Transcript Unsaturated Hydrocarbons I : Alkenes

Unsaturated Hydrocarbons 1
Alkenes
By:
Dr. Siham Lahsasni
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Unsaturated hydrocarbons that contain one or more double
carbon-carbon bonds.
Alkenes with just one double C-C bond form an homologous
series with the general formula CnH2n.
The first few in this series are ethene, propene, butene, and
pentene.
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HYBRIDISATION OF ORBITALS - ALKENES
2s22p2
The electronic
configuration of a
carbon atom is
1s22s22p2
2s12p3
If you provide a bit of energy
you can promote (lift) one of
the s electrons into a p orbital.
The configuration is now
1s22s12p3
3 x sp2
2p
Alternatively, only three orbitals
(an s and two p’s) combine or
HYBRIDISE to give three new
orbitals. All three orbitals are
equivalent. The remaining 2p
orbital is unchanged.
Hybridization Sp2
Bonding in Ethylene : Bond length =1.34 Å with
the atoms separated by bond angles of 120°.
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Nomenclature of alkenes
1. The ene suffix indicates an alkenes or cycloalkenes.
2. The longest chain chosen for the root name must include
both carbon atoms of the double bond.
3. The root chain must be numbered from the end nearest a
double bond carbon atom. If the double bond is in the center
of the chain, the nearest substituent rule is used to determine
the end where numbering starts.
4. In cycloalkenes the double bond carbons are assigned ring
locations C1 and C2. Which of the two is C1 may be
determined by the nearest substituent rule.
5. Substituent groups containing double bonds are:
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H2C=CH– Vinyl group
H2C=CH–CH2– Allyl group
CH2
H2C
Common
IUPAC
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Ethylene
Ethene
CH2
H3C
Propylene
Propene
CH2
H2C
Br
Vinyl bromide
Bromoethene
H2C
Cl
Allyl chloride
3-Chloro-1-propene
Vinyl cyclohexane
Cyclohexyl ethene
Alkenyl groups
1. Alkenyl groups are named by adding to alkenes suffix -yl. Ethene becomes a
ethenyl group, propene an propenyl group, etc.
•Exception case: Methene became Methylene group.
2. Alkenyl groups are named by dropping the -ene suffix of the alkenes and
adding the suffix -ylidene. Ethene becomes a ethylidene group, propene an
propylidene group, etc.
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Structure and Nomenclature of Dienes
Alkenes that contain two double bonds are called Dienes:
 If the double bonds are separated by only one single bond, the diene
is said to be conjugated.
 If the double bonds are separated by more than one single bond, the
diene is called non conjugated diene.
 Dienes are named by the IUPAC system in essentially the same way
as alkenes except that the suffix -adiene replaces the ending –ene of
the alkene.
 Two numbers are needed to indicate the locations of the double
bonds in the chain.
 In cyclic dienes one of the double bonds is always assigned the
number 1, and the other is given the lowest possible number.
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1,4-Pentadiene ( non conjugated)
1,3- Cyclohexadiene (conjugated)
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1,3-Hexadiene (conjugated)
1,2,3,4-Cyclooctatetraene (conjugated)
Geometric (cis-trans) isomerism
 The prefix cis- is used when the two arms of the longest
chain are on the same side of the double bond.
 The prefix trans- is used when they are on opposite sides of
the double bond.
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E-Z NOTATION FOR GEOMETRIC ISOMERISM
 If the two groups with the higher molecular weight are on
the same side of the double bond, that is described as the (Z)isomer. So you would write it is (Z)-name of compound.
 If the two groups with the higher molecular weight are on
opposite sides of the double bond, then this is the (E)isomer.
E
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Z
H 3C
C2H5
13
Cl
H
Br
H 3C
OH
H
Physical Properties of Alkenes
 Alkenes are non polar compounds.
 Insoluble in water.
 Soluble in non polar organic solvents.
 They are less dense than water.
 Range of physical states:
 ≤ 4 C's are gases
 5 - 17 C's are liquids
 ≥ 18 C's are solids
 The alkenes has a boiling point which is a small number of
degrees lower than the corresponding alkanes.
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Preparation of alkenes
1- Dehydration of alcohols:
CH 3CH 2OH
H2SO4 / heat
or H 3PO4 / Heat
Ethanol
OH
H2C
Ethene
+
H / heat
+ H2O
H
cyclohexanol
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CH2
cyclohexene
+
H2O
Dehydration of a Primary Alcohol: An E2 reaction
H+
OH
OH 2
E2
H
A
+ HA
+ H2O
Dehydration of a Secondary or tertiary Alcohol: An E1 reaction
HSO4
H
H
H
C
C
H
C
E1
C
OH2
C
H
C
- H2O
OH
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H
+
C
+
H2SO4
C
H
Saytzeff s Rule: In every instance in which more than one
alkene can be formed, the major product is the alkene with the
most alkyl substituents attached on the double bonded carbon.
H2C
-
H3C
CH3
CH3
+
H2O
1 Butene minor
HO
H3C
-
CH3
2 Butene major
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+
H2O
Rearrangement during Dehydration of Secondary Alcohols
OH2
OH
H+
- H2O
rearrangement
Secondary carbocation
less stable
Tertiary carbocation
more stable
+
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Minor
Major
2- Dehydrohalogenation of Alkyl Halides
H2C
+ KOH
CH2
Alcohol
H2C
heat
H
X
an alkyl halide
CH2
+
KX
+ H2O
Alkene
Br
H3C
CH3
H
KOH
alcohol / heat
H3C
CH3
2-Butene major
+ H3C
CH2
1-Butene minor
H
Br
H
CH3
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+
+
KOH
Alcohol / heat
CH3
1-methylcyclohexene
major
+
CH3
3methylcyclohexene
minor
OH
H
C
C
X
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H
E2
C
C
+ H2O
+ X-
2- Dehydrohalogenation of vicinal dihalides
H3C
H
C
H
C
Cl
Cl
H 3C
H
C
Br
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2 NaI
CH 3
H
C
Br
acetone
H 3C
C
H
C
H
CH 3
+
I2
+
CH 3
+
ZnBr2
Zn
CH 3
CH3COOH or EtOH
H 3C
C
H
C
H
2 NaCl
Reaction of alkenes
1- Additions to the Carbon-Carbon Double Bond
1-1- Addition of hydrogen: Hydrogenation (reduction)
A
A
+
A
Pt or Ni or Pd
H2
A
H3C
22
CH2
A
A
A
H
An alkene
H2C
A
H
An alkane
+
Pt
H2
CH2
+
H2
H3C
Pt
H3C
CH3
CH3
1-2- Addition of Halogens: Halogenation
A
A
A
+
A
X2
A
A
A
A
X
(X= Cl or Br)
X
Cl
H3C
CH3
+
Cl 2
CCl 4
H3C
CH3
Cl
Br
+
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Br2
CCl 4
Br
Br
+
Br
Br
Br
Br
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Br
1-3- Addition of Hydrogen Halides: Hydrohalogenation
A
A
A
+
A
A
A
HX
A
A
H
(X= Cl or Br or I)
X
Cl
H3C
CH3
+
HCl
CCl 4
H3C
CH3
H
H
+
HI
CCl 4
I
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H
+
HX
+
H
X
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X
f ormation of the more stable
carbocation
Markonikov’s rule: In addition of HX to unsymmetrical alkenes
the hydrogen halide adds to the double bonded carbon that
bears the greater number of hydrogen atoms and the negative
halide ion adds to the other double bonded carbon.
H3C
H3C
H3C
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CH3
major
Br
CH2
H3C
H3C
CH3
+
HCl
CH 2Br
minor
H3C
H3C
CH3
Cl
1-5- Addition of H2O: Hydration
A
A
+
A
A
+
H2O
H
A
A
A
A
H
OH
OH
H3C
CH3
+
+
H
H2O
H3C
CH3
H
H
+
+
CH3
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H2O
H
OH
CH3
2- OZONOLYSIS
A
A
A
+
A
A
A
O3
A
H2O, Zn
A
O
A
A
O
O
O
H3C
CH3
1) O 3
H3C
2) H 2O, Zn
1) O 3
H3C
CH3
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2) H 2O, Zn
O
2
H3C
O
+
O
CH3
+
A
O
A
3- Oxidation
KMnO 4 / OH
OH
-
OH
4- Epoxidation
H3 O
RCOOOH
+
OH
O
OH
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Electrophilic Addition to Conjugated Dienes:
1,4-Addition
 The addition of a reagent to a pair of adjacent carbons is
called 1,2-addition.
Br
+
Br 2
CCl4
1,2-addition
1,4-Pentadiene
Br
4,5-Dibromopentene
Br
Br
+
Br 2
Br
CCl4
1,2-addition
Br
Br
4,5-Dibromopentene
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Br
1,2,4,5-Tetrabromopentane
 Treatement of a conjugated diene with bromine under the
similar conditions gives, in addition to the exepected 1,2addition product, an unexcepted 1,4-addition product.
1,2-addition
expected product
1,4-addition
unexpected product
Br
+
1,3-Butadiene
Br 2
CCl4
+
Br
3,4-Dibromobutene
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Br
Br
1,4-Dibromo-2-butene
Resonance
The addition of bromine to 1,3-butadiene results in a secandary
carbocation. This carbocation is also called an allylic
carbocation.
H 2C
C
H
C
H
CH 2
H
C
H 2C
Br
C
H
CH 2
H 2C
Br
Br
Br
C
H
CH 2
Allylic carbocation (II)
Allylic carbocation (I)
H 2C
C
H
Br
1,2-addition Br -
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resonance
H
C
Br
C
H
Br -
1,4-addition
CH 2
H 2C
Br
C
H
C
H
CH 2
Br
+
H
Cl
CCl 4
1,3-Butadiene
Allylic carbocation equivalent to
+
(b)
1,2-addition
Cl -
+
+
(a)
(a)
Cl
1,4-addition
(b)
Cl
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+
The Diels-Alder reaction
The reaction is one between a conjugated diene and a
compound containing a double bond called a dienophile.
The product of a Diels-Alder reaction is offten called an
adduct.
+
Adduct
Dienophile
Diene
O
O
100°C
+
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O
Benzene
O
1,3-Butadiene
O
Maleic anhydride
O