catenation

Catenation occurs most readily with carbon, which forms covalent bonds 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 halogens.

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 sigma bonded 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.

Theories of the structure of water involve three-dimensional networks of tetrahedra and chains and rings, linked via hydrogen bonding.{{Cite journal |date=June 2006 |title=Tetrahedral structure or chains for liquid water |url=https://www.researchgate.net/publication/7082408 |doi=10.1073/pnas.0510593103 |pmid=16698934 |s2cid=20023418 |journal=Proceedings of the National Academy of Sciences|doi-access=free |last1=Head-Gordon |first1=Teresa |last2=Johnson |first2=Margaret E. |volume=103 |issue=21 |pages=7973–7977 |pmc=1472414 }}

A polycatenated network, with rings formed from metal-templated hemispheres linked by hydrogen bonds, was reported in 2008.{{cite journal |last1=Salaudeen |first1= A. Abibat|last2=Kilnera |first2=C.A.| last3=Halcrow |first3= M.A.|date=2008 |title=A crystalline hydrogen-bonded network with a poly-catenate topology |journal=Chem. Commun. |issue= 41|pages=5200–5202 |doi= 10.1039/B810393C|pmid= 18956068}}

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;{{cite journal |last1=Morris |first1= D.G. Abibat|last2=Ryder |first2=K.S.| last3=Walker |first3= A.|last4=Muir |first4=K.W |last5= Hix|first5=G.B |last6=Maclean |first6=E.J |date=2001 |title=Unusual synthesis and crystal structure of 4-tricyclanol |journal=Tetrahedron Letters |volume=47 |issue= 2|pages= 319–322|doi= 10.1016/S0040-4039(00)01903-1}} crystalline isophthalic acid C₈H₆O₄ is built up from molecules connected by hydrogen bonds, forming infinite chains.{{cite journal |last1=Derissen |first1=JL |date=1974 |title=The crystal structure of isophthalic acid |journal=Acta Crystallogr. |volume=B30 |issue=6 |pages=764–2765 |doi=10.1107/S0567740872004844 }}

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.{{cite journal |last1=Xia |first1= Y. Abibat|last2=Yang |first2=B.| last3=Jin |first3= F.|last4= Ma|first4=Y. |last5=Liu |first5=X. |last6=Zhao |first6=M. |date=2019 |title=Hydrogen confined in a single wall carbon nanotube becomes a metallic and superconductive nanowire under high pressure |journal=Nano Lett. |volume=19 |issue=4 |pages=2537–2542 |doi=10.1021/acs.nanolett.9b00258|pmid= 30884943|bibcode= 2019NanoL..19.2537X|s2cid= 83460656}}

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 hydrocarbons) 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 polysilanes (sometimes, erroneously called polysilenes) that are analogues of alkanes. These long chain compounds have surprising electronic properties - high electrical conductivity, for example - arising from sigma delocalization of the electrons in the chain.{{Cite journal | last1 = Miller | first1 = R. D. | last2 = Michl | first2 = J. | doi = 10.1021/cr00096a006 | title = Polysilane high polymers | journal = Chemical Reviews| volume = 89 | issue = 6 | pages = 1359 | year = 1989 }}

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 alkanes. Disilene and disilynes are quite rare, unlike alkenes and alkynes. Examples of disilynes, long thought to be too unstable to be isolated{{cite journal| journal= Silicon Chemistry |volume= 1 |issue= 1|date=January 2002 |pages= 59–65 |author1=Karni, M. |author2=Apeloig, Y. |title= The quest for a stable silyne, RSi≡CR′. The effect of bulky substituents |doi= 10.1023/A:1016091614005 |s2cid= 97098444 }} were reported in 2004.{{cite journal

| title = A Stable Compound Containing a Silicon-Silicon Triple Bond

|author1=Akira Sekiguchi |author2=Rei Kinjo |author3=Masaaki Ichinohe | journal = Science

|date=September 2004

| volume = 305

| issue = 5691

| pages = 1755–1757

| doi = 10.1126/science.1102209

|pmid=15375262 |bibcode=2004Sci...305.1755S |s2cid=24416825 }}

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, carboranes and metal dicarbollides.{{citation needed|date=August 2022}}

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 anion and triazoles.{{cite journal|journal=J. Am. Chem. Soc.|volume=123|issue=26|pages=6308–6313|date=9 Jun 2001|pmid=11427055|title=Polynitrogen Chemistry. Synthesis, Characterization, and Crystal Structure of Surprisingly Stable Fluoroantimonate Salts of {{chem|N|5|+}}|doi=10.1021/ja010141g|first=Ashwani|last=Vij|author2=William W. Wilson|author3=Vandana Vij|author4=Fook S. Tham|author5=Jeffrey A. Sheehy|author6=Karl O. Christe|url=http://www.dtic.mil/get-tr-doc/pdf?AD=ADA409721|access-date=21 August 2022|archive-date=23 September 2017|archive-url=https://web.archive.org/web/20170923051324/http://www.dtic.mil/get-tr-doc/pdf?AD=ADA409721|url-status=dead|url-access=subscription}}Forstel, Maksyutenko, Jones, Sun, Chen, Chang, & Kaiser. "Detection of the Elusive Triazane Molecule ({{Chem|[N|3|H|5|]}}) 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.{{Cite journal |last1=Klapötke |first1=Thomas M. |last2=Piercey |first2=Davin G. |date=2011-04-04 |title=1,1′-Azobis(tetrazole): A Highly Energetic Nitrogen-Rich Compound with a N 10 Chain |url=https://pubs.acs.org/doi/10.1021/ic200071q |journal=Inorganic Chemistry |language=en |volume=50 |issue=7 |pages=2732–2734 |doi=10.1021/ic200071q |pmid=21384800 |issn=0020-1669|url-access=subscription }}

Phosphorus chains (with organic substituents) have been prepared, although these tend to be quite fragile. Small rings or clusters are more common.{{Cite journal |last1=Jones |first1=R. O. |last2=Ganteför |first2=G. |last3=Hunsicker |first3=S. |last4=Pieperhoff |first4=P. |date=1995-12-08 |title=Structure and spectroscopy of phosphorus cluster anions: Theory (simulated annealing) and experiment (photoelectron detachment) |journal=The Journal of Chemical Physics |volume=103 |issue=22 |pages=9549–9562 |doi=10.1063/1.469969 |issn=0021-9606|doi-access=free }}

The versatile chemistry of elemental sulfur is largely due to catenation. In the native state, sulfur exists as S₈ 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 (polysulfides) 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.

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.

Except for fluorine that can only form unstable polyfluorides{{sfn|Wiberg|Wiberg|Holleman|2001|p=422}} at low temperature, all other stable halogens (Cl, Br, I) can form several isopolyhalogen anions 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.

{{wikt|catenation|catenate}}

• Backbone chain
• Chain-growth polymerization
• Macromolecule
• Aromaticity
• Polyhalogen ions
• Polysulfides
• Superatom
• Inorganic polymer
• Self-assembly

{{reflist}}

• {{cite book|last1=Wiberg|first1=Egon|last2=Wiberg|first2=Nils|last3=Holleman|first3=Arnold Frederick|title=Inorganic Chemistry|date=2001|publisher=Academic Press|isbn=978-0-12-352651-9|location=San Diego}}

Category:Organic chemistry

Category:Inorganic chemistry