Question

(a) Draw the Lewis structures for at least four species that have the general formula $$ [: X \equiv Y:]^{\pi} $$ where \(X\) and \(Y\) may be the same or different, and \(n\) may have a value from \(+1\) to \(-2\). (b) Which of the compounds is likely to be the strongest Bronsted base? Explain. [Sections 22.1, \(22.7\), and \(22.91\) eriodic Trends and Chemical Reactions Section 22.1)

Short answer

The four species that fit the general formula \([: X \equiv Y:]^{\pi}\) are the azide ion \((N_3^-)\), the cyanate ion \((OCN^-)\), the iso-cyanate ion \((NCO^-)\), and the diazenylium ion \((HNN^+)\). The Lewis structures for these species are: 1. Azide ion \((N_3^-)\): \(-N=N\equiv N^-\) 2. Cyanate ion \((OCN^-)\): \(-O\equiv C\equiv N^-\) 3. Iso-cyanate ion \((NCO^-)\): \(-N\equiv C=O^-\) 4. Diazenylium ion \((HNN^+)\): \(H-N=N^+\) Among these species, the azide ion \((N_3^-)\) is likely to be the strongest Bronsted base due to its higher negative charge and the resonance stabilization of the negative charge on the nitrogen atom.

Step-by-step solution

Identify species that fit the general formula

We will consider four different species that fit the general formula \([: X \equiv Y:]^{\pi}\) and have different values of \(n\). 1. Azide ion \((N_3^-)\): In this case, \(X = N\), \(Y = N\), and \(n = -1\). 2. Cyanate ion \((OCN^-)\): In this case, \(X = O\), \(Y = N\), and \(n = -1\). 3. Iso-cyanate ion \((NCO^-)\): In this case, \(X = N\), \(Y = O\), and \(n = -1\). 4. Diazenylium ion \((HNN^+)\): In this case, \(X = H\), \(Y = N\), and \(n = +1\).

Draw Lewis structures for each species

1. Azide ion \((N_3^-)\): \[ \begin{pmatrix} - & N & = & N & \equiv & N & - \\ \end{pmatrix} \] 2. Cyanate ion \((OCN^-)\): \[ \begin{pmatrix} - & O & \equiv & C & \equiv & N & - \\ \end{pmatrix} \] 3. Iso-cyanate ion \((NCO^-)\): \[ \begin{pmatrix} - & N & \equiv & C & = & O & - \\ \end{pmatrix} \] 4. Diazenylium ion \((HNN^+)\): \[ \begin{pmatrix} H & - & N & = & N & (+) \\ \end{pmatrix} \]

Determine the strongest Bronsted base

A stronger Bronsted base will have a more significant tendency to accept a proton. We can determine this tendency by looking at the stability of the negative charge, atomic size, and electronegativity of the atom that would accept the proton. When comparing the species with \(n = -1\), azide ion, cyanate ion, and iso-cyanate ion, the nitrogen atom of the azide ion is the most likely to accept a proton due to its higher negative charge and the resonance stabilization of the negative charge. Therefore, the azide ion \((N_3^-)\) is likely to be the strongest Bronsted base among the four species.

Key concepts

Azide Ion

The azide ion ( $N\_3^-$) is a polyatomic ion consisting of three nitrogen atoms linked by covalent bonds. The central nitrogen atom shares a double bond with one nitrogen and a triple bond with the other. Due to these alternating bonds, the azide ion exhibits resonance stabilization, distributing the negative charge across all three nitrogen atoms, thus increasing its stability.

Like all ions, understanding the azide ion's Lewis structure is essential as it reveals the arrangement of valence electrons and predicts its reactivity and interactions with other species. In line with the exercise solution, the azide ion's resonance grants it unique properties such as its ability to act as a nucleophile in various chemical reactions.

Cyanate Ion

The cyanate ion ( $OCN^-$) displays a different arrangement of atoms compared to the azide ion. In cyanate, an oxygen atom is double-bonded to a carbon atom, which is in turn triple-bonded to a nitrogen atom. The negative charge is localized on the oxygen atom as reflected in the Lewis structure.

This arrangement impacts the cyanate ion's chemical behavior, including its ability to act as a ligand in coordination chemistry. It also illustrates the versatility of carbon to form multiple bonds with different elements and the diverse ways in which valence electrons can be shared or transferred.

Iso-cyanate Ion

The iso-cyanate ion ( $NCO^-$), a structural isomer of the cyanate ion, involves a nitrogen atom triple-bonded to a carbon atom, which is double-bonded to an oxygen atom. The negative charge, in this case, is also on the oxygen. The specific ordering of atoms dictates the molecule's reactivity, and as seen in the iso-cyanate ion, affects its ability to interact with other chemicals, especially in organic synthesis and polymer industries.

Understanding the Lewis structure of the iso-cyanate ion not only allows us to visualize its electron distribution but also to predict its reactions and stability in different chemical environments.

Diazenylium Ion

The diazenylium ion ( $HNN^+$), an unusual species with a positive charge, contrasts with the negatively charged species previously discussed. This ion features a hydrogen atom bonded to a nitrogen atom, which is double-bonded to another nitrogen atom. The positive charge is located on the terminal nitrogen atom.

Due to its positive charge, diazenylium is less likely to act as a Bronsted base but instead may act as an acid, donating its attached hydrogen ion. The Lewis structure highlights its distinct role in chemical bonding and its potential reactivity.

Bronsted Base

A Bronsted base is characterized by its ability to accept protons (H⁺ ions). This concept is core to understanding acid-base reactions. The strength of a Bronsted base depends on factors like the stability of the negative charge after proton acceptance and the presence of resonance structures, making some bases stronger than others.

In the given exercise, when asked to identify which species is likely to be the strongest Bronsted base, the Lewis structures guide us to predict that the azide ion, with its resonance stabilization, would most readily accept a proton.

Resonance Stabilization

Resonance stabilization is a concept in chemistry where a molecule or ion has multiple valid Lewis structures, known as resonance structures. These structures differ only in the placement of electrons, not the arrangement of atoms. Resonance stabilization spreads out the charge over several atoms, reducing the energy and increasing the stability of the molecule or ion.

In the context of azide ion or other polyatomic species, resonance stabilization makes these ions less reactive and more inclined to engage in chemical reactions where stability is favored post-reaction.

Proton Acceptance

Proton acceptance refers to the capacity of a species to accept a proton, a key feature of Bronsted bases. Species with higher electron densities or those stabilized by resonance are typically better at accepting protons. The implication of proton acceptance is crucial when predicting the behavior of species in acid-base reactions and their relative strength as bases.

Proton acceptance plays a significant role in many biochemical processes and laboratory reactions and is often illustrated in the Lewis structures, which help visualize the sites of reactivity.

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules and compounds. The primary types of chemical bonds are ionic, where electrons are transferred between atoms, and covalent, where electrons are shared. The Lewis structures are a foundational tool in visualizing these bonds and understanding how compounds are constituted at the atomic level.

These bonds determine the physical and chemical properties of the substance, such as melting point, boiling point, and reactivity, which are all crucial concepts for students to comprehend.

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They play a critical role in bonding, as they are the electrons that participate in the formation of chemical bonds. Lewis structures help in predicting the arrangement of these electrons and thus assist in understanding how atoms will combine to form molecules.

Grasping valence electron concepts is essential not only for constructing Lewis structures but also for predicting the reactivity and properties of elements and their compounds in chemical reactions.