Alkene - Formula, Definition, Classification, Properties & Uses of Alkene with Examples & Videos

Edited By Team Careers360 | Updated on Jul 02, 2025 05:13 PM IST

Unsaturated compounds are referred to as alkenes. Alkenes, sometimes called olefins, are organic substances made up of carbon and hydrogen atoms with one or more double bonds connecting the carbon atoms in their chemical structure. An alkene's functional group is a carbon-carbon double bond. Because just carbon and hydrogen atoms make up hydrocarbons, alkenes are referred to as being unsaturated. This is because alkenes have fewer hydrogen atoms per carbon atom than is theoretically conceivable. Alkenes are more reactive than alkanes because of the double bond present in them. Ethene, also called ethylene, is an example of an alkene chemical.

This Story also Contains

Overview
Classification Of Alkenes
Structures Of Alkenes
Physical Properties Of Alkenes
Chemical Properties Of Alkenes
Uses Of Alkenes
Conclusion

Alkene - Formula, Definition, Classification, Properties & Uses of Alkene with Examples & Videos

Overview

A group of hydrocarbons are called alkenes. There is at least one carbon-carbon double bond among every member of this family. Instead of being found in significant amounts in crude oil, alkenes are discovered by cracking alkanes. The most pure alkene is ethene, and alkenes play a significant role in the chemical industry. Although ethene cannot be used as fuel on its own, it is utilised to create compounds such as plastics and other substances that can. Alkenes' chemical and physical characteristics, as well as their applications, will be covered in this article.

Classification Of Alkenes

The stability of the double bond depends on the alkyl groups attached to the alkenes' sp2 hybridised carbon atoms. The quantity of alkyl groups attached to the sp2 hybridised carbon atoms has a significant impact on the chemical reactivity of alkenes. As a result, it is helpful to categorise alkenes according to the quantity of alkyl groups connected to the C=C structural unit. The degree of substitution is the name of this characteristic.

A monosubstituted alkene is one in which the sp2 hybridised carbon atom of the double bond is joined to a single alkyl group. Another name for an alkene is a terminal alkene, which refers to an alkene whose double bond is at the end of the carbon atom chain.

Alkenes are classified as disubstituted, trisubstituted, or tetrasubstituted depending on how many alkyl groups there are attached to the double bond's carbon atoms.

Structures Of Alkenes

The only C-C single bonds found in an alkane make it the most basic hydrocarbon. The number of H atoms in chain alkanes reaches its maximum level, and the chain alkane fits the general formula of CnH2n+2 (n: positive integer).

Physical Properties Of Alkenes

Unsaturated hydrocarbons come in the shape of alkenes. Alkenes are flammable, colourless, and non-polar substances that have many physical characteristics with alkanes. Alkenes can have isomers because of their physical makeup. As they frequently have same or very similar boiling temperatures, isomeric alkenes are challenging to distinguish by boiling. Additionally, they exist in the gaseous, liquid, and solid states at room temperature, which is due to the carbon-carbon double bond. Because of their weak van der Waal forces, alkenes are lighter than water and insoluble in water. However, when alkenes come into contact with organic solvents, additional van der Waal forces form, making the alkenes soluble. Alkenes' boiling points are directly inversely correlated with their molecular structures; the longer the molecular chain, the higher the boiling point. Alkenes' boiling points are directly inversely correlated with their molecular structures; the longer the molecular chain, the higher the boiling point.

Chemical Properties Of Alkenes

Because they are unsaturated molecules, alkenes are very reactive chemically. Alkenes and alkanes are both flammable, but alkenes are often far more reactive than alkanes because the majority of the chemical processes inside of alkenes take place at the carbon-carbon double bonds. Alkenes typically undergo three main chemical reactions: first, when hydrogen is added, they become alkanes; second, when halogens are added, alkenes produce vicinal dihalides; and third, when hydrogen halides are added, the hydrogen attaches at the carbon-carbon double bond to the atom that is attached to more hydrogen atoms, and the halide ion attaches to the atom that is attached to fewer hydrogen atoms. The Markovnikov rule states that.

Uses Of Alkenes

Alkenes are very helpful to people, and they are utilised in many different industries to make a wide variety of goods. First, and maybe most significantly, they are used to make polythene. The plastic that is used to make several products, including plastic cups, plates, bowls, chairs, etc., is also made from polythene, which is also used to make polythene bags. Alkenes also produce ethane-1,2-diol, a substance utilised in automobile radiators as an anti-freezing agent. Alkenes and polystyrene are both used in the production of battery covers and radiator components for automobiles. Alkenes are used to create acrylic fibres. Alkenes are used to create polypropene, which is then utilised to create a plastic that is used to package a variety of goods as well as to create ropes. Propanol and acetone, sometimes known as nail polish remover and acetone, respectively, are made from alkenes and propanol, respectively. Alkenes are also utilised to create ethanol, or drinking alcohol, which is produced in the production of synthetic fibres.

Conclusion

Due to the many products that may be made utilising alkenes, this family of hydrocarbons is immensely significant and valuable to humans. Alkanes must be cracked in order to find them because they are not widely distributed. Since they always contain a carbon-carbon double bond, this greatly influences their chemical characteristics. Because of the double bond in the carbon atom, they have physical characteristics that are quite similar to those of alkanes and can exist at room temperature in solid, liquid, and gaseous forms.

Frequently Asked Questions (FAQs)

1. What are the uses of alkenes?

Alkenes have a variety of uses in manufacturing. They are used as the primary building blocks in the production of fuels, alcohols, polymers, lacquers, and detergents. The three most important alkenes for the chemical sector are ethene, propene, and 1,3-butadiene.

2. Where can you find alkene?

Alkenes and alkanes share several physical characteristics, such as being colourless, nonpolar, and flammable. At room temperature, alkenes can be found in all three states: solids, liquids, and gases. Alkenes and alkanes both have melting and boiling points, however, cis-alkene isomers have lower melting points than trans isomers.

3. Are boiling points higher for alkenes?

The intermolecular mass increases the boiling point by a factor of 2. Alkenes' intermolecular forces increase as a function of the size of the molecules. Each time, the alkene has a boiling point that is marginally lower than the matching alkane.

4. Can you utilise alkenes as fuel?

Alkenes burn easily compared to alkanes, producing water and carbon dioxide for example if combustion is complete. They are NOT utilised as fuels, nevertheless, for two reasons. They are far too precious to use as is; instead, they are used to manufacture plastics, anti-freeze, and many other important compounds.

5. What is the difference between cis and trans isomers in alkenes?

In cis isomers, the two larger groups are on the same side of the double bond, while in trans isomers, they are on opposite sides. This difference in spatial arrangement can lead to significant differences in physical and chemical properties between the isomers.

6. How does the reactivity of alkenes compare to that of alkanes?

Alkenes are generally more reactive than alkanes due to the presence of the carbon-carbon double bond. This pi bond is a region of high electron density, making alkenes prone to addition reactions where the double bond is broken and new single bonds are formed.

7. What is the most characteristic reaction type for alkenes?

The most characteristic reaction type for alkenes is addition reactions. In these reactions, a reagent adds across the double bond, breaking the pi bond and forming two new single bonds. Examples include hydrogenation, halogenation, and hydration.

8. How does electrophilic addition work in alkene reactions?

Electrophilic addition in alkenes involves an electrophile (electron-seeking species) attacking the electron-rich double bond. This forms a carbocation intermediate, which then reacts with a nucleophile to complete the addition. This mechanism explains the regioselectivity observed in many alkene addition reactions.

9. What is Markovnikov's rule and how does it apply to alkene reactions?

Markovnikov's rule states that in the addition of HX (where X is a halogen) to an unsymmetrical alkene, the hydrogen atom adds to the carbon with more hydrogen substituents. This rule helps predict the major product in electrophilic addition reactions and is based on the stability of carbocation intermediates.

10. What is the difference between E and Z nomenclature for alkenes?

E and Z nomenclature is used to describe the configuration of groups around a double bond in alkenes. 'E' (entgegen) means the higher priority groups are on opposite sides of the double bond, while 'Z' (zusammen) means they are on the same side. This system is more precise than cis/trans nomenclature, especially for complex molecules.

11. How do cycloalkenes differ from linear alkenes in their properties and reactions?

Cycloalkenes, which have a double bond within a ring structure, can exhibit different reactivity compared to linear alkenes. Small cycloalkenes (3-4 carbons) are more strained and thus more reactive. Larger cycloalkenes may have restricted rotation that affects their stereochemistry in reactions. Some reactions, like ozonolysis, can lead to ring-opening in cycloalkenes.

12. What is the difference between addition and elimination reactions in alkenes?

Addition reactions involve adding atoms or groups to the alkene, breaking the double bond. Elimination reactions do the opposite, removing atoms or groups to form a double bond. Alkenes typically undergo addition reactions, while alkenes are often the products of elimination reactions.

13. What is the concept of resonance in conjugated alkene systems?

Resonance in conjugated alkene systems refers to the delocalization of electrons across multiple pi bonds. This electron delocalization stabilizes the molecule and can lead to unique reactivity. For example, in 1,3-butadiene, the electrons are delocalized over all four carbons, allowing for both 1,2- and 1,4-addition reactions.

14. How does the presence of electron-withdrawing or electron-donating groups affect alkene reactivity?

Electron-donating groups (like alkyl groups) increase electron density at the double bond, making the alkene more reactive towards electrophiles. Electron-withdrawing groups (like -NO2 or -COOH) decrease electron density, making the alkene less reactive towards electrophiles but more reactive towards nucleophiles.

15. What are alkenes and how do they differ from alkanes?

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. They differ from alkanes, which have only single bonds between carbon atoms. This double bond gives alkenes unique chemical properties and reactivity.

16. What is the importance of alkenes in the petrochemical industry?

Alkenes, particularly ethene and propene, are crucial in the petrochemical industry. They serve as starting materials for producing plastics, synthetic rubbers, detergents, and many other chemicals. Their reactivity makes them versatile building blocks for a wide range of products.

17. What is meant by the term "alkene metathesis" and why is it significant?

Alkene metathesis is a reaction where the carbon-carbon double bonds of two alkenes are broken and then reformed in a different arrangement. This reaction, catalyzed by certain transition metal complexes, is significant in organic synthesis as it allows for the creation of new carbon-carbon double bonds in ways not possible with other methods.

18. How does UV spectroscopy help in identifying and characterizing alkenes?

UV spectroscopy is useful for identifying alkenes because the pi electrons in the double bond can absorb UV light, causing electronic transitions. The wavelength of absorption depends on the extent of conjugation in the alkene system. More conjugated systems absorb at longer wavelengths, allowing for structural characterization.

19. What is the Diels-Alder reaction and why is it important in alkene chemistry?

The Diels-Alder reaction is a cycloaddition reaction between a conjugated diene and an alkene (called a dienophile). It forms a six-membered ring and is important because it creates two new carbon-carbon bonds and up to four new stereogenic centers in one step. This reaction is widely used in the synthesis of complex organic molecules.

20. How does the polarity of alkenes compare to that of alkanes?

Alkenes are generally more polar than alkanes due to the presence of the pi bond, which creates a region of higher electron density. However, symmetrical alkenes (like ethene) are non-polar overall. Unsymmetrical alkenes may have a small dipole moment due to uneven electron distribution.

21. How does the stability of alkenes relate to their structure?

The stability of alkenes generally increases with increasing substitution at the double bond carbons. This trend is: tetrasubstituted > trisubstituted > disubstituted > monosubstituted. This is due to hyperconjugation and the electron-donating effects of alkyl groups.

22. What is the hybridization of carbon atoms in alkenes?

In alkenes, the carbon atoms involved in the double bond are sp2 hybridized. This means they have three sp2 hybrid orbitals in a trigonal planar arrangement and one unhybridized p orbital perpendicular to this plane. The p orbitals from adjacent carbons overlap to form the pi bond of the double bond.

23. What is the heat of hydrogenation and how does it relate to alkene stability?

The heat of hydrogenation is the energy released when an alkene is hydrogenated to an alkane. Less stable alkenes release more energy upon hydrogenation. This property can be used to compare the relative stabilities of different alkene isomers.

24. How does the reactivity of alkenes change with increasing chain length?

Generally, the reactivity of alkenes decreases slightly with increasing chain length. This is due to increased steric hindrance around the double bond in larger molecules, which can make it harder for reagents to approach. However, the effect is usually small compared to the influence of substitution patterns.

25. How do you write the general formula for alkenes?

The general formula for alkenes is CnH2n, where n is the number of carbon atoms. This formula reflects that alkenes have two fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms due to the presence of a double bond.

26. How does the presence of a double bond affect the geometry of alkene molecules?

The carbon-carbon double bond in alkenes creates a planar structure around the double bond. The two carbon atoms involved in the double bond and the four atoms directly attached to them all lie in the same plane, resulting in a rigid, fixed geometry.

27. Why are alkenes considered unsaturated hydrocarbons?

Alkenes are considered unsaturated because they contain at least one carbon-carbon double bond. This means they have fewer hydrogen atoms than the maximum possible for that number of carbon atoms, leaving them "unsaturated" with hydrogen.

28. How do alkenes exhibit geometric isomerism?

Alkenes can exhibit geometric isomerism (also called cis-trans isomerism) when there are two different groups attached to each carbon of the double bond. This is due to the restricted rotation around the double bond, resulting in cis (same side) and trans (opposite side) isomers.

29. What is the IUPAC naming system for alkenes, and how does it differ from alkane naming?

The IUPAC naming system for alkenes is similar to alkanes, but with two key differences: 1) The suffix "-ene" is used instead of "-ane", and 2) The position of the double bond is indicated by the lowest possible number in the carbon chain. For example, CH3-CH=CH-CH3 is named 2-butene.

30. What is hydrogenation of alkenes and why is it important?

Hydrogenation is the addition of hydrogen to an alkene, converting it to an alkane. This reaction is typically catalyzed by metals like platinum or palladium. It's important in industry for producing saturated fats from unsaturated oils and in organic synthesis for reducing double bonds.

31. How do alkenes undergo polymerization reactions?

Alkenes can undergo polymerization reactions where many alkene molecules (monomers) join together to form a large molecule (polymer). This can occur through addition polymerization, where the double bonds open and link monomers together in a chain reaction.

32. What is the significance of conjugated dienes in alkene chemistry?

Conjugated dienes are systems with two double bonds separated by a single bond. They exhibit special reactivity due to the delocalization of electrons across the system. This leads to unique reactions like 1,4-addition in Diels-Alder reactions, which are important in organic synthesis.

33. How does ozonolysis help in determining the structure of alkenes?

Ozonolysis is a reaction where ozone (O3) cleaves the carbon-carbon double bond in alkenes. The resulting ozonide intermediate is then reduced to form carbonyl compounds (aldehydes or ketones). By identifying these carbonyl products, the original structure of the alkene can be deduced, making it a valuable tool in structure determination.

34. What is anti-Markovnikov addition and when does it occur?

Anti-Markovnikov addition is when the major product of an addition reaction to an alkene is opposite to what Markovnikov's rule predicts. This occurs in reactions like hydroboration-oxidation, where the boron atom initially adds to the less substituted carbon due to steric factors.

35. How do alkenes react with halogens, and what is the mechanism called?

Alkenes react with halogens (like Br2 or Cl2) in an addition reaction called halogenation. The mechanism is electrophilic addition, where the halogen molecule polarizes and one atom attacks the double bond, forming a cyclic halonium ion intermediate before the other halogen atom completes the addition.

36. How do alkenes react with water, and what is this reaction called?

Alkenes react with water in a process called hydration, forming alcohols. This reaction typically requires an acid catalyst and follows Markovnikov's rule. The overall reaction adds H and OH across the double bond, with the OH group attaching to the more substituted carbon.

37. How do alkenes participate in free radical reactions?

Alkenes can undergo free radical additions, where a radical species adds to the double bond. This can lead to polymerization or, in the presence of HBr and peroxides, anti-Markovnikov addition. The mechanism involves the formation of a carbon-centered radical intermediate.

38. What is epoxidation of alkenes and why is it important?

Epoxidation is the reaction of an alkene with a peroxy acid to form an epoxide (a cyclic ether). This reaction is stereospecific and can create chiral centers. Epoxides are important intermediates in organic synthesis due to their ability to undergo ring-opening reactions with various nucleophiles.

39. How do alkenes react with boron compounds, and what makes this reaction unique?

Alkenes react with borane (BH3) in a process called hydroboration. This reaction is unique because it proceeds with anti-Markovnikov regioselectivity. The resulting organoborane can be oxidized to form an alcohol, providing a method for anti-Markovnikov hydration of alkenes.

40. How do alkenes behave in electrophilic aromatic substitution reactions?

While alkenes themselves don't undergo electrophilic aromatic substitution, alkenes attached to aromatic rings (like styrene) can participate in these reactions. The alkene group is strongly activating and ortho/para directing in electrophilic aromatic substitution due to its ability to stabilize the intermediate carbocation through resonance.

41. What is the mechanism of alkene hydrohalogenation and how does it follow Markovnikov's rule?

Hydrohalogenation of alkenes (addition of HX) follows an electrophilic addition mechanism. The proton (H+) from HX acts as an electrophile, attacking the double bond to form a carbocation intermediate. The halide ion (X-) then attacks the more stable carbocation, resulting in Markovnikov orientation (X on the more substituted carbon).

42. How do alkenes participate in pericyclic reactions?

Alkenes can participate in various pericyclic reactions, including cycloadditions (like Diels-Alder reactions), electrocyclic reactions, and sigmatropic rearrangements. These reactions involve the concerted movement of electrons in a cyclic transition state and often follow specific stereochemical rules based on orbital symmetry.

43. What is the importance of alkenes in the production of polymers?

Alkenes are crucial in polymer production as they serve as monomers for many important plastics. For example, ethene polymerizes to form polyethylene, propene forms polypropylene, and styrene (an aryl alkene) forms polystyrene. The double bond in alkenes allows for chain-growth polymerization through addition reactions.

44. How do alkenes react with carbenes, and what products are formed?

Alkenes react with carbenes (divalent carbon species) in a cycloaddition reaction to form cyclopropanes. This reaction is stereospecific, meaning the stereochemistry of the alkene is preserved in the product. Carbene additions are useful for creating three-membered rings in organic synthesis.

45. What is the difference between thermodynamic and kinetic control in alkene reactions?

Thermodynamic control leads to the formation of the most stable product, while kinetic control leads to the fastest-forming product. In alkene reactions, this can affect product distribution. For example, in elimination reactions, thermodynamic control often favors the more substituted alkene, while kinetic control may favor the less substituted one.

46. How does the presence of a double bond affect the acidity of nearby hydrogen atoms?

The presence of a double bond increases the acidity of adjacent hydrogen atoms (allylic hydrogens). This is because the resulting allylic anion is stabilized by resonance, delocalizing the negative charge over multiple atoms. This principle is important in many organic reactions, including allylic halogenation.

47. What is the role of alkenes in the synthesis of organometallic compounds?

Alkenes can coordinate to metal centers, forming organometallic complexes. This coordination can activate the alkene towards various transformations. For example, in hydroformylation, an alkene coordinates to a metal catalyst before undergoing reaction with CO and H2 to form aldehydes.

48. How do alkenes behave in radical addition reactions?

Alkenes can undergo radical addition reactions, where a radical species adds to the double bond. This often leads to anti-Markovnikov products. An important example is the addition of HBr to alkenes in the presence of peroxides, which proceeds through a radical mechanism rather than the usual electrophilic addition.

49. What is the significance of alkenes in biological systems?

In biological systems, alkenes are found in many important molecules. For example, unsaturated fatty acids contain alkene groups, which affect their physical properties and biological functions. Many terpenes, which are important in plant biology and have various medicinal properties, also contain alkene functionalities.

50. How do alkenes participate in cross-metathesis reactions?

In cross-metathesis, two different alkenes exchange groups around their double bonds. This reaction, catalyzed by specific metal complexes, allows for the synthesis of new alkenes that might be difficult to prepare by other means. It's particularly useful in organic synthesis and polymer chemistry.