Alkenes are a homologous series of unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). Their general formula is C_nH_2n. They are more reactive than alkanes due to the presence of the double bond.

Alkenes are unsaturated hydrocarbons that contain a carbon–carbon double bond and belong to the general formula C_nH_2n.

Structure of Alkenes (General):

    H   H
    |   |
    C = C
    |   |
    R   R'

The carbon atoms in the double bond are sp² hybridized, giving alkenes a planar structure around the double bond with bond angles of about 120°.

First 10 Alkenes and Their Properties

• Unsaturated hydrocarbons with one double bond.
• More reactive than alkanes due to π-bond.
• Undergo addition reactions (characteristic).
• Insoluble in water but soluble in organic solvents.
• Burn with a more luminous flame than alkanes.
• Exhibit structural and geometric isomerism.

Ethene (Ethylene) is the first and most important alkene. It is used widely in the chemical industry and plant physiology (ripening hormone).

    H   H
    |   |
    C = C
    |   |
    H   H

Each carbon is sp² hybridized and the molecule is planar.

1. Dehydration of ethanol using concentrated H₂SO₄:
Alkenes are prepared generally in the laboratory by the dehydration of monohydric alkanols using dehydrating agent, conc. \(H_2SO_4 \).

The gas evolved is passed through sodium hydroxide to remove gaseous impurities such as \((CO_2 \text{ and} SO_2) \)

$$ C_2H_5OH \xrightarrow{} C_2H_4 + H_2O $$

Ethene is produced industrially mainly by the cracking of hydrocarbons, particularly the cracking of long-chain alkanes obtained from petroleum. This process breaks down larger, less useful hydrocarbons into smaller and more useful molecules such as ethene, propene, and hydrogen.

1. Steam Cracking of Alkanes
In this method, long-chain alkanes (e.g., naphtha or gas oil fractions) are heated with steam to very high temperatures (about 750–900°C). The hydrocarbon chains break (crack) to produce ethene.

Example equation:
$$ \text{C}_{10}\text{H}_{22} \; \rightarrow \; \text{C}_2\text{H}_4 \; + \; \text{C}_3\text{H}_6 \; + \; \text{C}_5\text{H}_{12} $$

2. Cracking of Ethane or Propane
Lighter alkanes can also be cracked to produce ethene. For example, ethane undergoes thermal cracking to produce ethene and hydrogen:

$$ \text{C}_2\text{H}_6 \; \rightarrow \; \text{C}_2\text{H}_4 \; + \; \text{H}_2 $$

3. Dehydration of Ethanol (Industrial Alternative)
Ethanol obtained from fermentation can be dehydrated using hot catalysts such as alumina (Al₂O₃) at about 350°C to produce ethene. This method is also used industrially in regions where ethanol is abundant.

Equation:
$$ \text{C}_2\text{H}_5\text{OH} \xrightarrow{} \text{C}_2\text{H}_4 \; + \; \text{H}_2\text{O} $$

In all industrial methods, the ethene produced is separated, purified, and compressed for transportation or further use in the manufacture of plastics, ethanol, detergents, and other important chemicals.

• Ethene is a colourless gas at room temperature and pressure, making it visually similar to most low-molecular-mass hydrocarbons.
• It has a faint sweet smell, especially when pure. This odour becomes more noticeable at slightly higher concentrations.
• Ethene is slightly soluble in water but dissolves readily in organic solvents such as ethanol, ether, benzene, and chloroform due to its non-polar nature.
• It is less dense than air (relative density ≈ 0.97), meaning it rises when released into the atmosphere.
• Ethene has a low boiling point (-103.7°C) and low melting point (-169.4°C), typical of small non-polar molecules with weak van der Waals forces.
• It burns with a luminous (yellow) flame due to its higher carbon-to-hydrogen ratio, which produces unburnt carbon particles during combustion.
• It diffuses rapidly because of its low molar mass (28 g/mol), allowing it to spread quickly through air.
• Ethene is highly reactive chemically because of the C=C double bond, although this is a chemical characteristic, it influences observable behaviour such as easy polymerisation.
• It is non-toxic at low concentrations, but high concentrations can act as an asphyxiant by displacing oxygen in poorly ventilated areas.
• Ethene is slightly flammable and forms explosive mixtures with air within certain concentration ranges (3%–36%).

1. Combustion

$$ C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O $$

2. Addition Reactions (Characteristic Reaction)

(a) Hydrogenation (Addition of Hydrogen)

In presence of Ni catalyst., Ethene reacts with hydrogen at 200°C, to form ethane. This reaction is used in the hardening of oils to produce margarine.

    H   H             H   H
    |   |             |   |
    C = C + 2H₂ → H - C - C - H
    |   |             |   |
    H   H             H   H

(b) Addition of Halogens (Br₂, Cl₂)

This is called halogenation. Ethane reats with bromine and chlorine at room temperature to yield 1,2- dibromoethane and 1,2-dichloroethane respectively.

    H   H              H   H
    |   |              |   |
    C = C + Br₂ → Br - C - C - Br
    |   |              |   |
    H   H              H   H
                1,2-dibromoethane
                   (colourless)

Reddish brown bromine vapor is decolourised — test for unsaturation. It ie used to distinguish alkenes from alkanes

    H   H              H   H
    |   |              |   |
    C = C + Cl₂ → Cl - C - C - Cl
    |   |              |   |
    H   H              H   H
              (1,2 dichloroethane)

(c) Addition of Hydrogen Halides (HX)

The addition of ethene and Hydrogen halides leads to the formation of haloalkanes.

    H   H              H   H
    |   |              |   |
    C = C + HCl → Cl - C - C - H
    |   |              |   |
    H   H              H   H
                 (chloroethane)

    H   H             H   H
    |   |             |   |
    C = C + Hl →  H - C - C - I
    |   |             |   |
    H   H             H   H
                   (iodoethane)

The reaction between propene (CH₃CH=CH₂) and hydrogen chloride (HCl) is an example of an electrophilic addition reaction, resulting in the formation of 2-chloropropane (CH₃CHClCH₃), also known as isopropyl chloride.

Markovnikov's rule states that in an electrophilic addition reaction, the hydrogen atom adds to the carbon atom with the most hydrogen atoms, while the functional group (in this case, Cl) adds to the carbon atom with fewer hydrogen atoms. Applying Markovnikov's rule to propene (CH₃CH=CH₂) + HCl: H (from HCl) adds to CH₂ (more H's) and Cl (from HCl) adds to CH (fewer H's), resulting in CH₃CHClCH₃ (2-chloropropane or isopropyl chloride).

      H   H              Cl  H
      |   |              |   |
  C - C = C + HCl →  C - C - C - H
      |   |              |   |
      H   H              H   H
                 (2-chloropropane)

(d) Addition of bromine water and chlorine water

The addition of ethene and bromine water (HOBr) and chlorine water (HOCl) leads to the formation of 2-bromoethanol and 2-chloroethanol respectively.

    H   H               H   H
    |   |               |   |
    C = C + HOCl → Cl - C - C - OH
    |   |               |   |
    H   H               H   H
                 (2-chloroethanol)

  H   H                 H   H
  |   |                 |   |
  C = C + HOBr  →  HO - C - C - Br
  |   |                 |   |
  H   H                 H   H
                  (2-bromoethanol)

(e) Reaction with Concentrated H₂SO₄

Ethene (C₂H₄) reacts with concentrated sulfuric acid (H₂SO₄) to form ethyl hydrogen sulfate (C₂H₅HSO₄), which is an example of an electrophilic addition reaction.

C₂H₅HSO₄ + H₂O → C₂H₅OH + H₂SO₄

(e) Oxidation with \(KMnO_4 \)

On reaction with KMnO₄, ethene is oxidized to ethan-1,2-diol. The purple color of KMnO₄ becomes colorless if the reaction is acidic. If the reaction is alkaline, purple KMnO₄ is reduced to green.

H   H                   H   H
|   |                   |   |
C = C + H₂O + [O] → H - C - C - H
|   |                   |   |
H   H                  OH   OH
                  (Ethan-1,2-diol)

3. Polymerization Reaction

Ethene undergoes addition polymerisation to form poly(ethene).

$$ nC_2H_4 \xrightarrow{} [-CH_2-CH_2-]_n $$

• Used in producing polyethene for bags, bottles, buckets.
• Used in manufacturing ethanol (hydration process).
• Used in producing ethylene glycol (antifreeze).
• Used in making PVC, polystyrene, and other polymers.
• Used to ripen fruits (plant hormone).
• Used in welding (oxy-ethene flame).
• Used as a chemical feedstock in many organic syntheses.