Which one of the following is NOT a synthetic detergent ?
- A.CH3(CH2)10CH2OSO3⁻Na⁺
- B.[CH3(CH2)15–N⁺(CH3)3]Br⁻
- C.CH3(CH2)16COO⁻Na⁺
- D.CH3(CH2)16COO(CH2CH2O)nCH2CH2OH
Correct answer
Explanation
The correct answer is C, CH3(CH2)16COO⁻Na⁺. This is sodium stearate, the sodium salt of a long-chain fatty acid, which makes it a soap and not a synthetic detergent.
Soaps are sodium or potassium salts of fatty acids such as stearic, palmitic and oleic acid, made by saponification of oils and fats with an alkali. Their carboxylate head reacts with calcium and magnesium ions in hard water and forms an insoluble scum. Synthetic detergents carry other heads, such as sulphonate, sulphate or ammonium groups, whose calcium and magnesium salts stay dissolved, so they lather well even in hard water.
A is a detergent: sodium lauryl sulphate, an anionic detergent used in toothpastes and shampoos.
B is a detergent: cetyltrimethylammonium bromide, a cationic detergent used in hair conditioners.
D is a detergent: an ester of stearic acid with polyethylene glycol, a non-ionic detergent used in dishwashing liquids.
Exam tip: –COO⁻Na⁺ on a long chain means soap; sulphate, sulphonate or ammonium heads mean detergent.
Practice Questions
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- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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