In chemistry, the carbon–hydrogen bond (C−H bond) is a chemical bond between carbon and hydrogen atoms that can be found in many organic compounds. This bond is a covalent, single bond, meaning that carbon shares its outer valence electrons with up to four hydrogens. This completes both of their outer shells, making them stable.
Carbon–hydrogen bonds have a bond length of about 1.09 Å (1.09 × 10−10 m) and a bond energy of about 413 kJ/mol (see table below). Using Pauling's scale—C (2.55) and H (2.2)—the electronegativity difference between these two atoms is 0.35. Because of this small difference in electronegativities, the C−H bond is generally regarded as being non-polar. In structural formulas of molecules, the hydrogen atoms are often omitted. Compound classes consisting solely of C−H bonds and C−C bonds are alkanes, alkenes, alkynes, and aromatic hydrocarbons. Collectively they are known as hydrocarbons.
In October 2016, astronomers reported that the very basic chemical ingredients of life—the carbon–hydrogen molecule (CH, or methylidyne radical), the carbon–hydrogen positive ion (CH+) and the carbon ion (C+)—are created, in large part, using energy from the ultraviolet light of nearby stars, rather than in other ways, such as turbulent events related to supernovae and young stars, as thought earlier.
Bond length
The length of the carbonhydrogen bond varies slightly with the hybridisation of the carbon atom. A bond between a hydrogen atom and an sp2 hybridised carbon atom is about 0.6% shorter than between hydrogen and sp3 hybridised carbon. A bond between hydrogen and sp hybridised carbon is shorter still, about 3% shorter than sp3 C-H. This trend is illustrated by the geometry of ethane, ethylene and acetylene.[citation needed]
| Molecule | Methane | Ethane | Ethylene | Acetylene |
|---|---|---|---|---|
| Formula | CH4 | C2H6 | C2H4 | C2H2 |
| Class | alkane | alkane | alkene | alkyne |
| Structure |  |  |  |  |
| Hybridisation of carbon | sp3 | sp3 | sp2 | sp |
| C-H bond length | 1.087 Å | 1.094 Å | 1.087 Å | 1.060 Å |
| Proportion of ethane C-H bond length | 99% | 100% | 99% | 97% |
| Structure determination method | microwave spectroscopy | microwave spectroscopy | microwave spectroscopy | infrared spectroscopy |
Reactions
The C−H bond in general is very strong, so it is relatively unreactive. In several compound classes, collectively called carbon acids, the C−H bond can be sufficiently acidic for proton removal. Unactivated C−H bonds are found in alkanes and are not adjacent to a heteroatom (O, N, Si, etc.). Such bonds usually only participate in radical substitution. Many enzymes are known, however, to effect these reactions.
Although the C−H bond is one of the strongest, it varies over 30% in magnitude for fairly stable organic compounds, even in the absence of heteroatoms.
| Bond | Hydrocarbon radical | Molar Bond Dissociation Energy (kcal) | Molar Bond Dissociation Energy (kJ) |
|---|---|---|---|
| CH3−H | Methyl | 104 | 440 |
| C2H5−H | Ethyl | 98 | 410 |
| (CH3)2HC−H | Isopropyl | 95 | 400 |
| (CH3)3C−H | tert-Butyl | 93 | 390 |
| CH2=CH−H | vinyl | 112 | 470 |
| HC≡C−H | ethynyl | 133 | 560 |
| C6H5−H | phenyl | 110 | 460 |
| CH2=CHCH2−H | Allyl | 88 | 370 |
| C6H5CH2−H | Benzyl | 85 | 360 |
| OC4H7−H | tetrahydrofuranyl | 92 | 380 |
| CH3C(O)CH2−H | acetonyl | 96 | 400 |
See also
- Carbon–carbon bond
- Carbon–nitrogen bond
- Carbon–oxygen bond
- Carbon–fluorine bond
References
- March, Jerry (1985). Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.). New York: Wiley. ISBN 9780471854722. OCLC 642506595.
- "Life Sciences Cyberbridge". Covalent Bonds. Archived from the original on 2015-09-18. Retrieved 2015-09-15.
- Landau, Elizabeth (12 October 2016). "Building Blocks of Life's Building Blocks Come From Starlight". NASA. Retrieved 13 October 2016.
- CRC Handbook of Chemistry and Physics, 88th edition
- Bollinger, J. M. Jr., Broderick, J. B. "Frontiers in enzymatic C-H-bond activation" Current Opinion in Chemical Biology 2009, vol. 13, page 51-7. doi:10.1016/j.cbpa.2009.03.018
- "Bond Energies". Organic Chemistry, Michigan State University. Archived from the original on 29 August 2016.
- Yu-Ran Luo and Jin-Pei Cheng "Bond Dissociation Energies" in CRC Handbook of Chemistry and Physics, 96th Edition
In chemistry the carbon hydrogen bond C H bond is a chemical bond between carbon and hydrogen atoms that can be found in many organic compounds This bond is a covalent single bond meaning that carbon shares its outer valence electrons with up to four hydrogens This completes both of their outer shells making them stable Carbon hydrogen bonds have a bond length of about 1 09 A 1 09 10 10 m and a bond energy of about 413 kJ mol see table below Using Pauling s scale C 2 55 and H 2 2 the electronegativity difference between these two atoms is 0 35 Because of this small difference in electronegativities the C H bond is generally regarded as being non polar In structural formulas of molecules the hydrogen atoms are often omitted Compound classes consisting solely of C H bonds and C C bonds are alkanes alkenes alkynes and aromatic hydrocarbons Collectively they are known as hydrocarbons In October 2016 astronomers reported that the very basic chemical ingredients of life the carbon hydrogen molecule CH or methylidyne radical the carbon hydrogen positive ion CH and the carbon ion C are created in large part using energy from the ultraviolet light of nearby stars rather than in other ways such as turbulent events related to supernovae and young stars as thought earlier Bond lengthThe length of the carbonhydrogen bond varies slightly with the hybridisation of the carbon atom A bond between a hydrogen atom and an sp2 hybridised carbon atom is about 0 6 shorter than between hydrogen and sp3 hybridised carbon A bond between hydrogen and sp hybridised carbon is shorter still about 3 shorter than sp3 C H This trend is illustrated by the geometry of ethane ethylene and acetylene citation needed Comparison of bond lengths in simple hydrocarbons Molecule Methane Ethane Ethylene AcetyleneFormula CH4 C2H6 C2H4 C2H2Class alkane alkane alkene alkyneStructureHybridisation of carbon sp3 sp3 sp2 spC H bond length 1 087 A 1 094 A 1 087 A 1 060 AProportion of ethane C H bond length 99 100 99 97 Structure determination method microwave spectroscopy microwave spectroscopy microwave spectroscopy infrared spectroscopyReactionsThe C H bond in general is very strong so it is relatively unreactive In several compound classes collectively called carbon acids the C H bond can be sufficiently acidic for proton removal Unactivated C H bonds are found in alkanes and are not adjacent to a heteroatom O N Si etc Such bonds usually only participate in radical substitution Many enzymes are known however to effect these reactions Although the C H bond is one of the strongest it varies over 30 in magnitude for fairly stable organic compounds even in the absence of heteroatoms Bond Hydrocarbon radical Molar Bond Dissociation Energy kcal Molar Bond Dissociation Energy kJ CH3 H Methyl 104 440C2H5 H Ethyl 98 410 CH3 2HC H Isopropyl 95 400 CH3 3C H tert Butyl 93 390CH2 CH H vinyl 112 470HC C H ethynyl 133 560C6H5 H phenyl 110 460CH2 CHCH2 H Allyl 88 370C6H5CH2 H Benzyl 85 360OC4H7 H tetrahydrofuranyl 92 380CH3C O CH2 H acetonyl 96 400See alsoCarbon carbon bond Carbon nitrogen bond Carbon oxygen bond Carbon fluorine bondReferencesMarch Jerry 1985 Advanced Organic Chemistry Reactions Mechanisms and Structure 3rd ed New York Wiley ISBN 9780471854722 OCLC 642506595 Life Sciences Cyberbridge Covalent Bonds Archived from the original on 2015 09 18 Retrieved 2015 09 15 Landau Elizabeth 12 October 2016 Building Blocks of Life s Building Blocks Come From Starlight NASA Retrieved 13 October 2016 CRC Handbook of Chemistry and Physics 88th edition Bollinger J M Jr Broderick J B Frontiers in enzymatic C H bond activation Current Opinion in Chemical Biology 2009 vol 13 page 51 7 doi 10 1016 j cbpa 2009 03 018 Bond Energies Organic Chemistry Michigan State University Archived from the original on 29 August 2016 Yu Ran Luo and Jin Pei Cheng Bond Dissociation Energies in CRC Handbook of Chemistry and Physics 96th Edition
