Hybridization s character

TABLE 10. Conventional strain energies (CSE), hybridizations, s-character, overlap values, overlap repulsions and geminal delocalizations of propane, cyclobutane, cyclopropane and their heterologues with X = NH, O, SiH2, PH, S from Reference 47 ... 

Hybridization (s character of the C-H bond)—higher % s character, lower C-H 50, C=C-H 44, C=C-H 25 Effect of substitution—lower carbanion stability, higher... 

Compound Conjugate Base Hybridization s Character Pifa ... 

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... 

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... 

Figure 8.5 A comparison of alkyl, vinylic, and acetylide anions. The acetylide anion, with sp hybridization, has more s character and is more stable. Electrostatic potential maps show that placing the negative charge closer to the carbon nucleus makes carbon appear less negative (red).

What accounts for the stability of conjugated dienes According to valence bond theory (Sections 1.5 and 1.8), the stability is due to orbital hybridization. Typical C—C bonds like those in alkanes result from a overlap of 5p3 orbitals on both carbons. In a conjugated diene, however, the central C—C bond results from conjugated diene results in part from the greater amount of s character in the orbitals forming the C-C bond. 

Trends in Au-X and Au-P bond lengths in complexes (PPh3) AuX should be noted (Table 4.10) the Au-Cl bond length varies more with changes in coordination number than does the Au-P bond and is, therefore, more sensitive to the decrease in s character as the hybrid orbitals used by gold change from sp to sp3. 

Noting that the bond angle of an sp hybridized atom is 109.5° and that of an sp1 hybridized atom is 120°, do you expect the bond angle between two hybrid orbitals to increase or decrease as the s-character of the hybrids is increased ... 

Bond distances for some important bond types are given in Table 1.5. As can be seen in this table, carbon bonds are shortened by increasing s character. This is most often explained by the fact that, as the percentage of s character in a hybrid orbital increases, the orbital becomes more like an s orbital, and hence is held more tightly by the nucleus than an orbital with less s character. However, other explanations have also been offered (see p. 37), and the matter is not completely settled. 

Acetylene, where the carbon is sp hybridized with 50% s character, is much more acidic than ethylene (sp, 33% s), which in turn is more acidic than ethane, with 25% 5 character. Increased s character means that the electrons are closer to the nucleus and hence of lower energy. As previously mentioned, cyclopropyl carbanions are more stable than methyl, owing to the larger amount of s character as a result of strain (see p. 181). 

In triaziridine 23, the o-relaxation is appreciable due to the lone pair effect (Sect. 2.1.4) as is the case with triphosphirane 14 which has almost the same SE as tetra-phosphetane 15. Triaziridine 23 has lower SE (31.8 kcal moF ) than tetraazetidine 24 (SE = 33.4 kcal moF ) (Scheme 12) [12]. Lone pairs on nitrogen atoms have high s-character. The hybrid orbitals for the N-N bonds have lower s-character (sp ) in 23 than that (sp ) in triazane 25 [12]. The geminal a-a interaction is less antibonding in 23 (IBP, = -0.004) than in 25 (IBP. = -0.014). The geminal... 

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