[lord farquaad]

SN2 is likely to occur when the alkyl halide has primary substitution. But will SN2 still occur if the nucleophile has tertiary substitution? I was surprised that the explanation for this reaction (attached) was that it occurred through SN2 mechanism. I thought the fact that O is so sterically hindered would prevent SN2.

[Babcock_Hall]

I remember looking up the behavior of tertiary-butoxide as a nucleophile one time.  It is a good nucleophile against iodomethane.  Do you think that there could be a reaction competing with the S_N2 process?

[rolnor]

Sn1 goes via a carbocation, the primary halide can hardly not form this. What will happen with a bulky nucleophile is that the reaction is slow or does not occur at all or you get elimination.

[Babcock_Hall]

When iodomethane is the electrophile and the solvent is DMF, the rate constant for tertiary-butoxide is about twice that of ethoxide (Daley and Daley, p. 537).  Unfortunately, I don't have this book, and I don't know what the rates are with other alkyl halides.  The text on this page implies that tert-butoxide reacts slowly with other alkyl halides.  There was a thread here that mentioned this in 2016.
https://www.chemicalforums.com/index.php?topic=88451.0

[rolnor]

Iodomethane is less sensitive to this problem, its so small. tert-butoxide is a stronger base and probably a more powerfull nucleophile then ethoxide. Sodium ethoxide has pka 15,5 potassium tert-butoxide has 17.