Electron Delocalization, Hybridization, and Geometry

To delocalize nonbonded electrons or electrons in 7C bonds, there must be p orbitals that can overlap. This may mean that the hybridization of an atom is different than would have been predicted using the rules first outlined in Chapter 1. 

For example, there are two Lewis structures (A and B) for the resonance-stabilized anion (CH3COCH2r. 

The C is sun-ounded by four groups—three atoms and one nonbonded electron pair. Is it sf hybridized  

Based on structure A, the indicated carbon is sp hybridized, with the lone pair of electrons in an sp hybrid orbital. Based on structure B, though, it is hybridized with the unhybridized p orbital forming the K portion of the double bond. 

Delocalizing electrons stabilizes a molecule. The electron pair on the carbon atom adjacent to the C=O can only be delocalized, though, if it has a p orbital that can overlap with two other p orbitals on two adjacent atoms. Thus, the terminal carbon atom is sp hybridized with trigonal planar geometry. Three adjacentp orbitals make the anion conjugated. 

Sample Problem 16.3 Deteirrfei iyiVbrid flot arouSrtliefn r eS ifcrn atom in the following 

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Electron pairs involved in the n component of multiple (i.e double or triple) bonds behave somewhat differently from those in single (a) bonds (Section 7.4.1). Not only are there differences in geometry, hybridization, and s character (Section 9.3), but 7i-bond electrons interact with (delocalize into) neighboring a molecular orbitals in the molecule (hyperconjugation). Because of this delocalization, 71-bond electrons can communicate nuclear spin information over distances further than three bonds. Several examples of this type of long-range coupling are listed in Table 9.5. 

The carbonyl carbon of an amide is sp hybridized and has trigonal planar geometry. A second resonance structure can be drawn that delocalizes the nonbonded electron pair on the N atom. Amides are more resonance stabilized than other acyl compounds, so the resonance structure having the C=N makes a significant contribution to the hybrid. 

If X + E =, the central atom will be s/7 -hybridized and the ideal geometry will have bond angles of 109.5°. In exceptional cases, atoms with X + E = 4 may be sp -hybiidized. This occurs if sp hybridization enables a lone pair to occupy a p orbital that overlaps a delocalized ir electron system, as in the heteroatoms of structures 1-30 through 1-33 in Example 1.12. 

Characteristics Planar molecule with hexagonal C framework trigonal-planar geometry and sp hybridization at C atoms a framework has C-C and C-H single bonds 2p-orbitals (one from each C atom) overlap side-by-side form three rr-bonds six 7t-electrons) delocalized over the entire six C atom ring... 

The vast majority of diazo (generically C—N=N) compounds studied by X-ray diffraction involve extensive delocalization through unsaturated groups involving the C atom. Very few diazoalkanes or azidoalkanes have been reported and even here the canonical forms contributing to the effective resonance hybrids involve considerable electron delocalization. For these reasons, the geometries of diazo and azido compounds for all types of substitution are covered in Section III.B.4. 

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