Ethylene (C2H4) is the simplest alkene and a unsaturated hydrocarbon containing a carbon-carbon double bond. It is a colorless, highly flammable gas with a faint, sweet, and musky odor when pure. Ethylene serves as a crucial building block in the petrochemical industry and acts as a vital natural plant hormone.
Chemical Properties and Molecular Structure
Molecular Geometry and Bonding
Ethylene possesses a planar (flat) molecular geometry. Each carbon atom undergoes sp2 hybridization.
- Bonding: The carbon-carbon double bond consists of one strong σ (sigma) bond formed by head-on overlapping of sp2 hybrid orbitals and one weaker π (pi) bond formed by lateral overlapping of unhybridized $2porbitals. </li> <li> <b>Bond Angle:</b> The H-C-H and H-C-C bond angles are approximately 121.3° and 117.4°, close to the ideal trigonal planar angle of 120°. </li> <li> <b>Bond Length:</b> The C = C double bond length is 1.34 Å, which is shorter than the C-C single bond in ethane (1.54 Å). </li> </ul> <h4>Chemical Reactions</h4> <p> Because of the presence of the electron-rich\pibond, ethylene primarily undergoes electrophilic addition reactions. </p> <ul> <li> <b>Hydrogenation:</b> Reacts with hydrogen in the presence of a catalyst (Nickel, Platinum, or Palladium) to form ethane. <div class = "math-display">C<sub>2</sub>H<sub>4</sub> + H<sub>2</sub> <span class = "math-xarrow math-xarrow-right"><span class = "math-xarrow-label">Ni/Pt/Pd</span><span class = "math-xarrow-body">→</span></span> C<sub>2</sub>H<sub>6</sub></div> </li> <li> <b>Halogenation:</b> Reacts readily with halogens. The addition of bromine water is used as a laboratory test for unsaturation. The reddish-brown color of bromine is discharged when it reacts with ethylene to form 1,2-dibromoethane. <div class = "math-display">C<sub>2</sub>H<sub>4</sub> + Br<sub>2</sub> → CH<sub>2</sub>Br-CH<sub>2</sub>Br</div> </li> <li> <b>Hydrohalogenation:</b> Reacts with hydrogen halides (likeHClorHBr) to form alkyl halides. <div class = "math-display">C<sub>2</sub>H<sub>4</sub> + HCl → C<sub>2</sub>H<sub>5</sub>Cl (Ethyl chloride)</div> </li> <li> <b>Oxidation (Baeyer’s Test):</b> Reacts with cold, dilute, alkaline potassium permanganate (KMnO_4), decoloring the purple solution and producing ethylene glycol. This is another classic test for unsaturation. </li> <li> <b>Polymerization:</b> Under high temperature and pressure in the presence of a catalyst, thousands of ethylene molecules link together to form polyethylene (polythene), a widely used plastic. <div class = "math-display">n(CH<sub>2</sub> = CH<sub>2</sub>) <span class = "math-xarrow math-xarrow-right"><span class = "math-xarrow-label">Catalyst</span><span class = "math-xarrow-body">→</span></span> -[-CH<sub>2</sub>-CH<sub>2</sub>-]<sup>-</sup><sub>n</sub></div> </li> </ul> <h4>Biological and Industrial Applications</h4> <h4>Plant Hormone (Phytohormone)</h4> <p> Ethylene is unique because it is the only gaseous plant hormone. It regulates various physiological processes: </p> <ul> <li> <b>Fruit Ripening:</b> It triggers and accelerates the ripening process in climacteric fruits (e.g., bananas, mangoes, tomatoes, apples). </li> <li> <b>Abscission and Senescence:</b> It promotes the shedding of leaves, flowers, and fruits (abscission) and speeds up the aging process of plant tissues (senescence). </li> <li> <b>Triple Response:</b> In seedlings, it inhibits stem elongation, increases stem thickening, and causes horizontal growth habit modification to help seedlings navigate around obstacles in soil. </li> </ul> <h4>Industrial Utility</h4> <ul> <li> <b>Plastic Production:</b> Serves as the raw material for polyethylene (LDPE and HDPE), Polyvinyl Chloride (PVC), and polystyrene. </li> <li> <b>Chemical Synthesis:</b> Used to manufacture ethylene glycol (automotive antifreeze), ethanol (industrial solvent), and ethylene oxide (sterilizer and surfactant precursor). </li> </ul> <h4>Acetylene (Ethyne)</h4> <h4>Introduction to Acetylene</h4> <p> Acetylene (C_2H_2) is the simplest alkyne and an unsaturated hydrocarbon characterized by a carbon-carbon triple bond. It is a colorless, highly unstable, and volatile gas. While pure acetylene is odorless, commercial grades possess a distinct, garlic-like odor due to impurities like phosphine (PH_3) and arsine (AsH_3). </p> <h4>Chemical Properties and Molecular Structure</h4> <h4>Molecular Geometry and Bonding</h4> <p> Acetylene features a strictly linear molecular geometry. Each carbon atom undergoessphybridization. </p> <ul> <li> <b>Bonding:</b> The carbon-carbon triple bond consists of one strong\sigma(sigma) bond formed by head-onsp-spoverlap and two weaker\pi(pi) bonds formed by the lateral overlap of two pairs of unhybridized %%MONEYBLOCK1%%p orbitals.
- Bond Angle: The H–C–C bond angle is exactly 180°.
- Bond Length: The C≡C triple bond length is 1.20 Å, making it shorter and stronger than both ethylene and ethane carbon bonds.
Acidity of Terminal Alkynes
A unique chemical property of acetylene is its weak acidic character. The hydrogen atoms attached to the sp hybridized carbon atoms have high s-character (50%), making the carbon atom highly electronegative. Consequently, the C–H bond is polarized, allowing the hydrogen to be removed by strong bases.
- Sodium Acetylide Formation: Reacts with sodium metal or sodamide (NaNH2) to liberate hydrogen gas.HC≡ CH + Na → HC≡ C-Na^+ + 1/2H2
- Silver Mirror Test: Reacts with ammoniacal silver nitrate (Tollens’ reagent) to form a white precipitate of silver acetylide (Ag2C2). This distinguishes alkynes from alkenes and alkanes.
Key Chemical Reactions
- Oxy-Acetylene Combustion: Burns in an excess of oxygen with an intensely hot, luminous flame, releasing tremendous thermal energy.2C2H2 + 5O2 → 4CO2 + 2H2O + Heat (≈ 3300°C)
- Addition of Water (Hydration): Reacts with water in the presence of mercuric sulfate (HgSO4) and dilute sulfuric acid (H2SO4) at 60°C to form acetaldehyde via keto-enol tautomerism.HC≡ CH + H2O Hg2+/H^+→ [CH2 = CH-OH] → CH3CHO
- Cyclic Polymerization: When passed through a red-hot iron tube at 873 K, three molecules of acetylene polymerize to form benzene (C6H6).3C2H2 Red-hot Fe, 873 K→ C6H6
Comparison Matrix: Ethylene vs. Acetylene
| Chemical Property / Feature | Ethylene (C2H4) | Acetylene (C2H2) |
| Hydrocarbon Class | Alkene (Olefin) | Alkyne |
| Carbon Hybridization | sp2 | sp |
| Molecular Shape | Planar | Linear |
| Carbon-Carbon Bonds | 1 σ bond, 1 π bond | 1 σ bond, 2 π bonds |
| Acidity of C-H bond | Non-acidic | Weakly Acidic |
| Ammoniacal AgNO3 Test | No reaction / No precipitate | Forms a white precipitate (Ag2C2) |
| Primary Industrial Use | Plastics, polymers, chemical synthesis | Welding, metal cutting, fuel gas |
Industrial Applications and Safety Frameworks
Metal Working (Oxy-Acetylene Welding)
The primary industrial application of acetylene is in oxy-acetylene welding and cutting torches. The combustion of acetylene with pure oxygen generates temperatures exceeding 3300°C, which is hot enough to melt, cut, and weld thick steel plates.
Artificial Fruit Ripening (Calcium Carbide Controversy)
Acetylene is frequently used illegally as an artificial ripening agent. Traders use moisture-exposed Calcium Carbide (CaC2) to generate acetylene gas, which mimics the ripening effects of natural ethylene.
CaC2 + 2H2O → Ca(OH)2 + C2H2
- Health and Regulatory Issues: Calcium carbide often contains traces of toxic arsenic and phosphorus hydride. The Food Safety and Standards Authority of India (FSSAI) has strictly banned the use of Calcium Carbide for fruit ripening under the Food Safety and Standards (Prohibition and Restrictions on Sales) Regulations. Ethylene gas is recommended as a safe and legal alternative.
Safe Storage and Transport
Acetylene gas is highly explosive under pressure and cannot be stored as a compressed gas in ordinary cylinders.
- Storage Solution: It is stored in specialized cylinders packed with a porous material (like asbestos or diatomaceous earth) saturated with acetone. Acetylene dissolves efficiently in acetone under pressure, rendering it stable and safe for transport.
Key Facts and Trivia for Prelims
- Edmund Davy: Discovered acetylene in 1836, identifying it as a “new carburet of hydrogen.” It was later rediscovered and named “acetylene” by French chemist Marcellin Berthelot in 1860.
- The Ripening Indicator: Bananas and mangoes ripened naturally produce ethylene endogenously, showing uniform ripening, skin browning, and development of sweetness. Conversely, carbide-ripened fruits using acetylene often show artificially bright yellow skin but possess unripe, sour pulp.
- Illuminant Gas: Before the widespread adoption of electricity, acetylene generated from small calcium carbide lamps (carbide lamps) was widely used by miners, cavers, and early automobile headlights due to its bright, white flame.