Physical and Chemical Properties of Alkenes with Structure and Reaction Examples
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 and Their Physical and Chemical Properties
1. What are the properties of alkenes?
Alkenes are unsaturated hydrocarbons that contain at least one carbon–carbon double bond (C=C) and follow the general formula CnH2n.
- They are nonpolar and insoluble in water but soluble in organic solvents.
- They have lower boiling points than corresponding alcohols due to weak intermolecular forces.
- They undergo addition reactions because of the reactive C=C bond.
- They burn with a more luminous, sooty flame than alkanes.
2. What is the general formula of alkenes?
The general formula of open-chain alkenes is CnH2n, where n ≥ 2.
- This formula shows they have two fewer hydrogen atoms than alkanes (CnH2n+2).
- Example: Ethene is C2H4 and propene is C3H6.
- The formula applies only to non-cyclic alkenes with one double bond.
3. Why are alkenes more reactive than alkanes?
Alkenes are more reactive than alkanes because the π (pi) bond in the C=C double bond is weaker and easily broken.
- A double bond consists of one σ bond and one π bond.
- The π bond has higher electron density, attracting electrophiles.
- This makes alkenes undergo addition reactions such as hydrogenation and halogenation.
4. What type of reactions do alkenes undergo?
Alkenes mainly undergo addition reactions due to the presence of a carbon–carbon double bond.
- Hydrogenation: C2H4(g) + H2(g) → C2H6(g)
- Halogenation: C2H4(g) + Br2(l) → C2H4Br2(l)
- Hydration: C2H4(g) + H2O(g) → C2H5OH(l)
- They can also undergo polymerization and combustion.
5. How do you test for the presence of an alkene?
Alkenes can be identified by the bromine water test, where the orange solution turns colorless.
- Reaction: C2H4(g) + Br2(aq) → C2H4Br2(aq)
- The decolorization occurs because bromine adds across the double bond.
- Alkanes do not decolorize bromine water under normal conditions.
6. What is the difference between alkanes and alkenes?
The main difference between alkanes and alkenes is that alkanes are saturated hydrocarbons with only single bonds, while alkenes are unsaturated hydrocarbons with at least one C=C bond.
- Alkanes follow CnH2n+2; alkenes follow CnH2n.
- Alkanes undergo substitution reactions; alkenes undergo addition reactions.
- Alkenes are generally more chemically reactive.
7. What are the physical properties of alkenes?
The physical properties of alkenes depend on molecular size but are generally similar to alkanes.
- Lower alkenes (C2–C4) are gases at room temperature.
- They are nonpolar and insoluble in water.
- Boiling points increase with increasing molar mass.
- They are less dense than water.
8. What is hydrogenation of alkenes?
Hydrogenation of alkenes is the addition of hydrogen (H2) across the C=C double bond to form an alkane.
- It requires a metal catalyst such as Ni, Pt, or Pd.
- Example: C2H4(g) + H2(g) → C2H6(g)
- This reaction is used industrially to convert vegetable oils into margarine.
9. Do alkenes show isomerism?
Yes, alkenes show both structural isomerism and geometrical (cis–trans) isomerism.
- Structural isomerism occurs due to different positions of the double bond (e.g., but-1-ene and but-2-ene).
- Geometrical isomerism occurs because rotation around the C=C bond is restricted.
- Example: cis-but-2-ene and trans-but-2-ene.
10. How do alkenes undergo combustion?
Alkenes undergo combustion in oxygen to produce carbon dioxide and water in a complete reaction.
- Example (complete combustion of ethene): C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l)
- Incomplete combustion forms CO and soot.
- Alkenes burn with a more luminous flame than alkanes due to higher carbon content.
