Question

Borazine, \(\mathrm{B}_{3} \mathrm{~N}_{3} \mathrm{H}_{6},\) has the skeleton Draw the resonance forms of the molecule.

Short answer

Answer: The resonance structures of borazine can be represented by drawing two forms of the molecule with alternating double bonds between opposite boron and nitrogen atoms in the hexagonal ring. Each resonance structure has the same connectivity of atoms but has double bonds between different pairs of opposite B and N atoms. The delocalization of electrons within the molecule can be shown by placing a double-headed arrow between the two resonance structures.

Step-by-step solution

Identify the structure of borazine

Borazine has a hexagonal structure, alternating between boron (B) and nitrogen (N) atoms, with each atom connected to a hydrogen atom (H).

Determine the bonds and electron distribution

In borazine, each boron atom forms three bonds: one with the neighboring nitrogen atom, one with the hydrogen atom, and one double bond with the nitrogen atom that lies opposite to the boron atom in the ring. Similarly, each nitrogen atom forms three bonds: one with the neighboring boron atom, one with the hydrogen atom, and one double bond with the boron atom that lies opposite to the nitrogen atom in the ring.

Draw the first resonance structure

Draw the hexagonal structure of borazine with alternating boron and nitrogen atoms, and hydrogen atoms connected to each B and N atom. Show single bonds between B and N, and double bonds between opposite B and N atoms.

Draw the second resonance structure

In the second resonance structure, the position of double bonds is shifted by one position clockwise or counterclockwise. This will shift the double bond between a different pair of opposite boron and nitrogen atoms, while keeping the same connectivity between bonds in the molecule.

Indicate delocalization of electrons

In the resonance structures of borazine, the double bonds (and their associated electrons) are not fixed in a single position, but they can move between different positions in the ring. This delocalization of electrons can be represented by drawing a double-headed arrow between the two resonance structures.

Key concepts

Chemical Bonding

Chemical bonding is an integral part of understanding how atoms come together to form molecules. Borazine, with its formula equivalent to benzene, borazine showcases a unique bonding arrangement between boron (B) and nitrogen (N) atoms. Boron, being less electronegative, tends to form electron-deficient centers, while nitrogen, being more electronegative, tends to share its electrons.

In borazine, each boron atom forms three bonds, as does each nitrogen atom. Hydrogen atoms are connected to both types of atoms, completing their valence shell requirements. The B-N bonding can be visualized as a mixture of single and double bonds due to resonance structures. This bonding pattern leads to 'electron delocalization' which is essential for understanding the stability and reactivity of borazine.

Resonance in Chemistry

Resonance in chemistry is a concept that helps explain the stability of certain molecules that exhibit multiple bonding possibilities. It involves structures with the same arrangement of atoms but different distributions of electrons, known as 'resonance structures'. Borazine is an exemplary molecule for illustrating resonance because it can be drawn with several equivalent structures – each showing double bonds in different locations.

To truly capture the nature of borazine's bonding, it's necessary to consider all resonance structures and not just a single diagram. This reveals that the actual distribution of electrons is an average of these structures – a phenomenon that grants borazine a degree of stability similar to that of benzene, a well-known stable aromatic compound. Thus, the molecule doesn't have fixed double bonds; rather, the bonding picture is a hybrid of all possible structures.

Electron Delocalization

Electron delocalization is a phenomenon that occurs when electrons are shared between more than two atoms, often leading to increased stability in molecules. In the case of borazine, electron delocation is what underpins the concept of resonance – the electrons associated with the double bonds are not confined to a single B-N pairing but are spread out over the entire ring.

The depiction of electron delocalization in borazine requires more than one Lewis structure, also known as resonance forms. By drawing double-headed arrows between these forms, chemists signify that the true electron distribution is a blend of these configurations. Delocalization in borazine endows the molecule with a certain rigidity and planarity, traits that evoke the chemical behavior and characteristics of benzene – giving it the nickname 'inorganic benzene'. Understanding electron delocalization is crucial not only for grasping the bonding in borazine but also for predicting the properties and reactivity of a wide range of other molecules.