Front-side approach

Front-side approach is disfavored both because the density of the a orbital is less in the region between the carbon and the leaving group and because front-side approach would involve both a bonding and an antibonding interaction with the tr orbital since it has a nodal surface between the atoms. 

As shown in Fig. 1, the big lobes of these hybrids point toward each other. Therefore, if the nucleophile approaches the substrate from the front side, its HOMO overlaps in phase with the big lobe of 0c and out-of-phase with the big lobe of 0x-Numerical calculations show that the unfavourable (nucleophile-leaving group) interaction usually overrides the favourable (nucleophile - reaction center) interaction in this front-side approach, so that back-side attack is finally preferred, leading to inversion of configuration. 

Fig. 1. Overlaps of the nucleophile s HOMO with the substrate s LUMO in a front-side approach leading to retention of configuration...

Figure 9.1. Two-electron, two-orbital interaction of an alkyl halide a orbital (LUMO) and the nonbonded orbital, n, of a nucleophile (a) most favorable approach (b) unfavorable front-side approach on nodal surface.

The stereochemical consequences of front- and back-side displacements are different. With cyclic compounds, the two types of displacement lead to different products. For example, an SN2 reaction between cis-3-methylcyclopentyl chloride and hydroxide ion would give the cis alcohol by front-side approach but the trans alcohol by back-side approach. The actual product is the trans alcohol, from which we know that reaction occurs by backside displacement ... 

It should be noted that in spite of some difference in transition states geometries, all used approached give close values of In contrast to reaction with HF the alternative reaction with front-side approach of ammonia molecule to oxirane energetically is less favorable (E =200.7 kJ/mol (B3LYP/6-31G(d) [51])). 

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