Final answer:
A reagent adds across the double or triple bonds in an unsaturated hydrocarbon to form a new compound. Hydrogenation and hydration are common addition reactions, but aromatic hydrocarbons like benzene are an exception to this rule.
Step-by-step explanation:
Understanding how reagents react with unsaturated hydrocarbons to form various compounds is crucial in organic chemistry. An addition reaction occurs when atoms or molecules add across the double or triple bonds of unsaturated hydrocarbons like alkenes or alkynes. For instance, hydrogenation involves adding hydrogen (H2) to an alkene in the presence of a platinum catalyst, resulting in an alkane.
Hydration, another addition reaction, involves adding water (H2O) to an alkene, leading to the formation of an alcohol. However, not all hydrocarbons with double or triple bonds undergo addition reactions readily. Aromatic hydrocarbons like benzene (C6H6) typically do not respond to addition due to their unique bonding structure. Thus, to answer the student's question accurately, one must know the structure of the target compound and the type of unsaturated hydrocarbon involved to select the appropriate reagent for the addition reaction.