Carbon s character

In this form, eq. (3.15) yields only positive 13C—X couplings however, negative one-bond couplings (e.g. Jc 19p) are also observed [115]. Moreover, interpretation of carbon one-bond coupling constants in terms of carbon s character or hybridization (Fig. 3.14) assumes the factor Sc(0)Sx(0) (d e) 1 to be constant. In fact, AEC and s3 (0) are found to vary considerably in substituted methanes [116, 117]. 

Fig. 3.14. Ranges of one-bond carbon-13-proton coupling constants for sp3, sp2, and sp carbons (dotted rectangles) and linear correlation between JCH and carbon s character of comparable compounds (coupling constants from Ref. [115]). The empirical relation is. /CH 500 s (Hz).

Couplings between carbon and many heteronuclei, e.g. the JCF data shown in Table 3.14, do not depend simply on carbon s character as is the case for JcH and Jcc. Nevertheless, electron withdrawal at the coupling carbon often increases Jcx. This trend is obvious when the JCF values of mono- di-, and trifluoroethanol, or those of mono- and trifluoroacetic acid are compared (Table 3.14). 

CBr4 I , has been rationalized whereas the positive charge on carbon and, thus, the carbon s-character of the C-I bonds increases with n in the former series, it stays relatively constant in the latter series (because of the similar electronegativities of Br and I) [54],... 

The view of variable hybridization has some experimental support. The magnitude of C- H NMR coupling constants is expected to be proportional to the amount of carbon s character in the bond, because only s orbitals have density at the carbon nucleus and can affect neighboring nuclear spin states. In several systems, a clear correlation has been observed between NMR coupling constants and percent s character, as predicted from the geometry and the associated hybridization index. 

Thus in methane (sp -hybridized carbons s-character, 0.25) Jch = 125 Hz, in ethylene (sp -hybridized carbons s-character, 0.33) Vch = 167 Hz while in acetylene (sp-hybridized carbons s-character, 0.5) CH = 250 Hz. The values of Vch couplings thus allow one to probe the extent of s-character in a given bond. For example, in cyclopropane Vch = 162 Hz, which is evidence of sp hybridization in this molecule. 

For positive A (XY) it is actually possible to do comparative studies of coupling constants using only localized orbitals and formal hybridization. In the simplest description using McConnell s equation (20), the Fermi contact term of the coupling is proportional to P] J AE. If we further simplify to localized bonds, the bond order proportional to the s characters of X and Y in the XY bond. If we assume equivalent hybrids, the carbon s character in CH bonds in HjC—CHj, H2C=CH2, and HC = CH will be roughly K p ), sp ) and (ip), so that comparative values of /(CH) will be... 

Because each carbon m acetylene is bonded to two other atoms the orbital hybridization model requires each carbon to have two equivalent orbitals available for CT bonds as outlined m Figure 2 19 According to this model the carbon 2s orbital and one of Its 2p orbitals combine to generate two sp hybrid orbitals each of which has 50% s character and 50% p character These two sp orbitals share a common axis but their major lobes are oriented at an angle of 180° to each other Two of the original 2p orbitals remain unhybridized... 

All of these trends can be accommodated by the orbital hybridization model The bond angles are characteristic for the sp sp and sp hybridization states of carbon and don t require additional comment The bond distances bond strengths and acidities are related to the s character m the orbitals used for bonding s Character is a simple concept being nothing more than the percentage of the hybrid orbital contributed by an s orbital Thus an sp orbital has one quarter s character and three quarters p an sp orbital has one third s and two thirds p and an sp orbital one half s and one half p We then use this information to analyze how various qualities of the hybrid orbital reflect those of its s and p contributors... 

An easy way to keep track of the effect of the s character of carbon is to associ ate It with electronegativity As the s character of carbon increases so does that carbon s electronegativity (the electrons m the bond involving that orbital are closer to carbon) The hydrogens m C—H bonds behave as if they are attached to an increasingly more electronegative carbon m the series ethane ethylene acetylene... 

Using the relationship from the preceding section that the effective electronega tivity of carbon m a C—H bond increases with its s character (sp < sp < sp) the order of hydrocarbon acidity behaves much like the preceding methane ammonia water hydrogen fluonde series... 

The acidity mcreases as carbon becomes more electronegative Ionization of acetylene gives an anion m which the unshared electron pair occupies an orbital with 50% s character... 

At 146 pm the C 2—C 3 distance m 1 3 butadiene is relatively short for a carbon-carbon single bond This is most reasonably seen as a hybridization effect In ethane both carbons are sp hybridized and are separated by a distance of 153 pm The carbon-carbon single bond m propene unites sp and sp hybridized carbons and is shorter than that of ethane Both C 2 and C 3 are sp hybridized m 1 3 butadiene and a decrease m bond distance between them reflects the tendency of carbon to attract electrons more strongly as its s character increases... 

A considerable body of data is available on the acidity of substituted benzoic acids Ben zoic acid Itself is a somewhat stronger acid than acetic acid Its carboxyl group is attached to an sp hybridized carbon and ionizes to a greater extent than one that is attached to an sp hybridized carbon Remember carbon becomes more electron withdrawing as its s character increases... 

One-bond CH coupling constants Jqh ( Jch) proportional to the s character of the hybrid bonding orbitals of the coupling carbon atom, (Table 2.6, from left to right)according to... 

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