For instance, when you have a primary halide with a weak nucleophile, do you think that the reaction mechanism would be SN1 or SN2, neither or would it even be both, like sometime leaving group can leave by itself creating carbocation and nucleophile because it is late later attack in second step. Same when you have a tertiary halide and really strong nucleophile and polar aprotic solvent (which we know facilitates SN2 rxn). I am not sure how to think of the process. Like when it comes to primary and tertiary halides, does the substrate nauture determine mode of mechanism from the get go, with no input of the strength of nucleophiles, or type of solvent. Or do nucleophiles and type of solvent have a role in determining whether sn1 or sn2 would occur also?

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    $\begingroup$ You should never invoke $S_{\mathrm{N}}1$ on a primary halide unless it's heavily stabilized. And in cases where it's truly not clear, the answer is that they're both operating. $\endgroup$ – Zhe May 3 '17 at 15:16