Why is the potassium cyanide in aqueous alcoholic solution?

11

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.

Reply 1

charco

Study Forum Helper

18

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 ...

Reply 2

Mr.noname

12

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.

Reply 3

Mathstermind

OP

11

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?

Reply 4

Mr.noname

12

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.

Reply 5

Mr.noname

12

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.

Reply 6

Mathstermind

OP

11

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?

Reply 7

Mr.noname

12

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.

Reply 8

Aronisah8

7

This makes a lot of sense
It's a shame its not emphasised like this in class

Why is the potassium cyanide in aqueous alcoholic solution?

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