Other systems with -hybridized carbon atoms

Other systems with sp -hybridized carbon atoms 

A hypothetical planar square lattice (an infinite fenestrane sheet) would have an enormous strain because each carbon atom would be forced to have two opposite valence angles reduced from 109.5° to 90°, and then these two pairs of bonds would have to be twisted to become coplanar. 

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C-Alkylations of hindered aldehyde enamines can be effected with a variety of alkylating agents, but only allylic and benzylic reagents are useful for alkylations of unhindered systems.Acyclic, homoan-nular and heteroannular dienamines undergo alkylation primarily at the a-positions. ° As discussed above, reactions of enamines with electrophiles containing sp -hybridized carbon atoms have numerous limitations. On the other hand, enamine reactions with electrophilic alkenes are highly useful and have received wide coverage in the literature. 

Compounds contsining a halogen atom covalently bonded to an sp hybridized carbon atom are named haloalkanes, or, in the common system of nomenclature, allindustrial solvents. In addition, haloalkanes are invaluable building blocks for organic synthesis because of the variety of ways in which the halogen may be substituted with other groups or eliminated to produce tt bonds. 

In this section, we considered two different mechanisms by which a substitution reaction at an sp hybridized carbon atom occurs. It is important to remember that (1) 5 1 and 5 2 reactions compete with each other, and (2) the exact details of any particular reaction might place it somewhere between these two mechanistic extremes. With an appropriate selection of reactants and reaction conditions, we might be able to push a particular reaction toward one mechanism or the other. Ultimately, the molecules in a system will always react by the lowest energy pathway. Depending on the reactants and reaction conditions, the lowest energy pathway for a substitution reaction may be the SnI reaction or the Sn2 reaction or something in between these two extremes. Table 27.2 summarizes some key ideas from this section and identifies the combinations of reactants and reaction conditions that push a substitution reaction toward either the SnI or Sn2 mechanism. 

This bonding scheme permits two pairs of electrons to be shared between two atoms so diat each pair occupies a different region of space and does not violate the Pauli exclusion principle. Since only two p orbitals are used in the hybridization and they are orthogonal and define a plane, the sp2-hybridized carbon is planar with bond angles of 120°. The remaining p orbital, which is left unhybridized to form the n bond, is perpendicular to the molecular plane. Once formed, the n bond keeps the entire system rigid and planar, because rotation of one end of die -bonded system relative to the other end requires diat die it bond be broken. 

Because experimental data shows that the benzene molecule is planar, that all carbon atoms bond to three other atoms, and that all bond angles are 120°, the benzene molecule must possess sp2 hybridization. With sp2 hybridization, each carbon atom has an unhybridized atomic p orbital associated with it. The overlap of the sp2 hybrid orbitals would create the o bonds that hold the ring together, while the side-to-side overlap of the atomic p orbitals can occur in both directions, leading to complete delocalization in the n system. This complete delocalization adds great stability to the molecule. Figure 1-1 illustrates this idea. 

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