Carbon–hydrogen bond

The carbon-hydrogen bond (C–H bond) is a bond between carbon and hydrogen atoms that can be found in many organic compounds.[1] This bond is a covalent 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	Ethane	Ethylene	Acetylene
Formula	C2H6	C2H4	C2H2
Class	alkane	alkene	alkyne
Structure
Hybridisation of carbon	sp^3	sp^2	sp
C-H bond length	1.094 Å	1.087 Å	1.060 Å
Structure determination method	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 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

References

1. March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7
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