The correct option is
C Ethanol
When certain
organic polyhydroxy compounds such as glycerol, mannitol or sugars are added,
B(OH)3 behaves as a strong monobasic acid.
Here,
cis-diol forms a very stable complex
with
[B(OH)4]–, removing it from the solution.
Thus, the removal of one of the products shifts the equilibrium in the forward direction.
B(OH)3+2H2O⇌B(OH)−4+H3O+
Hence, it makes the
B(OH)3 strong monobasic acid.
Ethanol is not a diol so it does not form complex with
[B(OH)4]–.
THEORY
Properties of orthoboric acid
- White crystalline solid with soapy touch.
- Sparingly soluble in water but highly soluble in hot water.
- It has a layer structure in which planar BO3 units are joined by hydrogen bonds.
- Boric acid is a weak monobasic acid.
- It is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion.
B(OH)3+2H2O→B(OH)−4+H3O+
If certain organic polyhydroxy compounds such as glycerol, mannitol, or sugars are added,
B(OH)3 behaves as a strong monobasic acid.
The cis-diol forms a very stable complex with
[B(OH)4]–, removing it from the solution. The reaction is reversible. Thus, the removal of one of the products shifts the equilibrium in the forward direction.
B(OH)3+2H2O⇌B(OH)4+H3O+
Ethanol does not form similar complexes, but catechol, salicylic acids, and mannitol form similar complexes.
Polymeric metaborate species are formed at higher concentrations.
3B(OH)3⇌H3O++[B3O3(OH)4]–+H2O
On heating, orthoboric acid above 370K forms metaboric acid,
HBO2 which on further heating yields boric oxide,
B2O3.
H3BO3△→HBO2△→B2O3
Test for Borate radical:
When
H3BO3 is heated with
C2H5OH, the evolved gas is burned to form a green-edged flame.
H3BO3+3C2H5OH→B(OC2H5)3ethyl borate+3H2O