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] 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.

Reactions

The C−H bond in general is 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. Another reaction type involving C−H bonds is so-called C−H bond activation mediated by metals and carbene C−H insertion. C–H bond oxidation suffers from lack of selectivity. Enzymes are known for selective oxidations and some organic procedures exist [2]

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.[3]

Bond Hydrocarbon radical Bond dissociation energy (kcal/mole)
CH3−H Methyl 104
C2H5−H Ethyl 98
(CH3)2HC−H Isopropyl 95
(CH3)3C−H tert-Butyl 93
CH2=CH−H vinyl 112
C6H5−H phenyl 110
CH2=CHCH2−H Allyl 88
C6H5CH2−H Benzyl 85
OC4H7−H tetrahydrofuranyl 92

Nomenclature

The C−H unit is formally named carbogen.

CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs style="background-color:#98FF98" border-collapse:collapse" | CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr CRa Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
↓
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac CTh CPa CU CNp CPu CAm CCm CBk CCf CEs Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown

References

  1. ↑ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7
  2. ↑ Adding Aliphatic C–H Bond Oxidations to Synthesis M. Christina White Science 17 February 2012: Vol. 335 no. 6070 pp. 807–809 doi:10.1126/science.1207661
  3. ↑ http://www.cem.msu.edu/~reusch/OrgPage/bndenrgy.htm#dissbe