Lecture (5)
(III)-Alkenes (Olefins)
1. Introduction:
Alkenes are hydrocarbons that contain a carbon – carbon double bond, C = C.
Historically called olefins. Alkenes occur abundantly in nature, and many have
important biological roles, they have the general formula CnH2n, and the double bond
is known as ethylinic bond, the C=C is made up of a strong σ bond and a weak П
bond.
2. Nomenclature:
a- Common Method:
Rules for common names are the same as alkanes except that the ending (ylene) is
used instead of (ane):
Ethane
CH3 – CH3 → ethylene CH2 = CH2
Propane CH3 – CH2 – CH3 → propylene CH2 = CH – CH3
The name of univalent groups derived from alkenes have the ending –enyl, and
the carbon atom with the free valence is number (1).
Ex.1:
CH2=CH-
ethenyl(vinyl)
CH3CH=CH-
1-Propenyl
CH2=CHCH2-
2-Propenyl
Ex.2:
Ex.3:
b- IUPAC:
1. It is similar to alkane but the ending (ane) is replaced by (ene), also alkyl
groups ending (yl) is replaced by (enyl).
2. Choose the longest continuous chain contains the double bond and number
the chain to give the double bond the lowest possible number.
3. Substituents are ordered alphabetically.
4. Multiple double bonds indicated by diene, triene…….. at the end.
Ex.1:
CH3 – CH2 – CH = CH2
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1-butene
Ex.2:
CH3
3
CH3─2C = 1CH2
2-methyl-1-propene
Ex.3:
CH3
1
CH3─ 2CH= 3C ─4CH25CH3
3-methyl-2-pentene
Ex.4:
CH3
1
CH2= 2C─3CH2=4CH2
2-methyl-1,3-butadiene
c- Geometric Isomers:
1- Due to restricted rotation around double bond, Alkenes exist in two forms
(cis) & (trans); cis means that both substituents on the same side, and if
substituents on opposite sides it shows transform.
Ex.1:
H
CH3
CH3
CH3
C
C
H
CH3
H
H
C
≠
C
(b) trans - 2 butene
(a) cis - 2 - butene
Ex.2:
Cl
≠
C
H
H
Cl
C
C
Cl
H
Cl
C
H
(a) cis 1,2 dichloroethene
2- Cis
and
trans
are
geometrical
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(b) trans 1,2 dichloroethene
isomers; have the same molecular formula and different arrangement, such
isomers have different physical and chemical properties.
3- If one of the double bond carbons is attached to two identical groups; cis –
trans isomers is not possible.
Ex.:
A
B
D
C
B
D
=
C
C
D
C
D
A
4- If three or four different groups attached to the carbon atoms of the double
bond; E,Z system of nomenclature is applied; (Z) means together or groups
with higher priority on the same side while (E) means across or the groups
with higher priority on opposite sides. Priority depends on the atomic
number of the attached atoms.
Ex.:
lower
higher
C
C
lower
higher
(E)
31
higher
higher
C
C
lower
lower
(Z)
Atomic No.:
35
Br
17
Cl
›
8
O
›
›
7
N
6
C
›
›
1
H
Ex.:
(35)
(53)
Br
(9)
F
C
C
I
Cl
(17)
Z - 1 bromo - 2 - chloro - 2 - floro - 1 - iodoethene
(35)
(53)
Br
Cl
C
C
I
F
(17)
(9)
E - 1 bromo - 2 - chloro - 2 - floro - 1 - iodoethene
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3.Preparation of alkenes (Olefins):
(a)-Thermal cracking(pyrolysis):
When long chain hydrocarbons from petroleum refining are submitted to the
cracked process significant amounts of propene and ethylene are produced.
nC6H14
700-900°C.
CH4 + CH2=CH2+CH3CH=CH2+others
P,cat.
(15%)
(40%)
(20%)
(25%)
(b)Dehydration of alcohols:
1. By passing the alcohol vapor over heated alumina (aluminum oxide):
CH3 - CH - CH3
Al2O3
OH
isopropylalcohol
CH2 = CH – CH3 + H2O
Heat
propene
The hydrogen required to combine with OH is taken from the adjacent carbon atom
carrying the fewer number of hydrogen atoms.
2. By heating alcohol with conc. Sulphuric acid:
95% H2SO4
+
CH3CH2CH2CH2OH + H
CH3CH2CH = CH2 + H2O
∆ 170°C
n-butylalcohol
1-butene
(c)Removal of halogen acid from alkyl halide by heating with
suitable reagent(dehydrohalogenation):
The alkenes is prepared by simply heating together the alkyl halides and a solution
of potassium hydroxide in alcohol.
─C─ C─
H
KOH (alcoholic)
─C=C─ + KX + H2O
X
Ease of dehydrogenation of alkyl halides 3°>2°>1° hydrogen
 CH3CH2Br
KOH (alcoholic)
CH2=CH2
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Ethyl bromide
ethene
 CH3CH2CH2CL KOH (alcoholic) CH3CH=CH2 KOH (alcoholic) CH3CHCLCH3
n-Propyl chloride
Propylene
iso-propyl chloride
 CH3CH2CH2CH2Cl
n-butyl chloride
KOH (alcoholic)
CH3CH2CH=CH2
1-butene
 CH3CH2CHCLCH3 KOH(alcoholic) CH3CH=CHCH3 + CH3CH2CH=CH2
Sec-butyl chloride
2-butene(80%)
1-butene(20%)
in case where a mixture of isomeric alkene can be formed, the preferred product is
the alkene that has the greater number of alkyl groups attached to the doubly-bonded
carbon atoms.the stability of alkenes follows exactly the same sequence of ease of
formation of alkenes.
R2C=CR2 > R2C=CRH > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2
(d)Hydrogenation of alkynes:
CH3CH2C≡CH
2H
1-butyne
Ni
CH3CH2CH = CH2
1-butene
4-Physical properties of alkenes:
The melting and boiling points and their densities are rising with increasing of the
molecular weight of un branched alkenes.
Lecture (6)
5-Reactions of alkenes: (chemical properties):
there are four particulary broad types of organic reactions:addittions,eliminations,
substitution and rearrengemnts .
Examples:
(1)-Addition reactions:
H H
C=C + HCL
H H
H CL
H─C─C─H
H H
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Ethylene
chloro ethane
(2)-Elimination reactions:
H CL
H H
H─C─C─H + NaOH
C=C + HCL
H H
H H
Chloro ethane
ethylene
(3)-Subistitution reactions:
H
H
H─C ─ H+CL─CL UV
H
H─C ─ CL + HCL
H
Reactions of C=C : in alkenes the functional group is the C=C.
The bond is consists of a stonge σ bond and a weak П bond which is broken undergo
addition reactions. The C=C acts as a source of electrons, that is acts as a base. The
compounds with which it reacts are those that are deficient in electrons, called
ELECTROPHILIC REAGENT. The typical reaction of an alkenes is electrophilic
addition.
H Cl
H─C=C─H + HCl
H─C─C─H
H H
H H
The CARBONIUM ION, is a group of atoms that contains a carbon atom bearing
only six electrons, carries a positive charge. Carbonium ions are classified as
primary, secondry or tertiary after the carbon bearing the positive charge.
 CH3+
Methyl Cation
 CH3CH2+
Ethyl Cation (primary)
 CH3─ CH+
Isopropyl Cation(secondry)
CH3
CH3
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 CH3─ C+
Tert-Butyl Cation(tertiary)
CH3
CH3
CH3
H─CH2─C+
H+ CH2=C
CH3
CH3
Mechanism of electrophilic addition:
Addition of acidic reagent HCl is belived to proceed by two step:
1-Two electrons from the ethylene П bond move to form a new σ bond between the
H+ and one of the ethylene carbon atoms. The other ethylene carbon atom, having
last its share of the П electrons, is now left with a vacant P orbital and only six
valence electrons. This carbon atom carries a positive charge and is called a carbon
cation or carbocation.
:
C
H
C
+ H
+
C
+
C
2-Nucleophilic Chlorine ion donates an electron pair to form a new σ bond with
the carbocation.
H
¨
:C l :
¨
+
C+
Cl
C
H
C
(a)-Addition of hydrogen (hydrogenation):
Ex.:
H2
CH3CH2CH=CH2
1-butene
CH3CH2CH2CH3
Pt / Cat.
n-butane
The step can be controlled by the amount of H2
36
C
(b)Addition of halogens (halogenation):
Br
Br
CH3CH=CHCH3 + Br2
2-butene
CCl4 (solvent)
Room temp.
CH3 - CH - CHCH3
(in dark)
2,3-dibromobutane
1) The step can be controlled by the amount of halogen used.
2) Br2 & Cl2 are often used, F2 is too reactive & I2 is not reactive.
(C)Addition
│ │
C=C
│ │
+
of halogen acid (hydrohalogenation):
│ │
─C=C ─
│ │
HX
H
Olefin
X
alkylhalide
Note that:
(1) -The addition of HX, X2 and HOH to the double bond is electrophilic addition
reaction. The reaction must start with positively attacking species & forming
carbonium ion.
─ C=C─ + (H+)
─C ─ C+
H
(Electrophilic center)
─ C ─ C─
and XH
X
carbonium ion intermediate
(2)-Electrophilic addition of HX to double bond is oriented according to
Markovnikov's rule. It stated that:
In addition of HX to alkene, the hydrogen goes to the carbon with the most
number of hydrogen atoms i.e. less alkyl
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(d)Addition of water (hydration): (using dil. Sulphuric acid):
H
H
CH3─CH=CH2 cold H2SO4 CH3─C─CH3 hydrolysis
H+/-OSO3H
OSO3H
H2O/heat
CH3─C─CH3 + H2SO4
OH
Iso propyl alchohol
(e)Oxidation of alkenes:
Most olefins are readily attacked by permangannate, dichromate, and ozone.
(1)-Oxidation of olefins with permanganate:
when oxidation of alkene is carried out with KMNO4 in acidic rather than basic
solutions, cleavage of the double bond occurs and carbonyl-containing products
are obtained. If the double bond is tetrasubstituted,the two carbonyl- contaning
products are ketons; if a hydrogen is present on the double bond,one of the
carbonyl-containing products is carboxylic acids; and if two hydrogens are present
on one carbon, CO2 is formed.
CH3 – CH = CH2
KMnO4
CH3COOH + CO2 + H2O
1-propene
acetic acid
CH3
=C
CH3
KMnO4
= O + O=C
CH3
Cyclohexanone
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CH3
acetone
(2)-Oxidation of olefins with ozone:
RCH = CHR1 + O3
Zn
dil . acid
RCOH + R1COH (or ketone or mixture of both)
aldehydes
(f)Polymerization:
Under certain conditions olefins molecules can be polymerized i-e they are large
molecules that made up of many repeating units. The units that are used to synthesize
polymers are called monomers, ex., ethylene molecules is a monomer for preparin
polyethylene polymer.
mH2C = CH2
Ethylene monomer
catalyst
Heat / P.
CH3CH2 ( - CH2 – CH2 - )n CH2CH3
polyethylene
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