Which of the following reactions is common to all hydrocarbones?

Analyze the question: We need the reaction that all hydrocarbons (alkanes, alkenes, alkynes, arenes) undergo.

Explain the concepts:

• Combustion: Every hydrocarbon burns in oxygen to give carbon dioxide and water (or carbon monoxide/carbon if oxygen is limited).
• Addition: Only unsaturated hydrocarbons (alkenes and alkynes) with double or triple bonds do this. Alkanes are saturated and do not.
• Polymerization: Mainly alkenes and some alkynes, because they need unsaturation to join monomers together.
• Condensation: This joins molecules with the loss of a small molecule such as water. It involves functional groups like -OH, -COOH and -NH₂, which hydrocarbons do not have.

Solution:

General equation for combustion:

CxHy + (x + y/4) O₂ → x CO₂ + (y/2) H₂O

Examples:

• Alkane: CH₄ + 2O₂ → CO₂ + 2H₂O
• Alkene: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
• Alkyne: 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O

Since every hydrocarbon, saturated or unsaturated, can burn, combustion is the only reaction common to all of them.

Answer: A. Combustion

Exam tip: When a question asks what is "common to all" a group, pick the reaction that does not depend on a special structure. Addition and polymerization need double or triple bonds, so they are not common to all hydrocarbons. Combustion always works.