[robocop12]

Alright, so heres the deal. Me and my study group were wondering what the differences were, exactly, between using like pure/conc. H2SO4 vs diluted H2SO4 with water solution. Is the only main difference is with H2SO4 concentrated, there will be an alkyl created that has a hso4- attached to the carbocation creating an alkyl hydrogen sulfide? I know that when you treat that AHS with water, there will be an alcohol as the final product, where as with diluted h2so4 in water you just get an alcohol with no weird hso4- intermediate right??

[Hunter2]

It is how you said. But the product is Alkyl hydrogen sulfate not sulfide.

[curiouscat]

It is an interesting point. Are there certain alkenes where one must go via the Alkyl Hydrogen Sulfate and others that we can hydrate directly?

Does this depend on how hard a alkene is to hydrate? Is the mechanism similar for both routes? Although it seems like in one case one would need a stoichimetric quantity of H2SO4 and in the other case a catalytic quantity.

[robocop12]

So if theres no real difference then whats the point of having an alkyl hydrogen sulfate (thanks for the correction)? Is it used for anything besides being treated for water?

Why not use diluted H2SO4 and get an alcohol? Both do the same. For Organic 1 do I just need to know that they get the same results if treated with water? Thanks.

[orgopete]

Good question. All alkenes are not equal. Isobutylene is more reactive than propene and in turn more reactive than ethylene. Dehydration becomes easier with more substitution. So, it is not simple. Water will also compete with the alkene in the protonation.

Conc sulfuric will dehydrate t-butanol, but will be more reactive for an alkene. It is necessary for the reaction of ethylene. Because no water is present, bisulfate is the next best nucleophile and a bisulfate ester is formed. In this case, it is also much less basic than an alcohol which will prevent the reaction from reversing.

Perhaps alternate reaction sequences may seem more reasonable.

[curiouscat]

Good question. All alkenes are not equal. Isobutylene is more reactive than propene and in turn more reactive than ethylene. Dehydration becomes easier with more substitution. So, it is not simple. Water will also compete with the alkene in the protonation.

I think the OP was talking about hydration. And you dehydration. Maybe I am wrong....

Conc sulfuric will dehydrate t-butanol, but will be more reactive for an alkene. It is necessary for the reaction of ethylene. Because no water is present, bisulfate is the next best nucleophile and a bisulfate ester is formed. In this case, it is also much less basic than an alcohol which will prevent the reaction from reversing.

Oftentimes bisulfate is formed even  in the presence of water though. e.g. I've seen conditions of ~60% acid reported that form the AHS I think.

How does one rationalize that?

I think the OP's basic question remains unanswered: When does an alkene form an AHS and when does it directly hydrate to an alcohol?

[orgopete]

Good question…

I think the OP was talking about hydration. And you dehydration. Maybe I am wrong....

You will have to think about it. If conc sulfuric will dehydrate t-butanol, why should treating isobutylene with conc sulfuric acid give t-butanol? This isn't a one size fits all. That is why I tried to answer how different conditions were needed for different instances.

Oftentimes bisulfate is formed even in the presence of water though. e.g. I've seen conditions of ~60% acid reported that form the AHS I think.

How does one rationalize that?

I think the OP's basic question remains unanswered: When does an alkene form an AHS and when does it directly hydrate to an alcohol?

An alkene forms a bisulfate when conc sulfuric acid is used. Conc sulfuric acid is used with ethylene and propylene because dilute sulfuric acid does not add.

Again, I ask everyone to think about the reaction. Alcohols are more basic than most alkenes. I find it counter productive to think an alcohol should form except if the rate of addition is faster than the rate of elimination. A protonated alcohol should be favored over a protonated alkene.

The addition of acetic acid should be a better reaction because an acetate would be less basic than an alcohol. I can imagine that with something like 60% sulfuric acid, sufficient amounts of protonated alcohols would eliminate and with greater amounts of sulfuric acid would result in formation of bisulfate. A bisulfate would resist protonation because it is less basic. But why would one use 60% sulfuric anyway? I presume it is because lower percentages fail to add. We know that is the case with ethylene. High concentrations are needed for unreactive alkenes, but as a consequence bisulfate esters are formed.

When HBr is added, the alkyl bromide is less basic than the alkene. When water is added, the alcohol is more basic than the alkene. The addiction of water is an equilibrium reaction. It isn't possible to predict the equilibrium simply by giving the structure of the alkene or the concentration of the sulfuric acid.