@Abcd Why there and not on the carbon having halogen?
@Dante Why there? You answer: why not at the least hindered top carbon? Which is one of the main characteristic of SN2, nucleophile prefers least hindered electrophilic carbon
@Dante The aprotic solvent is even a supporter in this...because you know aprotic solvents favour SN2.
@Dante Also see the bottom of the above page pic.
I noted that from Clayden most probably.
That clearly shows that SN2' is strongly favoured at the least hindered carbon