Question

Boron trichloride on heating with ammonium chloride at $ {140^ \circ }C $ gives a compound (A), which on reduction with sodium borohydride followed by hydrolysis gives another compound (B).
Hence compound (A) and (B) are respectively.

Answer 1

Hint: Follow each and every step asked in the question to reach the solution. Only the terminal alkynes react with an ammoniacal solution of silver nitrate or cuprous chloride. Therefore this reaction can be used to distinguish between $ 2-alkynes $ and $ 1-alkynes $ . The terminal alkyne i.e. $ 1-alkyne $ will give this test while $ 2-alkynes $ will not give this test.

Complete answer:
Boron trichloride i.e. $ BeC{l_3} $ on heating with ammonium chloride gives trichloro- borazine. So, the compound (A) is trichloro-borazine.
 $ 3BC{l_3} + 3N{H_4}Cl \to {B_3}{N_3}{H_3}C{l_3} + 9HCl $
And then the reduction of compound (A) i.e. trichloro- borazine with sodium borohydride produces borazine and the reaction for the equation can be written as:
 $ 2{B_3}{N_3}{H_3}Cl + 6Na[B{H_4}] \to 2{B_3}{N_3}{H_6} + 6NaCl + 3{B_2}{H_6} $
Borazine has some similar properties as benzene but Borazine is kinetically and thermodynamically more stable than benzene.
Borazine on hydrolysis readily yields orthoboric acid, ammonia, and hydrogen.
 $ {B_3}{N_3}{H_6} + 9{H_2}O \to 3{H_3}B{O_3} + 3N{H_3} + 3{H_2} $
So the compound (B) is orthoboric acid.
Ortho boric acid is also known as boric acid or hydrogen borate with the molecular formula $ {H_3}B{O_3} $ . Ortho boric acid is a weak, monobasic Lewis acid of boron.

Note:
Borax or sodium tetraborate is an important compound of boron. Borax contains the tetranuclear units $ {[{B_4}{O_5}{(OH)_4}]^{2 - }} $ and therefore the correct formula is $ N{a_2}[{B_4}{O_5}{(OH)_4}].8{H_2}O $ . Borax is used in the laboratory for borax bead tests and it is also used in making optical glass and in softening of water and cleansing agents.
Boric acid is a trivial name for orthoboric acid. It is usually prepared by acidifying an aqueous solution of borax. The resulting solution upon concentration and cooling gives crystals of boric acid. Boric acid is used in medicines as an eyewash.