What is Alkene?
In inorganic chemistry, alkene is a compound, which belongs to the family of hydrocarbons. Additionally, it has a carbon-carbon double bond (C=C). Besides alkenes are also regarded as unsaturated hydrocarbons, because they have less than the maximum number of hydrogen atoms per carbon atoms.
Furthermore, the general formula of alkene is CnH2n. Moreover, alkene creates a homologous series comprising molecules that can increase their weight by adding methylene. Probably, the simplest compound in this alkene series is ethane or ethylene (C2H4). Other molecules of this series are propane (C3H6), butene (C4H8), etc.
Physical Properties of Alkene
The physical properties of alkene are as followed –
State Of The Compound: Alkene compounds are naturally odourless and colourless. However, ethane is an exception. Even though it is a gas without colour, it holds a slightly sweet smell. Additionally, the first three compounds of this family of alkene are gaseous, but the next fourteen are liquid, and the remaining ones are solid.
Solubility: Moreover, owing to their nonpolar characteristics, alkenes do not dissolve in water. Contrarily, they are entirely soluble in nonpolar solvents such as ligroin, benzene, etc.
Boiling Point: The boiling point of alkene is directly proportionate with its carbon atoms; if it increases, then the boiling point also rises. Additionally, when alkene and alkane’s boiling points are compared, it is found that they are almost similar. Moreover,they have a similar carbon structure. Besides, the boiling point of a straight-chain alkene is higher than the branched-chained ones.
Melting Point: The melting point of alkene depends on the positioning of its molecules. Moreover, the melting of this compound is similar to alkanes. On the other hand, cis-isomer molecules have an inferior melting point compared to trans-isomers, as they are packed in a U-bending shape.
Polarity: Furthermore, compared to alkanes, alkenes are weakly polar. However, alkenes are marginally more reactive owing to the existence of double bonds. On the other hand, you can easily remove the double bonds or add more, as they are not strongly held. Therefore, the dipole moment exists more in alkenes rather than in alkenes. Nevertheless, this polarity depends on the functional group attached to its chemical structure.
Chemical Properties of Alkene
As mentioned earlier alkene is an unsaturated compound which makes it a highly reactive substance. Moreover, these chemical reactions occur surrounding its carbon-carbon bond. Hence, it becomes more reactive than alkanes. To better understand the chemical properties of alkenes, read the following reactions.
Reactions of Alkene
The chemical properties of alkenes class 11 make it a relatively stable compound. Here are some of the prominent reactions that alkene takes part.
Addition Reactions
In this type of reactions, two or more molecules join together to create a larger one. The end product of these reactions is called an additive product. Furthermore, a number of such reactions follow the mechanism of electrophilic addition. Some of the prominent examples of such reactions are –
Hydrogenation: This reaction requires a temperature of 200 degree Celsius and the presence of a metallic catalyst. In case of industrial requirements, catalysts based on palladium, nickel, or platinum are used. Moreover, for laboratory synthesis, Raney nickel is used. One of the most common examples of this reaction is catalytic hydrogenation of ethylene to produce ethane. The equation of this reaction is, CH2=CH2 + H2 → CH3–CH3.
Hydration: With the help of this process, water is added across double bonds of alkenes. As a result, it produces alcohol. Moreover, this reaction is catalysed by either sulphuric or phosphoric acid. Additionally, hydration is used in industries to produce synthetic ethanol. The equation here is, CH2=CH2 + H2O → CH3–CH2OH. Furthermore, Mukaiyama hydration, oxymercuration–demercuration reaction, or hydroboration–oxidation reaction is used to produce alcohol from alkenes.
Halogenation: In this process, elemental chlorine or bromine is added to alkenes to produce vicinal dibromo. Moreover, the decolouration of bromine solution in water is a test to identify the presence of alkene. The equation of this reaction is, CH2=CH2 + Br2 → BrCH2–CH2Br. Additionally, related reactions like iodine number of bromine number are used as quantitative procedures of unsaturation.
Hydrohalogenation: This process helps in making haloalkanes by adding hydrogen halides like HI, HCI to alkenes. The equation here is CH3–CH=CH2 + HI → CH3–CHI−CH2–H. Moreover, in case the two carbon atoms in double bond are linked to different numbers of hydrogen atoms; thus, halogen is preferably located in carbon with fewer hydrogen substituents.
Furthermore, this process is regarded as Markovnikov’s rule. However, the use of radical initiators or any other compound can result in contrasting results. For instance, hydrobromic acid, in particular, is susceptible to producing radicals due to the presence of impurities or even atmospheric oxygen. Moreover, it moves against the principles of Markovnikov results. This hydrobromic acid reaction is, CH3–CH=CH2 + HBr → CH3–CHH–CH2–Br.
Elimination reaction
A popular alkene synthesis is by elimination reaction. This process helps gathering alkene from alcohol, alkyl halide, and others. Moreover, alcohol and alkyl halide goes through dehydrohalogenation dehydration for this purpose.
Oxidation
Oxidation of alkene is possible in many ways with assistance from various oxidising agents.
Moreover, in the existence of oxygen, alkene burns with a bright flame and creates water and carbon dioxide.
Furthermore, reaction with percarboxylix acid or catalytic oxidation process yields epoxides.
Additionally, reaction with hot and concentrated KMnO4 in an acidic solution will create carboxylic acid or ketones.
Lastly, ozonolysis in the presence of ozone helps in breaking the double bond, creating ketones and aldehydes.
The following reaction aids in determining the position of a double bond of an unknown alkene.
R1–CH=CH–R2 + O3 → R1–CHO + R2–CHO + H2O
Photooxygenation
Photosensitisers like methylene blue and light can help alkene to go through a reaction with reactive oxygen that generates photosensitiser. Some of the prominent examples of such oxygen are superoxide ion, singlet oxygen, and hydroxyl radicals.
Moreover, these photochemical intermediates are created in different types, such as Type I, Type II, and Type III, respectively. Furthermore, these reactions and processes can be controlled by opting for particular conditions. It aids in manufacturing various products. A popular example here is, [4+2]-cycloaddition of singlet oxygen along with diene like cyclopentadiene produces endoperoxide.
Application of Alkene
Alkene has an array of applications in industries. Typically, they are used as starting materials to synthesise alcohols, lacquers, detergent, plastics, etc. Following are some important industrial applications of an alkene.
Ethene is a vital organic feedstock in chemical industries. It is produced from crude oil and natural gas with the help of cracking. Moreover, ethene is used for the production of various chemical products like vinyl chloride, polyethylene, ethanol, styrene, acetaldehyde and several others.
Another product of alkene, propane, is largely used for the production of polypropylene. Additionally, various oxidation products like acrylic acid, butanol, acrylic acid ester, acrolein, glycerol, epichlorohydrin, and allyl chloride are also produced via this method.
Furthermore, butadiene, other products of alkene is primarily used for producing synthetic rubber.
Alkenes and Olefins
Even though alkenes and olefins are often used interchangeably, it is not accurate. According to IUPAC (International Union of Pure and Applied Chemistry), alkenes include every aliphatic hydrocarbon having one double bond. On the other hand, olefins have a bigger set of compounds, which includes alkenes.
Furthermore, olefins include aliphatic hydrocarbons, be it acyclic or cyclic. Moreover, they can have one or more carbon to carbon double bonds. Examples like alkene, polyenes, cycloalkens compounds exhibit more than one double bond.
However, if alkene comprises more than one double bond, then this nomenclature alters to alkadiene, alatriene, and so on. Additionally, alkadienes which are often found in fire debris, show that they are pyrolysis products of certain polymers.
Physical properties of alkene is an important chapter of chemistry. Moreover, it is an easy scoring one as well. Thus, students looking for assistance regarding this chapter can visit the official website or download our Vedantu app.
Furthermore, they can access live classes from subject experts and clear their doubts. Moreover, they can avail study material regarding physical properties of alkanes class 11 via this app with ease.
FAQs on Alkenes Properties
1. What are alkenes as per the Class 11 syllabus?
Alkenes are a class of unsaturated hydrocarbons that contain at least one carbon-carbon double bond (C=C). They are known as unsaturated because they have fewer hydrogen atoms than the maximum possible for their number of carbons. The general formula for an alkene with one double bond is CnH2n. The simplest example is ethene (C₂H₄).
2. What are the key physical properties of alkenes?
The main physical properties of alkenes are determined by their structure:
- State: The first three members (ethene, propene, butene) are gases at room temperature, the next fourteen are liquids, and higher members are solids.
- Solubility: Being nonpolar, alkenes are insoluble in water but readily dissolve in nonpolar organic solvents like benzene.
- Boiling Point: The boiling point increases with the size of the molecule (more carbon atoms). Straight-chain alkenes have higher boiling points than their branched-chain isomers.
- Polarity: Alkenes are considered weakly polar, slightly more so than alkanes, due to the presence of the pi-electron cloud in the double bond.
3. What are the major types of chemical reactions that alkenes undergo?
The presence of the electron-rich C=C double bond makes alkenes highly reactive. Their primary reactions include:
- Addition Reactions: This is the most characteristic reaction, where the double bond breaks to add atoms. Examples include hydrogenation (adding H₂), halogenation (adding Cl₂, Br₂), hydrohalogenation (adding HCl, HBr), and hydration (adding H₂O).
- Oxidation Reactions: Alkenes can be oxidised to form different products. For instance, ozonolysis breaks the double bond to form aldehydes and/or ketones, while strong oxidising agents like KMnO₄ can form glycols or carboxylic acids.
- Polymerisation: Alkene monomers join together to form long-chain polymers, such as the formation of polyethene from ethene.
4. Why are alkenes significantly more reactive than alkanes?
Alkenes are more reactive than alkanes primarily because of the nature of the carbon-carbon double bond. This double bond consists of one strong sigma (σ) bond and one weaker pi (π) bond. The pi bond is formed by the sideways overlap of p-orbitals, and its electrons are located above and below the plane of the atoms. This exposed and loosely held pi electron cloud is easily attacked by electrophiles (electron-seeking species), making alkenes susceptible to addition reactions where the pi bond breaks.
5. How does Markovnikov's rule predict the product of an addition reaction in an unsymmetrical alkene?
Markovnikov's rule provides a way to predict the outcome when an unsymmetrical reagent (like H-Br) is added to an unsymmetrical alkene (like propene, CH₃-CH=CH₂). The rule states that the negative part of the adding molecule (Br⁻) attaches to the carbon atom of the double bond that has fewer hydrogen atoms. Consequently, the positive part (H⁺) attaches to the carbon with more hydrogen atoms. For propene, this results in the formation of 2-bromopropane, not 1-bromopropane, as the major product.
6. How do cis- and trans-isomers of an alkene differ in their physical properties?
Cis- and trans-isomers (a type of geometric isomerism) have the same molecular formula but different spatial arrangements around the double bond. This difference impacts their physical properties:
- Melting Point: Trans-isomers generally have a higher melting point because their more symmetrical shape allows them to pack more efficiently into a crystal lattice, requiring more energy to break apart.
- Boiling Point & Polarity: Cis-isomers often have a higher boiling point because they are typically more polar. The bond dipoles in the cis- form do not cancel out, resulting in a net molecular dipole, which leads to stronger intermolecular forces.
7. How is the reaction with bromine water used as a chemical test for an alkene?
The reaction with bromine water (Br₂ dissolved in water) is a standard laboratory test to detect the presence of unsaturation (a C=C or C≡C bond). When an alkene is bubbled through or shaken with reddish-brown bromine water, it undergoes an addition reaction. The double bond breaks, and a bromine atom adds to each carbon, forming a colourless dibromoalkane. The rapid disappearance of the reddish-brown colour is a positive test, indicating the presence of an alkene.
8. What are some important industrial applications of alkenes?
Alkenes are fundamental starting materials in the chemical industry. Some key applications include:
- Ethene (C₂H₄): Used extensively to produce polyethylene (plastics), ethanol, vinyl chloride (for PVC), and antifreeze (ethylene glycol).
- Propene (C₃H₆): Primarily used to manufacture polypropylene (a versatile plastic), as well as other chemicals like propan-2-ol, acrylic acid, and acrylonitrile.
- Butadiene: A key monomer used in the production of synthetic rubbers like SBR (styrene-butadiene rubber).
