Alcohols HBr & PBr3

[deleted738762]

A reaction with 2-butanol with HBr produces 2-bromobutane with an enatiomer. An Sn1 Reaction. However,

2-butanol with PBr3/pyridine produces 2-bromobutane with an inversion. An Sn2 reaction.

Does this happen because HBr is a strong acid so a weak base thus being Sn1? Is PBr3 a strong base then for Sn2? Because I don't see why one is different then the other.
Thank You

I meant nucleophiles 🙂

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[rak173]

A reaction with 2-butanol with HBr produces 2-bromobutane with an enatiomer. An Sn1 Reaction. However,

2-butanol with PBr3/pyridine produces 2-bromobutane with an inversion. An Sn2 reaction.

Does this happen because HBr is a strong acid so a weak base thus being Sn1? Is PBr3 a strong base then for Sn2? Because I don't see why one is different then the other.
Thank You

The 2 butanol should be either R or S first for stereochemistry to occur. Sn1 produces racemic mixture; hence it will have enantiomers R and S. And like you said Sn2 will have inversion. I think your reasoning behind the strong acid is weak base is correct.

Is there any list of strong and weak nucleophiles that we need to memorize for sn1 and sn1 reaction?

 

[deleted738762]

I see nucleophile strength decreases as you go up a group so it makes sense the conjugate base B- is weak nuc hence SN1. But for the 2nd rxn wouldn't PBr3 turn into PBr2OH and thus a B- would attack.
I don't understand how I could distinguish 1 over the other because both use B- as a nuc.

The only logical explanation I have is that the Sn2 reaction uses pyridine which is aprotic, so I assume that's the only way to distinguish between the two but then wouldn't B- still be weak so why Sn2 if Sn2 requires strong nuc's

 

[DAT Destroyer]

A reaction with 2-butanol with HBr produces 2-bromobutane with an enatiomer. An Sn1 Reaction. However,

2-butanol with PBr3/pyridine produces 2-bromobutane with an inversion. An Sn2 reaction.

Does this happen because HBr is a strong acid so a weak base thus being Sn1? Is PBr3 a strong base then for Sn2? Because I don't see why one is different then the other.
Thank You

I meant nucleophiles 🙂

When you have an Sn1 process, a planar carbocation is formed. This specie can be attacked from the top or bottom face, which can give enantiomers. When using PBr3, NO carbocations are involved . Any text book will show you the reaction mechanism. Since no carbocation is involved, and a backside attack occurs,in an SN2-like process the inversion product is obtained. The SN2 process is indeed favored by strong bases such as CN-, SH-, and similar reagents, but not here. This is why I teach my students to call it an SN2-like process.

I hope this helps.

Dr. Romano