Stability and structure of carbocations
The most important factor in determining which mechanism (SN1 or SN2) applies to an organic compound is the structure of the carbon skeleton. A useful generalization is that most compounds that can form relatively stable cations generally do so and react via the SN1 mechanism, while the others have to react via the SN2 mechanism. Cations are more stable if they are heavily substituted, but this is bad for an SN2 reaction because the nucleophile would have to thread its way into the carbon atom through the alkyl groups.
(HINT: Think about the stabilizing effect alkyl groups can have, and also their role in terms of steric hinderence)
Click either SN1 or SN2 with your choice to find out the correct answer
(Red represents areas which are electron rich, blue represents areas that are electron poor)
Any charged organic intermediate is inherently unstable because of the charge. A carbocation can be formed only if it has some extra stabilization. The basic instability of the carbocation come from its electron deficiency, in that it has an empty orbital. The energy of the unfilled orbital is irrelevant to the overall stability of the cation, it's only the energy of the orbitals with electrons in that matter. For any cation the most stable arrangement of electrons in orbitals results from making filled orbitals as low in energy as possible to give the most stable structure, leaving the highest-energy orbital empty. Extra stabilization comes to the planar structure from weak donation of σ bond electrons into the empty p orbit of the cation. Three of these donations can occur at any one time in the t-butyl cation, and it doesn't matter if the C-H bonds point up or down, but one C-H bond on each methyl group much be parallel to one lobe of the empty p orbital at any one time.