Question

3 -Chlorocyclopropene, on treatment with AgBF \(_{4}\), gives a precipitate of AgCl and a stable solution of a product that shows a single \({ }^{1} \mathrm{H}\) NMR absorption at \(11.04 \delta .\) What is a likely structure for the product, and what is its relation to Hückel's rule?

Short answer

The product is the aromatic cyclopropenyl cation, following Hückel's rule (4n + 2).

Step-by-step solution

Analyze the Reaction Components

The reaction involves 3-chlorocyclopropene and AgBF₄, where AgBF₄ acts as a reagent that can facilitate the removal of chlorine as AgCl. Thus, the chlorine from 3-chlorocyclopropene is likely substituted by a positive charge, forming a cyclopropenyl cation.

Understand the Stability of the Cyclopropenyl Cation

The cyclopropenyl cation (\(C_3H_3^+\)) is aromatic due to having a planar ring with 3 conjugated pi electrons (the original double bond plus the positive charge). According to Hückel's rule (4n + 2 pi electrons), the cyclopropenyl cation (with n=0) becomes aromatic.

Predict the NMR Observation

Aromatic protons often give NMR signals in the region around 7-9 ppm, but the cyclopropenyl cation can cause shifts due to ring current effects or other factors. The observed 11.04 δ signal likely corresponds to the aromatic protons in the cyclopropenyl cation.

Conclude the Product Structure

Based on the reaction and NMR data, the likely structure of the product is the aromatic cyclopropenyl cation (\(C_3H_3^+\)), a stable ion due to its adherence to Hückel's rule.

Key concepts

Hückel's rule

Hückel's rule is a fundamental concept in organic chemistry used to determine the aromaticity of a molecule. According to this rule, a planar, cyclic molecule with \(4n + 2\) π electrons possesses aromatic stability, where \(n\) is a non-negative integer. This rule provides insight into why certain molecules have unique stability and electronic properties. Aromatic compounds, due to resonance, exhibit lower energy states compared to their non-aromatic counterparts, making them more stable.

• **Planarity:** The molecule must be flat or planar to allow for effective overlap of p orbitals.
• **Cyclic structure:** The molecule must form a closed loop of overlapping p orbitals.
• **Conjugated π system:** There should be continuous overlapping p orbitals across the entire structure.
• **(4n + 2) π electrons:** This specific number of π electrons leads to aromaticity.

For the cyclopropenyl cation (\(C_3H_3^+\)), \(n=0\), which results in \(4(0) + 2 = 2\) π electrons. This adherence to Hückel's rule means it is aromatic, explaining its stability despite being a cation.

cyclopropenyl cation

The cyclopropenyl cation is a fascinating molecule primarily due to its status as one of the simplest aromatic ions. Represented as \(C_3H_3^+\), this cation arises from 3-chlorocyclopropene when the chlorine atom is replaced by a positive charge during reaction with AgBF₄. This cation possesses a perfectly planar triangular structure with a net positive charge distributed evenly due to delocalization over the three carbon atoms.
The cyclopropenyl cation is aromatic because it satisfies Hückel’s rule with its \(2\) π electrons. The circular arrangement of these electrons allows them to delocalize around the ring, granting the molecule added stability despite the inherent strain in such a small, triangular structure. Its aromaticity also influences its reactivity and the types of interactions it can participate in, often favoring stabilization via resonance.

NMR spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool used to study the structure of organic compounds. This technique relies on the magnetic properties of certain atomic nuclei. In the context of the cyclopropenyl cation, NMR plays a crucial role in confirming its structure and stability.
The surprising \(11.04\, \delta\) signal observed in the \(^1H\) NMR spectrum of the cyclopropenyl cation is due to its aromatic nature and the specific electronic environment of the protons. Typically, aromatic protons resonate downfield (high chemical shift) due to deshielding effects caused by the ring current associated with aromaticity. This downfield shift is generally seen in the region of 7-9 ppm for typical aromatics but can extend to unusual values as seen here.
The distinctive NMR shift further reinforces the conclusion that the cyclopropenyl cation exhibits a classic case of aromatic behavior, verifying its unique chemical nature.

organic chemistry reactions

Organic chemistry reactions form the basis of transforming one molecule into another, leveraging the diverse functional groups within organic compounds. In this particular example, we're looking at a substitution reaction where 3-chlorocyclopropene reacts with silver tetrafluoroborate (AgBF₄).
Here, the chlorine atom in 3-chlorocyclopropene is removed as silver chloride (AgCl), which precipitates out of solution, while the remaining structure becomes a cyclopropenyl cation. This reaction highlights several key aspects:

• **Formation of stable cations:** The cyclopropenyl cation is stable due to aromaticity, as described by Hückel’s rule.
• **Role of catalysts and reagents:** AgBF₄ not only aids in removing chlorine but also stabilizes the cationic form by contributing to aromaticity.

This specific transformation showcases the ways in which organic reactions can result in stable, unique intermediates that offer fantastic insights into the behavior and characteristics of organic structures.