SN2 question

[kabtq9s]

I was given the below reaction and asked about whether its sn2 or sn1. I'm having trouble understanding the provided explanation

the only alkly halide in that reaction is the Ch3X and its the Nucleophile, I'm confused why they are referring to it as the Electrophile

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[chiddler]

The alkyl halide seeks the negatively charged tertiary alkoxide. It seeks negative! It is electrophile!

 

[milski]

Another way to think about it: Sn1 would mean a carbocation formed. CH3+ is not going to happen nor is forming any sort of cation on the negatively charged alcohol.

 

[kabtq9s]

The alkyl halide seeks the negatively charged tertiary alkoxide. It seeks negative! It is electrophile!

How did you know its an alkoxide with a negative charge ? I see a K+ there so the whole molecule will have 0 charge

 

[SaintJude]

This how I would think about it on the MCAT

1. The tertiary alcoxide is written as an ion and so is the K+ ion. So they want me to understand they exist in solution as separate ions)

2.) Will the tertiary alcoxide undergo SN1? No it's got no good leaving group. What about the alkyl halide? No, primary alkyl halides do Not undergo SN1 reaction's [Imagine the alkyl halide undergoing SN1, it would just leave a "CH3" carbocation group hanging?!"Not happening]

So it's must be SN2.

But a few extra notes for you...

SN2 favor primary alkyl halides. And what I've noticed from this problem is that's crucial for us to know how functional groups are written. I had a similar problem that was kind of based on functional groups.

 

[kabtq9s]

Part of my confusion is that I'm used to figuring out if a reaction is Sn2 or Sn2 by the reactivity of the R-OH reactant. SN1 favor 3°>2°>>1° while Sn2 favor 1°>2°>>3°. And I see that the R-OH reactant here fits the description of an Sn1 reaction. (taken straight from my Princeton Review book pg 510 & 512)

 

[SaintJude]

Another thing regarding the designation of an electrophile vs. nucleophile...Alcoxide are some of the strongest nucleophiles: RO-> HO-> ROH> H20.

Yeah, a lot of the test preps are trying to feed us formulas, that are all too easy to memorize and not question. But the more study, the more I realize that only understanding the concepts fully is a sure way not to get tripped up.

Thanks for posting!

 

[kabtq9s]

Thanks SaintJude,

I think I get it now. I just looked at the fact that alkoxide was tertiary and assumed that the reaction would proceed as an Sn1 since Sn1 favor tertiary electrophilic reactants. However in this case the alkoxide was actually the Nucleophile since it had a negative charge and that would make the alkyl halide the Electophile and since it was primary the reaction would proceed as an Sn2

Thanks for all your help guys/ gals

 

[hopeful22213]

I think it's just easiest to look at the CH3X. SN1 = a carbocation and that's not going to happen on a primary carbon.

 

[Class1P]

It is easy to miss that the CH3-X is an electrophile since the halide will have a negative charge and the carbon a positive and you're probably used to seeing it as a grignard CH3-MgX. the Grignard carbon would be a nucleophile.