Alkenes Formula and First 10 Examples with Structures
Alkenes are a key group of hydrocarbons in organic chemistry and are central to many reactions, both in laboratories and industry. Understanding alkenes is important not only for board exams but also for competitive tests like NEET and JEE. This topic helps students recognize, name, and use alkenes in real-life and scientific applications.
What is Alkenes in Chemistry?
An alkene refers to a class of unsaturated hydrocarbons that contain at least one carbon-carbon double bond. They fit into the group of organic compounds called unsaturated hydrocarbons and are sometimes called olefins. This concept appears in chapters related to hydrocarbons, organic chemistry, and polymerization, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The molecular formula of alkenes is CnH2n. Alkenes consist of carbon and hydrogen atoms, where at least two carbon atoms are linked by a double bond. This double bond makes them much more reactive than alkanes (single-bonded hydrocarbons) and less saturated with hydrogen. Alkenes are classified as unsaturated hydrocarbons and include compounds such as ethene (C2H4), propene (C3H6), and butene (C4H8).
Preparation and Synthesis Methods
There are several common ways to prepare alkenes:
- Industrial Method: Cracking of large hydrocarbon molecules obtained from petroleum to form smaller alkenes like ethene and propene.
- Laboratory Methods: Dehydration of alcohols using concentrated sulfuric acid to produce alkenes, and dehydrohalogenation of alkyl halides with alcoholic KOH.
- Catalytic Methods: Passing alkanes over heated catalysts (alumina or silica) or using nickel, palladium, or platinum as catalysts for hydrogenation or dehydrogenation reactions.
Physical Properties of Alkenes
Alkenes have unique physical properties due to their double bond:
- Physical State: The first three (ethene, propene, butene) are gases; with more carbon atoms, they become liquids and then solids.
- Boiling Point: Increases with carbon chain length; higher alkenes are harder to vaporize.
- Solubility: Insoluble in water but soluble in organic solvents like benzene and ether.
- Density: Less dense than water.
- Odor: Many low molecular mass alkenes are odorless, while some larger alkenes may have a faint smell.
Chemical Properties and Reactions
Alkenes show several very important chemical reactions:
- Addition Reactions: Alkenes add hydrogen, halogens, and hydrogen halides across the double bond. For example, ethene reacts with bromine water to form dibromoethane, causing the orange color of bromine water to disappear — a standard test for unsaturation.
- Polymerization: Small alkene molecules (monomers) join to form long-chain polymers, like polyethene (polythene) and polypropene.
- Oxidation: In the presence of oxidizing agents like potassium permanganate (Baeyer’s reagent), alkenes form glycols.
- Combustion: Alkenes burn with a luminous, sometimes smoky, flame to produce carbon dioxide and water.
- Markovnikov Rule: In addition of hydrogen halides, the negative part (halide) attaches to the more substituted carbon.
Frequent Related Errors
- Confusing alkenes with alkanes (single bonds) or alkynes (triple bonds), especially in reactions.
- Forgetting that alkenes are unsaturated and thus more reactive than alkanes.
- Drawing the wrong structure for isomers or missing cis-trans (geometric) isomerism in alkenes.
- Incorrectly counting hydrogen atoms due to overlooking the general formula CnH2n.
Uses of Alkenes in Real Life
Alkenes are widely used in modern industries and everyday life:
- Production of plastics like polyethylene (from ethene) and polypropylene (from propene).
- Manufacture of ethanol, antifreeze, detergents, and synthetic fibers.
- Alkenes are the building blocks for the industrial synthesis of chemicals such as alcohols, aldehydes, and acids.
- Used in making rubber, packaging materials, ropes, and car battery cases.
Relevance in Competitive Exams
Students preparing for NEET, JEE, and Olympiads frequently encounter alkene questions, especially in reaction mechanisms, tests for unsaturation, isomerism, and naming. Alkenes are also important in understanding polymerization and in answering questions about hydrocarbons in board practicals and theory exams.
Relation with Other Chemistry Concepts
Alkenes are closely related to alkanes and alkynes; all are part of the hydrocarbon family. Understanding alkenes also helps in studying isomerism, electrophilic addition reactions, and polymerization, which are building blocks for deeper organic chemistry concepts.
Step-by-Step Reaction Example
1. Start with ethene (C2H4) and bromine (Br2) for the test of unsaturation.2. Write the balanced equation:
3. Bromine adds across the double bond, turning orange Br2 colorless.
4. Condition: Room temperature, no catalyst needed.
5. Final product: 1,2-dibromoethane.
Lab or Experimental Tips
Remember alkenes by their double bond — always draw or imagine two lines between two carbon atoms to spot them in structures. Vedantu educators recommend using colored pens for double bonds during note-making and using bromine water in lab tests for easy confirmation of alkenes.
Try This Yourself
- List the first five alkenes, their formulas, and draw their structures.
- Name an alkene with four carbon atoms and a terminal double bond.
- Explain why alkenes undergo addition reactions more easily than substitution reactions.
Final Wrap-Up
We explored alkenes—their definition, structure, synthesis, physical and chemical properties, uses, and connections in chemistry. Alkenes are both a testing ground for exam questions and a gateway to industry applications, especially in plastics and synthetic materials. For more support and tricks on mastering organic chemistry topics like alkenes, check Vedantu’s live classes and exam prep resources.
See also: Alkenes Properties | Nomenclature of Alkene, Alkane and Alkyne | Isomerism | Polymerization
FAQs on Alkenes in Chemistry: Concept, Structure, Reactions & Examples
1. What are alkenes and what is their general formula?
Alkenes are a class of unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). Because of this double bond, they have two fewer hydrogen atoms than the corresponding alkane. Their general formula is CnH2n, where 'n' is the number of carbon atoms (n ≥ 2).
2. How are alkenes structurally different from alkanes and alkynes?
The primary structural difference lies in the type of carbon-carbon bonds they contain:
- Alkanes are saturated, possessing only single bonds (C-C).
- Alkenes are unsaturated, featuring at least one double bond (C=C).
- Alkynes are also unsaturated, characterised by at least one triple bond (C≡C).
3. What are the first three members of the alkene series and their structures?
The first three and most common alkenes are:
- Ethene (C2H4): The simplest alkene, with the structure CH2=CH2.
- Propene (C3H6): The next member, with the structure CH2=CH-CH3.
- Butene (C4H8): This exists as isomers, with the simplest being But-1-ene (CH2=CH-CH2-CH3).
4. What are the key physical properties of alkenes?
The physical properties of alkenes are similar to alkanes. Key properties include:
- State: The first few members (ethene, propene, butene) are gases at room temperature, intermediate ones are liquids, and higher alkenes are waxy solids.
- Solubility: They are insoluble in water but soluble in non-polar organic solvents like benzene or ether.
- Boiling Point: The boiling point increases with the increase in molecular mass or chain length.
- Density: They are less dense than water.
5. What are the main types of chemical reactions that alkenes undergo?
Due to the presence of the electron-rich pi (π) bond, alkenes primarily undergo electrophilic addition reactions. In these reactions, the double bond breaks to form two new single bonds. Common examples include:
- Hydrogenation: Addition of hydrogen (H2).
- Halogenation: Addition of halogens (e.g., Cl2, Br2).
- Hydrohalogenation: Addition of hydrogen halides (e.g., HBr, HCl).
- Hydration: Addition of water (H2O).
- Polymerisation: Joining of many monomer units.
6. What is the significance of the pi (π) bond in determining the structure and reactivity of alkenes?
The pi (π) bond is crucial because it is formed by the sideways overlap of unhybridized p-orbitals, placing its electron cloud above and below the plane of the sigma (σ) bond. This makes the π-electrons exposed and loosely held. Consequently, the π-bond is weaker than the σ-bond and acts as a source of electrons, making it susceptible to attack by electrophiles (electron-seeking species). This explains why addition reactions are the characteristic reactions of alkenes.
7. Why do alkenes exhibit geometric (cis-trans) isomerism while alkanes do not?
Geometric isomerism in alkenes arises due to the restricted rotation around the carbon-carbon double bond. Unlike a single bond (as in alkanes) which allows free rotation, the double bond locks the attached atoms or groups into fixed positions. If each carbon atom of the double bond is attached to two different groups, two distinct spatial arrangements are possible: cis (similar groups on the same side) and trans (similar groups on opposite sides).
8. What is Markovnikov's rule and how does it predict the outcome of addition reactions in unsymmetrical alkenes?
Markovnikov's rule provides a way to predict which product will be major when an unsymmetrical reagent (like HBr) adds to an unsymmetrical alkene (like propene). The rule states that the negative part of the addendum (the molecule being added, e.g., Br⁻) attaches to the carbon atom of the double bond that has fewer hydrogen atoms. This is because this pathway forms a more stable carbocation intermediate.
9. Under what conditions is Markovnikov's rule not followed in alkene reactions, and why?
Markovnikov's rule is not followed during the addition of HBr to an alkene in the presence of peroxide (like benzoyl peroxide). This is known as the peroxide effect or anti-Markovnikov addition. The reason for this reversal is a change in the reaction mechanism from an electrophilic addition to a free radical mechanism. In this mechanism, the bromine free radical adds first to form the more stable carbon free radical, leading to the bromine attaching to the carbon with more hydrogens.
10. How does the substitution of alkyl groups on the double bond affect the stability of an alkene?
The stability of an alkene increases as the number of alkyl groups attached to the double-bonded carbons increases. This is due to hyperconjugation, where electrons from the C-H bonds of the alkyl groups help delocalise and stabilise the π-system. The general order of stability is: Tetra-substituted > Tri-substituted > Di-substituted > Mono-substituted > Ethene. This increased stability also influences their heat of hydrogenation and reactivity.
11. How can one chemically distinguish between an alkene and an alkane in a laboratory test?
A simple and common test is the bromine water test. When an alkene is bubbled through reddish-brown bromine water, the alkene undergoes an addition reaction across its double bond. This consumes the bromine, causing the solution to be rapidly decolourised. Alkanes, being saturated and unreactive towards bromine under these conditions, will not decolourise the bromine water.
12. What are some important industrial applications and examples of alkenes in daily life?
Alkenes are fundamental starting materials in the chemical industry. Their main applications include:
- Polymer Production: Ethene is polymerised to make polyethene (plastic bags, bottles), and propene is used to make polypropene (containers, carpets, car parts).
- Synthesis of Alcohols: The hydration of ethene produces ethanol, an important industrial solvent and fuel additive.
- Artificial Ripening of Fruits: Ethene gas is a plant hormone used commercially to ripen fruits like bananas and tomatoes.
- Production of Antifreeze: Ethene is used to synthesise ethylene glycol, a major component of antifreeze.
