Catenation
Encyclopedia
Catenation is the ability of a chemical element
to form a long chain-like structure via a series of covalent bond
s. Catenation occurs most readily in carbon
, which forms covalent bonds with other carbon atoms. Catenation is the reason for the presence of a large 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 (otherwise known as catenae). 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.
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
hybridization 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.
The remarkable properties of elemental sulfur are 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. Selenium and tellurium also show variants of these structural motifs.
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 SinH2n+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. 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. (Ref. R.D. Miller and J. Michl. Chem Rev 89 (1989), pp. 1359–1410.)
Phosphorus chains (with organic substituents) have been prepared, although these tend to be quite fragile. Small rings or clusters are more common.
Even silicon–silicon pi bonds are possible. However, these bonds are less stable than the carbon analogues. Disilane
is quite reactive compared to ethane
. Disilylenes are quite rare, unlike alkene
s. Examples of disilyne
s, long thought to be too unstable to be isolated were reported in 2004.
In recent years a variety of double and triple bonds between the semi-metallic elements have been reported, including silicon, germanium, arsenic, bismuth and so on. The ability of certain main group elements to catenate is currently the subject of research into inorganic polymer
s.
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
to form a long chain-like structure via a series of covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s. Catenation occurs most readily in carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, which forms covalent bonds with other carbon atoms. Catenation is the reason for the presence of a large number of organic compounds in nature. Carbon is most well known for its properties of catenation, with organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
essentially being the study of catenated carbon structures (otherwise known as catenae). 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.
The ability of an element to catenate is primarily based on the bond energy
Bond energy
In chemistry, bond energy is the measure of bond strength in a chemical bond. It is the heat required to break one Mole of molecules into their individual atoms. For example, the carbon-hydrogen bond energy in methane E is the enthalpy change involved with breaking up one molecule of methane into...
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
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
of the element in question, the molecular orbital
Molecular orbital
In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
hybridization 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.
The remarkable properties of elemental sulfur are 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. Selenium and tellurium also show variants of these structural motifs.
Silicon can form sigma bonds to other silicon atoms and (disilane
Disilane
Disilane is a chemical compound with chemical formula Si2H6 that was identified in 1902 by Henri Moissan and . Moissan and Smiles reported disilane as being among the products formed by the action of dilute acids on metal silicides...
is the parent of this class of compounds). However, it is difficult to prepare and isolate SinH2n+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
Polysilicon hydride
Polysilicon hydrides x are silicon backbone polymeric solids. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that, like silanes, are easily hydrolyzed and ignite readily in air.-Synthesis:The silanes are much less thermally stable than alkanes and they are...
and hydrogen. But with a suitable pair of organic substituents in place of hydrogen on each silicon it is possible to prepare polysilanes
Silanes
Silanes are chemical compounds of silicon and hydrogen, which are analogues of alkane hydrocarbons. Silanes consist of a chain of silicon atoms covalently bonded to each other and to hydrogen atoms. The general formula of a silane is SinH2n+2...
(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. (Ref. R.D. Miller and J. Michl. Chem Rev 89 (1989), pp. 1359–1410.)
Phosphorus chains (with organic substituents) have been prepared, although these tend to be quite fragile. Small rings or clusters are more common.
Even silicon–silicon pi bonds are possible. However, these bonds are less stable than the carbon analogues. Disilane
Disilane
Disilane is a chemical compound with chemical formula Si2H6 that was identified in 1902 by Henri Moissan and . Moissan and Smiles reported disilane as being among the products formed by the action of dilute acids on metal silicides...
is quite reactive compared to ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
. Disilylenes are quite rare, unlike alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s. Examples of disilyne
Disilyne
A disilyne is a chemical compound that contains a formal silicon - silicon triple bond and as such is formulated R2Si2 and is the silicon analogue of an alkyne. The hypothetical parent hydride, Si2H2 does not actually exist, all attempts to synthesize it results in a diradical disilene, with the...
s, long thought to be too unstable to be isolated were reported in 2004.
In recent years a variety of double and triple bonds between the semi-metallic elements have been reported, including silicon, germanium, arsenic, bismuth and so on. The ability of certain main group elements to catenate is currently the subject of research into inorganic polymer
Inorganic polymer
Inorganic polymers are polymers with a skeletal structure that does not include carbon atoms. Polymers containing inorganic and organic components are named hybrid polymers. One of the best known examples is polydimethylsiloxane, otherwise known commonly as silicone rubber...
s.