ORGANIC
CHEMISTRY 171
Section 201
Alkenes,Chapter 3
2
Alkenes and Cycloalkenes
Unsaturated hydrocarbons can be
1-open-chain (linear and branched alkenes)
2- cyclic (cycloalkenes)
3
Unsaturated Hydrocarbons
Hydrocarbons that contain at least one C=C (
alkenes) are called unsaturated hydrocarbons
CH2
CH3
C
CH2
CH3
CH2
4
Alkenes are acyclic unsaturated hydrocarbons
that contain at least one C=C
C2H4
5
• Generic formula: Start with CnH2n+2 and
minus two for each C=C
• one C=C  CnH2n e.g., C2H4, C3H6, etc.
C2H4
6
Nomenclature of Alkenes
Common Names
• Usually used for small molecules.
• Examples:
CH3
CH2
CH2
ethylene
CH2
CH CH3
propylene
CH2
C CH3
=>
isobutylene
8
IUPAC Nomenclature of
Alkenes
• 1. Find the longest continuous chain containing the
double bond.
• 2. Name the corresponding alkane and change the
“ane” ending to “ene” for alkenes.
• 3. Number the chain so as to give the double bond
the lowest number. Place a numerical prefix in
front of the parent name to indicate the position of
the first carbon in the double bond.
• Number and name alkyl groups as with alkanes.
Nomenclature
• alkenes: parent chain contains C=C
C=C gets lowest numbers
position of C=C indicated by lower of the two numbers
CH3 CH2 CH CH2
1-butene
4-methyl-1-butene
3-bromocyclohexene
Br
10
Nomenclature
• C=C and OH: alkenol
higher priority group (OH) gets last suffix
and lowest number
OH
2-propen-1-ol
5-methyl-4-hexen-2-ol
OH
OH
2-cyclohexenol
11
Nomenclature
• as side groups:
H2C
H2C
CH
CH CH2
Examples
vinyl chloride
ethenyl (vinyl)
2-propenyl (allyl)
Cl
allyl alcohol
H2 C
1-methylethenyl (isopropenyl)
C
OH
isopropenyl bromide
H3 C
methylene
CH2
CH2
1-vinylcyclohexene
methylenecyclopentane
Br
12
Name the following compound:
H3C
H2C
C
H2C
CH 3
CH
CH 3
The longest continuous chain containing the
double bond is 5 carbons long and is indicated in
blue, below:
The parent compound is derived
H3C
H2C
C
H2C
CH
CH 3
CH 3
from pentane. The parent alkene is
pentene.
Nomenclature
E-Z notation
1.
2.
Determine the higher priority group on each end of the alkene.
If the higher priority groups are:
on opposite sides: E (entgegen = opposite)
on the same side: Z (zusammen = together)
H
CH3 > H
Cl > CH2CH3
Cl
H
C C
H3C
CH2CH3
C C
CH2CH3
(E)-3-chloro-2-pentene
H3C
Cl
(Z)-3-chloro-2-pentene
14
Name These Alkenes
CH2
CH CH2
CH3
1-butene
CHCH2CH3
CH3
C CH CH3
CH3
H3C
2-sec-butyl-1,3-cyclohexadiene
2-methyl-2-butene
CH3
3-methylcyclopentene
3-n-propyl-1-heptene
=>
15
ALKENE
STRUCTURE AND
BONDING
16
ALKENE STRUCTURE AND BONDING
sp2
R
R
C
R
sp2
C
R
SHAPE IS TRIGONAL PLANAR
17
Orbital Description
•
•
•
•
•
Sigma bonds around C are sp2 hybridized.
Angles are approximately 120 degrees.
No nonbonding electrons.
Molecule is planar around the double bond.
Pi bond is formed by the sideways overlap of
parallel p orbitals perpendicular to the plane of
the molecule.
=>
18
THE BOND ANGLE OF AN ALKENE
120o
120o
120o
19
Bond Lengths and Angles
• Hybrid orbitals have more s character.
• Pi overlap brings carbon atoms closer.
• Bond angle with pi orbitals increases.
– Angle C=C-H is 121.7
– Angle H-C-H is 116. 6
=>
20
 bond
2p
H
H
C C
H
H
trigonal planar
sp2
H
H
C C
H
H
overlap
p orbitals
(sp2C + 1sH)
(sp2C + sp2C)
H
H
C C
H
H
no free rotation
21
Isomerism in alkenes
22
Cis and Trans Isomers
• Some alkenes can have the same
connection of atoms, but have a different
arrangement in three dimensional space.
• This is due to the lack of free rotation
about the double bond.
• The different arrangements are geometric
isomers.
• One of the isomers is cis- the other is
trans.
CIS / TRANS ISOMERS
substituents on
the same side of
main chain
substituents on
opposite sides of
main chain
cis
trans
C
C
C
H
H
C
C
C
H
H
C
C
24
COMPARE cis / trans ISOMERS IN RING COMPOUNDS
R
R
C
H
C
C
H
C
R
H
cis
R
H
R
trans
R
R
R
In alkenes and rings cis / trans isomers
are called stereoisomers or geometric isomers.
25
2-butene
H3C
CH 3
C
cis-2-butene
CH3 groups same side
CH 3
C
C
H
mp = -139oC
H
H
H3C
C
H
trans-2-butene
CH3 groups opposite sides
mp = -106oC
Geometric Isomers of 2butene
Insert figure 19.11
Physical Properties
•
•
•
•
Low boiling points, increasing with mass.
Branched alkenes have lower boiling points.
Less dense than water.
Slightly polar
– Pi bond is polarizable, so instantaneous dipoledipole interactions occur.
– Alkyl groups are electron-donating toward the pi
bond, so may have a small dipole moment.
=>
28
Polarity Examples
H3C
CH3
H
C C
H
C C
H
cis-2-butene, bp 4 °C
 = 0.33 D
CH3
H3C
H
trans-2-butene, bp 1 °C
=0
=>
29
Preparation
I.
Preparation
• 1. Dehydration
excess conc. H2SO4 at 170oC
C C
or Al2O3/SiO2 at 350oC
OH
+ H2O
C C
What kinds of dehydration?
Another reaction occur!
conc.H2SO4
H H
H
H
H OH
140 oC
H H
H
H H
O
H H
This is intermolecular dehydration.
H
H H
+
OH2
I.
Preparation - Dehydration
C C C C
H OH H
C C C C
(major)
+ C C C C
(minor)
How do you which one is major product?
Saytzeff Rule:
Hydrogen is preferably removed from the carbon
with least no. of hydrogen since the alkene
formed is more highly branched and is
energetically more stable.
Dehydration Mechanism
H
H O H
C C
H O H
O
H O S
O H
C C
_
HSO4
O
H
H O H
H
C C
C C
H2O:
C C
+
H3O
=>
33
I.
Preparation
• 2. Dehydrohalogenation
H H
H
strong base
Press
H
H
H
H X
alcoholic reflux
H
H
+
HX
Example:
KX + OH2
alcoholic KOH
EtO- (ethoxide ion) in EtOH (ethanol)
EtOH
weak
conjugate acid
+
EtO + H
strong
conjugate base
Hofmann Product
• Bulky bases abstract the least hindered H+
• Least substituted alkene is major product.
H CH3
CH3 C C CH2
H Br H
_
CH3CH2O
CH3 CH3CH2
H3C
C C
C C
CH3CH2OH
CH3
H
CH3 C C CH2
H Br H
_
(CH 3)3CO
CH3CH2OH
29%
CH3 CH3CH2
H3C
H
C C
C C
H
H
H3C
71%
H CH3
H
CH3
28%
H
H3C
=>
72%
35
I.
Preparation
3.Dehalogenation
C C
X
alcoholic
+ Zn dust
reflux
C C
+ ZnX 2
X
(vicinal dihalide)
X
(c.f. gem-dihalide
)
X
I. Preparation dehalogenation (application)
[O]
H H H H
H
H H H O
H
OH
H
H H
Zn dust
alcoholic reflux
X2
KMnO4 / H+
H H H H
H
OH
X
X H H
OH
H H H O
H
OH
X X H
I.
Preparation
• 4. Hydrogenation
Pd/BaSO4
C C
+
H2
H H
C C
– This makes use of a catalyst which activity
has been decreased by sulphur containing
compound. E.g. Pd (palladium) in BaSO4
Reactions of Alkenes
Reactivity of C=C
• Electrons in pi bond are loosely held.
• Electrophiles are attracted to the pi
electrons.
• Carbocation intermediate forms.
• Nucleophile adds to the carbocation.
• Net result is addition to the double bond.
=>
Chapter 8
40
Markownikoff’s rule
– The more electronegative atom (or group of
atoms) attached to carbon having least no.
of H.
In general, the greater the no. of alkyl grops
present, or the larger is the alkyl group, the
more stable is the carbonium ion.
– Stability of carbonium ion:
– 3ry C+ > 2ry C+ > 1ry C+ > CH3+
(It undergoes addition reaction.)
• Electrophilic Addition Reactions
1-With HX
C C
+
HX
C C
H X
(Mechanism of Addition Reactions)
(I)
CH3CH CH2
+
+
(II)
CH3CH2CH2
CH3CH2CH2X
X
H X
+
CH3CHCH3
CH3CHCH3
X
X
• Electrophilic Addition Reactions (cont’d)
2-With conc. sulphuric acid
+ H OSO3H
C
C
+
H .H SO
4
alkyl hydrogen sulphate
C
C
H
O SO H
3
b o il
Hence, this is used in
preparation of alcohol.
H O
2
C
C
H
OH
3-Addition of halogen to alkene (Halogenation) Br
C
C
Br
C
+
C
C
C
Br
Br
Br
Br
C
Br
C
+
Proof for the formation of brominium ion
CCl4 / C2H5OH
+
Br
Br2
Br
Trans-addition
(anti-addition)
The bromide ions attack carbon of the ring from the
side opposite to that of the “positive” brominium ion.
• Addition Reactions (NOT electrophilic)
4-Hydrogenation
C
C
+
H2
C
C
H
H
a. It is used analytically to find the number of
mole of double bond or triple bond by the number
of mole of hydrogen absorbed per mole of molecule.
b. It is used in converting vegetable oil.
5-Ozonolysis
O
R'
R''
C
R
+
C
ice-cold
O3
R'''
chloroform
O
R'
R''
C
C
O
R
ozonide
H2O /H
R'
R''
C
Zn dust
R
O
O
C
R'''
R'''
6-Oxidation
a.at room temperature (Hydroxylation)
(addition)
C
C
+ [O]
+ H 2O
from MnO4-/OH -
C
C
OH OH
b.at vigorous condition (bond breaking)
MnO4- / H+
H H
R C C R'
+ [O]
H
H
R C O
+
O C R'
Further oxidation
OH
R C O
In acidic condition, the products will be
oxidised to acid or ketone.
7-Addition Polymerization
– This is a process by which simple molecules are
joined up to form large molecule with same
empirical formula.
n
C C
R'
n
R'
R C C
– condition :
–
( C C )
n
( C C)
n
R
temperature and
pressure
high
with Ziegler’s catalyst
• Free radical addition mechanism
Initiation:
2RO
OR
RO
R'
R'
RO
C
RO
C
C
C
Propagation:
R'
RO
C
C
R'
R'
C
C
RO
C
C
R'
C
C
Termination:
RO
R'
RO
C
RO OR
OR
R'
C
C
R'
C
RO
OR
C
C
C
C
n
n
R'
RO
R'
C
C
C
R'
R'
C
C
R'
C
C
C
RO
m
n
R'
RO
C
C
OR
n+m+2
OR
e.g. polythene
moleclar mass: 50000g
melting point: 126oC – 135oC (having diff.
Isomer)
Properties:
Light, inert (strong sigma bonds) and water-resistance (do
not form H-bonding), tough and capable of moulding. It
is a thermo-plastic since chains of hydrocarbons causes
the chain to move apart and to come closer again on
cooling.
Uses:
1. Make water-proof sheeting
2. Electrical cable insulator