Alkenes – Formula, Definition, Structure, Properties
Alkenes are unsaturated hydrocarbons that contain carbon-carbon double bond. They are represented by the general formulae CnH2n where ‘n’ stands for number of carbon atoms in the molecule. Alkenes are also known as olefins (in Latin oil maker) because the first member ethene combines with chlorine gas to form an oily liquid as a product.
(i) Nomenclature of Alkenes:
Let us write the IUPAC name for the below mentioned alkanes by applying the general rules of nomenclature that we already discussed in unit No.11
IUPAC NAMES FOR ALKENES
Presence of double bond in Alkene provides the possibility of both structural and geometrical isomerism.
The first two member’s ethene C2H4 and propene C3H6 do not have isomers because the carbon atoms in the molecules can be arranged only one distinct way. However from the third member of alkene family butene C4H8, structural isomerism exists.
structures (i) & (ii) are position isomers. structures (i) & (iii), (ii) & (iii) are chain isomers
It is a type of stereoisomerism and it is also called cis-trans isomerism. Such type of isomerism results due to the restricted rotation of doubly bounded carbon atoms. If the similar groups lie on the same side, then the geometrical isomers are called Cis-isomers. When the similar groups lie on the opposite side, it is called a Trans isomer.
The geometrical isomers of 2-Butene is expressed as follows
General methods of preparation of alkenes:
(1) Preparation of alkene by dehydration of alcohol:
When an alcohol is heated at 430-440 K with excess of concentrated sulphuric acid, a molecule of water from alcohol is removed and an alkene is formed. This reaction is called elimination reaction.
Ethene can also be prepared in laboratory by catalytic dehydration of alcohol.
(2) Preparation of alkenes from alkynes:
Alkynes can be reduced to cisalkenes using Lindlar’s catalyst (CaCO3 supported in palladuium partially deactivated with sulphur (or) gasoline). This reaction is stero specific giving only the cis-alkene.
Alkynes can also be reduced to transalkenes using sodium in liquid ammonia. This reaction is stereospecific giving only the trans-alkene.
(3) Preparation of alkenes by dehydrohalogenaton of halo alkanes.
Halo alkanes react with alcoholic KOH and eliminate hydrohalide resulting in the formation of alkene.
(4) Preparation of alkenes from vicinal dihalogen derivative of alkanes or vicinal dihalides
The compound in which two halogen atoms are attached to adjacent carbon-atoms are called as vicinal dihalides. When vicinal dihalides are warmed with granulated zinc in methanol, they lose a molecule of ZnX2 to form an alkene.
(5) Preparation of ethene by kolbe’s electrolytic method:
When an aqueous solution of potassium succinate is electrolyzed between two platinum electrodes, ethene is produced at the anode.
Physical properities of alkenes:
The first three members (Ethene, Propene and Butene) are gases, next fourteen members are liquids and the higher alkenes are waxy solids. They are all colourless and odourless except ethene which has a sweet smell.
1. The melting and boiling point of alkenes increases along the homologous series. Like alkanes, straight chain alkenes have high boiling point compared to its isomeric branched alkenes.
2. Alkenes are slightly soluble in water but readily in organic solvents.
Chemical Properties of Alkenes:
Alkenes are more reactive than alkanes due to the presence of a double bond. The σ- bond is strong but the π- bond is weak. The typical reactions of alkenes involve addition of an electrophile across the double bonds proceeding through ionic mechanism. However addition reactions proceed through free-radical mechanism also. Ozonolysis and polymerization are some of the characteristic reactions of alkenes.
(i) Addition Reactions
(ii) Addition of hydrogen: (Hydrogenation of alkenes)
Hydrogen adds on to alkenes in the presence of a metal catalyst (Ni, Pd (or) Pt) to yield corresponding alkanes. This is known as catalytic hydrogenation. This process is of great importance in the manufacture of vanaspathi from vegetable oil. This helps to prevent rancidity of vegetable oils.
(ii) Addition of halogens: (Halogenation of alkenes)
When alkene is treated with halogens like chlorine or bromine, addition takes place rapidly and forms 1, 2 – dihalo alkane (or) vicinal dihalide
Iodine reacts very slowly to form 1, 2 – diiodo alkane which are unstable and regenerate the original alkene by elimination of iodine.
TEST FOR ALKENE:
Bromine in water is reddish brown colour. When small amount of bromine water is added to an alkene, the solution is decolourised as it forms dibromo compound. So, this is the characteristic test for unsaturated compounds.
Markovnikoff ’s Rule:
“When an unsymmetrical alkene reacts with hydrogen halide, the hydrogen adds to the carbon that has more number of hydrogen and halogen add to the carbon having fewer hydrogen”. This rule can also be stated as in the addition reaction of alkene / alkyne, the most electro negative part of the reagent adds on to the least hydrogen attached doubly bonded carbon.
(iii) Addition of water:- (Hydration of alkenes)
Normally, water does not react with alkenes. In the presence of concentrated sulphuric acid, alkenes react with water to form alcohols. This reaction follows carbocation mechanism and Markovnikof ’s rule.
(iv) Addition of hydrohalides: (Hydrohalogenation of Alkenes)
Hydrogen halides (HCl, HBr and HI) add to alkene to yield alkyl halides. The order of reactivity of different hydrogen halides is HI>HBr>HCl. It is an example for electrophilic addition.
(a) Addition of HBr to symmetrical alkene:
Addition of HBr to symmetrical alkene (similar groups are attached to double bond) yields alkyl halides (haloalkanes)
(b) Addition HBr to unsymmetrical alkene:
In the addition of hydrogen halide to an unsymmetrical alkene, two products are obtained.
Consider addition of HBr to propene
Formation of electrophile:
In H-Br, Br is more electronegative than H. When bonded electron moves toward Br, polarity is developed and creates an electrophile H+ which attacks the double bond to form carbocation, as shown below.
Secondary carbocation is more stable than primary carbocation and it predominates over a the primary carbocation.
The Brθ ion attacks the 2° carbocation to form 2-Bromo propane, as the major product.
Consider addition of HBr to 3-methyl-1-butene. Here the expected product according to markovnikoff ’s rule is 2-bromo-3-methyl butane but the actual major product is 2-Bromo-2-methyl butane. This is because, the secondary carbocation formed during the reaction rearranged to more stable tertiary carbocation. Attack of Br– on this tertiary carbocation gives the major product 2-bromo-2-methyl butane.
The addition of HBr to an alkene in the presence of organic peroxide, gives the anti Markovnikof ’s product. This effect is called peroxide effect.
The reaction proceeds via free radical mechanism.
The weak O-O single bond linkages of peroxides undergoes homolytic cleavage to generate free radical.
The radicals abstracts a hydrogen from HBr thus generating bromine radical.
The Bromine radical adds to the double bond in the way to form more stable alkyl free radical.
Addition of HBr to secondary free radical
Addition of HBr to secondary free radical
The H-Cl bond is stronger (430.5 kJmol-1) than H-Br bond (363.7 kJmol-1), thus H-Cl is not cleaved by the free radical. Thus H-I bond is weaker (296.8 kJ mol-1), than H-Cl bond. Thus H-I bond breaks easily but iodine free radicals combine to form iodine molecules instead of adding to the double bond and hence peroxide effect is not observed in HCl & HI.
Metal catalysed free radical addition of CXCl5 Compounds to alkene is called Kharash addition reaction.
(v) Addition of sulphuric acid to alkenes
Alkenes react with cold and concentrated sulphuric acid to form alkyl hydrogen sulphate accordance with Markownikof ‘s rule. Further hydrolysis yields alcohol.
(i) With cold dilute alkaline KMnO4 solution (Baeyer’s Reagent)
Alkenes react with Baeyer’s reagent to form vicinal diols. The purple solution (Mn7+) becomes dark green (Mn6+), and then produces a dark brown precipitate (Mn4+).
(ii) With acidified KMnO4 Solution:
Alkenes react with acidified KMnO4 solution and are oxidised to ketones or carboxylic acid depends on the substituent at the olefinic carbon atom. The purple solution becomes colourless. This is one of the test for unsaturation.
Ozonolysis is a method of oxidative cleavage of alkenes or alkynes using ozone and forms two carbonyl compounds. Alkenes react with ozone to form Ozonide and it is cleaved by Zn/H2O to form smaller molecules. This reaction is often used to identify the structure of unknown alkene or alkyne by detecting the position of double or triple bond.
A polymer is a large molecule formed by the combination of larger number of small molecules. The process in known as polymerisation. Alkenes undergo polymerisation at high temperature and pressure, in the presence of a catalyst.
Extensive use of polymers clogs up landfills and polute the environment. Because of diversity of polymers in consumer products, recycling requires sorting the polymers into various sub-types, labels with codes and symbols, which are then recycled separately.
Table shows the codes and symbols used in recycling of ethene-based additionpolymers.
(Lower the number, greater the ease of recycling the material)
Uses of Alkenes
1. Alkenes find many diverse applications in industry. They are used as starting materials in the synthesis of alcohols, plastics, liquors, detergents and fuels.
2. Ethene is the most important organic feed stock in the polymer industry. E.g. PVC, Sarans and polyethylene. These polymer are used in the manufacture of floor tiles, shoe soles, synthetic fires, raincoats, pipes etc.,
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