Overlap in phase

In phase overlap of two Is orbitals gives new orbital encompassing both hydrogen atoms... 

FIGURE 4 11 Combi nation of tert butyl cation and chloride anion to give tert butyl chloride In phase overlap between a vacant p orbital of (CHbIbC and a filled p orbital of Cr gives a C—Cl (T bond... 

The chaiacteristic feature of valence bond theory is that it pictures a covalent bond between two atoms in tenns of an in-phase overlap of a half-filled orbital of one atom with a half-filled orbital of the other, illustrated for the case of H2 in Figure 2.3. Two hydrogen atoms, each containing an electron in a I5 orbital, combine so that their orbitals overlap to give a new orbital associated with both of them. In-phase orbital overlap (constructive interference) increases the probability of finding an electron in the region between the two nuclei where it feels the attractive force of both of them. 

In valence bond theory a covalent bond is described in temns of in-phase overlap of a half-filled orbital of one atom with a half-filled orbital of another. When applied to bonding in H2, the orbitals involved are the I5 orbitals of two hydrogen atoms and the bond is a a bond. 

Changes in molecular shape which increase the in-phase overlap between two or more AOs in a MO tend to lower the energy of that MO. Conversely, changes in shape which decrease in-phase overlap or increase out-of-phase overlap among AOs in a MO tend to raise the energy of the MO. This can be called the overlap rule. 

Fig. 6.16 Molecular Orbital pictures and qnalilalive energies of linear and bent AB molecules. Open and shaded areas represent differences in sign (+ or ) of the wave functions. Changes in shape which increase in-phase overlap lower the molecular orbital energy- From Gimarc.

In the case of we can say that there is positive (in-phase) overlap between the hydrogen Is orbitals, because the product is positive in the internuclear region (the amplitudes of both orbitals are positive) this leads to bonding between the two atoms. In the case of the amplitudes... 

An in-phase overlap is bonding and lowers the MO energy, whereas an out-of-phase overlap is antibonding and raises the MO energy. 

This means that the nucleophile is pushed away more strongly in the alkene case than in the carbonyl case. At the same time, the attractive in phase overlap between Nu and Q is smaller with n cc than with 7t co, which tends to maintain Nu nearer to the vertical position in the carbonyl case. Both factors contribute to give a larger angle for the reaction with alkenes or alkynes. 

In order to understand it let us consider the interference of waves. Now if the crest of one wave overlaps with the crest of the other, the two waves interact in a constructive interference and therefore the new resulting wave is reinforced, i.e. add up. In other words, there is in phase overlap or addition overlap. In the similar way, addition overlap of atomic orbitals with same signs leads to the formation of bonding molecular orbital. 

The formation of the C(l)-C(4) a bond via the participation of two 7T electrons of the HOMO requires that the 2p,r orbitals in atoms C(l) and C(4) undergo rotation in the same sense conrotatory mode) so that positive (in-phase) overlap occurs (bonding effect). If they rotate in opposite directions (disrotatory mode) then there will be negative overlap and an anti-bonding effect. Accordingly, the conrotatory closure of (a) in a thermally controlled reaction leads to the cis isomer of dimethylcyclobutene (b) and not to (c). 

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