Question

\(\left[\mathrm{NS}_{2}\right]\left[\mathrm{SbF}_{6}\right]\) reacts with nitriles, \(\mathrm{RC} \equiv \mathrm{N},\) to give \([\mathrm{X}]\left[\mathrm{SbF}_{6}\right]\) where \([\mathbf{X}]^{+}\) is a cycloaddition product. Propose a structure for \([\mathbf{X}]^{+}\) and show that it is a \(6 \pi\) -electron system. Do you expect the ring to be planar or puckered? Give reasons for your answer.

Short answer

The structure \([\mathbf{X}]^{+}\) is a six-membered, aromatic, 6\(\pi\)-electron system and expected to be planar.

Step-by-step solution

Analyzing Reactants and Products

The reactant \([\mathrm{NS}_{2}][\mathrm{SbF}_{6}]\) interacts with a nitrile such as \(\mathrm{RC} \equiv \mathrm{N}\) forming \([\mathrm{X}][\mathrm{SbF}_{6}]\). Here, \([\mathrm{X}]^{+}\) is a cycloaddition product, suggesting a [4+2] cycloaddition involving two electron-deficient species.

Propose the Structure of \([\mathbf{X}]^{+}\)

Cycloaddition often involves forming a new ring. Considering a general nitrile \(\mathrm{RC} \equiv \mathrm{N}\) interacts with \([\mathrm{NS}_{2}]^{+}\), a plausible structure for \([\mathbf{X}]^{+}\) could be a six-membered ring, where the nitrogen atoms form a bridge and help close the ring system. The initial nitrile triple bond participates by opening to form part of this ring.

Determine the Electron Count

To check the \(6\pi\)-electron system, consider the number of double bonds in the proposed ring. Typically, each double bond contributes two \(\pi\)-electrons. In our proposed structure, if there are three double bonds involved, we indeed have \(6\pi\)-electrons, satisfying the Huckel's rule for aromaticity.

Assess Ring Planarity

For a ring to maintain a planar structure, conjugation and aromaticity are key. A \(6\pi\)-electron system fits Huckel's rule and tends to be planar to maximize \(\pi\)-overlap. Thus, the ring in \([\mathrm{X}]^{+}\), being potentially aromatic with delocalized electrons, is expected to be planar.

Key concepts

6 pi-electron system

A 6 pi-electron system is an essential concept in organic chemistry, especially when discussing aromatic compounds. In cycloaddition reactions, such as the one presented in the problem, a cycloaddition product, denoted as \([\mathbf{X}]^{+}\), forms through the interaction of \([\mathrm{NS}_{2}]\) with a nitrile.

To qualify as a 6 pi-electron system, this new structure must contain a total of 6 \(\pi\)-electrons. Pi electrons are typically found in double bonds. In a typical six-membered aromatic ring, there are usually three double bonds. Each double bond contributes two \(\pi\)-electrons, equating to a total of 6 \(\pi\)-electrons. These electrons play a crucial role in stabilizing the molecular structure through electron delocalization, enabling aromaticity.

Huckel's rule

Huckel's rule is a fundamental guideline used for determining the aromaticity of cyclic compounds. It states that for a molecule to exhibit aromaticity, it must be planar, cyclic, conjugated, and contain \((4n+2)\) \(\pi\)-electrons, where \(n\) is a non-negative integer.

In this context, our cycloaddition product \([\mathbf{X}]^{+}\) should follow Huckel's rule to affirm its aromatic character. Given that the proposed structure holds 6 \(\pi\)-electrons (where \(n=1\)), it satisfies the \((4n+2)=6\) requirement. Thus, the potential aromaticity of \([\mathbf{X}]^{+}\) is intrinsically linked to Huckel's rule, lending the compound favorable stability. This aromatic stability is one of the main reasons such cyclic compounds form and persist in nature.

Nitrile interaction

The interaction of nitriles in chemical reactions is pivotal, particularly in forming specific structures through cycloaddition reactions. Nitriles tend to partake due to their high electron-withdrawing nature, originating from the carbon-nitrogen triple bond.

When a nitrile \(\mathrm{RC} \equiv \mathrm{N}\) meets the \([\mathrm{NS}_{2}]\), it forms a bond, leading to the cycloaddition product \([\mathbf{X}]^{+}\). In this reaction, the triple bond in the nitrile opens up, participating in the reformation of the new ring structure.

• The nitrile's role is crucial as it helps close the ring by providing necessary electrons.
• This coupling often results in a six-membered ring, a common motif in aromatic compounds.

The nitrile's involvement gives insights into electron movement and redistribution in molecular reactions.

Aromaticity

Aromaticity is a geometrical and electronic configuration common to a class of cyclic compounds that includes benzene. Such compounds hold a resonant stability far greater than any expected from their simple structure experiment.

In order to exhibit aromaticity, a molecule must be cyclic, planar, completely conjugated, and adhere to Huckel's rule. In the case of \([\mathbf{X}]^{+}\), the aromatic nature is anticipated due to the presence of 6 \(\pi\)-electrons and a cyclic structure.

• Aromaticity results in a significant stabilization due to the delocalization of electrons across the molecular orbitals.
• This characteristic results in notable chemical properties like resistance to reactions that would typically break the aromatic ring.

The stability arising from aromaticity is a key determinant in the molecule's chemical behavior and interactions.

Planar vs puckered

Determining whether a cyclic structure is planar or puckered is crucial, as planarity influences aromaticity. A planar structure allows for continuous overlap of \(\pi\)-orbitals above and below the plane of the ring, which is essential for aromaticity.

The ring structure in \([\mathbf{X}]^{+}\) likely remains planar due to its aromatic character and Huckel's requirement for planarity for maximum \(\pi\)-overlap.

• In a planar system, all atoms in the ring lie in a single plane, facilitating resonance.
• In contrast, a puckered conformation disconnects conjugated systems, disrupting potential aromatic properties.

Thus, the cycloaddition product's shape—and therefore its properties—is heavily influenced by the demand for planarity to sustain aromaticity.