thane, ethylene and acetylene are textbook examples of molecules with sp3, sp2 and sp hybridization respectively. As gases, they are impractical for use in undergraduate laboratory experiments. 4-t-Butylphenylacetylene [TBPA] [C12H14], a readily available liquid, has all three types of C-H hybridization. Theory predicts 1JCH measures fractional s character because an electron in an s orbital has a finite chance of being located in the nucleus of an atom. In contrast, an electron in a p orbital has a node at the nucleus and a zero probability of being located there - Fermi contact interaction. The 1H coupled 13C NMR spectrum of TBPA yields 1JCH values of ~250 Hz for the ethynyl proton, ~160 Hz for phenyl ring protons, and ~125 Hz for methyl protons of the t-butyl group. These 1JCH values are very little affected by changes in solvent. Accordingly, Gaussian calculations of 1JCH more closely agree with experiment than do the calculated chemical shifts, which depend on solvent. An sp hybridized bond has 1/2 s character, an sp2 has 1/3 s character and sp3 has 1/4 s character. A plot of 1JCH vs the fraction of s character is linear, yielding the relation: 1JCH = 500 x (fraction of s character) which enables measuring the hybridization of C-H bonds. The ethynyl H of TBPA can be exchanged for D by adding a crystal of NaOH to its solution in CD3OD and heating and hence the 1JCH can be obtained using the standard 13C spectrum [1H decoupled] since 1JCH = 6.51 x [1JCD] which is the ratio of their gyromagnetic constants. This value in turn can be obtained from the 1H coupled 13C spectrum of C6H6 added to C6D6. It is noteworthy that hybridization of C-H bonds does not necessarily correctly predict internal bond angles. Cyclooctatetraene, 1JCH = 156 Hz, a commonly cited nonaromatic compound, has sp2 hybridized C-H bonds. It is nonplanar and does not have 120o bond angles. Azulene, [C10H8], an isomer of naphthalene, is aromatic and so planar. Its 5 and 7 membered rings have quite different bond angles despite the fact that all its C-H bonds are sp2 hybridized. 1JCH values confirm this. Hence the claim that hybridization predicts bond angles, or its converse, is false. .

Experimental approach to teaching hybridization: Hybridization does not determine internal bond angles

2019 Spring National Meeting

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