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Topic: Does the substitution of the nucleophile matter during an SN2 reaction?  (Read 947 times)

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Offline lord farquaad

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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.

Offline Babcock_Hall

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Re: Does the substitution of the nucleophile matter during an SN2 reaction?
« Reply #1 on: January 24, 2022, 10:47:39 AM »
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 SN2 process?

Offline rolnor

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Re: Does the substitution of the nucleophile matter during an SN2 reaction?
« Reply #2 on: January 24, 2022, 03:49:51 PM »
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.

Offline Babcock_Hall

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Re: Does the substitution of the nucleophile matter during an SN2 reaction?
« Reply #3 on: January 24, 2022, 05:04:29 PM »
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
« Last Edit: January 24, 2022, 06:08:20 PM by Babcock_Hall »

Offline rolnor

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Re: Does the substitution of the nucleophile matter during an SN2 reaction?
« Reply #4 on: January 24, 2022, 11:59:43 PM »
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.

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