N, P, As, Sb, Bi: The Elements
G&E Ch. 11, 12, 13
Nitrogen Cycle
Nitrogen is the most abundant uncombined element accessible to man – 78.1% volume of
the atmosphere. It was first isolated in pure form in 1772. The greatest anthropogenic
impact on the natural nitrogen cycle comes from the industrial fixation of N2 to produce
ammonia for crop fertilizer. The present global population would be unsustainable without
ammonia produced by the Haber Bosch process (see next slide)!
Fritz Haber
"for the synthesis of ammonia from its elements"
It may be that this solution is not the final one. Nitrogen bacteria teach us that Nature, with
her sophisticated forms of the chemistry of living matter, still understands and utilizes
methods which we do not as yet know how to imitate. Let it suffice that in the meantime
improved nitrogen fertilization of the soil brings new nutritive riches to mankind and that
the chemical industry comes to the aid of the farmer who, in the good earth, changes
stones into bread.
N vs. P
N2 is rather inert due to the strength of its triple bond, its large HOMO-LUMO gap, its
symmetrical electron distribution, and the absence of bond polarity.
The molecular form of phosphorus, on the other hand, is quite reactive. We can
understand why P4 forms a structure with all P-P single bonds and why N2 forms a dimer
with a triple bond by looking at bond enthalpies:
N-N: 39 kcal/mol, N≡N: 226 kcal/mol
P-P: 50 kcal/mol, P≡P: 117 kcal/mol
P4 is strained due to the really acute P-P-P angles (60 degrees) enforced by the tetrahedral
structure, but it gains stability through aromaticity.
Spherical Aromaticity
Aromaticity was recognized to exist in 3D in 1959 by Lipscomb in B12H122-. It’s difficult to
define aromaticity in 3D because we don’t have anything like a resonance stabilization
energy to help. One defining feature of 2D aromatic, however, is there unusual magnetic
properties – large electron densities at the center of rings, leading to heavy shielding of
internal protons or other nuclei. This is manifest in the NMR spectra of such molecules. Take
18-annulene (top left), for example. The internal protons have a chemical shift of -3ppm
whereas the protons on the outside have a chemical shift of 9.3ppm. Helium has been
found to be particularly shielded using NMR when trapped inside fullerenes. People have
now defined a nucleus independent chemical shift as an indicator of aromaticity in 3D
molecules. Large negative NICS values indicate aromaticity. P4 is doubly aromatic!
Elemental Phosphorus
In 1669, on the hunt for the philosopher’s stone, Henning Brand obtained a white waxy
solid that glowed in the dark and spontaneously ignited on exposure to air after he boiled
putrefied urine. This marked the discovery of phosphorus. Phosphorus is named for its
most distinguishing feature – the bright emission of light upon oxidation. This was also the
first recorded example of chemiluminescence.
The Sordid Tale…
It is difficult if not impossible to
obtain P4 in the United States due
to its use as a chemical weapon
(both as a smoke screen and as a
burn agent) and due to its use as a
reductant in the production of
methamphetamine.
Industrial Production: Wöhler Process
For a century after its discovery, the only source of P4 was urine. Now we obtain P4 on
large scales by heating calcium phosphate with sand and coke using the Wohler process.
The high temperatures required (1400-1500 C) results in 15 MWh of electricity
consumption per tonne of P4. 80-90% of the P4 we produce goes into making phosphoric
acid (thanks coca cola) and some is converted to red P. The rest gets turned into
phosphorus oxides, sulfides, and halides, and eventually organophosphorus compounds.
Other Phosphorus Allotropes
As, Sb, Bi
The three elements As, Sb, and Bi were among the earliest elements to be isolated and
were known long before N2 (1772) and P4 (1669). The properties of arsenic compounds
have been known to physicians and professional poisoners since the 5th century BC.
Global Arsenic Production
As, Sb, and Bi are chalcophiles. Arsenic minerals are widely distributed and the free
element is also naturally occurring. We obtain pure arsenic metal industrially from
smelting FeAs2 or from As2O3 obtained as a byproduct of the Pb and Cu industries. The
largest use of arsenic metal is in alloying with other metals (more next class).
Making Yellow Arsenic to Play With
Bismuth in Your Medicine Cabinet
One of the most familiar uses of bismuth is in bismuth subsalicylate – a colloidal
suspension with an approximate formualtion. A colloid is a substance that is
microscopically dispersed in another substance. The dispersed particles have diameters
ranging from 1-1000 nm. Milk is another common example of a colloid. Quantum dots are
yet another (more soon!).
Zintl Phases
NaCl
NaTl
Zintl phases are intermetallics that demonstrate complete charge transfer from the more
electropositive element to the more electronegative one. The difference with ionic solids
like NaCl is that on transfer of the electron from Na to Cl, Cl obtains a complete octet and
is satisfied to exist as a single unit. In zintl phases, the anions do not obtain a closed shell
electronic configuration and complex bonding arrangements are found between the
anionic atoms. This property was first described in NaTl – notice the Tl atoms (blue) are
interconnected in the lattice.
Solid State and Molecular Anions
These types of phases are formed commonly in intermetallics of group 14 and 15. The
elaborate structures observed for the anions is a reflection of their drive to achieve an
octet configuration. Interestingly, isolated zintl clusters also exist. The P7 trianion is shown
at the upper right!
Why P4 Is So Darn Cool