Parallel orbitals

In contrast, the chemical shifts of the axial methyl groups in 53 are nearly independent of X (slope = -0.7) (Figure 4). From that finding these authors (164) concluded tht electronegativity information is transmitted to a y-gauche carbon atom only through a rigid pathway of parallel orbitals and that methyl rotation protects this carbon atom from electronic interaction. 

When orbitals overlap, bonds form. When the overlap is along the internuclear axis, this is known as a a (sigma) bond. Sigma bonds can form between s, p, or hybridized orbitals. When the overlap is between parallel orbitals that lie perpendicular to the nuclear axis, this is known as a it (pi) bond. 

The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. In most cases, the structure must be planar (or nearly planar) for effective overlap to occur. 

Why is this important Well, if you cast your mind back to Chapter 38, you will remember that tlie overlap of parallel orbitals was very important in fragmentation reactions. Here, for example, is a fragmentation reaction that goes very well, but that can take place only if the nitrogen s lone pair is equatorial, because only an equatorial (black) lone pair can overlap with the antibonding orbital of the C-C bond that breaks. The chloride leaving group must be equatorial as well. 

Other situations you have met where overlap between parallel orbitals is important are ... 

In order to estimate roughly the forces exerted on an electron in one orbit (A in the figure of the previous note) due to the electrons in the parallel orbit, I assumed that they were the same as if the charges of the electrons in the orbit BC were concentrated, half at the nearest point... 

Note that cr bonds are formed from orbitals whose lobes point toward each other, but tt bonds result from parallel orbitals. A double bond consists of one cr bond, where the electron pair is located directly between the atoms, and one tt bond, where the shared pair occupies the space above and below the cr bond. 

When an electron pair is shared by the direct overlap of bonding orbitals, a sigma bond results. The overlap of parallel orbitals forms a pi bond. Single bonds are sigma bonds. Multiple bonds involve both sigma and pi bonds. 

In the last chapter, we looked at some stereospecific eliminations to give double bonds, and you know that E2 elimination reactions occur best when there is an anti-periplanar arrangement between the proton and the leaving group, largest -Ufrom parallel orbitals... 

The pi bond A multiple covalent bond consists of one sigma bond and at least one pi bond. A pi bond, represented by the Greek letter pi (tv), forms when parallel orbitals overlap and share electrons. The shared electron pair of a pi bond occupies the space above and below the line that represents where the two atoms are joined together. 

Orbitals overlap directly in sigma bonds. Parallel orbitals overlap in pi bonds. A single covalent bond is a sigma bond but multiple covalent bonds are made of both sigma and pi bonds. 

Coupling is a through-bond phenomenon, as we know from the couplings in cis and trans alkenes, where trans alkenes have much larger coupling constants as their orbitals are perfectly parallel. Another case of perfectly parallel orbitals occurs with trans-diaxial protons in cyclohexanes. Typical coupling constants are 10-12 Hz for trans-diaxial protons, but much smaller (2-5 Hz) for axial/equatorial and equatorial/equatorial protons. 

Figure 2.19 n-Overiap between parallel orbitals is responsible for double and triple bond formation. Various changes in geometry and overlap account for stronger or weaker tr-bonds. 

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