Carbon–hydrogen bond

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Carbon–hydrogen bond - Wikipedia

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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 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.^[3]

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 molecular geometry of ethane, ethylene and acetylene.^{[citation needed]}

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

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.^[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

^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 3rd edition, 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

Molecules detected in outer space

Molecules

Diatomic	Aluminium monochloride Aluminium monofluoride Aluminium(II) oxide Argonium Carbon cation Carbon monophosphide Carbon monosulfide Carbon monoxide Cyano radical Diatomic carbon Fluoromethylidynium Helium hydride ion Hydrogen chloride Hydrogen fluoride Hydrogen (molecular) Hydroxyl radical Iron(II) oxide Magnesium monohydride Methylidyne radical Nitric oxide Nitrogen (molecular) Imidogen Sulfur mononitride Oxygen (molecular) Phosphorus monoxide Phosphorus mononitride Potassium chloride Silicon carbide Silicon monoxide Silicon monosulfide Sodium chloride Sodium iodide Sulfanyl Sulfur monoxide Titanium(II) oxide
Triatomic	Aluminium(I) hydroxide Aluminium isocyanide Amino radical Carbon dioxide Carbonyl sulfide CCP radical Chloronium Diazenylium Dicarbon monoxide Disilicon carbide Ethynyl radical Formyl radical Hydrogen cyanide (HCN) Hydrogen isocyanide (HNC) Hydrogen sulfide Hydroperoxyl Iron cyanide Isoformyl Magnesium cyanide Magnesium isocyanide Methylene N₂H⁺ Nitrous oxide Nitroxyl Ozone Methylidynephosphane Potassium cyanide Trihydrogen cation Sodium cyanide Sodium hydroxide Silicon carbonitride c-Silicon dicarbide SiNC Sulfur dioxide Thioformyl Thioxoethenylidene Titanium dioxide Tricarbon Water
Four atoms	Acetylene Ammonia Isocyanic acid Cyanoethynyl Formaldehyde Fulminic acid HCCN Hydrogen peroxide Hydromagnesium isocyanide Isocyanic acid Isothiocyanic acid Ketenyl Methylene amidogen Methyl cation Methyl radical Propynylidyne Protonated carbon dioxide Protonated hydrogen cyanide Silicon tricarbide Thioformaldehyde Tricarbon monoxide Tricarbon monosulfide Thiocyanic acid
Five atoms	Ammonium ion Butadiynyl Carbodiimide Cyanamide Cyanoacetylene Cyanoformaldehyde Cyanomethyl Cyclopropenylidene Formic acid Isocyanoacetylene Ketene Methane Methoxy radical Methylenimine Propadienylidene Protonated formaldehyde Silane Silicon-carbide cluster
Six atoms	Acetonitrile Cyanobutadiynyl radical E-Cyanomethanimine Cyclopropenone Diacetylene Ethylene Formamide HC₄N Ketenimine Methanethiol Methanol Methyl isocyanide Pentynylidyne Propynal Protonated cyanoacetylene
Seven atoms	Acetaldehyde Acrylonitrile Vinyl cyanide Cyanodiacetylene Ethylene oxide Glycolonitrile Hexatriynyl radical Propyne Methylamine Methyl isocyanate Vinyl alcohol
Eight atoms	Acetic acid Aminoacetonitrile Cyanoallene Ethanimine Glycolaldehyde Hexapentaenylidene Methylcyanoacetylene Methyl formate Acrolein
Nine atoms	Acetamide Cyanohexatriyne Dimethyl ether Ethanol Methyldiacetylene Octatetraynyl radical Propene Ethanethiol Propionitrile N-Methylformamide
Ten atoms or more	Acetone Benzene Benzonitrile Buckminsterfullerene (C₆₀, C₆₀⁺, fullerene, buckyball) C₇₀ fullerene Cyanodecapentayne Ethylene glycol Ethyl formate Methyl acetate Methyl-cyano-diacetylene Methyltriacetylene Propionaldehyde Butyronitrile Pyrimidine Heptatrienyl radical

Deuterated
molecules

Unconfirmed

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References