A Level Chemistry OCR Practice Exam

Which of the following reactions forms a saturated molecule?

1. Dehydration reaction

2. Substitution reaction

3. Addition reaction

4. Oxidation reaction

The correct answer is: Addition reaction

The formation of a saturated molecule occurs when carbon atoms in a hydrocarbon chain are bonded with the maximum number of hydrogen atoms, meaning there are no double or triple bonds present. In the context of the reactions listed, an addition reaction specifically involves the adding of atoms across a multiple bond, converting an unsaturated compound (which contains double or triple bonds) into a saturated one by fully saturating the carbon atoms with hydrogen. For example, when an alkene reacts with hydrogen in the presence of a catalyst, the double bond is broken, and additional hydrogen atoms are added, resulting in a saturated alkane. This is a characteristic feature of addition reactions, as they involve the conversion of unsaturated compounds to saturated compounds by adding elements like hydrogen or halogens. In contrast, dehydration reactions typically remove a water molecule, frequently resulting in the formation of double bonds and thus creating or maintaining unsaturation. Substitution reactions replace one atom or group in a molecule with another without increasing the saturation level. Oxidation reactions often lead to the formation of multiple bonds or may involve the loss of hydrogen, also potentially leading to unsaturation. Therefore, the addition reaction is the only process among the choices that directly leads to the formation of a saturated molecule.