Boron-nitrogen chemistry has attracted significant attention in part because a B–N unit is isoelectr — Physical Chemistry — Thermodynamics Chemistry Question
Boron nitride and its solid state structure
Boron-nitrogen chemistry has attracted significant attention in part because a B–N unit is isoelectronic with C–C. Furthermore, the radius of carbon and its electronegativity are roughly the average of those properties for B and N.
One of the simplest boron-nitrogen compounds is H3N–BH3, the ammonia-borane adduct. Pyrolysis of this compound leads to the generation of H2 gas and poly-borazylene.
H3N–BH3(s) → 2.5 H2(g) + (polyborazylene, BNH)
(If an efficient and low-cost method can be found to regenerate H3N–BH3 from BNH, the substance could be used to generate hydrogen in fuel-cell powered applications.) Further heating polyborazylene results in boron nitride, BN.
Boron nitride exists in several forms, the most common polymorph being one that is similar to graphite. Another, formed by heating the graphite-like form under pressure, has the same structure as zinc blende, ZnS. Boron nitride is thermally and chemically stable and is used in high temperature ceramics. Most recently, layers of the graphite-like form, hexagonal BN, have been combined with sheets of graphene to produce new materials.
A model of a portion of hexagonal boron nitride is illustrated below. How is it similar to or different from the structure of graphite?
Model Answer
In graphite the C atoms in the C6 rings of one layer lie over the middle of a C6 ring in the layer below. In BN, a boron atom in one layer is directly over an N atom in the layer below. In the following table, note the great similarity in bond distances and in the interlayer distance.
Graphite Boron Nitride
Interlayer distance 335.4 pm 333 pm
C-C bond distance 141.5 ---
B-N bond distance --- 144.6 nm
The ZnS-like structure of BN, illustrated below, is a face-centered cube of nitrogen atoms with boron atoms in one half of the tetrahedral holes of the lattice. If the density of this form of BN is 3.45 g cm–3, what is the B–N bond length?
Model Answer
There are 4 BN units in each unit cell, so the mass of a unit cell is 1.648 ⋅ 10–22 g. Using the density, the volume of the unit cell is found to be 4.777 ⋅ 10–23 cm3.
Edge of the unit cell = 3.629 ⋅10–8 cm
Diagonal distance across face of cube (dd) = 5.132 ⋅ 10–8 cm or 513.2 pm.
The BN distance is calculated as illustrated below. BN distance = 157 pm.