Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: souro10 on January 23, 2013, 04:58:37 PM
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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.
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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).
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well, in dilute acid solution, the contact between reagent and organic compound is limited too, just like the KCN case isn't it?
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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.
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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?
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@curiouscat- because alkenes are insoluble in water and hence there would be very limited contact between the first reactant ( H+) and the alkene?
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@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.
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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?
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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.
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But what about liquid alkenes? In the KCN case shouldn't reaction take place at the junction between the two layers following the logic?
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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.
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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.
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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?
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@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.
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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 SN1. Plus the alkene has a huge negative source of electrons, the pi bond. This just loves protons.
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@souro
…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…
…if the halide is small, like Bromoethane, it is water-soluble and hence reacts…
It appears both sides are covered.
Cyanide reactions are SN2 and not aided by polarization, but are aided by aprotic solvents. Are you surprised that a gas phase reaction would be slower?
The more basic methyl flouride is much more soluble than other halides. Methanol is miscible in water. Ethylene is not, even I can discern a difference in an interaction with water. You have to use an acid to increase the solubility of an alkene.
Are you trolling or do you have an actual question?
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Are you trolling or do you have an actual question?
+1
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Not trolling. Not sure why some people perceive cross-questioning as trolling.
Coming back to it, my actual question is, why adding acid in water increases the solubility of alkenes, just what you stated without reasoning ? Is it soluble because it is reactive to acid? Isn't it solubility first, reactivity next?
Secondly, why does adding acid to water increase the solubility of alkenes in water more than it would increase the solubility of the corresponding alkyl halide, derived from the same alkene?
I do understand that I have weak concepts in Chemistry, hence the questions :)
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Isn't it solubility first, reactivity next?
Does CaCO3 dissolve in HCl or react?
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Have a look at the reaction in the attachment. Hence the solubility in water.
Can this happen with an alkyl halide?
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Thank you so much , I understand now.
But if the alkyl halide is tertiary then it can form solvated ions of alkyl cation and halide anion , isn't it?
So tertiary halides are soluble in water, isn't it?
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A tertiary halide will not dissociate in water. I think their water solubity is low to non existant.
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" But what can we say about an SN1 reaction? Here, the rate-determining step is simple heterolysis - bond breaking without apparently bond making to balance it. In the gas phase, bond dissociation energies show, heterolysis of an alkyl halide would require a greate deal of energy : 149 kcal/mol for tert-butyl bromide, and even more for other substrates. Yet in an SN1 reaction heterolysis occurs at moderate temperatures with an Eact of only 20 to 30 kcal/mol. This leaves a difference of 130 kcal or more to be provied. Where does this very large amount of energy come from?
The answer is, once again, from bond formation: not formation of one bond as in SN2 reaction , but formation of many bonds- bonds between the ions produced and the solvent. The ions are not generated as naked particles in the near emptiness of the gas phase; instead , they are generated as solvated ions..
The reactant has a dipole moment, and forms dipole-dipole bonds to solvent molecules. The transition state, we have seen has a stretched carbon-halogen bond and well-developed positive and negative charges. It has much greater dipole moment than the reactant and forms much stronger dipole dipole bonds to the solvent. The solvent thus stabilizes the transition state more than it does the reactant, lowers the Eact and speeds up the reaction.
... On the basis of this, then, the ionizing power of the solvent depends upon how well it solvates the ions. In turn, the ability of solvate ions depends in part on the polarity of the solvent- other things being equal, the more polar the solvent, the stronger the ion-dipole bonds. Thus, SN1 reactions of netural substrates go faster in water than in ethanol; they go faster in say 20% ethanol (20:80 ethanol:water mixture) than in 80% ethanol .
... Cations are solvated chiefly through unshared pairs of electrons anions are solvated chiefly through hydrogen bonding. Now , here, the cations are carbocations because of their dispersed charge they form weaker ion-dipole bonds than smaller metal cations. In the ionization of these organic substrates , therefore, solvation of the cation is relatively weak; it is the solvation of the anion that is particularly important. For this we want solvents capable of hydrogen bonding, that is protic solvants. Thus SN1 reactions proceed more rapidly in water, alcohols and mixtures of water and alcohol than in aprotic solvents like DMF , DMSO and HMPT "
I think the above text means tertiary halides get solvated by water molecules?
The text is from Morrison and Boyd.
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I think the above text means tertiary halides get solvated by water molecules?
Why don't you look a bit for data.
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I looked for data. Wikipedia says " It is sparingly soluble in water, with a tendency to undergo spontaneous solvolysis when dissolved into it ... When tert-butyl chloride is dissolved in water, a polar and protic solvent, the bulky chloride substituent is carried away by it, and isolated from the aliphatic chain, causing an heterolytic rupture of the compound, giving rise to a carbocation which eventually becomes a tertiary alcohol after a water molecule reacts with it, releasing hydrochloric acid as the final product "
Am I misinterpreting the text or the data? Am I misunderstanding you statement "A tertiary halide will not dissociate in water. I think their water solubity is low to non existant." I think your statement contradicts the book and the data. These sources say it does dissociate, doesn't it? Correct me if I'm wrong.
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I don't understand. If a tertiary halide does not dissociate in water, how does the solvolysis reaction occur?
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I also read that. I think the solvolysis will be very slow and reversible.
You may have picked a special case in tertiary butyl chloride, what is the case in the general series of alkyl halides?
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I don't understand. If a tertiary halide does not dissociate in water, how does the solvolysis reaction occur?
Presumably it occurs by H bonding between the halide and the water weakening the C-Cl bond, eventually water may attack the slightly more positively charges C atom, producing the alcohol, and HCl, which will promote the reverse reaction.
As I said tertiary butyl chloride may be a special case, what about the rest of the alkyl halides?
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I did state some time back that small alkyl halides like Ethylbromide are soluble in water.
Larger primary halides shouldn't be soluble in water.
I was talking about tertiary halides, and your statement was specific to tertiary halides too, I think.
If the attack by water occurred before the C-X bond was completely broken, as you presume, many things cannot be explained. If the reaction was carried out with an optically active tertiary halide, there should've been a much much higher amount of inversion product in that case- which isn't the case.
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Tertiary butyl chloride is sparingly soluble in water, what sparingly means I do not know. If the reaction is carried out with an optically active halide you may expect racemisation if an SN1 process is active.
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Precisely. The mechanism you proposed isn't typical of SN1 , is it?
If the water attacks the carbon before the bromide ion is completely detached, racemisation shouldn't have occurred. Inversion should have occurred.
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So where are we now with this?
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I was trying to make the point that, in the KCN case I posted in the first post of this thread, the reaction does not occur not because all alkyl halides are (wrongly) insoluble in water- and in fact some alkyl halides do undergo spontaneous solvolysis in water- but because the alkyl halides would prefer the organic solvent in preference to water, for entropic reasons. This point was never raised.
But if we directly dissolve the same alkyl halide in water and raise the temperature to moderate levels, the reaction will occur, even if a bit slow.
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I did state some time back that small alkyl halides like Ethylbromide are soluble in water.
Solubility is less than 1% w/w. I'd call that slightly soluble but I'm not sure what accepted terminology is.
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Yeah, approximately 9.1 grams/l of water.
That's low solubility according to me too, but then, its still soluble even if in small proportions. There is a difference between insolubility and low solubility. Low soluble means soluble- although it doesn't describe how much soluble it is. Insolubility means not soluble.
Ethylene too has low solubility in water. If ethylene reacts with water with a bit of acid mixed with it because the acid loves the electrons of the pi bond, then tertiary halides should react with CN- in water too, although there should be a significant amount of tertiary butyl alcohol.
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I didn't know wood and sand are at all soluble in water in any proportion! But we're deviating from the topic.
It'll be helpful if someone commented on the KCN case.
EDIT: The above post was a reply to a post by Curiouscat that almost everything is soluble in some proportions, which I now find is deleted.
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I didn't know wood and sand are at all soluble in water in any proportion! But we're deviating from the topic.
It'll be helpful if someone commented on the KCN case.
EDIT: The above post was a reply to a post by Curiouscat that almost everything is soluble in some proportions, which I now find is deleted.
Yes, that's right and I still stand by that statement.
Reason I deleted that post is that I'm afraid, by adding any fodder to the fire I'm only encouraging a troll here. Hence I will refrain from any further comments in this thread no matter how tempting. Have fun.
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I think the usual people here mostly avoid contexts and cross questions about a topic.
A very unpleasant experience in this forum, thanks to some of the Senior members.
I'm leaving this forum, thank you. Have fun too.