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In organic chemistry, a methylene bridge, methylene spacer, or methanediyl group
is any part of a molecule with formula −CH2−; namely, a carbon atom bound to two hydrogen atoms and connected by single bonds to two other distinct atoms in the rest of the molecule. It is the repeating unit in the skeleton of the unbranched alkanes.
A methylene bridge can also act as a bidentate ligand joining two metals in a coordination compound, such as titanium and aluminum in Tebbe's reagent.[1]
A methylene bridge is often called a methylene group or simply methylene, as in "methylene chloride" (dichloromethane CH2Cl2). As a bridge in other compounds, for example in cyclic compounds, it is given the name methano. However, the term methylidene group (not to be confused with the term methylene group, nor the carbene methylidene) properly applies to the CH2 group when it is connected to the rest of the molecule by a double bond (=CH2), giving it chemical properties very distinct from those of a bridging CH2 group.
Reactions
Compounds possessing a methylene bridge located between two strong electron withdrawing groups (such as nitro, carbonyl or nitrile groups) are sometimes called active methylene compounds.[2] Treatment of these with strong bases can form enolates or carbanions, which are often used in organic synthesis. Examples include the Knoevenagel condensation and the malonic ester synthesis.[3]
Examples
Examples of compounds which contain methylene bridges include:
Malonic acid Malonic acid
Acetylacetone
Malononitrile
See also
Methyl group
Methylene group
Methyne
References
W. A. Herrmann (1982), "The methylene bridge". In Advances in Organometallic Chemistry, volume 20, pages 195-197.
"Active methylene compound".
House, Herbert O. (1972). Modern Synthetic Reactions. Menlo Park, CA.: W. A. Benjamin. ISBN 0-8053-4501-9.
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Functional groups
Hydrocarbons
(only C and H)
Alkyl
Methyl Ethyl Propyl Cyclopropyl Butyl Pentyl Methylene
Bridge Methine Alkene
Vinyl Allyl 1-Propenyl Crotyl Allene Cumulene Phenyl Benzyl Alkyne Carbene
Only carbon,
hydrogen,
and oxygen
(only C, H and O)
R-O-R
Acetal Alkoxy
Methoxy Ether
Enol ether Epoxide Peroxy
Hydroperoxy Dioxiranes Ethylenedioxy Alcohol Methylenedioxy
carbonyl
Acyl
Acetyl Acryloyl Benzoyl Aldehyde
Ketene Ketone Ynone
carboxy
Carboxyl
Acetoxy Anhydride Ester
Orthoester
Only one
element,
not being
carbon,
hydrogen,
or oxygen
(one element,
not C, H or O)
Nitrogen
Amine
Enamine Ammonium Hydrazo Nitrene Imine Oxime Hydrazone Azo Amide Imidate Amidine Carbamate Imide Nitrile Isonitrile Cyanate Isocyanate Nitrate Nitrite Nitro Nitroso
Phosphorus
Phosphate
Phosphodiester Phosphonate
Phosphite Phosphonous Phosphinate Phosphine oxide Phosphine
Phosphonium Phosphaalkene Phosphaalkyne
Sulfur
Thiol Sulfide
Sulfonium Persulfide Disulfide Sulfenic acid Thiosulfinate Sulfoxide Thiosulfonate Sulfinic acid Sulfone Sulfonic acid Thioketone Thial Thioester Thionoester Thioxanthate Xanthate
Boron
Boronic acid
Selenium
Selenol Selenonic acid Seleninic acid Selenenic acid Selone
Tellurium
Tellurol Telluroketone
Halo
Haloalkane
Fluoroethyl Trifluoromethyl Trichloromethyl Trifluoromethoxy Hypervalent iodine Vinyl halide
Iodide Acyl halide
Chloride Perchlorate
Other
Isothiocyanate Phosphoramides Sulfenyl chloride Sulfonamide Thiocyanate
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