tertiary alcohols plus strong acid

[myusernam3]

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Why do tertiary alcohols react faster with HCl than primary and secondary alcohols?

 

[eric51]

Tertiary alcohols can follow the SN1 reaction pathway because of the stability of the carbocation intermediate.

 

[boaz]

Tertiary alcohols can follow the SN1 reaction pathway because of the stability of the carbocation intermediate.

👍

 

[crazymedgirl]

Thanks for posting, this! I was tripped up by this one too--it's from one of the AAMC exams, right?

I'm still not sure why we should assume that it's undergoing an SN1 reaction here. If it was undergoing SN2, then primary would obviously react fastest? I remember being confused about this on the passage because it didn't seem to make this clear....

Maybe I'm remembering wrong.

 

[Isoprop]

This is how I reasoned it out:

Cl- is a moderate/poor nucleophile. But the protonated hydroxy group is a very good leaving group.

With tert alcohols, the step when the L.G. leaves (1st step) is the rate determining step via SN1. In this case, the RTS happens very fast because the LG is so good and because the carbocation is tertiary.

With primary alcohols, it can only leave via SN2. But how well the L.G. is doesn't matter as much because it all happens in 1 step. And the nucleophile isn't that great (halogens are only moderate to weak nucleophiles). So the reaction is slower.

To summarize, let's look at the reaction rate laws.

SN1 (tert): rate = k[alcohol]
SN2 (primary): rate = k[alcohol][nucleophile]

The first reaction will go fast because it only depends on how good the alcohol is. The second reaction will not go as fast because it depends on how good both the alcohol and the nucleophile is.

This might not be the correct way to think about it.

 

[eric51]

This is how I reasoned it out:

Cl- is a moderate/poor nucleophile. But the protonated hydroxy group is a very good leaving group.

With tert alcohols, the step when the L.G. leaves (1st step) is the rate determining step via SN1. In this case, the RTS happens very fast because the LG is so good and because the carbocation is tertiary.

With primary alcohols, it can only leave via SN2. But how well the L.G. is doesn't matter as much because it all happens in 1 step. And the nucleophile isn't that great (halogens are only moderate to weak nucleophiles). So the reaction is slower.

To summarize, let's look at the reaction rate laws.

SN1 (tert): rate = k[alcohol]
SN2 (primary): rate = k[alcohol][nucleophile]

The first reaction will go fast because it only depends on how good the alcohol is. The second reaction will not go as fast because it depends on how good both the alcohol and the nucleophile is.

This might not be the correct way to think about it.

No. I think that's exactly the way to think about it. Good explanation.

 

[crazymedgirl]

wow, that's really impressive logic!

ok thanks a lot, that really helped.