SuperSaiyan3

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May 13, 2009
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I wrote a Kaplan FL test today and they asked what would happen if a 2,3-dimethyl-2-butanol was reacted with HBr.

I assumed that it would undergo a Sn1 reaction because it was a tertiary alcohol and I was unsure about the strength of a bromide anion. But either way, it would be Sn1 because Sn2 cannot proceed with a tertiary carbon.

I was wrong. The answer was Sn2.

I am referring to this website: http://www.brynmawr.edu/Acads/Chem/mnerzsto/SN2vsE2_flowchart.htm

What's going on? Why is this Sn2? Is Kaplan just wrong (as usual)?

Furthermore, when do you know if something is base or a nucleophile? For example, HBr is an acid. I would assume that the Br- anion would be a base since it is technically a conjuage base of HBr. But in this case, the Br- anion acts as a nucleophile because no double bonds are made..

SO CONFUSED!!
 

RogueUnicorn

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I wrote a Kaplan FL test today and they asked what would happen if a 2,3-dimethyl-2-butanol was reacted with HBr.

I assumed that it would undergo a Sn1 reaction because it was a tertiary alcohol and I was unsure about the strength of a bromide anion. But either way, it would be Sn1 because Sn2 cannot proceed with a tertiary carbon.

I was wrong. The answer was Sn2.

I am referring to this website: http://www.brynmawr.edu/Acads/Chem/mnerzsto/SN2vsE2_flowchart.htm

What's going on? Why is this Sn2? Is Kaplan just wrong (as usual)?

Furthermore, when do you know if something is base or a nucleophile? For example, HBr is an acid. I would assume that the Br- anion would be a base since it is technically a conjuage base of HBr. But in this case, the Br- anion acts as a nucleophile because no double bonds are made..

SO CONFUSED!!
this is not true

that being said i would assume this should be sn1.
 

Accalia

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Aug 3, 2009
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Hey SuperSaiyan3, you're right! Did the Kaplan test give any sort of explanation for why they thought it would be Sn2? 2,3-dimethyl-2-butanol is a tertiary alcohol, so it would undergo an Sn1 reaction with HBr.

As for the difference between a base and a nucleophile--this depends depend on what the species does in the reaction. The same species can be both a good base and a good nucleophile, but nucleophiles donate electron pairs while bases accept protons. So, if the species pulls off a proton,then it's acting as a base. If it attacks an electrophile, then it's acting as a nucleophile.

In this reaction, the bromide anion is acting as a nucelophile because it donates electrons to the tertiary carbocation that forms in Sn1 after water leaves.

Hope that helps!
 

Omni

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So a nucleophile is a Lewis Base?
 

RogueUnicorn

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it could also be that sn2 was typo. did the rest of the explanation go into carbocations and such? that would imply that they meant to write sn1
 

Accalia

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So a nucleophile is a Lewis Base?
Hey Omni--a Lewis base can act as a nucelophile, but they don't exactly correspond. Not all nucleophiles are Lewis bases, because lewis bases donate lone pairs, whereas a nucleophile can have either structural lone pairs or a negative charge.

Here is a good answer from another board regarding the differences between acidity/basicity and electophilicity/nucleophilicity:

"The essential difference is that the acidity/basicity is a thermodynamical property, measured with the equilibrium constant, while the electrophilicity/nucleophilicity is a cinetic property, measured with the kinetic constant. So, a strong lewis acid, for ex., can react slowly with a lewis base and so be a weak electrophile; or a strong electrophile, which reacts fast with a nucleophile, can have a low equilibrium constant and so be a weak lewis acid. Usually a strong lewis acid/base is also a strong electrophile/nucleophile, but it's not always so, for the reason I've written."