- Alkenes are unsaturated organic compounds containing carbon – carbon double bonds.
- They are important feedstock for series of industrial chemicals specially polymers.
- Simple examples are ethene and propene that are important industrial organic chemicals.
- The chemical activity of alkenes are mainly because of carbon – carbon double bond,
- Alkenes undergo addition reactions at site of double bonds.
- The pi bond act as source of electrons and may have electrophilic addition reaction to synthesize saturated compounds.
Alkenes are unsaturated hydrocarbons. The presence of carbon - carbon double bond (C =C) is the salient feature of this group of compounds. The double bond is greatly responsible for the physical and chemical properties of these compounds. Simple examples for this class of compounds are ethene and propene as shown below. Carbon atoms attached to each other by a double bond are sp2 hybridized compared to sp3 hybridization in saturated alkanes.
Ethene is the simplest alkene. In an ethene molecule, three out of four p orbitals of each carbon atom and s orbitals of two hydrogen atoms give rise to three sp2 hybrid orbitals. Their overlapping gives two C – H single bonds and one C – C sigma (σ) bond. Each sp2 orbital lie in plane of carbon nucleus and are directed towards three corners giving a trigonal geometry. While carbon atom lies in the center of this geometry. The remaining fourth p orbital of each carbon atom lies above and below the plane of sigma C – C bond. The two p orbitals of each carbon atom overlap above and below the plane and form a pi (π) bond. This bond due to weaker interactions between the orbitals is relatively weak compared to sigma bond. The electrons in pi bond are more accessible and responsible for most of the chemical reactions of alkenes.
The double bond is stronger (bond energy = 146 kcal/mol) than single bond of alkanes (bond energy = 88 kcal/mol). Also, the double is shorter as two carbon atoms are more tightly bonded to each other due to an extra bond between them. The bond length for double in ethene is 1.53Ao compared to 1.34Ao for single carbon-carbon bond in its saturated analogue, ethane.
In short, the nature of bonding in alkenes, their unsaturation, bond energy of double and presence of pi electrons drive their chemical activity. Alkenes undergo addition reactions. The double bond act as source of electrons and hence alkenes react with electron deficient species. These species are known as electrophiles and may lead to electrophilic addition. Alkene can also undergo free-radical addition reaction and polymerization. The details of these reactions are discussed in more details in sub-sections below.
Addition reactions of alkenes
Hydrogenation: Addition of hydrogen
Double bond of alkene undergoes addition of hydrogen in the presence of metal catalyst. This hydrogenation is an exothermic reaction as two sigma bonds (C – H) are formed at the expense of one sigma bond (H – H) and pi bond of carbon – carbon. The amount of heat evolved when one mole of an unsaturated compound is hydrogenated is called heat of hydrogenation.
Electrophilic addition reactions of alkenes
Addition of hydrogen halides
The pi bond is not as strong as sigma bond and electron cloud above and below the plane is polarizable. This way, double bond can act as a nucleophile. A typical example of this mechanism is addition of hydrogen halides where proton from strong acid may yield a carbocation. In the presence of strong nucleophile, carbocation undergoes electrophilic addition to the double bond site.
In simple alkenes e.g. ethene, addition of hydrogen halide is simple and does not give multiple products. However, in case of branched alkene e.g. propene, there could be two possible products i.e. 2-chloro propene and 1-chloro propene. However in practical, only product is formed i.e. 2-chloro propene.
The answer is described by Markovnikov’s Rule which states “in the addition of an acid to the carbon – carbon double bond of an alkene, the hydrogen of the acid attaches itself to the carbon that already holds the greater number of hydrogens”. This rule defines the regioselectivity in electrophilic addition reactions for alkenes.
Halogenation: Addition of halogens
Alkenes are easily converted by halogens to alkayl halides where two halogen atoms are attached to two carbons at adjacent positions. The reaction is relatively easy carried out at room temperature and does not require light. Two reactants are mixed in an insert solvent such as carbon tetrachloride.
In example of bromination of ethene, both reactants are not polar but can be polarized. Hence induced dipole creates slight polarity at bromine atoms of bromine molecule. The reaction proceeds with electrophilic mechanism forming an positively charged intermediate which subsequently with other bromine atom gives saturated di-bromo ethane. Two bromines are attached to two adjacent carbons, such halides are known as vicinal dihalides.
Addition of bromine is an extremely useful tool employed to detect the presence of double bond in an unknown organic compound. Bromine solution in carbon tetrachloride is red, addition of an unsaturated molecule rapidly decolourizes bromine. This a characteristic test to confirm the presence of a carbon – carbon double bond.
Iodine does not react with alkenes at all.
Addition of Water
Water adds to reactive alkenes in the presence of an acid to form alcohol. This reaction is carried out in dilute acids e.g. 50% water: H2SO4 solution. This addition reaction has significant industrial importance in manufacturing alcohols.
For branched bigger alkenes, the reaction follows markovnikov’s rule and thus hydroxyl anion goes to carbon center with least hydrogen attached.
Addition of sulfuric acid
This is another method of converting alkenes into alcohols. Alkenes react with cold concentrated sulfuric acid to form alkayl hydrogen sulfate ester. This product is formed by addition of hydrogen of acid to one carbon of alkene double bond and bisulfate ion to the other. On diluting the reaction mixture and warming it up, sulfate ester is hydrolyzed to form alcohol. This method is used for large scale manufacturing of alcohols.
Hydroxylation: Formation of 1,2 diols
Alkenes are oxidized with certain oxidizing agents such as potassium permanganate to add hydroxyl groups at double bonds of alkenes. This addition of hydroxyl groups is called hydroxylation and it gives vicinal diols. This is an important reaction for the synthesis of diols.
This reaction forms the basis of very useful analytical test known as the Baeyer test to detect the presence of double bond (unsaturation) in a molecule.
Three membered ring containing oxygen are called epoxide. The epoxide formation is a major organic reaction. They are formed by the reaction of alkenes with a source of electrophilic oxygen. The most important epoxide at industrial scale is actually the simplest of all, ethylene oxide. It has numerous industrial applications and is formed by catalytic oxidation of ethene by air.
Ozone (O3) is a triatomic molecule and is a powerful electrophile. It undergoes reaction with alkenes cleaving both sigma and pi carbon-carbon bonds. This reaction is known as ozonolysis or ozonation and the products are called ozonides.
Ozone is passed through alkene solution in an inert solvent. The reaction leaves viscous ozonide that cannot be purified as it is unstable and explosive in nature. Ozonide is immediately treated with water and in the presence of a reducing agent to form carbonyl compounds (ketone or aldehyde).
Polymerization is a chemical reaction where small units or groups combine together to make a bigger molecular material. The small sub-units (or building blocks) are called monomers while the resultant bigger molecular weight material is called polymer and the process is called polymerization. During polymerization, thousands of monomer unite together to make a very large molecule.
Ethene is an important feedstock for the synthesis of various polymers in chemical industry. PVC and PE are commonly known words in our everyday life. PVC stands for polyvinyl chloride while PE is polyethylene. Both are plastics and are in use around us.
Polyethylene is made up by the polymerization of numerous ethylene units. The reaction proceeds via free radical mechanism where oxygen or peroxide is used to initiate the chemical reaction.
Another good example of polymerization is synthesis of polyvinyl chloride. Here, ethene undergoes halogenation first and then subsequent polymerization to form our popular plastic.
Similarly other examples are polyvinyl alcohol and polystyrene used in many industrial applications.
Books for further study
- Morrison, R. T., and R. N. Boyd. "Organic chemistry 5th edition." (1987).
- Cary, A, F. Organic Chemistry, 3rd edition, (1996).
- Volhardt K, P, C. Organic Chemistry, (1987).
- Smith M, B and March, J. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th edition (2001).