What is the name of the reaction that converts alkynes into alkanes? (A) Halogenation, (B) hydrogenation, (C) hydrohalogenation, (D) alkylation, or (E) hydration.
Each of the reactions listed is an example of an addition reaction. Addition reactions are a type of chemical reaction where two or more molecules combine to form a larger molecule without any by-products forming. The name of each addition reaction indicates the type of molecule that is added to the initial organic starting material.
For example, halogenation involves the addition of one or more halogens to a compound. This type of reaction is typically performed using diatomic chlorine, bromine, or iodine. Hydrogenation involves the addition of hydrogen. This type of reaction is typically carried out using molecular hydrogen in the presence of a catalyst. Hydrohalogenation involves the addition of hydrogen and a halogen. This type of reaction typically involves the use of a hydrogen halide, like hydrogen chloride, hydrogen bromide, or hydrogen iodide. Alkylation involves the addition of an alkyl group or a hydrocarbon. A common alkylation reaction used in oil refining involves the addition of propene in the presence of a catalyst. It’s worth mentioning here that not all alkylation reactions are true addition reactions, as some of these reactions do produce by-products. Hydration involves the addition of water. An acid catalyst is often used during this reaction.
Now that we can recognize the basics of each reaction, we need to identify which of these reactions can be used to convert alkynes into alkanes. An alkyne is an unsaturated hydrocarbon that contains at least one carbon-carbon triple bond. Ethyne is the simplest alkyne. An alkane is a saturated hydrocarbon, meaning that the molecule consists only of carbon and hydrogen atoms that are single-bonded together. Ethane is a two-carbon alkane.
Looking at the two structures, we can see that ethane has four more hydrogen atoms than ethyne. So, in order to convert from ethyne to ethane, an alkyne to an alkane, four hydrogen atoms must be added to the alkyne. In fact, to carry out this specific reaction, two equivalents of hydrogen gas are needed for each equivalent of ethyne. And the reaction is often performed with a nickel catalyst. Since the reaction involves the addition of hydrogen, the name of this reaction is hydrogenation. So the name of the reaction that converts alkynes into alkanes is hydrogenation, answer choice (B).