🌟🔥 Alkenes Complete Notes 2026 – Ultimate Guide for MDcat & Chemistry Lovers! 🌿🧪

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🌟🔥 Alkenes Complete Notes 2026 – Ultimate Guide for MDcat & Chemistry Lovers! 🌿🧪

📌 Topics Covered:

📘Definition & General Formula

📘Nomenclature of Alkenes (IUPAC rules)

📘Isomerism (Structural & Geometrical/Cis-Trans)

📘Physical Properties

📘Chemical Reactions (Addition, Polymerization, Oxidation)

📘Special Reactions (Ozonolysis, Markovnikov Addition)

📘Uses of Alkenes

📘Exam & Revision Tips

📘Summary Table

🌀🌟 Chemistry of Alkenes (Some Basics Topics)

📘Definition of Alkene

Alkenes are a class of aliphatic unsaturated hydrocarbons that contains one or more carbon-carbon double bonds (pi-bond) in the chain. Alkenes have two hydrogen atoms less than the corresponding alkanes. The functional group of alkene is olefinic double bond (i.e. >C=C<).

📘Reason for Calling Alkenes as Olefins

Alkenes are commonly called Olefins, a term derived from Latin (oleum and affins) means oil forming. The name olefins is assigned to alkene because of some early discovered alkenes were oil like (and higher members of alkenes are present in fats). [Another reason for this this name is due to the formation of Oily products by the combination of alkene with reagents like halogens and halogen acids. e.g. ethene reacts with chlorine producing an oily product ethylene dichloride (C₂H₄Cl₂) which is also known as Dutch-Liquid].

The name olefins is assigned to alkene because of following reasons:

(i) Higher members of alkenes are present in fats.

(ii) Oily products are often obtained by the combination of alkene with reagents like halogens and halogen acids.

e.g.

Ethene reacts with chlorine producing an oily product ethylene dichloride (C₂H₄Cl₂) which is also known as Dutch-Liquid.

📘Characteristics of Alkenes

1. In alkenes, at least two carbon atoms are sp²-hybridized whereas rest of the carbon atoms may be sp³-hybridized.

2. They form homologous series with general formula CₙH₂ₙ [n = 2–∞]. Alkenes and cylcoalkanes are functional or ring-chain isomers.

3. They are highly reactive.

4. They undergo addition reaction and oxidation reactions.

5. They can add one molecule (two atoms) to change double bond into single bond.

📘Isomerism in Alkenes

⚛️a) Structural Isomerism

🌟Position isomerism: Double bond in different positions.

Example: But-1-ene & But-2-ene

🌟Chain isomerism: Different carbon skeletons.

Example: 2-Methylpropene & But-1-ene

⚛️b) Geometrical (Cis-Trans) Isomerism

Occurs due to restricted rotation around C=C.

Cis: Similar groups on the same side

Trans: Similar groups on opposite sides

Example: Cis-But-2-ene vs Trans-But-2-ene

📘Physical Properties of Ethene

1. It is a colourless gas with pungent smell.

2. It is slightly sweet in taste.

3. It is almost insoluble in water but readily soluble in organic solvents such as alcohol and ether.

4. Its density is almost equal to that of air.

5. It boils at –105°C.

6. When inhaled, it causes unconsciousness acting as anaesthetic.

📘Uses of Ethene

It is used:

1. For the manufacture of polythene (a plastic).

2. As a general anaesthetic.

3. In the preparation of war gases like mustard gas.

4. For welding and cutting of metals.

5. To prepare ethylene oxide, ethyl halides, ethyl alcohol and glycol.

📘Nomenclature of Alkenes

IUPAC Rules:

Find the longest chain containing the double bond.

Number the chain from the end nearest the double bond.

Name substituents and their positions.

Use the suffix “-ene”.

Examples:

CH₂=CH–CH₃ → Propene

CH₃–CH=CH–CH₃ → But-2-ene

📘Common names of alkenes

1. The carbon atoms sharing the double bond can be referred to as the "vinyl carbons".

2. The carbon atoms adjacent to the vinyl carbon atoms are called "allylic carbons".

3. These carbon atoms have unique reactivity because of the potential for interaction with the pi bond.

Overall, common names remove the -ane suffix and add -ylene. There are a couple of unique ones like ethenyl's common name is vinyl and 2-propenyl's common name is allyl that need to be memorized.

1. vinyl substituent H₂C=CH-

2. allyl substituent H₂C=CH-CH₂-

3. allene molecule H₂C=C=CH₂

4. isoprene is shown below

📘Structure of alkene (Hybrid Orbital Structure of Ethene)

In alkene, doubly bonded carbon atoms are sp²–hybridized, each carbon atom is trigonally bonded with hydrogen atoms and other carbon atoms.

Ethene is composed of two carbon atoms and four hydrogen atoms (C₂H₄). Each carbon atom in ethene is sp²–hybridized containing three sp³–hybrid orbitals containing one electron each which are arranged in a trigonal fashion at the mutual angle of 120° and one unhybrid 2pz orbital which lies perpendicularly to the sp²-hybrid orbitals.

two out of three sp²-hybrid orbitals of each carbon atom overlaps linearly with 1s orbital of two hydrogen atoms forming four C–H s-bonds through sp²-s sigma bonding. The remaining third sp²-hybrid orbital of each carbon atom overlaps linearly with each other to form a C–C s-bond through sp²-sp² sigma bonding, giving trigonal shape to ethene. The unhybrid 2pz orbitals of each carbon undergoes lateral overlapping to form a pi bond between two carbon atoms.

The molecule of ethene consists of two trigons and has total five s-bonds. Each C–C bond length is 134 pm (1.34Å) and each C–H bond length is 109 pm and the bond angle between carbon, carbon and hydrogen is 120°.

📘Physical Properties of Alkenes

Alkenes are non-polar hydrocarbons. The dominant intermolecular forces shared by alkenes are the London dispersion forces. These interactions are weak and temporary, so they are easily disrupted. In general, the physical properties of alkenes are similar to those of alkanes because aIkanes are also subject only to van der Waals attractive forces. Geometrical isomers (cis and trans) behave differently because of their geometry.

1. Physical State

The physical states reflect the weak attractive forces between molecules.

The first three members of the alkene group (Ethene, propene, and butene) are gaseous in nature, the next fourteen members (alkenes with 5 to 14 carbons) are liquids and the remaining alkenes (alkenes with 15 carbons or more) are solids.

2. Colour and Odour

They are colourless and odourless in nature. However, ethene is an exception because it is a colourless gas but has a faintly sweet odour.

3. Solubility

The alkenes are insoluble in water due to their non-polar characteristics. But they are completely soluble in non-polar solvents such as benzene, ligroin, etc.

4. Boiling Point

The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. The boiling points of the compounds increase as the number of carbon atoms in the compound increases. The boiling point of straight-chain alkenes is more that branched-chain alkenes just as in alkanes.

Boiling points of alkenes depend on more molecular mass (chain length). The more intermolecular mass is added, the higher the boiling point. Intermolecular forces of alkenes get stronger with increase in the size of the molecules. In each case, the alkene has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are van der Waal’s dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains.

5. Melting Point

Melting points of alkenes depends on the packaging of the molecules so the stereochemistry of the carbon-carbon double bond has a strong influence on the relative melting points.

Alkenes have similar melting points to that of alkanes, however, in cis isomers molecules are package in a U-bending shape, therefore, will display a lower melting points to that of the trans isomers. This effect is notable when comparing the melting points of fats and oils. The difference in the melting points is strongly influenced by the long hydrocarbon tails. Oils have a greater number of cis double bonds and exist as liquids at room temperature. Whereas, fats are primarily saturated and exist as solids at room temperature.

6. Density

Alkenes are less dense than water with most densities in the range of 0.6 to 0.7 g/mL. Alkenes float on top of water.

7. Polarity

Alkenes are weakly polar just like alkanes but are slightly more reactive than alkanes due to the presence of double bonds. The π electrons which make up the double bonds can easily be removed or added as they are weakly held. Hence, the dipole moments exhibited by alkenes are more than alkanes.

The polarity depends upon the functional group attached to the compounds and the chemical structures.

📘Relative Stability of Alkenes

There are three factors that influence alkene stability:

1. Degree of Substitution

More highly alkylated alkenes are more stable. Hence stability of alkenes decreases in the following order:

Tetra-substituted alkene > tri- substituted alkene > di-substituted alkene > mono- substituted alkene

2. Stereochemistry

Trans alkenes are more stable than cis alkenes due to reduced steric interactions when R groups are on opposite sides of the double bond.

3. Conjugation

Conjugated alkenes with alternate double bonds are more stable than isolated alkenes. e.g. 1,3-butadiene is more stable than 1,4-butadiene.

Conjugation

The word “conjugation” is derived from a Latin word that means “to link together”. In organic chemistry, it is used to describe the situation that occurs when p-systems are linked together.

An isolated p-system exists only between a single pair of adjacent atoms (e.g. C=C).

An extended p-system exist over a long series of atoms (e.g. C=C-C=C or C=C-C=O).

An extended p-system results in a extension of the chemical reactivity.

The fundamental requirement for the existence of a conjugated system is revealed if one considers the orbital involved in the bonding within the system.

(i) A conjugated system required that there is a continuous array of “p” orbitals that can align to produce a bonding overlap along the whole system.

(ii) If a position in the chain does not provide a “p” orbital or if geometry prevents the correct alignment, then the conjugation is broken at that point.

🧪⚡General Methods of Preparation of Alkenes

Alkenes are generally synthesized through elimination reactions involving the removal of two atoms or groups from two adjacent carbon atoms of the reagent (saturated substituted alkanes).

Following are the general methods of preparation of alkenes:

1. By the dehydration of alcohols by dehydrating agents (E₁ reaction)

2. By the dehydrohalogenation of alkyl halides by alcoholic potash (E₂ reaction)

3. By the dehalogenation of vicinal dihalide by zinc dust

4. By controlled (partial or Semi) hydrogenation of alkynes

Summary of general methods of preparation of alkenes

🔥📘1. Preparation of Alkene By the acid-catalyzed Dehydration of Alcohols Through Uimolecular Elimination or E₁ reaction Mechanism

Definition of Dehydration

Dehydration is the elimination of water molecule from an organic compound by dehydrating agent like conc. H₂SO₄ or alumina (Al₂O₃) or H₃PO₄.

Reagent Used and Method

Alkene can be prepared in the laboratory by dehydration of alcohols losing a water molecule by dehydrating agent like excess conc. H₂SO₄ at 170°C (or H₃PO₄ or alumina, Al₂O₃ at 300-360°C) at elevated temperatures. (H₃PO₄ is a better dehydrating agent). In this reaction, –OH group is removed from α–carbon and H atom from β–carbon.

Other common strong acids such as HCl, HBr or HI are less suitable catalysts as nucleophilic substitution reactions will probably interfere.

Order of Reactivity of Alcohols

The order of reactivity (or the ease dehydration) of various alcohols is tertiary Alcohols > secondary Alcohols > primary Alcohols.

e.g. primary alcohols like ethanol gives only about 2-4 % yield of ethene.

General Reaction and Examples

Complete Reaction

On mixing ethanol sulphuric acid in ratio of 1:2 by volume, first ethyl hydrogensulphate is formed in cold which on heating decomposes into ethene regenerating sulphuric acid.

E₁ Reaction (uni-molecular b-Elimination reaction)

Dehydration of alcohol is an example of uni-molecular β-Elimination (E₁) reaction. The E₁ reaction is defined as an elimination reaction in which a β-hydrogen atom from β-carbon and OH group from a-carbon atom are removed and which occurs in two steps and only one molecule determines the rate of reaction.

Regioselectivity

The major product is usually the more highly substituted alkene (alkene stability) Saytzef's Rule.

Stereoselectivity

trans- > cis- again controlled by stability

Summary

Mechanism of Acid-Catalyzed Dehydration (Elimination Reaction) of Alcohol

Reaction usually proceeds via an E₁ mechanism which proceeds via a carbocation intermediate, that can often undergo rearrangement. Primary alcohols will proceed via an E₂ mechanism since the primary carbocation is highly unfavourable.

Dehydration of alcohols follows a three-step mechanism.

1. Formation of protonated alcohol

2. Formation of carbocation

3. Formation of alkenes

🔥📘2. Preparation of Alkene By Dehydrohalogenation of Alkyl Halides Through Bimolecular Elimination or E₂ reaction Mechanism

Definition of Dehydrohalogenation

Dehydrohalogenation is the elimination of hydrogen as well as halogen atom from adjacent carbon atoms of a compound.

Reagent Used and Method

Alkenes can be prepared by dehydrohalogenation of alkyl halide by heating it with alcoholic potash (alcoholic solution of KOH). When alkyl halides are heated with alcoholic solution of KOH (alcoholic potash), a hydrogen atom and a halogen atom from adjacent carbon atoms are removed forming alkene. In this reaction, –X group is removed from a-carbon and H atom is removed from b-carbon.

General Reaction and Examples

Order of reactivity or the ease dehydrohalogenation of various alkyl halides

The order of reactivity or the ease dehydrohalogenation of various alkyl halides is

tertiary alkyl halides > secondary alkyl halides > primary alkyl halides.

E₂-Reaction (1,2-elimination reaction)

Dehydrohalogenation of alkyl halide is an example of bimolecular b-elimination (E₂) reaction. The Bimolecular b-elimination (E₂) reaction is defined as an elimination reaction in which a b-hydrogen atom from b-carbon and X group from a-carbon atom are eliminated and which occurs in one step and two molecules determine the rate of reaction.

Saytzeff Rule/Zaitsev’s rule or Z-rule

Saytzeff Rule and Hofmann’s Rule are applied when there are more than one b-carbons containing different number of b-hydrogens. Saytzeff’s Rule is also called Zaitsev’s rule, Saytzev’s rule or Z-rule. A Russian Chemist Alexander Zaitsev analyzed different elimination reactions and observed a general pattern in the resulting alkenes.

According to Saytzeff Rule, H is more preferentially removed from that b-carbon which has fewer b-hydrogen atoms bonded to it i.e. stable alkene is formed when the removal of hydrogen from β-carbon has a low number of hydrogen substituents. The Saytzeff Product dominates when X in the substrate is halogen and base is small sized (e.g. OH-, CH₃O-). Predominant formation of a substituted alkene is formed according to Saytzeff’s rule. The corresponding olefin is known as the Saytzeff’s product.

When more than one alkene (product) can be formed during an elimination reaction, the major product is the one that has more alkyl groups (greater substitution) attached to the double-bonded carbon atoms.In simple words, “The alkene with more substituted (more stable) double bond is the major product.”

During elimination reactions, Saytzeff’s Rule comes into the picture. The most substituted product would be the most stable and most preferred one. This rule does not generalize about the product stereochemistry, but only the regiochemistry of the elimination reaction.

Elimination reactions of some alkyl halides and alcohol will result in different alkenes and Saytzeff’s Rule is used to predict the major product. The major and minor products are predicted based on the number of alkyl groups attached to the alkene.

Dehydrohalogenation and dehydration of secondary- and tertiary-alkyl halides proceeds by the preferential removal of the β-hydrogen from the carbon that has the smallest number of hydrogens.

Important concept behind Saytzeff’s Rule

If more than one elimination product is possible, the most substituted alkene is the most stable product (major product).

CH₂ = CHR < RCH = CHR < R₂C = CHR < R₂C = CR₂

Mono < di < Tri < Tetra

Summary

1. Saytzeff’s rule predicts the regioselectivity of the olefin (alkene), formed by the elimination reaction of secondary and tertiary alkyl halides.

2. During the elimination reaction proton is removed from the carbon atom having less number of substituents.

Examples

1. Dehydrohalogenation of 2-chlorobutane

When 2-chlorobutane undergoes a dehydrohalogenation reaction it gives two products 1-butene and 2-butene. Out of these two, 2-Butene is a major product since it is highly substituted and more stable.

2. Dehydration of alcohols

When 2-butanol undergoes dehydration, it gives two products 1-Butene (minor product) and 2-Butene (major product).

Hofmann’s Rule

According to Hofmann’s rule, H is preferably removed from that b-carbon which is richer in b-hydrogen. The Hofmann’s product dominates when X in the substrate is large sized (e.g. I) and base is large sized.

Summary

🔥📘3. Preparation of Alkene By Dehalogenation of Vicinal Dihalide (dihaloalkanes) by Zinc Dust

Dihalogen derivative of alkane having two same halogen atoms on adjacent carbon atoms are called Vicinal Dihalide which on treatment with zinc dust (powder), loses one molecule of halogen (X₂) forming alkenes.

Summary

🔥📘4. By Controlled Hydrogenation of Alkynes

Alkynes react with limited quantity of hydrogen in the presence of Ni or Pt as catalyst to form alkenes. [To stop the reaction at alkene, catalyst is poisoned (inactivated) partially by the addition of salts of heavy metals.]

Summary

🔥📘Summary of General Methods of Preparation of Alkenes

🧪⚡Chemical Reactions of Alkenes

Alkenes are highly reactive hydrocarbons. Alkenes are more reactive than alkanes. The higher reactivity of alkenes as compared to alkane is attributed to the presence of a relatively weaker pi bond between the carbon atoms that requires less energy to break. Additionally the electron density associated with the pi bond is distributed above and below the carbon-carbon axis, making the pi electrons more exposed to an electrophilic species producing marked nucleophilic character in alkene. Consequently, alkenes readily react with electrophile making them more reactive than alkanes.

The high reactivity of alkenes is due to:

1. Presence of double bonds between two adjacent carbon atoms (>C=C<). The double bond consists of a sigma bond and a pi bond. The s-bond is stable and delocalized while p-bond is less stable, mobile and exposed (localized). Thus p-bond is broken easily during chemical reactions.

2. The availability of p-electrons is responsible for high reactivity of alkenes. The p-electrons of p- bond are located much farther from carbon nuclei and thus less firmly bound to them. Hence they are easily donated. Hence alkenes have a marked nucleophilic nature.

3. In terms of molecular orbital theory (M.O.T), p-bond stabilizes itself by changing to s-bond which is acquired by addition reaction.

Following types of reactions occur in alkenes:

(a) Addition Reactions

(b) Oxidation Reactions

Summary of Reactions of Alkenes

🧪⚡Addition Reactions of Alkenes

Definition

Addition Reaction involves the combination of two (or more) molecules of substances to form a single molecule of product. Such reactions proceed by the addition of reacting molecules along the double bond. In organic chemistry, addition reactions are those in which two or more atoms or groups are simply added across a double or triple (multiple) bond without the elimination of any atom or molecule.

Course of Addition Reaction

In these reactions, at least one p-bond is lost while two new σ-bonds are formed. During addition reactions double bonds become saturated to single bond and triple bonds are converted into double bond or may become saturated by further addition.

Reason of Addition Reactions

Alkenes are nucleophilic in character due to presence of readily available p-electrons. Thus alkenes undergo addition reactions by providing p-electrons to electrophilic reagents. Hence the reactions are called Electrophilic Addition Reactions (E.A.R. or AE).

Families undergoing Addition Reactions

These reactions are characteristic of unsaturated organic compounds containing >C=C> or -C≡C- bonds like alkenes, alkynes, benzenes and carbonyl compounds containing carbonyl group (>C=O) like aldehydes and ketones.

Types of Addition Reaction

Addition reactions are of following types:

1. Electrophilic Addition Reactions (EAR or AE)

2. Nucleophilic addition reactions (NAR) Or Nucleophilic Carbonyl addition, NCA OR AN)

3. Free radical addition (AF) OR Addition Polymerization

Electrophilic Addition Reactions (EAR)

🔥 When Electrons Get Ambushed: Electrophilic Addition Unleashed!

The type of addition reactions in which the species that first attack the multiple bond is an electrophile (electron seeker) are called Electrophilic Addition Reactions (EAR). These reactions proceed by the addition of reacting molecules along the double (or triple) bond.

⚡ Why It’s Called Electrophilic Addition: The Proton Strikes First!

It is called Electrophilic Addition because the reaction is triggered by the attack of an electrophile (an acid or a proton) on the p-electrons of the C-C multiple bonds of alkenes or alkynes (which are weak bases or nucleophiles).

🟣 Alkenes as Electron Donors: The π-Cloud That Attracts Attackers!

Alkenes (or alkynes) are nucleophilic in character due to presence of readily available π-electrons. Thus alkenes (or alkynes) undergo EAR by providing π-electrons to electrophilic reagents.

General Representation

General Pattern of Mechanism of Electrophilic Addition Reactions (E.A.R.)

Electrophilic Addition Reactions (E.A.R.) proceeds through two steps mechanism. The first step, which is normally the rate-determining step, involves the transfer of the two π–electrons of alkene to an electrophile of unsymmetrical reagent to generate a carbocation called carbonium ion as a reactive intermediate which react with nucleophile of unsymmetrical reagent to form final addition product.

Examples of Electrophilic Addition Reactions (E.A.R.)

1. Hydrogenation (Addition of Hydrogen; H–H ) [Alkane]

2. Halogenation (Addition of Halogen; X–X ) [Vicinal dihalide]

3. Hydrohalogenation (Addition of Hydrogen halide; H–X ) [Alkyl halide]

4 Hydration (Addition of water; H–OH ) [Alcohol]

5. Hydrobisulphation (Addition of Sulphuric Acid; H–OSO₃H ) [Alkyl bisulphate]

6. Halohydration (Addition of Hypohalous Acid; HO–X/) [Halohydrin]

7. Epoxidation (Addition of O₂) [Exoxides]

8. Ozonolysis (Addition of O₃ followed by decomposition through hydrolysis)

9. Oxymercuration reaction

10. Hydroboration–oxidation reaction

11. Prins reaction

1. Hydrogenation

Definition of Hydrogenation

Hydrogenation is the addition of H₂ molecule across the multiple bonds of unsaturated organic compounds in the presence of transition metal catalyst like Ni or Pt or Pd at high temperature and pressure. That is why reaction is also termed as catalytic hydrogenation. The reaction is exothermic and the amount of heat evolved is called heat of hydrogenation.

Reagent Used and Conditions for Hydrogenation

At a temperature of 200–250°C in the presence of metal catalyst such as Pt, Pd or Ni, alkene adds one molecule of H2 across its carbon-carbon double bond to form saturated hydrocarbon; alkane. This reaction is known as Catalytic Hydrogenation. It is an exothermic process. [The amount of heat evolved during hydrogenation is called Heat of Hydrogenation.]

General Reaction and Examples

Raney nickel

An especially active form of Raney nickel is prepared from a Ni-Al alloy; NaOH solution is added to dissolve the Al and nickel remains as black residue.

Sabatier and Senderens Contribution

Pioneer work in catalytic hydrogenation was carried out by Sabatier and Senderns in the early twentieth century. The process is used in the ‘hardening’ of fish, whale and vegetable oils. These are esters of unsaturated long chain carboxylic (fatty) acids and are liquids at ordinary temperatures with disagreeable smells; the corresponding saturated compounds have higher melting points (hence the term hardening) and inoffensive smell and are used in the manufacture of margarine. In soft margarine, some of the double bonds remain; the degree of softness can be controlled by regulating the amount of hydrogenation.

Summary of Hydrogenation of Alkene

2. Halogenation

Definition of Halogenation

Halogenation is the addition of one or more molecules of halogen X₂ across the multiple bonds of unsaturated organic compounds.

Reagent Used and condition

Alkenes readily add one molecule of halogens in the presence of an inert solvent such as carbon tetrachloride or chloroform forming dihalides called Vicinal Dihalide (dihaloalkane).

Order of Reactivity of Halogens

The order of reactivity of addition of halogens is Cl₂ > Br₂ > I₂.

e.g. thus the reaction is rapid with chlorine, moderate with bromine at room temperature but very slow with iodine and takes place in the presence of ethanol.

Test for Unsaturation

This reaction is rapid and serves as a single diagnostic method for unsaturation. The reaction of alkene with bromine is used as a test for unsaturation because the brown colour of bromine gets discharged as the vicinal dibromide is formed. when ethene gas is bubbled through bromine in a solvent such as CCl₄ held under a layer of water, the brown colour of bromine disappears with the formation of a colourless oily liquid, vicinal dibromide. The decolourization of bromine is a test for unsaturation.

Dutch liquid

Chlorine gas reacts with ethene at room temperature without catalysis by light or peroxides to form an oily liquid called Dutch liquid (ethylene vicinal dichloride).

Mechanism of Reaction

The reaction of halogen (X₂) with alkene is similar to HX. But what is the source of the electrophile in X₂? Although Cl₂, and Br₂ are non-polar, as they approach the pi-electron cloud of the double bond, the repulsion between pi-electron cloud and the non-bonding electrons in the outer shell of the halogen molecule, momentarily polarizes the halogen molecule (Xδ⁺–Xδ⁻).

Summary of Halogenation of Alkene

3. Hydrohalogenation

Definition

Hydrohalogenation is the addition of hydrogen halides or hydrohalic acid; HX across the multiple bonds of unsaturated organic compounds like an alkene.

Reagent Used and condition

Alkene adds one molecule of halogen acid (HX) to form monohalogen derivatives of alkanes called alkyl halide or monohaloalkane. The order of reactivity of different halogen acid is HI > HBr > HCl.

Order of Reactivity

The order of reactivity of different halogen acid is HI > HBr > HCl; thus the reaction with HI is rapid, moderate with HBr and very slow with HCl.

General Reaction

Addition in Symmetrical Alkenes

Addition of HX in symmetrical alkenes like ethene, 2-butene etc. gives only one addition product.

Addition in Unsymmetrical Alkenes

More than one addition products are possible from unsymmetrical alkenes, like propene or 1-butene on treatment with any unsymmetrical reagents like H–X, H–HSO₄, H–OH, HO–X etc. Addition of HX in unsymmetrical alkenes like propene, 1-butene etc. may give more than one addition products

Addition of any unsymmetrical reagents like H–X, H–HSO₄, H–OH, HO–X etc. to an to an unsymmetrical alkene like propene or 1-butene is governed by Markownikoff’s Rule or Markovnikov’s Rule (Vladimir Markownikoff in 1870), which states that:

In the addition of unsymmetrical reagents to unsymmetrical alkenes (or alkynes), the electronegative group of adding reagent becomes attached to the more highly-substituted of the unsaturated carbon atoms containing the least number of hydrogen atoms (while electropositive group joins the less highly-substituted carbon atom) i.e. In additions of unsymmetrical reagents to unsymmetrical alkenes, the H of unsymmetrical reagent goes to that double-bonded carbon which already has the greater number of hydrogens (rich get richer).

When an unsymmetrical reagent adds to unsymmetrical alkene (or alkyne), the negative part of attacking reagent attaches to the double-bonded carbon atom that has lesser number of hydrogen atoms while the positive part (hydrogen atom) is attached to the carbon atom with the highest number of hydrogen substituents .

For example

Propene may give 1-bromopropane or 2-bromopropane with HBr. The addition of hydrogen bromide to propene gives 2-bromoporpane as a major product in accordance with Markovnikov’s Rule.

MECHANISM

Step I (Nucleophilic Attack of p-electrons of Alkene on Electrophilic Part of Reagent)

Step II (Attack of Leaving Nucleophile on Carbonium Ion)

FREE RADICAL ADDITION (ADDITION OF HBr)

In the absence of peroxides and in polar media, hydrogen bromide undergoes a slow addition to propene to form only 2-bromopropane. This product is in complete accord with Markownikov’s rule. When peroxides, light or other free radical initiators are present, rapid addition occurs to give 1-bromopropane, and the direction of addition is exactly opposite to that found in electrophilic addition.

Thus in the presence of light or peroxides, the addition occurs anti to the Markovnikov rule.

This phenomenon of anti-Markovnikov addition caused by the presence of light or peroxide is known as Kharasch Peroxide effect (discovered by M.S. Kharasch). This effect is observed with HBr and not observed with HCI and HI. When peroxide is present or reaction is carried out in the presence of UV light, HBr addition proceeds through a free radical mechanism instead of an ionic one.

Summary

4. Addition of Water/Acid-catalyzed Hydration

Definition of Hydration

The addition of water across the multiple bond of unsaturated organic compounds is called hydration. It is the acid-catalyzed addition of water to alkene.

Reagent Used and condition

Water adds to alkene under acid catalysis at 80-100oC forming alcohol. Alkene reversibly adds one molecule of water under acid-catalyzed conditions (65-70% H₂SO₄) to form corresponding alcohol. (The reaction is reversible). Addition in unsymmetrical alkene is governed by Markownikov’s Rule.

Addition in unsymmetrical alkene

Addition in unsymmetrical alkene is governed by Markownikoff’s Rule.

General Reaction and Examples

Summary

5. Halohydration/Halohydroxylation/Addition of Hypohalous Acid(Confirmatory Test for alkene)

Definition of halohydration

The addition of a halogen and water to an alkene is called halohydration.

Product of hydrohydration

The product of this reaction is known as halohydrin.

Reagent used and Method

Alkene reacts with Hypohalous acid; HO–X like HOCl, HOBr to form alkylene halohydrin (haloalkanol). Markownikoff’s rule is followed in addition to unsymmetrical alkenes.

Bromine water test (Test for Unsaturation)

Alkenes discharges the brown colour of bromine water (Hypobromous acid; HOBr) with the formation of bromoalkanol. The decolourization of bromine water is a test for unsaturation indicating the presence of double bond. This is known as bromine water test.

For example

Hypobromous acid (bromine water) reacts with ethene to form ethylene bromohydrin. In this reaction, brown colour of Br₂-water is disappeared indicating the presence of double bond.

Summary

6. Epoxidation/ Catalytic Air Oxidation

Definition of epoxidation

The catalytic addition of oxygen to the double bond of an alkene to form an epoxide (oxirane) is called epoxidation.

Product of epoxidation

The product of epoxidation is epoxide (IUPAC: oxirane) which is a three membered cyclic ether with an oxygen atom in a ring. The epoxidation of an alkene is clearly an oxidation, since an oxygen atom is added. The simplest epoxide is epoxyethane (ethylene oxide). Epoxide belongs to a class of chemicals called heterocyclic compounds having cyclic structures in which one (or more) of the ring atoms is a hetero atom (i.e. an atom of an element other than carbon).

Reagent for Epoxidation/Epoxidation Reagent

The epoxidation reaction of an alkene is carried out with an epoxidation reagent, usually a peroxyacid or peracids, a carboxylic acid that has an extra oxygen atom in a -O-O- (peroxy) linkage to give an oxygen-containing three-membered ring called an epoxide or oxirane. (Another way to say it is epoxidation is the electrophilic addition of oxygen to the double bond of the alkene).

Peroxyacid or peracids

Peroxyacids are derivatives of carboxylic acids that contain an additional O-O bond. The peroxyacid reagent forms an acid as by-product, while the epoxide is formed. Peroxyacids are highly selective oxidizing agents.

There are several types of commonly used peroxyacid such as peroxy trifluoroacetic acid, peroxyacetic acid, hydrogen peroxide, and mCPBA. The most common peroxyacid used for the epoxidation of alkenes (like propene) is meta-chloroperoxybenzoic acid, or mCPBA.

Some simple peroxyacids and their corresponding carboxylic acids are shown next.

General Reaction

Examples

Book Reaction

Summary

7. Ozonolysis or Ozoniation (Ozonide Reaction)

Definition of Ozonolysis or Ozoniation

The addition of ozone or trioxygen (O₃) to an alkene or alkyne to give an adduct called ozonide is known as ozoniation. The hydrolysis of ozonide (product of ozoniation) is called ozonolysis. The process allows for each carbon-carbon double or triple bonds to be replaced by double bonds with oxygen.

In other words,

the process of preparing the ozonide by the addition of ozone involving the cleavage of an alkene or alkyne followed by its subsequent decomposition by hydrolysis in which the pi bonds between carbon-carbon atoms are broken down in the presence of reducing agent such as zinc or oxidizing agent such as hydrogen peroxide is called ozonolysis.

Ozonolysis is a chemical reaction involving oxidative cleaving of the unsaturated bonds of alkenes, alkynes and azo compounds (compounds with the functional diazenyl functional group) using ozone (O₃; a reactive allotrope of oxygen). It is an organic redox reaction. This reaction is often used to identify the structure of unknown alkenes by breaking them down into smaller, more easily identifiable pieces.

Products of Ozonolysis or Ozoniation

▶ Oxidation of alkenes with the help of ozone can give alcohols, aldehydes, ketones, or carboxylic acids.

The products of ozonolysis are aldehyde(s) and/or ketone(s), and the exact structures of the products depend on the structure of the initial alkene:

Disubstituted alkene carbons are oxidatively cleaved to ketone.

Monosubstituted alkene carbons are oxidatively cleaved to aldehyde.

Unsubstituted alkene carbons are oxidatively cleaved to formaldehyde (HCHO).

▶ Alkynes undergo ozonolysis to give acid anhydrides or diketones. If water is present in the reaction, the acid anhydride undergoes hydrolysis to yield two carboxylic acids. For external alkynes, the ozonolysis results in a carboxylic acid and carbon dioxide: The ozonolysis of internal alkynes, on the other hand, produces two carboxylic acids. Alkynes are less reactive than alkenes towards O₃.

▶ For azo compounds, the ozonolysis yields nitrosamines.

Once the ozone is added to the reaction mixture, a reagent must be added to convert the ozonide to the required carbonyl derivative. For this conversion, 2 techniques can be employed:

▶ Reductive Workup

▶ Oxidative Workup

However, reductive workup conditions seen a lot more in use when compared to oxidative workup conditions. In these workup conditions, triphenylphosphine, thiourea, zinc dust, and dimethyl sulfide can be used to produce aldehydes or ketones. On the other hand, hydrogen peroxide can be used to produce carboxylic acids.

The ozonolysis of an alkene involves addition of ozone in the presence of ether which initially gives an ozonide intermediate which are unstable undergoes decomposition (either with Zn or H₂O₂) to form carbonyl compounds (an aldehyde or ketone or both). The ozonolysis of Alkenes with ozone (O₃) involves oxidative cleavage giving carbonyl compounds, cleaving the C=C bond

The reaction generates an ozonide intermediate, which is then treated with a reducing agent (e.g. dimethyl sulfide or zinc) gives aldehydes or ketones depending on the structure of the starting alkene.

One of the advantages of ozonolysis compared to other oxidative cleavage reactions is that it does not over-oxidize the alkene to carboxylic acid unlike, for example, the potassium permanganate (KMnO₄):

General Reaction

Formaldehyde is formed when ozonide is treated with a reducing agent like zinc dust in the presence of boiling water.

Application

Ozonolysis is used to detect the position of double bond.

Polymerization/Self-Addition (Formation of Polyethene)

Polymerization involves the linking of many small molecules of monomers together to form a long chain polymer molecule. At 200°C and 2000 atm. pressure, and in the presence of 0.01% oxygen, ethene polymerizes to polythene or polyethylene, which is white tough flexible plastic.

🌿📘 Ultimate Alkenes MCQs Quiz | MDCAT Concept Smashers 2026! 📘🌿

1. The general formula of alkenes is:
🟩 A. CₙH₂ₙ
🟦 B. CₙH₂ₙ+₂
🟨 C. CₙH₂ₙ−₂
🟥 D. CₙHₙ

2. The simplest alkene is:
🟦 A. Propene
🟩 B. Ethene
🟥 C. Butene
🟨 D. Pentene

3. The double bond in alkenes consists of:
🟩 A. One sigma, one pi
🟨 B. Two sigma bonds
🟥 C. Two pi bonds
🟦 D. One sigma only

4. Hybridization of carbon atoms in an alkene is:
🟦 A. sp³
🟨 B. sp
🟩 C. sp²
🟥 D. dsp²

5. Electrophile attacks which part of an alkene?
🟩 A. π-Electron cloud
🟥 B. Sigma bond
🟦 C. Lone pair
🟨 D. Vacant orbital

6. Test used to detect alkenes:
🟦 A. Bromine water test
🟥 B. Fehling’s test
🟨 C. Tollens test
🟩 D. Iodine test

7. Addition of HBr to propene follows:
🟩 A. Markovnikov’s rule
🟥 B. Wurtz reaction
🟦 C. Saytzeff rule
🟨 D. Anti-Markovnikov’s rule (always)

8. Anti-Markovnikov addition occurs in presence of:
🟥 A. H₂SO₄
🟩 B. Organic peroxides
🟦 C. Water only
🟨 D. UV light

9. Hydrogenation of alkene gives:
🟩 A. Alkane
🟥 B. Alkyne
🟦 C. Alcohol
🟨 D. Acid

10. cis-trans isomerism is shown by:
🟦 A. Alkanes
🟩 B. Alkenes
🟥 C. Alkynes
🟨 D. Arenes

11. Which alkene gives only one product on hydration?
🟥 A. But-1-ene
🟩 B. 2-methylpropene
🟦 C. Pent-1-ene
🟨 D. Hex-1-ene

12. Complete dehydrohalogenation of 2-bromopropane gives:
🟩 A. Propene
🟥 B. Propane
🟦 C. Propyne
🟨 D. Ethene

13. Ozonolysis of propene gives:
🟥 A. CO₂
🟦 B. Ethanal only
🟩 C. Ethanal + Methanal
🟨 D. Propanone

14. Number of structural isomers of C₄H₈ (alkenes) is:
🟦 A. 1
🟩 B. 3
🟥 C. 4
🟨 D. 5

15. π-bond is formed by overlap of:
🟩 A. Parallel p-orbitals
🟥 B. Head-on p-orbitals
🟦 C. s-s overlap
🟨 D. s-p overlap

16. Which reagent decolourizes alkenes fastest?
🟩 A. Br₂ in CCl₄
🟥 B. Fehling solution
🟦 C. Acidified KMnO₄ (warm)
🟨 D. Ammoniacal AgNO₃

17. Strongest electrophile among the following for alkene attack:
🟥 A. Br–
🟩 B. H⁺
🟦 C. OH–
🟨 D. H₂O

18. Alkenes burn with:
🟩 A. Sooty flame
🟥 B. Non-luminous flame
🟨 C. Blue flame
🟦 D. Sparkling flame

19. Catalytic hydrogenation uses:
🟥 A. CaCO₃
🟩 B. Ni or Pt
🟦 C. Cu
🟨 D. NaOH

20. Hydroboration-oxidation converts alkene into:
🟦 A. Ketone
🟥 B. Alkane
🟨 C. Aldehyde
🟩 D. Alcohol

21. Which does NOT undergo electrophilic addition?
🟥 A. Propene
🟦 B. Cyclohexene
🟨 C. Ethyne
🟩 D. Ethane

22. The reagent used to convert alkene into vicinal diol:
🟩 A. Cold KMnO₄
🟥 B. Hot KMnO₄
🟦 C. HBr
🟨 D. HI

23. Which alkene shows geometrical isomerism?
🟩 A. But-2-ene
🟥 B. Propene
🟦 C. Ethene
🟨 D. 2-methylpropene

24. Dehydration of ethanol forms:
🟩 A. Ethene
🟥 B. Ethyne
🟦 C. Methene
🟨 D. Propene

25. Lindlar catalyst converts alkyne into:
🟥 A. Alkane
🟩 B. Cis-alkene
🟦 C. Trans-alkene
🟨 D. Alcohol

26. Electrophilic addition in alkenes occurs because:
🟩 A. High electron density in π bond
🟥 B. Low reactivity
🟦 C. Strong sigma bond
🟨 D. Absence of electrons

27. Polymerization of ethene produces:
🟥 A. PVC
🟩 B. Polyethylene
🟦 C. Teflon
🟨 D. Nylon-6

28. Major product in dehydration follows:
🟩 A. Saytzeff rule
🟥 B. Markovnikov rule
🟦 C. Hund’s rule
🟨 D. Fajan’s rule

29. Alkenes react with cold dilute KMnO₄ to give:
🟥 A. Carboxylic acid
🟦 B. Ketones
🟩 C. Glycols
🟨 D. Esters

30. Which alkene gives two products with HBr?
🟦 A. Ethene
🟩 B. But-1-ene
🟥 C. 2-methylpropene
🟨 D. Propene

✅ ANSWERS + SHORT REASONS
1. 🟧A – Alkenes follow CₙH₂ₙ.
2. 🟧B – Ethene is the first member.
3. 🟧A – Double bond = σ + π.
4. 🟧C – sp² hybridization.
5. 🟧A – Electrophile loves π electrons.
6. 🟧A – Bromine water instantly decolourizes.
7. 🟧A – Normal HBr addition follows Markovnikov.
8. 🟧B – Peroxides cause Anti-Markovnikov.
9. 🟧A – Hydrogenation adds H₂ to form alkane.
10. 🟧B – Alkenes can show cis-trans.
11. 🟧B – 2-methylpropene gives same carbocation.
12. 🟧A – Removal of HBr forms propene
13. 🟧C – Propene splits into ethanal + methanal.
14. 🟧B – Three alkenes exist for C₄H₈.
15. 🟧A – π bond forms via parallel p-orbitals.
16. 🟧A – Bromine in CCl₄ reacts fastest.
17. 🟧B – H⁺ is strongest electrophile.
18. 🟧A – Due to high C content alkenes burn sooty.
19. 🟧B – Ni/Pt required for hydrogenation.
20. 🟧D – Hydroboration forms alcohol.
21. 🟧D – Ethane doesn’t have π electrons.
22. 🟧A – Cold KMnO₄ makes vicinal diols.
23. 🟧A – But-2-ene shows cis/trans.
24. 🟧A – Ethanol loses H₂O to form ethene.
25. 🟧B – Lindlar stops at cis-alkene.
26. 🟧A – π bond attracts electrophiles.
27. 🟧B – Polymerization gives polyethylene.
28. 🟧A – Saytzeff gives more substituted alkene.
29. 🟧C – Glycols form by syn-addition.
30. 🟧B – But-1-ene gives Markovnikov + minor product.

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