Carbon–oxygen bond

A carbon–oxygen bond is a covalent bond between carbon and oxygen and one of the most abundant in organic chemistry and biochemistry. Oxygen has 6 valence electrons and prefers to share two electrons in bonding with carbon, leaving the remaining 4 nonbonding electrons in 2 lone pairs. The simplest representatives can be thought of organic derivatives of water: the alcohols.

A C–O bond is strongly polarized towards oxygen (electronegativity C vs O = 2.55:3.44). Bond lengths for paraffinic C–O bonds are in the range of 143 picometer – less than those of C–N or C–C bonds. Shortened single bonds are found with carboxylic acids (136 pm) due to partial double bond character and elongated bonds are found in epoxides (147 pm). The C–O bond strength is also larger than C–N or C–C. For example, bond strengths are 91 kcal/mol (at 298 K) in methanol, 87 kcal/mol in methylamine, and 88 kcal/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, but occur mainly as reactive intermediates. 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 bonds in acyl halides have partial triple bond character and subsequently 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 (112.8 pm). 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

Important carbon–oxygen bond forming reactions are the Williamson ether synthesis, nucleophilic acyl substitutions and electrophilic addition to alkenes. The Paternò–Büchi reaction is the carbonyl equivalent of a metathesis reaction.

Oxygen functional groups

Carbon–oxygen bonds are present in these functional groups:

Chemical class	Bond order	Formula	Structural Formula	Example
Alcohols	1	R3C–OH		Ethanol
Ethers	1	R3C–O–CR3		Diethyl ether
Peroxides	1	R3C–O–O–CR3		Di-tert-butyl peroxide
Esters	1	R3C–CO–O–CR3		Ethyl acrylate
Carbonate esters	1	R3C–O–CO–O–CR3	Carbonate ester	Ethylene carbonate
Ketones	2	R3C–CO–CR3		Acetone
Aldehydes	2	R3C–CHO		Acrolein
Furans	1.5			Furfural Furfural
Pyrylium salts	1.5			Anthocyanins

See also

• The chemistry of carbon bonded to other elements in the periodic table: