Carbon–oxygen bond

A carbon–oxygen bond is a polar covalent bond between atoms of carbon and oxygen.[1][2][3]: 16–22  Carbon–oxygen bonds are found in many inorganic compounds such as carbon oxides and oxohalides, carbonates and metal carbonyls,[4] and in organic compounds such as alcohols, ethers, carbonyl compounds and oxalates.[5]: 32–36  Oxygen has 6 valence electrons of its own and tends to fill its outer shell with 8 electrons by sharing electrons with other atoms to form covalent bonds, accepting electrons to form an anion, or a combination of the two. In neutral compounds, an oxygen atom can form up to two single bonds or one double bond with carbon, while a carbon atom can form up to four single bonds or two double bonds with oxygen.

Bonding motifs

Bonding at oxygen

In ethers, oxygen forms two covalent single bonds with two carbon atoms, C–O–C, whereas in alcohols oxygen forms one single bond with carbon and one with hydrogen, C–O–H.[5]: 32  In carbonyl compounds, oxygen forms a covalent double bond with carbon, C=O, known as a carbonyl group.[5]: 136  In ethers, alcohols and carbonyl compounds, the four nonbonding electrons in oxygen's outer shell form two lone pairs.[5]: 108  In alkoxides, oxygen forms a single bond with carbon and accepts an electron from a metal to form an alkoxide anion, R–O−, with three lone pairs. In oxonium ions, one of oxygen's two lone pairs is used to form a third covalent bond which generates a cation, >O+– or =O+– or ≡O+, with one lone pair remaining.[5]: 343, 410 

Bonding at carbon

A carbon atom forms one single bond to oxygen in alcohols, ethers and peroxides, two in acetals,[3]: 524 [5]: 35, 340–348  three in ortho esters,[5]: 345  and four in orthocarbonates.[6] Carbon forms a double bond to oxygen in aldehydes, ketones and acyl halides. In carboxylic acids, esters and anhydrides, each carbonyl carbon atom forms one double bond and one single bond to oxygen. In carbonate esters and carbonic acid, the carbonyl carbon forms one double bond and two single bonds to oxygen. In carbon dioxide, carbon forms two double bonds to oxygen.

Electronegativities and bond lengths

The C–O bond is strongly polarized towards oxygen (electronegativity of C vs O, 2.55 vs 3.44). Bond lengths[4] 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 (136 pm) due to partial double bond character and elongated bonds are found in epoxides (147 pm).[7] The C–O bond strength is also larger than C–N or C–C. For example, bond strengths are 91 kilocalories (380 kJ)/mol (at 298 K) in methanol, 87 kilocalories (360 kJ)/mol in methylamine, and 88 kilocalories (370 kJ)/mol in ethane.[7]

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 (112.8 pm), and acylium ions, R–C≡O+ (typically 110-112 pm).[8][9][10] Such triple bonds have a very high bond energy, even higher than N–N triple bonds.[11] Oxygen can also be trivalent, for example in triethyloxonium tetrafluoroborate.[5]: 343 

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 class Bond order Formula Structural Formula Example
Alcohols1 ROH Alcohol Ethanol
Ethanol
Ethers1 ROR′ Ether Diethyl ether
Diethyl ether
Peroxides1 ROOR′ Hydroperoxide Di-tert-butyl peroxide
Di-tert-butyl peroxide
Cyanate esters1 ROCN Cyanate ester Bisphenol A cyanate ester
Bisphenol A cyanate ester
Aldehydes2 RCHO Aldehyde Acrolein
Acrolein
Ketones2 RCOR′ Ketone Acetone
Acetone
Carboxylic acids1 & 2 RCOOH
(or RCO2H)		 Carboxylic acid Acetic acid
Acetic acid
Esters1 & 2 RCOOR
(or RCO2R)		 Ester Ethyl acrylate
Ethyl acrylate
Carbonate esters1 & 2 ROCOOR
(or ROCO2R)		 Carbonate ester Ethylene carbonate
Ethylene carbonate
Isocyanates2 RNCO Isocyanate Phenyl isocyanate
Phenyl isocyanate
Nitrite esters1 RONO Nitrite ester Amyl nitrite
Amyl nitrite
Nitrate esters1 RONO2 Nitrate ester Isopropyl nitrate
Isopropyl nitrate
Carbamate esters1 & 2 ROCONR2 Carbamate ester Carbaryl
Carbaryl
Furans1.5 R4C4O Furan Furfural
Furfural
Pyrylium salts1.5 R5C5O+ X− pyridine Anthocyanin
Anthocyanins

See also

References
  1. Organic Chemistry John McMurry 2nd Ed.
  2. Advanced Organic Chemistry Carey, Francis A., Sundberg, Richard J. 5th ed. 2007
  3. Smith, Michael B.; March, Jerry (2007). March's Advanced Organic Chemistry (6th ed.). John Wiley & Sons. ISBN 978-0-471-72091-1.
  4. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 292, 304–314. ISBN 978-0-08-037941-8.
  5. Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. ISBN 978-0-19-850346-0.
  6. Laniel, Dominique; Binck, Jannes; Winkler, Björn; Vogel, Sebastian; Fedotenko, Timofey; Chariton, Stella; Prakapenka, Vitali; Milman, Victor; Schnick, Wolfgang; Dubrovinsky, Leonid; Dubrovinskaia, Natalia (2021). "Synthesis, crystal structure and structure–property relations of strontium orthocarbonate, Sr2CO4". Acta Crystallogr. B. 77 (1): 131–137. doi:10.1107/S2052520620016650. ISSN 2052-5206. PMC 7941283.
  7. CRC Handbook of Chemistry and Physics 65Th Ed.
  8. Chevrier, B.; Carpentier, J. M. Le; Weiss, R. (1972). "Synthesis of two crystalline species of the Friedel–Crafts intermediate antimony pentachloride-p-toluoyl chloride. Crystal structures of the donor–acceptor complex and of the ionic salt". J. Am. Chem. Soc. 94 (16): 5718–5723. doi:10.1021/ja00771a031.
  9. Davlieva, Milya G.; Lindeman, Sergey V.; Neretin, Ivan S.; Kochi, Jay K. (2004). "Structural effects of carbon monoxide coordination to carbon centers. π and σ bindings in aliphatic acyl versus aromatic aroylcations". New J. Chem. 28: 1568–1574. doi:10.1039/B407654K.
  10. Hermannsdorfer, André; Driess, Matthias (2021). "Silicon Tetrakis(trifluoromethanesulfonate): A Simple Neutral Silane Acting as a Soft and Hard Lewis Superacid". Angew. Chem. Int. Ed. 60 (24): 13656–13660. doi:10.1002/anie.202103414.
  11. "Standard Bond Energies". Department of Chemistry, Michigan State University. Archived from the original on 29 August 2016.
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