Question

3-Chlorocyclopropene, on treatment with AgBF₄, gives a precipitate of AgCl and a stable solution of a product that shows a single ¹HNMR absorption at 11.04 d. What is a likely structure for the product, and what is its relation to Hückel’s rule?

3-Chlorocyclopropene

Short answer

The reaction of 3-chlorcylopropene with AgBF₄

Step-by-step solution

Aromatic

Aromatic: A molecule must be cyclic planar, completely conjugated, must satisfy huckle’s ruleHuckle’s rule$\left(4n+2\right)\mathbf{\pi }$electrons, and contains a particular number of pi electrons.

3-chlorocyclopropene

The compound 3-chlorocyclopropene readily reacts with silver tetrafluoroborate(AgBF₄) gives a stable salt (product)

The reaction of 3-chlorcylopropene with AgBF₄

The cycloprep-2-en-1-ilium combines with $B{F_4}^{-}$and forms the stable salt. Here, Ag⁺ combines with Cl^-and forms AgCl precipitate.

The formed product shows a single absorption in ¹HNMR at 11.04 ppm because of its aromatic nature. The carbocation formed in the product is sp² hybridized with a vacant p orbital, so it is planar and satisfies Huckel’s rule (here n=0). Thus, the cyclopropenyl cation that is formed as a product is aromatic. Its resonance structures are as shown:

Resonance structures