Skip to main content

📝 Alkenes

🎧️ Listen to the recording and mind pronunciation of words.#


Alkenes are unsaturated hydrocarbons with one carbon–carbon double bond consisting of a σ bond and a π bond. Their general formula is CnH2n.
Cycloalkanes and alkenes show functional isomerism. Pent-1-ene and 2-methylbut-1-ene show chain isomerism. But-1-ene and but-2-ene show – positional isomerism.
Alkenes are more reactive than alkanes because they contain a π bond. The characteristic reaction of the alkenes is addition, which occurs across the π bond. Addition is the combination of two or more molecules to form a single molecule. Addition reactions are generally faster than substitution reactions since only weak π-bonds are broken, rather than stronger σ-bonds.
The π-bond in an alkene is an area of high electron density. It can thus attract electrophiles and undergo heterolytic fission. Heterolytic fission is the breaking of a covalent bond which results in both electrons going to the same atom. This is in contrast to alkanes which can only react with free radicals and undergo homolytic fission. An electrophile is a species which can accept a pair of electrons from a species with a high electron density. Electrophilic addition reactions of alkenes – making halogenoalkanes, dihalogenoalkanes, and alcohols.
Ethene produces ethane when reacted with hydrogen over a nickel catalyst.
Ethene produces 1,2-dibromoethane when reacted with bromine at room temperature.
Ethene produces chloroethane when reacted with hydrogen chloride at room temperature.Ethene produces ethanol when reacted with steam in the presence of a concentrated H3PO4 catalyst.
The mechanism of the reaction of bromine with ethene is electrophilic addition. Electrophiles accept a pair of electrons from an electron-rich atom or centre, in this case, the π bond. A carbocation intermediate is formed after the addition of the first bromine atom. This rapidly reacts with a bromide ion to form 1,2-dibromoethane. Unsymmetrical alkenes are those in which the two carbon atoms in the double bond are not attached to the same groups. If unsymmetrical alkenes (propene) react with unsymmetrical electrophiles (hydrogen bromide), there are two possible products: 2-bromopropane and 1-bromopropane. The likelihood of one product being formed over the other depends on the stability of the carbocation intermediate.
Secondary carbocations are more stable than primary carbocations. Tertiary carbocations are even more stable than secondary cations. Therefore, the product 2-bromopropane is a more likely product than the product 1-bromopropane. The more stable carbocation will be the one which is more highly substituted. The more electronegative part of the electrophile will thus always attach itself to the more highly substituted carbon atom.
Mild oxidation of alkenes by cold, dilute acidified manganate (VII) solution gives a diol.
Alkenes produce many useful polymers by addition polymerization. For example, poly(ethene) is made from CH2 = CH2 and poly(chloroethene) is made from CH2=CHCl. The disposal of poly(alkene) plastic waste is difficult, as much of it is chemically inert and nonbiodegradable. When burnt, waste plastics may produce toxic products, such as hydrogen chloride from PVC (poly(chloroethene)).