Alkenes are unsaturated hydrocarbons that have double bonds shared by two carbons. This means that not all of the carbon atom bonds are connected to hydrogen atoms. Therefore, more hydrogen atoms can be added to make the hydrocarbon saturated.
Alkenes can have one or more double bonds. More than one double bond may occur in longer-chains. Alkenes can also be either straight-chain (aliphatic) or closed-ring (cyclic). The double bonds in cyclic alkenes occur in one of the carbons in the ring.
The term ‘alkene’ is often interchangeable with olefin, but the systematic IUPAC nomenclature is preferred in organic chemistry.
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What Are the Classifications of Alkenes?
Although alkenes can have more than one double bond along their chains or rings, those with just one double bond are more common. These are the acyclic or open-chain monoalkenes with the general formula CnH2n.
The position of the double bond is important. It can either be terminal or internal. Terminal monoalkenes, also known as α-olefins, have more applications in industries and pharmaceuticals. For example, they’re used as precursors to plastic polymerisation. Some medicines and vitamin supplements are also derived from alkenes.
Alkenes can be classified based on the number of alkyl groups attached to the double-bonded carbons. Below is a list of the types of alkenes with their general formulas. The R represents the additional alkyl group.
- Monosubstituted alkenes: Only one alkyl group is bonded to one of the double-bonded carbons. They have the general formula of RCH=CH2. Propene is the simplest example of a monosubstituted alkene, as shown below:
- Disubstituted alkenes: These have two alkyl groups attached to the double-bonded carbons. The general formula can be written either as RCH=CHR or R2C=CH. This means that the two alkyl groups can either be bonded to just one of the double-bonded carbons, or they are equally distributed to the two double-bonded carbons. One of the simplest examples is 2-Methyl-1-propene (isobutylene). This is a terminal substituted alkene, which means that the double-bonded carbons are located on the end of the chain.
- Trisubstituted alkenes: These have a total of three alkyl groups attached to the double-bonded carbons. The general formula can be written R2C=CHR. One of the simplest examples of this is 2-Methyl-2-butene, as shown below:
- Tetrasubstituted alkenes: A total of four alkyl groups are attached to the double-bonded carbons, replacing all the hydrogens. The general formula for these can be written as R2C=CR2. One of the simplest examples is 2,3-Dimethyl-2-butene.
What Are the Properties of Alkenes?
- State of matter under standard conditions
Alkenes have very similar properties to alkanes in that they’re flammable and hydrophobic. The main difference is that they have stronger bonds. Alkenes with two or four carbon atoms are in the form of a gas at room temperature. Meanwhile, those that have five to 17 carbon atoms are in liquid form under standard conditions. Finally, those that have at least 18 carbon atoms are sufficiently heavy and have such strong intermolecular forces that they take solid form.
- Density and solubility in water
Since alkenes are nonpolar, they’re not soluble (miscible) in water. They’re also less dense than water, with densities ranging from 0.6 to 0.7 g/mL. By comparison, pure water has a density of 0.9998395 g/ml at 4.0°C. This tells us that alkenes will always float on water at room temperature.
- Melting points and boiling points
Both the melting points and the boiling points of alkenes tend to increase as the number of carbon atoms increase. Larger molecules have greater intermolecular forces, which means that their molecular bonds are harder to break. Therefore, they require higher energy to boil.
What Are the Chemical Reactions of Alkenes?
Alkenes can undergo several types of reactions either with other substances (elements and compounds) or with themselves in the presence of catalysts. The three main types of reactions are addition reactions, polymerisation, and metal complexation.
These reactions involve the addition of substituents by opening up the double bond. Elements or functional groups are added in the process. Most of these reactions are basically electrophilic. The reactions are further subdivided into six types: hydrogenation, hydration, halogenation, hydrohalogenation, halohydrin formation, and oxidation.
- Hydrogenation and related hydroelementations: This process involves high temperature, high pressure, and catalysts such as platinum and palladium. The main industrial application is the large-scale production of margarine.
- Hydration: This reaction is made possible by using an acidic catalyst, which can either be phosphoric acid or sulphuric acid. Water is added to an alkene to make an alcohol. The large-scale industrial production of ethanol is achieved in this manner. See the balanced equation below:
CH2=CH2 + H2O → CH3–CH2OH
- Halogenation: An alkene can react with either chlorine or bromine to form either vicinal dibromo- or dichloroalkanes, respectively. The reaction is used as an analytical method for determining the presence of alkenes in water or in other mixtures. For example, if the reddish-brown colour of bromine solution disappears when added to water, it means that an alkene is present. The reaction can be written as:
CH2=CH2 + Br2 → BrCH2–CH2Br
- Hydrohalogenation: This reaction involves the addition of hydrogen halides to alkenes, which produce haloalkanes. For example, hydrogen iodide can be added to propene to produce 1-iodopropane, as shown below:
CH3–CH=CH2 + HI → CH3–CHI−CH2–H
- Halohydrin formation: A halohydrin is a functional group that has a halogen and a hydroxyl (-OH) bonded to a carbon atom. This reaction happens between an alkene, a halogen, and water, as shown in the general equation below. The reaction is best described by Markovnikov’s regiochemistry and anti-stereochemistry.
CH2=CH2 + X2 + H2O → XCH2–CH2OH + HX
- Oxidation reactions: These involve several types, namely, hydroxylation, epoxidation, and ozonolysis.
- Hydroxylation is a reaction of an alkene with an oxidising agent and water.
- An epoxide is a three-member ring that contains oxygen. Epoxidation reaction occurs at high temperatures and with the help of catalysts.
- Ozonolysis is the reaction of an alkene with ozone, producing a transitory, unstable product known as ozonide. It is treated with water to form carbonyl compounds like ketones.
Terminal alkenes, or those with double bonds on one end of the chain, are very useful as precursors to polymers. These polymers are made into different types of materials and final products such as plastics and polyethylene. Various methods or chemical pathways can be used for polymerisation.
The following are some mechanisms whereby alkenes can be polymerised:
- Anionic polymerisation: A nucleophilic reagent is used to initiate the process.
- Cationic polymerisation: In this case, acidic reagents are used.
- Radical polymerisation: The alkene, such as ethene, is subjected to high pressure and temperature. It’s then polymerised using peroxide catalysts.
- Coordination polymerisation: Polymerisation at normal pressure is achieved with the use of aluminum-molybdenum oxide catalyst.
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