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GATE 2024 CY – Question 36

Inorganic Chemistry · Main Group Elements: Shapes and structures of molecules using VSEPR postulates, Hydrides, halides, oxides, oxoacids, nitrides, sulfides - shapes and reactivity. Structure and bonding of boranes, carboranes, silicones, silicates, boron nitride, borazine and phosphazenes: electron counting in polyhedral boranes, isolobal analogy. Allotropes of carbon, phosphorus and sulfur. Industrial synthesis of compounds of main group elements such as NH3, H2SO4 and HNO3. Chemistry of noble gases and interhalogen compounds. Acid-base concepts and principles (Lewis, Brønsted, HSAB and acid-base catalysis). · 2 marks · Multiple choice

Borax on treatment with NaOH and $\mathrm{H_2O_2}$ forms X. Compound X on reaction with PhCN at 60 °C in methanol–water gives Y as major product. X and Y, respectively, are

  1. $\mathrm{NaB(O)(OH)_2\cdot nH_2O}$ and $\mathrm{PhCONH_2}$
  2. $\mathrm{NaB(O)(OH)_2\cdot nH_2O}$ and $\mathrm{PhCOOH}$
  3. $\mathrm{Na_2B_2(O_2)_2(OH)_4\cdot nH_2O}$ and $\mathrm{PhCONH_2}$
  4. $\mathrm{Na_2B_2(O_2)_2(OH)_4\cdot nH_2O}$ and $\mathrm{PhCOOH}$

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Correct answer: (C) $\mathrm{Na_2B_2(O_2)_2(OH)_4\cdot nH_2O}$ and $\mathrm{PhCONH_2}$

Explanation

**Step 1: borax with NaOH and H₂O₂.** This is the way sodium perborate is made. The product X is the dimeric peroxoborate containing two peroxide bridges, $\text{Na}_2[\text{B}_2(\text{O}_2)_2(\text{OH})_4]\cdot n\text{H}_2\text{O}$. (Option C and D have this formula; options A and B show a monomeric formula.)

**Step 2: X with benzonitrile.** Perborate acts as a mild source of nucleophilic peroxide (as in the Radziszewski reaction). Hydroperoxide adds to the nitrile and the peroxyimidic acid transfers oxygen, giving the **amide**, **benzamide** ($\text{PhCONH}_2$). The reaction stops at the amide stage and does not hydrolyse it to benzoic acid under these mild conditions.

So X $=\text{Na}_2\text{B}_2(\text{O}_2)_2(\text{OH})_4\cdot n\text{H}_2\text{O}$ and Y $=\text{PhCONH}_2$ (option **C**).