In chemistry, catenation is the chemical bond of of the same Chemical element into a series, called a chain.[ Oxford English Dictionary, 1st edition (1889) [http://www.oed.com/view/Entry/30197 s.v. 'chain', definition 4g] A chain or a ring may be open if its ends are not bonded to each other (an open-chain compound), or closed if they are bonded in a ring (a cyclic compound). The words to catenate and catenation reflect the Latin root , "chain".
Carbon
Catenation occurs most readily with
carbon, which forms
with other carbon atoms to form long chains and structures. This is the reason for the presence of the vast number of organic compounds in nature. Carbon is most well known for its properties of catenation, with organic chemistry essentially being the study of catenated carbon structures (and known as
catenae). Carbon chains in
biochemistry combine any of various other elements, such as
hydrogen,
oxygen, and biometals, onto the backbone of carbon.
However, carbon is by no means the only element capable of forming such catenae, and several other main-group elements are capable of forming an expansive range of catenae, including hydrogen, boron, silicon, phosphorus, sulfur and Halogen.
The ability of an element to catenate is primarily based on the bond energy of the element to itself, which decreases with more diffuse orbitals (those with higher azimuthal quantum number) overlapping to form the bond. Hence, carbon, with the least diffuse valence shell p orbital is capable of forming longer p-p chains of atoms than heavier elements which bond via higher valence shell orbitals. Catenation ability is also influenced by a range of steric and electronic factors, including the electronegativity of the element in question, the molecular orbital n and the ability to form different kinds of covalent bonds. For carbon, the sigma overlap between adjacent atoms is sufficiently strong that perfectly stable chains can be formed. With other elements this was once thought to be extremely difficult in spite of plenty of evidence to the contrary.
Hydrogen
Theories of the structure of water involve three-dimensional networks of tetrahedra and chains and rings, linked via
hydrogen bonding.
A polycatenated network, with rings formed from metal-templated hemispheres linked by hydrogen bonds, was reported in 2008.
In organic chemistry, hydrogen bonding is known to facilitate the formation of chain structures. For example, 4-tricyclanol C10H16O shows catenated hydrogen bonding between the hydroxyl groups, leading to the formation of helical chains; crystalline isophthalic acid C8H6O4 is built up from molecules connected by hydrogen bonds, forming infinite chains.
In unusual conditions, a 1-dimensional series of hydrogen molecules confined within a single wall carbon nanotube is expected to become metallic at a relatively low pressure of 163.5 GPa. This is about 40% of the ~400 GPa thought to be required to metallize ordinary hydrogen, a pressure which is difficult to access experimentally.
Silicon
Silicon can form sigma bonds to other silicon atoms (and
disilane is the parent of this class of compounds). However, it is difficult to prepare and isolate Si
nH
2n+2 (analogous to the saturated alkane
) with n greater than about 8, as their thermal stability decreases with increases in the number of silicon atoms. Silanes higher in molecular weight than disilane decompose to polymeric polysilicon hydride and
hydrogen.
[W. W. Porterfield, Inorganic Chemistry: A Unified Approach, 2nd Ed.", Academic Press (1993), p. 219.][Inorganic Chemistry, Holleman-Wiberg, John Wiley & Sons (2001) p. 844.] But with a suitable pair of organic substituents in place of hydrogen on each silicon it is possible to prepare
silanes (sometimes, erroneously called polysilenes) that are analogues of
. These long chain compounds have surprising electronic properties - high electrical conductivity, for example - arising from sigma
delocalization of the electrons in the chain.
Even silicon–silicon pi bonds are possible. However, these bonds are less stable than the carbon analogues. Disilane and longer silanes are quite reactive compared to Alkane. Disilene and disilynes are quite rare, unlike and . Examples of , long thought to be too unstable to be isolated were reported in 2004.[
]
Boron
In dodecaborate(12) anion, twelve
boron atoms covalently link to each other to form an icosahedral structure. Various other similar motifs are also well studied, such as
boranes,
Carborane and
Dicarbollide.
Nitrogen
Nitrogen, unlike its neighbor carbon, is much less likely to form chains that are stable at room temperature. But, there do exist nitrogen chains; for example, in solid nitrogen,
triazane,
Azide and
Triazole.
[Forstel, Maksyutenko, Jones, Sun, Chen, Chang, & Kaiser. "Detection of the Elusive Triazane Molecule () in the Gas Phase", ChemPhysChem, 2015, 16, 3139.] Longer series with eight or more nitrogen atoms, such as 1,1'-Azobis-1,2,3-triazole, have been synthesized. These compounds have potential use as a convenient way to store large amount of energy.
Phosphorus
Phosphorus chains (with organic substituents) have been prepared, although these tend to be quite fragile. Small rings or
Atom cluster are more common.
Sulfur
The versatile chemistry of elemental
sulfur is largely due to catenation. In the native state, sulfur exists as S
8 molecules. On heating these rings open and link together giving rise to increasingly long chains, as evidenced by the progressive increase in
viscosity as the chains lengthen. Also, sulfur polycations, sulfur polyanions (
Polysulfide) and lower sulfur oxides are all known.
[Shriver, Atkins. Inorganic Chemistry, Fifth Edition. W. H. Freeman and Company, New York, 2010; pp 416] Furthermore,
selenium and
tellurium show variants of these structural motifs.
Semimetallic elements
In recent years, a variety of double and triple bonds between semi-metallic elements have been reported, including
silicon,
germanium,
arsenic and
bismuth. The ability of certain main group elements to catenate is currently the subject of research into inorganic polymers.
Halogens
Except for
fluorine that can only form unstable polyfluorides at low temperature, all other stable halogens (Cl, Br, I) can form several
Polyhalogen ions that are stable at room temperature, of which the most prominent example being
triiodide. In all these anions, the halogen atoms of the same element bond to each other.
See also
Bibliography