Carbon–hydrogen bond

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.[1] 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.[2]

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 the result, in large part, of ultraviolet light from stars, rather than in other ways, such as the result of turbulent events related to supernovae and young stars, as thought earlier.[3]

Bond length

The length of the carbon-hydrogen 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 molecular geometry of ethane, ethylene and acetylene.

Comparison of bond lengths in simple hydrocarbons[4]
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
Main article: Carbon–hydrogen bond activation

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 affect these reactions.[5]

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.[6][7]

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

References
  1. March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 3rd edition, New York: Wiley, ISBN 9780471854722, OCLC 642506595
  2. "Life Sciences Cyberbridge". Covalent Bonds. Archived from the original on 2015-09-18. Retrieved 2015-09-15.
  3. Landau, Elizabeth (12 October 2016). "Building Blocks of Life's Building Blocks Come From Starlight". NASA. Retrieved 13 October 2016.
  4. CRC Handbook of Chemistry and Physics, 88th edition
  5. 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
  6. "Bond Energies". Organic Chemistry, Michigan State University. Archived from the original on 29 August 2016.
  7. Yu-Ran Luo and Jin-Pei Cheng "Bond Dissociation Energies" in CRC Handbook of Chemistry and Physics, 96th Edition
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