SN1 Question

[AZFutureDoc]

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I'm confused on the SN1 reaction where you take a tertiary alcohol and halogenate it by adding HCl or HBr, by first protonating the alcohol and then an SN1 reaction. I know that in an SN1 reaction, the rate only depends on the concentration of substrate. But in this case, since to even begin the alcohol needs to be protonated, is the reaction now second order? Does increasing the amount of acid matter since more substrate will be protonated and able to react? There's a question in EK, (42 on page 52 of the Ochem manual) and I think I am overthinking the question.

 

[TieuBachHo]

I'm confused on the SN1 reaction where you take a tertiary alcohol and halogenate it by adding HCl or HBr, by first protonating the alcohol and then an SN1 reaction. I know that in an SN1 reaction, the rate only depends on the concentration of substrate. But in this case, since to even begin the alcohol needs to be protonated, is the reaction now second order? Does increasing the amount of acid matter since more substrate will be protonated and able to react? There's a question in EK, (42 on page 52 of the Ochem manual) and I think I am overthinking the question.

Well, I am guessing that the amount of acidic solution does not affect the reaction rate but rather it is the strength of its acidity (as in acid-base reaction). However, the slowest step is not the protonation of tertiary alcohol but the leaving group HOH. HBr is not a deficit. It doesn't matter how much more you add.

 

[sleepy425]

acid/base reactions are some of the most rapid chemical reactions. because of that, they are not rate-limiting and thus do not factor into the kinetics of the reaction.

think about it, why is Sn1 even unimolecular to begin with? it's because the rate-limiting step is leaving group dissociation. nucleophilic attack of the carbocation is more rapid, so it's not rate-limiting. thus, since the rate-limiting step is dissociation, and since the only reactant in a dissociation is the alcohol (or alkyl halide), the rate of reaction only depends on alcohol concentration.

 

[AZFutureDoc]

Ahhh true. In a two step reaction, a slow step proceeded by a fast step has a rate only dependent on the first step. Its fast steps proceeded by a slow step that gets the kinda tricky rate laws.

 

[sleepy425]

Ahhh true. In a two step reaction, a slow step proceeded by a fast step has a rate only dependent on the first step. Its fast steps proceeded by a slow step that gets the kinda tricky rate laws.

well, sort of, but really, the place where the slow step occurs doesn't really affect the rate law. it's what the slow step is that affects the rate law.

think of it this way:

you have a long pipe that is made up of 3 sections:
1. the first section allows water to flow at a max of 3 L/min
2. the second section allows water to flow at a max of 1 L/min
3. the third section allows water to flow at a max of 2 L/min

at what rate will water come out of the end of the third section?

well, that's 1 L/min, because no matter what, you have the bottleneck of the middle section. when you first introduce water into the system, you could get it up to 3 L/min while it's still in the first section, but if the second section only allows water to flow at 1 L/min the water will get backed up until the rate through the first, second and third pipe is all 1 L/min.

this is the concept of something being rate-limiting. it's rate-limiting because the slow step forms a bottleneck that cannot be overcome. no matter where I put that 1 L/min section of pipe, either at the beginning or at the end or in the middle, it'll still limit the rate to 1 L/min.

 

[AZFutureDoc]

Right right. I was just overthinking the question. So changing the concentration of acid that is required to protonate the alcohol has no effect. This just seems counterintuitive. Surely more acid means more protonated hydroxyl groups which means more actual substrate available. If you have 1 acid molecule for every 100 alcohol molecules, more/faster protonation could occur with say, 10 acid molecules to 100 alcohols. Lol sorry for the confusion here.