Alkenes

Full Notes

Introduction to Alkenes

Alkenes are hydrocarbons that contain one or more carbon-carbon double bonds. These unsaturated compounds are of immense importance in organic synthesis and industrial chemistry. Their reactivity is largely due to the presence of the π-bond in the C=C double bond, which is more reactive than a σ-bond.

Structure of Double Bond

The double bond in alkenes consists of one sigma (σ) and one pi (π) bond.

The σ-bond arises from head-on overlap of sp² orbitals, while the π-bond is formed by the sideways overlap of unhybridized p-orbitals on adjacent carbon atoms. This double bond restricts rotation and gives rise to geometrical isomerism.

Nomenclature

Alkenes are named using the IUPAC system, where:

• The longest carbon chain containing the double bond is chosen.
• Numbering starts from the end nearest the double bond.
• The suffix “-ene” is added, and the position of the double bond is indicated by the lowest possible number.

Isomerism

Note – Cis and Trans and E and Z isomerism has been covered in more detail here. Below is just what you need to know for NCERT 11 chemistry :)

Alkenes can show both structural and geometrical isomerism:

Structural isomerism:
Differing in the position of the double bond or branching (see isomerism 8.6).

Geometrical isomerism (cis-trans):
Due to restricted rotation about the C=C bond.

• To show cis/trans isomerism:
  • Each carbon in the double bond must have two different groups
  • Cis = the same groups on each carbon are on the same side of the double bond
  • Trans = the same groups on each carbon are on opposite sides of the double bond

Example: But-2-ene

Cis-but-2-ene: CH₃ groups on same side of C=C
Trans-but-2-ene: CH₃ groups on opposite sides of C=C

This type of isomerism is also common in cyclic compounds when rotation is restricted.

For Example: 1,2-dimethylcyclopropane

Has cis and trans isomers because the ring prevents free rotation, so the two methyl groups can be locked in position on the same side (cis) or opposite sides (trans) of the ring plane.

9.3.4 Preparation

Alkenes can be prepared through elimination reactions or reduction processes, depending on the starting material.

From Alkynes:

• Lindlar’s Catalyst (Pd/C):
  When alkynes are hydrogenated in the presence of partially deactivated palladised charcoal, known as Lindlar’s catalyst, they produce cis-alkenes (Z isomers).
  RC≡CR′ + H₂ → RCH=CHR′ (cis)
• Sodium in Liquid Ammonia:
  Reduction of alkynes using sodium in liquid ammonia (Na/NH₃) gives trans-alkenes (E isomers). This is called Birch reduction.
  RC≡CR′ + H₂ → RCH=CHR′ (trans)

From Alkyl Halides:

Dehydrohalogenation of alkyl halides using alcoholic KOH removes HX to form alkenes.

Example Dehydrohalogenation

CH₃CH₂Cl + alc. KOH → CH₂=CH₂ + HCl

From Vicinal Dihalides:

Reacting dihalides with zinc in alcohol removes halogens from adjacent carbons.

Example Dehalogenation

CHBr–CHBr + Zn → CH=CH + ZnBr₂

From Alcohols (acidic dehydration):

Alcohols on heating with conc. H₂SO₄ undergo dehydration to form alkenes.

Example Ethanol to ethene

CH₃CH₂OH → CH₂=CH₂ + H₂O (in presence of conc. H₂SO₄ and heat)

Physical Properties

• State: Lower alkenes (C₂ to C₄) are gases. C₅ to C₁₇ are liquids. Higher members are waxy solids.
• Solubility: Alkenes are insoluble in water due to their non-polar nature; soluble in non-polar solvents like benzene and ether.
• Boiling and Melting Points: Increase with molecular mass; isomeric alkenes may differ slightly due to branching and symmetry.
• Density and Odour: Less dense than water; many lower members possess distinct odours.

Chemical Properties and Reactions

Alkenes are highly reactive due to the electron-rich π-bond, which is readily attacked by electrophiles. This makes them undergo a wide variety of electrophilic addition reactions. They can also be oxidized, cleaved, or polymerised under suitable conditions.

Addition of Dihydrogen (Hydrogenation)

Alkenes react with hydrogen gas in the presence of metal catalysts like Ni, Pd, or Pt to form alkanes. (see 9.2.2)
CH₂=CH₂ + H₂ → CH₃–CH₃
(This reaction is used industrially for hydrogenation of oils.)

Addition of Halogens

Alkenes react with halogens (Cl₂, Br₂) to form vicinal dihalides.
CH₂=CH₂ + Br₂ → BrCH₂–CH₂Br
This is also used as a test: bromine in CCl₄ gets decolourised.

Addition of Hydrogen Halides

Alkenes react with HCl, HBr, or HI to form alkyl halides.

Example Hydrohalogenation

CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (Markovnikov)

Markovnikov’s Rule: H attaches to the carbon with more hydrogen atoms (see below for more detailed explanation).

Anti-Markovnikov Addition (Peroxide Effect): In presence of peroxides, HBr adds such that Br goes to the less substituted carbon.

Addition of Sulphuric Acid

Alkenes react with cold, concentrated H₂SO₄ to give alkyl hydrogen sulphates, which can then be hydrolyzed to alcohols.

Example CH₂=CH₂ + HOSO₃H → CH₃CH₂–OSO₃H

Addition of Water (Hydration)

In the presence of an acid catalyst (like H⁺), alkenes react with water to form alcohols.

Example CH₂=CH₂ + H₂O → CH₃CH₂OH (Follows Markovnikov’s Rule)

Oxidation

Baeyer’s Test (cold dilute KMnO₄): Forms glycols (1,2-diols) and turns purple KMnO₄ colourless.

Hot KMnO₄ / Acidic Cleavage: Cleaves double bond to give ketones or carboxylic acids, depending on substitution.

Ozonolysis

Alkenes react with ozone (O₃), forming ozonides, which are then reduced to carbonyl compounds using Zn/H₂O.

Example CH₂=CH₂ → 2HCHO (formaldehyde)
Useful in determining the position of double bonds.

Polymerisation

Alkenes undergo addition polymerisation to form long-chain polymers.

Example CH₂=CH₂ → –[CH₂–CH₂]_n–
(Polyethene is formed this way; used in plastics.)

Markovnikov’s Rule

When adding HX to an unsymmetrical alkene, two possible products can form.

• The two possible products won’t be formed in equal amounts. The product formed most is called the major product and the one formed the least is the minor product.
• We can predict the major product based on the carbocation intermediate formed in the reaction.
• The major product forms from the most stable carbocation.
• Alkyl (carbon) groups bonded to the positively charged carbon in the intermediate stabilise the positive charge by ‘giving’ electron density to the positively charged carbon. This is called the positive inductive effect. The more alkyl groups there are bonded to the positively charged carbon, the more stable the carbocation is and the more likely it is to form.

Carbocation stability order: Tertiary > Secondary > Primary

Example: Propene + HBr

Secondary carbocation → 2-bromopropane (major)
Primary carbocation → 1-bromopropane (minor)

This explains Markovnikov’s rule (Major product will be the one where H from HX bonds to carbon in C=C that is bonded to the most hydrogens).

Anti-Markovnikov’s Rule

When HBr is added to unsymmetrical alkenes (like propene) in the presence of peroxide (e.g., benzoyl peroxide), the product formed does not follow Markovnikov’s rule.

• Instead of the Br⁻ adding to the more substituted carbon, it adds to the less substituted carbon.
• This is called the Kharash effect or Peroxide effect.
• Only observed with HBr (not HCl or HI).

Mechanism: Free Radical Chain Mechanism

• Initiation: Benzoyl peroxide decomposes: C₆H₅CO–O–OC₆H₅ → 2 C₆H₅• + 2 CO₂
• Propagation:
  • C₆H₅• + HBr → C₆H₆ + Br•
  • Br• adds to propene → two possible free radicals:
    • Primary (less stable)
    • Secondary (more stable) → favoured
  • Secondary radical reacts with HBr again: CH₃–CH•–CH₂Br + HBr → CH₃–CH₂–CH₂Br + Br•
  • Chain continues...

Why not with HCl or HI?

• H–Cl bond is stronger → difficult to break (430.5 kJ mol⁻¹)
• H–I bond is too weak (easily forms I₂) → radicals recombine
• Only HBr has the ideal bond strength for this free radical mechanism

Summary

• Alkenes contain a C=C double bond comprising one σ and one π bond.
• They show structural and geometrical isomerism because rotation about C=C is restricted.
• Key preparations include reduction of alkynes and eliminations from halides and alcohols.
• Electrophilic additions follow Markovnikov’s rule; peroxides give anti-Markovnikov addition with HBr.
• Typical reactions include halogenation, hydration, oxidation, ozonolysis and polymerisation.