The Student Room Group

Why is the potassium cyanide in aqueous alcoholic solution?

1) For nucleophilic substitution why does the potassium cyanide have to be in aqueous alcoholic solution?
2) Why isn’t it one or the other?
3) I thought water was for substitution and alcohol was for elimination? Why is alcohol used if it’s a substitution reaction.
Original post by Mathstermind
1) For nucleophilic substitution why does the potassium cyanide have to be in aqueous alcoholic solution?
2) Why isn’t it one or the other?
3) I thought water was for substitution and alcohol was for elimination? Why is alcohol used if it’s a substitution reaction.


It's a solubility thing ...
If you are talking about the nucleophilic substitution of cyanide ions with halogenoalkanes then the cyanide ions are in alcoholic conditions because water is a weak nucleophile, so it has the ability to also react with halogenoalkanes and form an alcohol. So basically, in alcoholic conditions you avoid this reaction and get a nitrile only.
Original post by Mr.noname
If you are talking about the nucleophilic substitution of cyanide ions with halogenoalkanes then the cyanide ions are in alcoholic conditions because water is a weak nucleophile, so it has the ability to also react with halogenoalkanes and form an alcohol. So basically, in alcoholic conditions you avoid this reaction and get a nitrile only.


But it’s in both water and alcohol?
For your third question, one of the reaction your are talking about is the nucleophilic substitution of hydroxide ions(OH- ions) with halogenoalkanes.
When you use aqueous hydroxide ions (hydroxide ions dissolved in water) you will form an alcohol.
When you use hydroxide ions in alcoholic conditions then you for an Alkene -This is NOT a nucleophilic substitution reaction it is an elimination reaction.

Hydroxide ions are not the same as cyanide ions so they have different reactions.
Original post by Mathstermind
But it’s in both water and alcohol?

Aqueous means dissolved, so it's not in both it is only in ethanol.
Original post by Mr.noname
For your third question, one of the reaction your are talking about is the nucleophilic substitution of hydroxide ions(OH- ions) with halogenoalkanes.
When you use aqueous hydroxide ions (hydroxide ions dissolved in water) you will form an alcohol.
When you use hydroxide ions in alcoholic conditions then you for an Alkene -This is NOT a nucleophilic substitution reaction it is an elimination reaction.

Hydroxide ions are not the same as cyanide ions so they have different reactions.


Yes but why use aqueous alcoholic solution. Why not just an alcoholic solution?
Original post by Mathstermind
Yes but why use aqueous alcoholic solution. Why not just an alcoholic solution?

Basically, the "aqueous" part means that the KCN is dissolved in the solution it doesn't mean that there is water in the solution. KCN is a salt so it fully ionizes in a solvent like an alcohol that has a short carbon chain e.g. Ethanol because it behaves like water and it can also form hydrogen bonds so it can also dissolves substances that water would dissolve. Some text books say "aqueous" alcoholic solution to emphasize the fact that the salt is ionized ,so it exists as ions and the cyanide ion is reacting, the potassium ion is a spectator ion.
This makes a lot of sense
It's a shame its not emphasised like this in class

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