ALKENES
The alkenes are acyclic unsaturated
organic compounds made up of carbon and hydrogen only. These compounds are
unsaturated, have a double bond along with a single bond. They have fewer hydrogens than alkanes.
Another name of alkene is
olefins (mean oil forming compound) because the starting member of alkene make
oily products when they react with halogens( bromine or chlorine). They contain
no triple bond in their molecules and have single and double covalent bonds in the molecule. Some alkene has only one double
bond these are known as mone-ene and some compounds have two double bonds are
known as dienes.
Properties of alkenes
1. Alkenes are more
reactive as compared to alkane due to
their unsaturated nature. The name of alkanes ends with –ene.
2. The general formula
of alkenes is CnH2n, where n shows the number of carbon atom
present in a molecule.
3. The lower member
of alkene class are gases but higher member exit as liquid and solids.
4. They have at
least one C=C
5. They have a double bond along with a single bond in
a molecule and no triple bond is present.
6. These are
non-polar compounds, and insoluble in water but soluble in alcohol because they are weekly polar compounds due to
the presence of Sp2 hybridization.
7. Among all compounds
of alkene, only ethene have a smell and other members are colorless and
odorless.
8. When burning in the
air, they burn with a luminous flame.
9. They are weekly
polar compounds they do not conduct electricity.
10. The density of
alkene is less than water.
Methods for the preparation of alkenes
Alkenes are
prepared by different methods which are as follows.
v Deydrohalogenation
of alkyl halide
When alkyl halide reacts with alcoholic potassium hydroxide, it
undergoes an elimination reaction from adjacent carbon atoms. Hydrogen and halogen are both eliminated in the form of halogen acid. This reaction is known as dehydrohalogenation.
v Dehydration of an alcohol
The release of water molecules from
alcohol results in the formation of alkene. Different alcohol forms alkenes in
different conditions. This reaction occurs in the presence of sulphuric acid and
high temperatures. The ease of dehydration of tertiary alcohol is greater as
compared to a primary alcohol.
 |
| dehydration |
Alcohol also undergo dehydration
reaction in another catalyst that is alumina.
In this reaction vapors of alcohol pass over alumina.
 |
| dehydration |
v Dehalogenation of vicinal Dihalide
When vicinal
dihalide (the compound which has two halogen atoms on two adjacent carbon
atoms) react with zinc dust in an anhydrous solvent like acetic acid and methanol,
undergo elimination reaction and ZnCl2
formed as a by-product.
 |
| dihalide |
v Partial hydrogenation of alkyne
The controlled hydrogenation of alkyne in different conditions gives cis and trans alkene. Alkyne gives cis-alkene in the presence of Lindlar’s catalyst. Alkyne gives trans alkene when it reacts with sodium in liquid ammonia at -33 º C.
 |
| reduction |
v By Kolbe’s Electrolysis method
When metallic salt
of decarboxylation undergoes an electrolysis reaction in water, alkene will be formed.
 |
| electrolysis |
Reactions of alkenes
Alkenes are more reactive as
compared to alkane due to their unsaturated nature. A Sigma bond is stronger than a pi bond. So this bond breaks very easily and undergo different types of
reactions. Alkenes mostly give an electrophilic reaction. It undergoes two types of
reactions.
v Addition
reaction
v Oxidation
reaction
1.
Addition
reactions
v
Addition of hypohalus acid
When alkene
react with halogen in aqueous solution, a new product haloalcohol is formed, which
is known as halohydrin.
 |
| halohydrin |
v
Addition of sulphuric acid
When alkene reacts with cold con. H2SO4, it dissolves in it and forms alkyl hydrogen sulfate. When these alkyl hydrogen halides react with hot water undergo decomposition and alcohol is formed.
 |
| with sulphuric acid |
v
Hydrogenation
When molecules of hydrogen
add across the double bond of alkene in
the presence of a catalyst, corresponding alkanes are formed. This reaction is
known as hydrogenation. A large amount of heat is also evolved during this
reaction. This heat is known as the heat of hydrogenation. This reaction is used in
the laboratory for the preparation of vegetable ghee from oil.
 |
| hydrogenation |
v
Halogenation
When alkene reacts with
halogens in a solvent (carbon tetrachloride) at 25ºC give vicinal dihalides. The reaction of different halogens is different; iodine does not react. Fluorine is too reactive, it is difficult to control reaction. Bromine and chlorine give
electrophilic reactions.
 |
| halogenation |
v
Addition of hydrogen
When alkene reacts with hydrogen
halide, hydrogen, and halogen both add in the double bond and alkyl halide is
formed. This reaction occurs in two steps. In this first step, carbocation is
formed then it reacts with halogen ion to
form alkyl halide.
 |
| product |
v
Addition of water
When alkene reacts with
water, this reaction is known as hydration. This reaction occurs in the presence
of strong acid. Hydroxyl alkane is formed during this reaction.
2.
Oxidation
reaction
o
Hydroxylation
When alkene reacts with Bayer's reagent (dilute alkaline
KMnO4) at low temperature, hydroxylation of double bond occurs and
glycol ids are formed. This reaction is used as a test in the laboratory for the
detection of unsaturation in a compound. During this reaction pink color of KMnO4
is discharged.
 |
| hydroxylation |
o
Addition of oxygen
When alkene reacts with
oxygen in the presence of catalyst silver oxide at high temperature and
pressure, epoxide will be formed in this reaction.
 |
| ethylene oxide |
o
Ozonolysis
Ozone is an allotropic form of oxygen that is highly reactive and present in the outer atmosphere and made a
shield around the whole earth. It reacts with alkene and forms an unstable compound that is known as molozonide. This product
rearranges quickly and forms ozonide.
 |
| ozonolysis |
Ozonide are also unstable compounds,
when reacting with water and zinc,
reduction occurs and formaldehyde is formed.
 |
| formaldehyde |
o
Combustion
When alkene is treated with oxygen, it
will burn with luminous flame and
produce carbon dioxide and water, and heat is also evolved in this reaction.
 |
| combustion |
3.
Polymerization
It is a process in
which monomers combine with each other and the resulting product will be polymer. At
high temperature and pressure alkenes (ethane) undergo polymerization reaction and
polythene is formed.
 |
| polymerization |
Uses of Alkenes
Alkenes are used as artificial ripening of fruits.
Alkenes are
used in the preparation of polythene plastic, which is used for the
manufacturing of kids' toys, bags, cables, chairs, and buckets.
They are also used for the manufacturing of many
industrial products like ethyl alcohol, glycols, and ethyl halide.
We can also use
alkenes as a local anesthetic.
They are also
used for the manufacture of polystyrene, which is used in making different parts of refrigerator and car battery cases.
They are used for the preparation of vegetable ghee
from oil.
They are also used in the preparation of lubricating oil.
alkene is also used for the preparation of mustard gas.
 |
| mustard gas |
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