The electronegativity order of hybridized carbons is $\mathrm{s}\mathrm{p}>\mathrm{s}{\mathrm{p}}^2>\mathrm{s}{\mathrm{p}}^3$ . So in an allylic halide, such as $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}\mathrm{H}-\mathrm{C}{\mathrm{H}}_2-\mathrm{I}$ , the alpha carbon is much more partially positive than the alpha carbon in $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2\mathrm{I}$ . Therefore, the attacking nucleophile will be attracted to the alpha carbon more in the allylic halide compound than in the primary halide. Hence, allylic halides tend to follow ${\text{S}}_{\text{N}}\text{2}$ pathways more than primary halides.