${{S}_{{{N}^{1}}}}$ also depends upon the degree of'shielding' of the front side of the reacting carbon.
$(1)\,\,(CH_3)_3C-Br$ $(2)\,\,(C_6H_5)_2CH-Br$
$(3)\,\,(C_6H_5)_2C(CH_3)Br$ $(4)\,\,(CH_3)_2CH-Br$
$(5)\,\,C_2H_5Br$
[જ્યા $\left.Et = C _2 H _5\right]$