A heterocumulene is a molecule or ion containing a chain of at least three

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

s between consecutive atoms, in which one or more atoms in the doubly bonded chain is a

heteroatom In chemistry, a heteroatom () is, strictly, any atom that is not carbon or hydrogen. Organic chemistry In practice, the term is usually used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecula ...

. Such species are analogous to a

cumulene In organic chemistry, a cumulene is a compound having three or more ''cumulative'' (consecutive) double bonds. They are analogous to allenes, only having a more extensive chain. The simplest molecule in this class is butatriene (), which is al ...

in which the chain of doubly bonded atoms contains only carbon, except that at least one carbon is replaced by a heteroatom. Some authors relax the definition to include species with chains of only two double bonds between consecutive atoms, also known as '' heteroallenes''. Because of the

double bond rule In chemistry, the double bond rule states that elements with a principal quantum number greater than 2 for their valence electrons ( period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). The double ...

, heterocumulenes are rarely isolated. Instead they tend to polymerize. Many are however common in the interstellar medium, where they exist as a dilute gas. Most of the longer ones are very unstable and reactive, and thus have a transient existence, or can only survive when dilute or in an inert matrix. Molecular clouds in space are very dilute and allow heterocumulenes to exist long enough to be detected. Some simple heterocumulenes are common chemicals or ions. These include

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is a neurotoxic, colorless, volatile liquid with the formula and structure . The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical n ...

carbon diselenide Carbon diselenide is an inorganic compound with the chemical formula CSe2. It is a yellow-orange oily liquid with pungent odor. It is the selenium analogue of carbon disulfide (CS2) and carbon dioxide (CO2). This light-sensitive compound is insolu ...

cyanate Cyanate is an anion with the structural formula , usually written . It also refers to any salt containing it, such as ammonium cyanate. It is an isomer of the much less stable fulminate anion .William R. Martin and David W. Ball (2019): "Sma ...

, and

thiocyanate Thiocyanate (also known as rhodanide) is the anion . It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in pyr ...

. Some definitions of heterocumulenes include compounds that contain concatenated double bonds with more than one element, but may have other parts to them. This class includes

ketene In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary monovalent chemical groups (or two separate substitution sites in the same molecule). The name may also refer to the specific compound ethen ...

, sulfur diimide,

sulfine Sulfinylmethane or sulfine is an organic compound with molecular formula H2CSO. It is the simplest sulfine. Sulfines are chemical compounds with the general structure XY=SO. IUPAC considers the term 'sulfine' obsolete, preferring instead thiocarb ...

, and

dicyclohexylcarbodiimide ''N'',''N''′-Dicyclohexylcarbodiimide (DCC or DCCD) is an organic compound with the chemical formula (C6H11N)2C. It is a waxy white solid with a sweet odor. Its primary use is to couple amino acids during artificial peptide synthesis. The low ...

. Some heterocumulenes can act as

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...

s with various metals.

Reactions

Some energised heterocumulenes can cyclise by bending into a circle and bonding the two ends of the chain. Molecules that can do this are CCCB, CCCAl, CCCSi, CCCN, and CCCP. Other four-atom heterocumulenes include CCBO, tricarbon monoxide (CCCO), and CCCS. Four-atom heterocumulenes when cyclised can have two forms. In the kite (or rhombic) form, a triangle of carbon has two of its atoms bonded to the heteroatom. In the fan form the hetero atom links to three carbon atoms arranged in a fan shape. CCCSi has linear, rhombic or fan isomers. The rhombic form is known in space near the

carbon star A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes mo ...

IRC+10216. CCCCO cyclises to a three-member ring. CCCCN undergoes an

isonitrile An isocyanide (also called isonitrile or carbylamine) is an organic compound with the functional group –. It is the isomer of the related nitrile (–C≡N), hence the prefix is ''isocyano''.IUPAC Goldboo''isocyanides''/ref> The organic fragme ...

conversion.

Molecules

Other known five-atom heterocumulenes include CCBCC, CCCCB, CCOCC, CCCCSi, CNCCO, HCCCO, HCCCS, and NCCCN. CCCCSi is known as a linear molecule in space. CCCCBO turns into a six-member ring. Other six atom heterocumulenes include OCCCCN and HCNCNH. Seven atom heterocumulenes include NCCCCCN, HCCBCCH. A known nine atom heterocumulene is HCCCCCCCH. Thiocumulenes have a sulfur atom. They include dicarbon monosulfide CCS and tricarbon monosulfide CCCS, both known from

molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...

s. SC_nS chains can be made by laser ablation with n up to 27.

Table of molecules

This table lists heterocumulene molecules. Heterocumulenes are supposed to be straight, but some combinations of elements result in bent or cyclic molecules. Two different hetero atoms

References

{{Reflist * Carbon compounds Inorganic carbon compounds