Which of the following is the most reactive hydrocarbon?

The reactivity of hydrocarbons depends on the type of bonding between carbon atoms — single (σ), double (σ + π), or triple (σ + 2π). The presence of π-electrons makes a compound more reactive because these electrons are more exposed and can interact easily with electrophiles.

Ethene (C₂H₄), an alkene, contains a carbon–carbon double bond composed of one sigma (σ) and one pi (π) bond. The π-bond is weaker and lies above and below the molecular plane, making it easily accessible to reagents. This allows ethene to undergo a wide range of addition reactions such as:

• 
  

  Hydrogenation: C₂H₄ + H₂ → C₂H₆

  
  


• 
  

  Halogenation: C₂H₄ + Br₂ → C₂H₄Br₂

  
  


• 
  

  Hydrohalogenation: C₂H₄ + HCl → C₂H₅Cl

  
  

Comparison with other hydrocarbons:

• 
  

  Ethane (C₂H₆): An alkane with only single σ-bonds. These bonds are very stable and require high activation energy for reactions. Hence, alkanes are the least reactive.

  
  


• 
  

  Ethyne (C₂H₂): An alkyne with a triple bond (σ + 2π). Though it has two π-bonds, the carbon atoms are sp-hybridized and hold the electrons tightly, reducing reactivity in addition reactions compared to ethene.

  
  


• 
  

  Benzene (C₆H₆): An aromatic compound with delocalized π-electrons, which are stabilized by resonance. This delocalization lowers its reactivity toward addition reactions, making it less reactive than ethene.

  
  

Order of reactivity:
Ethane < Benzene < Ethyne < Ethene

Ethene’s high reactivity arises from the easy availability of its π-electrons and its tendency to form stable addition products. It readily reacts with halogens, hydrogen, acids, and oxidizing agents.

Thus, among the given hydrocarbons, ethene is the most reactive due to the presence of a reactive double bond capable of participating in multiple addition reactions.

✅ Correct Option: C) Ethene