Bond lengths carbon-hydrogen

Aromaticity reveals itself in benzene both by the structural phenomena of equal carbon-carbon and equal carbon-hydrogen bond lengths and a characteristic pattern of reactivity. For five-membered heteroaromatics the bond length equivalence found in benzene is not attainable. This is clearly seen in the structure of thiophene (1) as shown the structure is derived from microwave data <6lJSP(7)58). Nevertheless, aromaticity indisputably influences the reactions undergone and the spectroscopic properties. 

Jeffrey GA, Ruble JR (1984, publ. 1985) Effect of thermal motion on carbon-hydrogen bond lengths. Thins Am Cryst Assoc 20 129-132... 

We have seen that the carbon-hydrogen bond length decreases as we proceed along the series ethane, ethylene, acetylene, and we attributed this to changes in hybridization of carbon (see Table 8.3). As the p character of the bonding orbital... 

Spectroscopic studies of methane have shown that the carbon-hydrogen bond length is 1.10 A. The six H—C—H bond angles have the value 109.5° that is characteristic of the regular tetrahedron. 

The structure of the ethane molecule illustrates further the structural principles mentioned in the discussion of the methane molecule and the diamond crystal. The carbon-carbon bond length is 1.54 A, as in diamond, and the carbon-hydrogen bond length is 1.10 A, as in methane. The H—C—H and H—C—C bond angles have been found by experiment to have the regular tetrahedral value 109.5° to within the experimental uncertainty, about one-half a degree. 

The structural features of the isobutane molecule are otherwise essentially the same as those of n-butane carbon-carbon bond length 1.54 A, carbon-hydrogen bond length 1.10 A, bond angles close to 109.5°, and staggered orientation about the carbon-carbon bonds. 

Currently accepted units of measurement are used in this book. This generally means the SI system. An exception is the use of Angstrom (A) rather than nanometre (nm). Although the latter is the SI standard the A is the unit in common use. Inevitably therefore the A is adopted here also. Indeed this is a reasonable unit of length because a carbon-hydrogen bond length is of the order of 1 A (10 10m). 

Fig. 2. 3-2IG fully-optimized structures for trans conformers of 2-vinylthiophene (a), 2-vinylfuran (b), and 2-vinylpyrrole (c). Bond lengths are given in A and angles in degrees. Carbon-hydrogen bond lengths have values around 1.07 A and are not indicated. 

Martinez-Mayorga, K., Juaristi, E., Cuevas, G. (2004). Manifestation of Stereoelectronic Effects on the Calculated Carbon-Hydrogen Bond Lengths and One-Bond 17C-H NMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (C = 0), Thiocarbonyl (C = S) and Methylidene (C = CH2) Groups toward C-H Donor Bonds. Journal of Organic Chemistry, 69, 7266-7276. 

Stereoelectronic effects on the calculated carbon-hydrogen bond lengths and one-bond /hc couplings in derivatives of cyclohexane have been studied by Martlnez-Mayorga el al. In particular, relative acceptor ability of the C=0, C=S and C=CH2 groups toward C-H donor bonds has been investigated by the authors. 

The length of a bond between a carbon atom and another atom is the shortest for a carbon atom with sp hybrid orbitals. The carbon-hydrogen bond lengths for ethyne, ethene, and ethane are 105, 109, and 111 pm, respectively. The C—H bond energy of ethyne is 536 kj moleA larger than the 470 kJ mole C—H bond energy of ethene and the 422 kJ mole" C—H bond energy of ethane. 

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