 2-Ethyl-4-methyl pentene CH3 CH3 6 1 5 6 5 4 2 4 1 3 3 2 Cl Cl CH3 CH3 4-Chloro-3,6-dimethylcyclohexene 3-Chloro-2,5-dimethylcyclohexene 1 H3C CH3 Br Br H Cl C C C C C 2 C 3 H H H Cl H CH2CH3 4 1,1-Dichloroethene 5 E-3-Methyl-2-pentene Cis-1,2-Dibromoethene Geometrical isomerism not geometrical isomerism 2 4 6 1 3 5 7 Trans-trans-2,4-heptadiene Trans-1,3,5-heptatriene 20 Cis-cis-2,5-heptadiene Home work
Alkenes Physical Properties of Alkenes Alkenes are nonpolar compounds thus: Insoluble in water Soluble in nonpolar solvents ( hexane, benzene, ) The boiling point of alkenes increase as the number of carbons increase. 21
Alkenes Preparation Of Aalkenes 1- Dehydration of alcohols ( removal of OH group and a proton from two adjacent carbon atoms ) using mineral acids such as H2SO4 or H3PO4 Conc.H2SO4 CH3CH2OH CH2 + H2 C H2 O 180 oC Ethanol Ethene OH H+/ heat + H2 O H cyclohexanol cyclohexene 22
Alkenes Zaitsev sRule If there are different protons can be eliminated with the hydroxyl group or with halogen atom, in this case more than one alkene can be formed, the major product will be the alkene with the most alkyl substituents attached to the double bonded carbon. H2C CH3 + H2O H3C H / Heat 1- Butene Minor CH3 OH H3C CH3+ H2O 2- Butene Major Zaitsev rule: an elimination occurs to give the most stable, more highly substituted alkene 23
Alkenes 2- Dehydrohalogenation of alkyl halides using a base or NaOH 24
Alkenes 3. Dehalogenation of vicinal dihalides Zn/AcOH Br Br For example: Dehalogenation of 1,2-Dibromobutane leads to the formation of 1- Butene. In the presence of catalyst ( AcOH acetic acid). 25
Alkenes Reactions Of Alkenes Oxidation Reactions KMnO4 Ozonolysis Reactions of Alkenes Addition(Electrophilic) reaction: - Hydrogenation - Halogenation - Hydrohalogenation - Halohydrin formation -Hydration 26
Alkenes An electrophile, an electron-poor species,(from the Greek words meaning electron loving). It is a species (any molecule, ion or atom) that accept a pair of electrons to form a new covalent bond. , H , Br , Cl , I , etc., AlCl3 , BF3 , FeCl3 , FeBr3 , etc. C A nucleophile, an electron-rich species, ,(from the Greek words meaning nucleus loving). It is a species (any molecule, ion or atom) that donate an electron pair to form a new covalent bond. , OH , Br , Cl , I , etc., H2N, HS, etc., H2O , C , etc. CH3OH , RNH2 , R2NH , R3N , 27
Alkenes Electrophilic Addition Reaction 1- Additions To The Carbon-Carbon Double Bond 1.1 Addition Of Hydrogen: Hydrogenation (Reduction) A A + A A Pt or Ni or Pd H2 A A A A H H An alkane An alkene Pt CH2 + H2 H2 C H3 C CH3 H3 C CH2+ H2 H3 C Pt CH3 CH3 CH3 Pt + H2 CH3 CH3 Cis-1,2-Dimethyl cyclohexane 28
Alkenes 1.2.Addition of Halogens( Halogenation) A A A A + X2 A A (X= Cl or Br) A A X X Cl CCl4 CH3+ H3 C Cl2 H3 C CH3 Cl Br CCl4 + Br2 Br CH3 Br CH3 + CCl4 Br2 Br CH3 CH3 29 Trans-1,2- Dibromo-1,2-Dimethyl cyclohexane
Alkenes 1.3. Addition of Hydrogen Halides Only one product is possible from the addition of these strong acids to symmetrical alkenes such as ethene, 2-butene and cyclohexene. A A A A A A + HX A A (x= Cl or Br or I) H X Cl CH3 + HCl H3C H H + HI I However, if the double bond carbon atoms are not structurally equivalent, i.e. unsymmetrical alkenes as in molecules of 1- propene, 1-butene, 2-methyl-2-butene and 1- methylcyclohexene, the reagent may add in two different ways to give two isomeric products. This is shown for 1-propene in the following equation. 30
Alkenes Br CH3CHCH3 2o Carbocation CH3CHCH3 maijor HBr Br CH3CH=CH2 Br CH3CH2CH2 1o Carbocation CH3CH2CH2Br minor Stability of carbocation H3C CH3 H3C CH2CH2 C CH CH3 CH3 CH3 2o 1o 3o 31
Alkenes However when the addition reactions to such unsymmetrical alkenes are carried out, it was found that 2-bromopropane is nearly the exclusive product. Thus it said the reaction proceeded according to Markovnikov s rule Markovnikov s rule stats that : In addition of unsymmetrical reagent to unsymmetrical alkenes the positive ionaddsto the carbon of the alkene that bears the greater number of hydrogen atoms and the negative ion adds to the other carbon of the alkene. CH3 CH3Cl + HCl CH3 CH3 H3C H3C 32
Alkenes 1.4. Addition of HOX halogen in aqueous solution ( -OH, X+): Halohydrin formation Only one product is possible from the addition of HOX acids (formed from mixture of H2O and X2) to symmetrical alkenes such as ethene and cyclohexene. Symmetrical akenes A A A A A A (x= Cl or Br ) + H2O / X2 A A OH X Cl + H2O / Cl2 CH3 H3C OH 33
Alkenes However, addition reactions to unsymmetrical alkenes will result in the formation of Markovonikov s product preferentially. Unsymmetrical akenes OH + H2O / Cl2 Cl CH2Br + H2O / Br2 OH 34
Alkenes 1.5. Addition of H2O: Hydration Only one product is possible from the addition of H2O in presence of acids as catalysts to symmetrical alkenes such as ethene and cyclohexene. Symmetrical akenes A A A A H A A + H2O A A H OH OH H CH3 + H2O H3C H However, addition reactions to unsymmetrical alkenes will result in the formation of Markovonikov s product preferentially. Unsymmetrical akenes H H OH 35 + H2O CH3 CH3
Alkenes 2-Oxidation Reaction: 2.1- Ozonolysis: Oxidation with ozone (Oxidative cleavage): This reaction involves rupture of the C=C to give aldehydes or ketones according to the structure of the original alkene. A A A A A A A Zn /H2O - H2O2 A + O3 O O + A A A O O A O ( A= H or R) i) O3 ii) Zn /H2O O + O H i) O3 ii) Zn /H2O O + O O i) O3 ii) Zn /H2O 36 O
Alkenes 2- Oxidation with KMnO4(Oxidative addition): OH KMnO4 / OH/ H2O OH Cis- diol 37
Alkenes Thank You for your kind attention ! Questions? Comments 38
