Carbon–oxygen bond


A carbon–oxygen bond is a polar covalent bond between carbon and oxygen. Oxygen has 6 valence electrons and prefers to either share two electrons in bonding with carbon, leaving the 4 nonbonding electrons in 2 lone pairs :O: or to share two pairs of electrons to form the carbonyl functional group. =O: Simple representatives of these two bond types are the _OH in alcohols such as the ethanol in beverages and fuels and the C=O in ketones .

Electronegativities and bond lengths

The C–O bond is strongly polarized towards oxygen. Bond lengths for paraffinic C–O bonds are in the range of 143 pm – less than those of C–N or C–C bonds. Shortened single bonds are found with carboxylic acids due to partial double bond character and elongated bonds are found in epoxides. The C–O bond strength is also larger than C–N or C–C. For example, bond strengths are /mol in methanol, /mol in methylamine, and /mol in ethane.
Carbon and oxygen form terminal double bonds in functional groups collectively known as carbonyl compounds to which belong such compounds as ketones, esters, carboxylic acids and many more. Internal C=O bonds are found in positively charged oxonium ions. In furans, the oxygen atom contributes to pi-electron delocalization via its filled p-orbital and hence furans are aromatic. Bond lengths of C=O bonds are around 123 pm in carbonyl compounds. The C=O bond length in carbon dioxide is 116 pm. The C=O bonds in acyl halides have partial triple bond character and are consequently very short: 117 pm. Compounds with formal C–O triple bonds do not exist except for carbon monoxide, which has a very short, strong bond. Such triple bonds have a very high bond energy, even higher than N–N triple bonds. Oxygen can also be trivalent, for example in triethyloxonium tetrafluoroborate.

Chemistry

Carbon–oxygen bond forming reactions are the Williamson ether synthesis, nucleophilic acyl substitutions and electrophilic addition to alkenes. The Paternò–Büchi reaction involves carbonyl compounds.

Oxygen functional groups

Carbon–oxygen bonds are present in these functional groups:
Chemical classBond orderFormulaStructural FormulaExample
Alcohols1R3C–OH
Ethanol
Ethers1R3C–O–CR3
Diethyl ether
Peroxides1R3C–O–O–CR3
Di-tert-butyl peroxide
Carboxylic acids1 & 2R3C–CO–OH
Acetic acid
Esters1 & 2R3C–CO–O–CR3
Ethyl acrylate
Carbonate esters1 & 2R3C–O–CO–O–CR3
Ethylene carbonate
Ketones2R3C–CO–CR3
Acetone
Aldehydes2R3C–CHO
Acrolein
Furans1.5
Furfural
Pyrylium salts1.5
Anthocyanins