ALKENES
An alkene also known as olefin is an unsaturated hydrocarbon containing at least one carbon-to-carbon double bond. The simplest alkenes, with only one double bond, no rings, and no other functional groups, are hydrocarbons with the general formula CnH2n.
The first three are:
Ethene C2H4
Propene C3H6
Butene C4H8
Laboratory Preparation of Ethene
- Dehydration of alcohols using aluminium oxide as catalyst
This is a simple way of making gaseous alkenes like ethene. If ethanol vapour is passed over heated aluminium oxide powder, the ethanol is essentially cracked to give ethene and water vapour.
- The dehydration of ethanol to give ethane using concentrated sulphuric acid as catalyst
Ethanol is heated with an excess of concentrated sulphuric acid at a temperature of 170°C. The gases produced are passed through sodium hydroxide solution to remove the carbon dioxide and sulphur dioxide produced from side reactions.
The ethene is collected over water.
Physical Properties of Alkenes
(a) All are colourless and has pleasant smell.
(b) Lower members (C2 to C4) are gases, middle one (C3 to C17) are liquids, higher are solids.
(c) The boiling points, melting points, and specific gravities show a regular increase with increase in molecular weight, however less volatile than corresponding alkanes.
(d) A cis isomer has high boiling and melting point than trans isomer because of more polar nature.
(e) They are soluble in non polar solvents.
(f) Alkenes are weak polar. The polarity of cis isomer is more than trans which are either non polar or less polar. (e.g. trans butene-2 is non polar; trans pentene-2 is weak polar).
Chemical Properties
Alkenes are more reactive than alkanes due to the exposed pi-bonding electrons.
There are four types of common reactions:
1. Combustion Reactions
2. Addition Reactions
3. Oxidation Reactions of Alkenes
4. Polymerization of Alkenes
- Combustion Reactions
CnH2n + 1.5nO2 —> n CO2 + n H2O
C2H4 + 3 O2 —> 2 CO2 + 2 H2O
C3H6 + 4.5 O2 —> 3 CO2 + 3 H2O
As with alkanes, combustion of alkenes with insufficient oxygen also produces CO. - Addition Reactions
Addition reactions can be divided into two classes:
A. Symmetrical reactants, where the same substituent is added to each carbon of the double bound.
Hydrogenation generally requires a catalyst such as Pt, Pd and may also require heat and / or pressure.
Hydrogenation is used to convert liquid oils to solid fats. As the number of double bonds is reduced, the melting point of the fat increases. This type of reaction is called hardening. Platinum (pt) is a common catalyst.
2 b. Addition of Halogens = Halogenation
The double bond reacts rapidly at room temp with either Cl2 or Br2. No catalyst is needed (iodine hardly reacts, fluorine reacts explosively).
Halogenation can be a qualitative test.
Suspect compound + dilute solution of Br2. If red color disappears then the compound is probably an alkene.
2 c. Addition of water = Hydration
In the presence of acid, water adds to an alkene double bond.
Addition of water to an alkene follows Markovnikov’s rule.
- Oxidation Reactions of Alkenes
Oxidation can be defined as either a) The addition of oxygen to a molecule, or - b) The removal of hydrogen from a molecule.
Common oxidizing agents: oxygen, O2, ozone, O3, potassium permanganate, KMnO4, potassium dichromate, K2Cr2O7.
Reaction of an alkene with dilute aqueous solution of KMnO4 produces a dihydroxy alcohol.
Ethylene + KMnO4 —–> CH2(OH)CH2OH [fusion_builder_container hundred_percent=”yes” overflow=”visible”][fusion_builder_row][fusion_builder_column type=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”no” center_content=”no” min_height=”none”][ethylene glycol – used in antifreeze]
Oxidation of an alkene with potassium permanganate, KMnO4, can be a qualitative test. Suspect compound + aqueous solution of KMnO4. If purple color disappears then the compound is probably an alkene. Reaction with ozone in ozonolysis leads to the breaking of the double bond, yielding two aldehydes or ketones. Reaction with concentrated, hot KMnO4 (or other oxidizing salts) in an acidic solution will yield ketones or carboxylic acids.
R1-CH=CH-R2 + O3 → R1-CHO + R2-CHO + H2O
- Polymerization of Alkenes
Certain alkenes undergo self-addition reactions in the presence of specific catalysts to produce molecules called polymers. A polymer is a very large molecule (macromolecule) made up of repeating units. The reaction involves double bonds being converted to single bonds as hundreds or thousands of molecules bond and form long chains. The large number of molecules bonding is represented by “n” in the following example polymerization of ethene to from polyethylene.
Heat, pressure
n CH2=CH2 ——————–> (-CH2-CH2-)n
ethene catalysts polyethylene
The word “polymer” comes from the Greek “poly” (many) and “mer” (parts). An addition polymer is a polymer formed by linking together many alkene molecules through addition reactions. The monomer is the starting material that becomes the repeating units of polymers. It is not possible to give an exact formula for a polymer produced by a polymerization reaction because the individual polymer molecules vary in size. Polymers made from alkenes result in a very long-chain alkane. As a result, it has the chemical inertness of alkanes.
Uses of Ethene
- It is used in manufacturing ethanol
- Ethane is also used to produce synthetic rubber such as styrenebutadiene rubber which is formed by the polymerization of phenylethene.
- It is used in the process of ripening fruits
- It is also used in the manufacture of detergents, as well as glycol
- It is the raw materials for making many important plastics such as poly(ethene) poly(chloroethene) also known as polyvinyl chloride (PVC), etc.
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