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A carbon–oxygen bond is a covalent bond between carbon and oxygen and one of the most abundant in organic chemistry and biochemistry.[page needed] 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), or to share two pairs of electrons to form the carbonyl functional group. Simple representatives of these two bond types are alcohols such as the ethanol in beverages and fuels (and all sugars, sterols, etc.), and ketones like the solvent acetone and the substrates/products of enzymes like aldolase in primary metabolism (as well as many other related carbonyl compounds).
Electronegativities and bond lengths
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The C–O bond is strongly polarized towards oxygen (electronegativity of C vs O, 2.55 vs 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).[page needed] 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.[clarification needed] 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.[clarification needed]
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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
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Carbon–oxygen bonds are present in these functional groups:
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- The chemistry of carbon bonded to other elements in the periodic table: