DAT Destroyer organic chemistry #67

[sicemdude]

The question asks which reagents would be best to make isobutanol into isobutyl chloride. "SOCl2, Pyridine" was the correct answer. I chose an answer that included that along with HCl, but apparently HCl is wrong. The solution says that a shift would occur with HCl to give 2-chloro-2-methyl propane.

I'm wondering why the hydroxyl group couldn't be protonated and the Cl- couldn't come in and replace it via SN2. Any explanation would be awesome.

Thanks!

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[DMikes]

It would predominately react in an SN1 fashion.

If the hydroxyl is protonated, it will leave as water. Then, an unstable carbocation will form and a shift will occur. The Cl- will then add.

With SOCL2 only that product will form.

 

[sicemdude]

Thanks for the response.

So why won't it react via SN2?

 

[DMikes]

Thanks for the response.

So why won't it react via SN2?

Ya know, the more I think about it I may be incorrect to say it would "predominately act in an SN1 fashion," (Some SN1 will still definitely occur) but after looking at destroyer, the question specifies "which reagents would be best for this reaction" and that may be why it is SOCl2 w/pyridine.

Paging @orgoman22

 

[DAT Destroyer]

The question asks which reagents would be best to make isobutanol into isobutyl chloride. "SOCl2, Pyridine" was the correct answer. I chose an answer that included that along with HCl, but apparently HCl is wrong. The solution says that a shift would occur with HCl to give 2-chloro-2-methyl propane.

I'm wondering why the hydroxyl group couldn't be protonated and the Cl- couldn't come in and replace it via SN2. Any explanation would be awesome.

Thanks!

The SN2 reaction would have a hard time here because of the rather large isopropyl group. This will effectively compete with the faster SN1 reaction. We would protonate, and simultaneously do a shift as the water leaves.....Although rather rare, this would occur because we dont form primary carbocations, they are very high energy. Would there be any SN2 competition? Sure.....but SN1 wins out.....If you want more info, I can look up the reaction in the literature and give it to you.....I would estimate that 85% of the reaction took the SN1 path. Using SOCl2 would prevent any rearrangement from occurring, this is the correct answer.

Hope this helps.

Dr. Romano

 

[sicemdude]

The SN2 reaction would have a hard time here because of the rather large isopropyl group. This will effectively compete with the faster SN1 reaction. We would protonate, and simultaneously do a shift as the water leaves.....Although rather rare, this would occur because we dont form primary carbocations, they are very high energy. Would there be any SN2 competition? Sure.....but SN1 wins out.....If you want more info, I can look up the reaction in the literature and give it to you.....I would estimate that 85% of the reaction took the SN1 path. Using SOCl2 would prevent any rearrangement from occurring, this is the correct answer.

Hope this helps.

Dr. Romano

Thanks for the explanation! I guess this is just sort of a tricky question.

Btw I think it's pretty cool that you're active on this site. Don't see that kinda thing too often.

Take care