Alkylidene is a general term for divalent functional groups of the form R2C=, where each R is an alkane. They can be considered mono- or disubstituted carbenes, or as the result of removing two hydrogen atoms from the same carbon atom in an alkane.[1]
The simplest alkylidene group is the methylidene group, H2C=.
Nomenclature
editIn standard IUPAC nomenclature, alkylidene groups are named by replacing the -yl in the corresponding alkyl group with -ylidene.[2] This practice is also often extended to common names. For example, the isopropyl group (IUPAC: prop-2-yl) −CH(CH3)2 corresponds to the isopropylidene group =C(CH3)2 (IUPAC: prop-2-ylidene).
Common names of many classes of compounds use alkylidenes in their nomenclature, even though many of them do not formally contain the alkylidene group =CR2, instead containing a substituted methylene bridge −CR2−.
Examples include geminal disubstituted alkanes, such as 1,1-dichloroethane being known as ethylidene dichloride (compare 1,2-dichloroethane's common name of ethylene dichloride); solketal's common name of isopropylidene glycerol; and Meldrum's acid as isopropylidene malonate. Isopropylidene in particular is commonly found in names for products of the condensation of acetone.
Compounds
editKetenes are compounds of alkylidene and carbonyl.[1] Alkylidene ketenes are, as the name suggests, further composition of alkylidenes and ketenes, containing multiple consecutive C=C bonds.
The aldol condensation can be considered as introducing an alkylidene group adjacent to a carbonyl, yielding an enal. Whether the alkylidene group is explicitly named depends on the relative lengths of the substituents in the reagents.
Reactions
editThe distinction between alkylidenes and alkenes (or alkenyl groups) is semantic in organic chemistry. The compounds are isomeric, all defined by the presence of a carbon-carbon double bond; the names only indicate its relationship to the longest chain of the compound. As such, they undergo the same reactions as substituted alkenes, differing by the location of the reaction.
Isomerization of alkylidenes yields alkenes and alkenyls; alkenyl compounds is one route to alkylidene compounds, such as the production of ethylidene norbornene from vinyl norbornene.
In organometallic chemistry, alkylidene groups are the defining characteristic of transition metal carbene complexes, where they are typically referred to as carbene ligands. In this context, "alkylidene" refers specifically to Schrock carbenes
References
edit- ^ a b Moss, G. P.; Smith, P. A. S.; Tavernier, D. (1995). "Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)". Pure and Applied Chemistry. 67 (8–9): 8. doi:10.1351/pac199567081307.
- ^ IUPAC Chemical Nomenclature and Structure Representation Division (2013). "P-56.4". In Favre, Henri A.; Powell, Warren H. (eds.). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. IUPAC–RSC. ISBN 978-0-85404-182-4.