NDA (II) 2023 General Ability Test · Question 88 of 146

Correct — A, Ammonium cyanate. Ammonium cyanate is the ammonium salt of cyanic acid, an ionic compound built from an ammonium ion and a cyanate ion, and chemistry has always filed cyanates alongside carbonates, cyanides, carbides and the oxides of carbon on the inorganic side of the line. The question is really pointing at the single most famous experiment in the subject — in 1828 Friedrich Wohler described the conversion of ammonium cyanate into urea, turning a compound everyone called inorganic into one everyone called organic. The other three options are all carbon compounds of exactly the kind that experiment was contrasted with.

• (b)Marsh gas — Marsh gas is the old name for methane, CH4, released when plant matter rots without air in marshes and paddy fields. It is the simplest hydrocarbon and the first member of the alkane series, so it is organic.
• (c)Urea — Urea is the organic compound in this set — it is the nitrogenous waste that mammals excrete and the product side of the very reaction the question alludes to. Its formula is CO(NH2)2 and it is classified as an organic compound even though it holds no carbon-hydrogen bond.
• (d)Cane sugar — Cane sugar is sucrose, C12H22O11, a disaccharide of glucose and fructose. Carbohydrates are a core family of organic compounds, so it cannot be the exception.

Organic chemistry is the chemistry of carbon compounds, but the definition carries an inherited list of exceptions. Carbon monoxide, carbon dioxide, carbonates and bicarbonates, cyanides, cyanates, carbides and the metal-carbon salts are treated as inorganic by convention, even though every one of them contains carbon. Ammonium cyanate belongs to that exception list, which is what makes it the odd item here.

The trap works in two directions. A student who reasons 'it contains carbon, so it is organic' marks ammonium cyanate as organic and then has to force one of the other three into the answer. A student who reasons 'urea contains no carbon-hydrogen bond, so it cannot be organic' picks urea instead. Both readings miss the historical peg the question is built on. It is worth knowing the honest version of that history too: the popular claim that this single 1828 experiment demolished the doctrine of vitalism is disputed by historians of chemistry, since Dalton and Berzelius had already persuaded chemists that organic and inorganic matter obey the same laws, and the idea only really lost ground after Kolbe reported a further conversion in 1845. One survey found that about ninety per cent of chemistry textbooks still repeat some version of the simplified story.

• Ammonium cyanate, NH4OCN, is an inorganic ammonium salt; cyanates, cyanides, carbonates and carbides are all conventionally inorganic despite containing carbon.
• Friedrich Wohler described the conversion of ammonium cyanate into urea in 1828.
• Marsh gas is methane, CH4, the first member of the alkane series.
• Cane sugar is sucrose, C12H22O11, a disaccharide made of glucose and fructose.

• Assuming every carbon-containing compound is organic; the exception list is short but it is examined often.
• Rejecting urea because it has no carbon-hydrogen bond — it is still the standard textbook example of an organic compound.
• Not recognising marsh gas as methane, which turns a routine item into a guess.

Asked as a negative-form odd-one-out item — spot the compound that is not organic.

• practice — not a real PYQ

  Marsh gas is another name for which one of the following?

  • (a)Ethane
  • (b)Methane
  • (c)Carbon monoxide
  • (d)Nitrous oxide

  Answer(b) Methane — it is released by the anaerobic decay of plant matter in marshes and paddy fields.

• practice — not a real PYQ

  The synthesis of urea from ammonium cyanate was first described by

  • (a)Friedrich Wohler
  • (b)Antoine Lavoisier
  • (c)John Dalton
  • (d)Joseph Priestley

  Answer(a) Friedrich Wohler — he described the conversion in 1828, the reaction this question is built around.