Q.39 BF 3 reacts readily with ________.
(A) C5 H5 N (B) SnCl 2
(C) SO3 (D) (C5 H5 N)SnCl 2

BF3 reacts readily with pyridine (C5H5N), forming a stable Lewis acid-base adduct. This is a classic example from competitive exams like IIT JAM Biotechnology/Chemistry.

Option Analysis

Option (A) C5H5N (Pyridine):
Pyridine has a lone pair on nitrogen available for donation. BF3, with electron-deficient boron (6 electrons in valence shell), accepts this lone pair to form C5H5N→BF3 adduct, completing boron’s octet. This reaction occurs readily due to pyridine’s basicity and BF3’s strong Lewis acidity.​

Option (B) SnCl2 (Stannous chloride):
SnCl2 acts as a Lewis acid itself, with Sn having a lone pair but able to accept electrons via empty d-orbitals. No free lone pair donor exists for BF3 to accept, so no reaction occurs. Both are acids, not complementary.​

Option (C) SO3 (Sulfur trioxide):
SO3 functions as a Lewis acid, with sulfur electron-deficient. BF3 cannot accept electrons from SO3, preventing adduct formation. Their acidities differ, but no donor-acceptor interaction happens.​

Option (D) (C5H5N)−SnCl2:
This represents a pyridine-SnCl2 complex where pyridine’s lone pair coordinates to Sn (itself a Lewis acid). The nitrogen lone pair is unavailable, so BF3 cannot react readily with it.​

Correct Answer: (A) C5H5N​

Introduction to BF3 Reacts Readily with C5H5N

BF3 reacts readily with C5H5N due to classic Lewis acid-base chemistry, a key topic in IIT JAM Biotechnology and Chemistry syllabi. Boron trifluoride (BF3) accepts pyridine’s nitrogen lone pair, forming a stable adduct vital for understanding coordination compounds.​

Lewis Acid Properties of BF3

BF3 features sp²-hybridized boron with an empty p-orbital, making it electron-deficient. It readily forms adducts with Lewis bases like amines or pyridines, unlike protonic acids.​

Why Pyridine (C5H5N) Reacts

Pyridine’s sp² nitrogen lone pair donates to BF3:
C₅H₅N + BF₃ → C₅H₅N→BF₃
This stabilizes BF3 and is used in organic synthesis.​

Other Options Don’t React

• SnCl2: Mutual Lewis acids, no lone pair donation.​

• SO3: Lewis acid, no complementarity.​

• (C5H5N)−SnCl2: Lone pair blocked by Sn coordination.​