Liquid vegetable oils are converted to solid margarine by the use of
- (a)hydrogen gas
- (b)chlorine gas
- (c)carbon dioxide gas
- (d)oxygen gas
Correct — A, hydrogen gas. Liquid vegetable oils are rich in unsaturated fats, whose long carbon chains carry carbon-carbon double bonds. Bubbling hydrogen through the warm oil in the presence of a nickel catalyst adds hydrogen across those double bonds and saturates the chains. Saturated chains pack together far more closely, the melting point rises, and the liquid oil sets into a semi-solid fat — margarine abroad, vanaspati ghee in India.
- (b)chlorine gas — Chlorine cannot add hydrogen to the chain, and chlorination is not a food-processing step. Chlorine is a toxic, strongly reactive gas used for disinfecting water and for bleaching, not for hardening edible oils.
- (c)carbon dioxide gas — Carbon dioxide does not react with the double bonds in an oil at all. It is used to carbonate drinks and to extinguish fires, and passing it through an oil would leave the oil unchanged.
- (d)oxygen gas — Oxygen does the opposite of what is wanted — it attacks unsaturated chains and turns fats rancid, which is why oils are stored away from air rather than bubbled with oxygen.
An unsaturated compound contains carbon-carbon double or triple bonds, and its characteristic reaction is addition. Hydrogen adds across a carbon-carbon double bond in the presence of a catalyst such as nickel or palladium, and this reaction, called hydrogenation, converts unsaturated vegetable oils into saturated or partially saturated fats that are solid at room temperature.
The reaction is worth knowing for both its chemistry and its health story. Chemically it is an addition and not a substitution — nothing leaves the molecule, hydrogen simply adds on across the double bond, and the catalyst is recovered unchanged. Nutritionally, partial hydrogenation is the industrial source of trans fats, which raise LDL cholesterol and are associated with heart disease, and that is why food regulators have moved to restrict them and why manufacturers advertise products as free of trans fats. The same reaction therefore appears in exams as a public-health question as often as a chemistry one.
- Hydrogenation adds hydrogen across a carbon-carbon double bond and is classified as an addition reaction.
- Vegetable oils are unsaturated and liquid; hydrogenating them gives solid fats such as vanaspati and margarine.
- Nickel is the catalyst normally used for the hydrogenation of edible oils.
- Saturated fats are considered less healthy than unsaturated oils, and partial hydrogenation produces trans fats.
- Unsaturated compounds undergo addition reactions, while saturated hydrocarbons undergo substitution reactions.
- Choosing oxygen because it is the gas most associated with cooking and food.
- Calling hydrogenation a substitution reaction — nothing is replaced, hydrogen is added.
- Assuming the nickel catalyst is used up; it is recovered unchanged at the end.
As the reagent recall here, as the reaction type (addition), or as a health question on trans fats and vanaspati.
A company marketing food products advertises that its items do not contain trans-fats. What does this campaign signify to the customers? 1. The food products are not made out of hydrogenated oils. 2. The food products are not made out of animal fats/oils. 3. The oils used are not likely to damage the cardiovascular health of the consumers. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer(c) 1 and 3 only
UPSC prelims tested the consequence of the very same reaction — industrial trans fats come from partially hydrogenated vegetable oils, so a trans-fat-free claim means hydrogenated oils were not used.
Hydrogenation of vegetable oils using nickel catalyst is an example of
- (a) Substitution reaction.
- (b) Elimination reaction.
- (c) Addition reaction.
- (d) Free-radical polymerization.
Answer(c) Addition reaction.
NDA-I 2021 asked for the reaction type of exactly this process; between the two papers the reagent, the catalyst and the classification have all now been examined.
- practice — not a real PYQ
The catalyst normally used in the hydrogenation of vegetable oils is
- (a)iron
- (b)nickel
- (c)manganese dioxide
- (d)vanadium pentoxide
Answer(b) nickel — finely divided nickel is the standard catalyst for hardening edible oils.
- practice — not a real PYQ
Hydrogenation of an unsaturated oil is an example of
- (a)substitution reaction
- (b)addition reaction
- (c)oxidation reaction
- (d)neutralisation reaction
Answer(b) addition reaction — hydrogen adds across the carbon-carbon double bond and nothing is displaced.