# Hydrohalogenation vs Hydration of Alkenes

So, I am doing an investigation involving the hydration and the hydrohalogenation of ethylene. Essentially, to produce the ethylene, I heat ethanol, in the presence of concentrated sulfuric acid, to obtain ethylene gas, which I plan to bubble into a solution of hydrochloric acid at varying concentrations.

Now, when the ethylene is bubbled in, I imagine that it will react in the solution and there are two possible products it could potentially form; Ethyl Chloride (from the hydrohalogenation mechanism) and ethanol (from the hydration mechanism). The problem is that I want to make a prediction of which product would be found in greater proportion, though I'm not very sure how I might go about doing this.

I'm thinking that, because the HCl dissociates to form hydronium ions and chloride ions, there are more hydronium ions than HCl molecules and therefore, hydration is likely to take precedence.

However, I've also thought that, in both mechanisms, the first step is protonation. In that case, it would also be possible that the chloride might add to the positively charged ethyl carbocation to a greater extent than the water molecules. Again, this is because both the carbocation and the chloride are charged and attract more easily. The oxygen in water is only partially charged and therefore, does not act as a good nucleophile in this case. However, I'm not sure how valid my arguments are.

Could someone help me out with pointing out flaws in this scheme of work?

## 1 Answer

"I'm thinking that, because the HCl dissociates to form hydronium ions and chloride ions, there are more hydronium ions than HCl molecules and therefore, hydration is likely to take precedence."

Definitely don't go by the that logic. The first step of hydrohalogenation requires the dissociation of $HCl$ and produce $H^+$ (/hydronium), so that the $H^+$ can activate the alkene forming the carbocation intermediate. This is also the first step of the competing hydration mechanism, as you ended up realizing.

The key here is the second step, where a nucleophile attacks the carbocation to form the neutral product. The nucleophilicity of water and chloride aren't extremely different. One might argue that "in a vacuum", chloride is a better nucleophile due to the charge (though that charge is quite stabilized over a large volume). But, practically speaking, water tends to attack instead of chloride in most situations. This is because when both are present, water tends to be in much higher concentration which increases the rate of the water attack pathway.

So consider your conditions, and if water is in much higher concentration than chloride, I would assume the hydration pathway will dominate.