The type of reaction ethyne mainly undergoes is

Step 1: Analyze the question We need the main type of reaction ethyne (acetylene, C₂H₂) undergoes.

Step 2: Key concept Ethyne is an alkyne with a carbon-carbon triple bond (HC≡CH). The triple bond is unsaturated, with two π bonds that are electron-rich and easily broken. Unsaturated hydrocarbons (alkenes and alkynes) mainly undergo addition reactions, where the π bonds break and new atoms attach to the carbon atoms.

Step 3: Examples of ethyne addition reactions

• Hydrogenation: HC≡CH + H₂ → H₂C=CH₂ (Ni catalyst), then + H₂ → CH₃CH₃
• Halogenation: HC≡CH + Br₂ → CHBr=CHBr, then + Br₂ → CHBr₂CHBr₂
• Hydrohalogenation: HC≡CH + HCl → CH₂=CHCl (vinyl chloride)
• Hydration: HC≡CH + H₂O → CH₃CHO (ethanal)

Step 4: Check the other options

• B. Carboxylation: adding CO₂, not characteristic of ethyne.
• C. Esterification: reaction between an alcohol and a carboxylic acid.
• D. Saponification: alkaline hydrolysis of fats/oils or esters to make soap.
• E. Substitution: typical of alkanes (saturated). Ethyne can do some substitution (e.g., replacing its acidic hydrogen to form metal acetylides with ammoniacal silver nitrate), but this is not its main reaction.

Answer: A. addition

Exam tip: Remember the rule: Alkanes → substitution; Alkenes and alkynes → addition. If you see "unsaturated" or "double/triple bond" in a question, think addition (and polymerization for alkenes).