Tert butyl alcohol + HCl and ZnCl2

[reising1]

In reacting tert butyl alcohol (or any alcohol that undergoes SN1) with HCl and ZnCl2 it seems that the purpose of the ZnCl2 is to activate the alcohol and make it a better leaving group.

If it was an acid, I would understand this, because the OH could be protonated and then leave as water. But how does ZnCl2 activate the alcohol exactly?

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[sakabato93]

In reacting tert butyl alcohol (or any alcohol that undergoes SN1) with HCl and ZnCl2 it seems that the purpose of the ZnCl2 is to activate the alcohol and make it a better leaving group.

If it was an acid, I would understand this, because the OH could be protonated and then leave as water. But how does ZnCl2 activate the alcohol exactly?

I do believe Zn2+ acts as a Lewis acid.

 

[reising1]

But it doesn't donate a proton so how does this make the OH a better leaving group? It just leaves as OH-?

 

[desperadoflakes]

But it doesn't donate a proton so how does this make the OH a better leaving group? It just leaves as OH-?

Fairly certain that the HCl (a stong acid) dissociates into H+ and Cl-. The OH- picks up the hydrogen and is now H2O, a good leaving group. The newly formed tertiary carbocation now reacts with the ZnCl2 to give t-butyl chloride.

 

[sakabato93]

Fairly certain that the HCl (a stong acid) dissociates into H+ and Cl-. The OH- picks up the hydrogen and is now H2O, a good leaving group. The newly formed tertiary carbocation now reacts with the ZnCl2 to give t-butyl chloride.

I thought he meant that both of the reagents were being used separately, not together.

 

[reising1]

They are being used together. The purpose of the ZnCl2 is to activate the alcohol (as per Examkrackers), not to add to the tert butyl.

 

[Vurday]

ZnCl2 is your lewis acid in this situation, while the -OH will be your lewis base.

Firstly, you'll form a bond between OH and ZnCl2 giving you H -- O(+) -- (-)ZnCl2
This will basically add a degree of resonance to your Alcohol, allowing it to become a better leaving group. From here, Cl- from the dissociation of HCl will come in and attack the newly form carbocation, adding Cl.

If you're more of a visual person, this is drawn out here (Slide 20): http://www.utdallas.edu/~biewerm/11alc.pdf

And here (Page 7):
http://crab.rutgers.edu/~alroche/Ch11.pdf