Chapter 4 · Question 12
Distinguish between addition and substitution reactions with one chemical equation each. What is hydrogenation and why is it industrially important?
Q12
Distinguish between addition and substitution reactions with one chemical equation each. What is hydrogenation and why is it industrially important?
Answer Revealed
Direct Answer:
Addition reactions occur in unsaturated hydrocarbons where atoms (e.g., hydrogen) add across the double or triple bond to form a saturated compound, catalysed by nickel or palladium. Example: hydrogenation of ethene — . Substitution reactions occur in saturated hydrocarbons where an atom (e.g., chlorine) replaces a hydrogen atom, typically requiring sunlight. Example: . Hydrogenation is an addition reaction that converts unsaturated compounds to saturated ones by adding hydrogen. Industrially, it converts liquid vegetable oils (unsaturated) into solid vanaspati ghee (saturated fats) using a nickel catalyst. Vegetable oils with long unsaturated carbon chains are healthier for cooking; animal fats with saturated fatty acids are considered harmful.
Simple Explanation
Addition is for unsaturated compounds — you add atoms across the double/triple bond, like opening a door to let more people in. Ethene + hydrogen → ethane. Substitution is for saturated compounds — one atom gets kicked out and replaced. Methane + chlorine in sunlight → chloromethane + HCl (a hydrogen is replaced by chlorine). Hydrogenation is an addition reaction used in factories to turn liquid vegetable oils into solid vanaspati ghee — they pump hydrogen through the oil with a nickel catalyst, converting unsaturated fats to saturated ones.
Exam-Ready Structure
Addition and substitution are two distinct reaction types that distinguish the reactivity of unsaturated versus saturated hydrocarbons: 1. Addition reaction: (a) Definition — unsaturated hydrocarbons add hydrogen (or other atoms) in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons. The double or triple bonds open up, and atoms attach to the now-free valencies. (b) Equation — . (c) Catalyst — a catalyst is a substance that causes a reaction to occur or proceed at a different rate without itself being affected by the reaction. 2. Substitution reaction: (a) Definition — saturated hydrocarbons are generally quite unreactive and inert in the presence of most reagents. However, in the presence of sunlight, chlorine can replace hydrogen atoms one by one in a very fast reaction. (b) Equation — . The hydrogen atom of methane is substituted by a chlorine atom. Further substitution can yield , (chloroform), and (carbon tetrachloride). 3. Hydrogenation and its industrial importance: (a) Hydrogenation is the addition of hydrogen to an unsaturated hydrocarbon. (b) Industrial application — vegetable oils generally have long unsaturated carbon chains (contain double bonds). These liquid oils are converted into solid fats (vanaspati ghee) by hydrogenation using a nickel catalyst. The unsaturated bonds are saturated with hydrogen, making the product solid at room temperature. (c) Health context — vegetable oils with unsaturated fatty acids are recommended for cooking. Animal fats (containing saturated fatty acids) are considered harmful to health because saturated fats can raise cholesterol levels.
Key Points
- Addition reaction: unsaturated compounds add atoms across double/triple bonds → saturated
- Example: (hydrogenation)
- Substitution reaction: saturated compound — atom replaces H, needs sunlight for halogens
- Example:
- Hydrogenation: vegetable oils (unsaturated) vanaspati ghee (saturated)
- Catalysts (Ni, Pd) speed up addition without being consumed
Related Questions
Q5
Distinguish between saturated and unsaturated carbon compounds with the help of examples — ethane (), ethene (), and ethyne (). Draw their electron dot structures. Which type is more reactive and why?
Q11