[souro10]

Sulfuric acid is a polar medium. Alkenes have a small polarity. And hence they are soluble - I've heard this argument, but my question is then why aren't alkyl halides soluble in sulfuric acid? Alkyl halides have much larger dipole moment than many alkenes, in fact, many alkyl halides are even soluble in water!

Because sulfuric acid is a strong acid and a fruitful reaction results between the electrophile H⁺ and the pi-bond of the alkene, alkenes are soluble in sulfuric acid. - I've heard this argument as well. Well, if whether or not a fruitful reaction occurs determines solubility (!!) then we should expect a solution of an alkyl halide in an organic solvent to react with Aqueous solution of KCN , because a fruitful reaction should occur. However, the latter reaction does not occur , even after heating for very long times.

Again, if a liquid alkene is shaken with cold sulfuric acid, reaction occurs immediately.
So I'm searching for the proper answer to the question why alkenes are sol. in cold sulfuric acid?
Also, why do alkenes react with dilute acid to give hydrated products( alcohols ) ? Because since alkenes are insoluble in water, such a reaction shouldn't take place. But it does. So I must be wrong somewhere.

[tomek]

However, the latter reaction does not occur , even after heating for very long times.

This is because aqueous and organic won't mix. You have your KCN dissolved in aqueous so it will be a diluted solution. This comes into contact with organic phase and contact of the reagent is very limited. The classical procedure involves reacting alkyl halide with KCN in alcoholic solution (MeOH or EtOH). Another way would be to use phase transfer catalyst which would carry CN^- into a organic phase.

Also, why do alkenes react with dilute acid to give hydrated products( alcohols ) ?

Acid will protonate alkenes and now your carbocation will gladly add water (which is present in excess since this is dilute solution).

[souro10]

well, in dilute acid solution, the contact between reagent and organic compound is limited too, just like the KCN case isn't it?

[tomek]

Protonation is relatively easy so I guess even limited contact is enough. Although I admit there might be something else going on, which I would gladly like to know.

[curiouscat]

Also, why do alkenes react with dilute acid to give hydrated products( alcohols ) ? Because since alkenes are insoluble in water, such a reaction shouldn't take place. But it does.

Why? I don't see your reasoning here. Why shouldn't it take place?

[souro10]

@curiouscat- because alkenes are insoluble in water and hence there would be very limited contact between the first reactant ( H+) and the alkene?

[curiouscat]

@curiouscat- because alkenes are insoluble in water and hence there would be very limited contact between the first reactant ( H+) and the alkene?

There's many biphasic reactions out there which happen at interfaces.

[souro10]

What is the specialty of such reactions that they occur, while most other reactions do not.
What is the reasoning that the reaction with dilute acid and alkenes occurs , while the in the KCN and alkyl-halide case it doesn't?

 

[discodermolide]

In the KCN/halide case there is not enough CN^- transported into the organic phase so that is can react. In the alkene/acid reaction the reaction occurs at the gas/liquid interphase, the alkene is protonated by the acid giving a "salt" which has solubility in the aqueous phase causing reaction to occur.

[souro10]

But what about liquid alkenes? In the KCN case shouldn't reaction take place at the junction between the two layers following the logic?

[discodermolide]

No because KCN is ionic it does not react at the interface, you need to transport it across the barrier with a phase transfer catalyst.
Liquid alkenes react just the same as gaseous ones.

[orgopete]

I find these generalizations inadequate to the specifics. Without actually performing the experiment, I surmise bromoethane has sufficient water solubility and reactivity to react with KCN in water. However, neopentyl chloride will be much much less reactive. What is the structure of the unreactive halide?

Re: solubility
I suggest this question be reversed. Solubility is data. Some halides may be more soluble than some alkenes. The solubility generally reflects the intra and intermolecular forces. If a halide is more water soluble than its alkene, its interactions with water must be stronger than the alkene. If the solubility of an alkene can be increased with sulfuric acid, I would surmise a different interaction must be taking place than in water (or the strength of an interaction).

These are good and fundamental questions. They should rely upon the same forces as mp, bp, and bonding characteristics as found in more familiar problems as solubility of ethanol, for instance. The difference is we think we understand hydrogen bonding fairly well, but other weak forces may be less well understood.

[curiouscat]

The discussion might get more fruitful with a dose of empiricism:

Do we have any citations where:  "some Alkyl halide in an organic solvent that does react with Aqueous solution of KCN"

or any citations to studies that claim none react?

[souro10]

@discodermolide: KCN is ionic and hence it does not react at the interface. Agreed. Sulfuric acid is ionic too. So it shouldn't react at the interface too, should it?

@Orgopete: Unreactivity of halides in aq. solution may be for many different reasons. Firstly, if the halide is small, like Bromoethane , it is water-soluble and hence reacts. Then again, if we have a Isobutyl-chloride, it does react in aq. solvent in SN1 reaction. But the reason it does is different. The compound itself isn't quite soluble in water, but the compound is partly dissociated into R⁺ and Br^-. Here, the transition state is more polar, and hence a polar medium like water helps stabilize the transition state and helps to "pull apart" the Bromide ion and make the reaction occur. Conduct the same SN1 reaction in gas phase, and the reaction rate goes down by more than million times!

Hence, we cannot conclude anything general to alkyl halides or alkenes about solubility.

I think we missed a very important point in the KCN case.

When we have KCN dissolved in an aqueous solvent , and the alkyl halide dissolved in an organic layer, reaction rates will be very slow because the organic layer will not mix with the aq. layer, preventing the molecules to come in contact. However- I don't know if this is the case- but my common sense says if we bubble a gaseous alkyl halide into Aq. KCN , then the reaction should take place, and at appreciable rate. Furthermore, if we take a liquid alkyl halide, like isobutylbromide and make it react with KCN in an aq. solvent then also the reaction should occur- because here the molecules can come in contact, the aq. solvent can stabilize the transition state and the reaction thus should occur. So my point is, if they are dissolved in different layers - reaction becomes difficult. But if the organic compounds (in liquid or gas phase ) directly react with inorganic reagents - the reaction can occur in most instances. When we have two layers, the organic compound will always prefer to stay in the organic layer for entropy favored reasons. But its a complete different situation if the alkene or the alkyl halide ( conc. liquid solution ) directly reacts, isn't it? 

I think we are forgetting the difference between making the organic compounds directly react, and making the organic compounds in organic solvent react with the inorganic reagents.

 Correct me if I'm wrong.

[discodermolide]

KCN will be completely ionised and solvated in water. This will prevent reaction with a non-polar alkyl halide. The sulphuric acid is a different case, as you say S_N1. Plus the alkene has a huge negative source of electrons, the pi bond. This just loves protons.