Question

Bromo propane when reacts with sodium cyanide the reaction is faster in which of the solvent

Answer 1

Answer:

If a halogenoalkane is heated under reflux with a solution of sodium or potassium cyanide in ethanol, the halogen is replaced by a -CN group and a nitrile is produced. Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture.

The solvent is important. If water is present you tend to get substitution by -OH instead of -CN.

Note:  A solution of potassium cyanide in water is quite alkaline, and contains significant amounts of hydroxide ions. These react with the halogenoalkane.

For example, using 1-bromopropane as a typical primary halogenoalkane:

You could write the full equation rather than the ionic one, but it slightly obscures what's going on:

The bromine (or other halogen) in the halogenoalkane is simply replaced by a -CN group - hence a substitution reaction. In this example, butanenitrile is formed.

Note:  When you are naming nitriles, you have to remember to include the carbon in the -CN group when you count the longest chain. In this example, there are 4 carbons in the longest chain - hence butanenitrile.

The mechanism

Here is the mechanism for the reaction involving bromoethane:

This is an example of nucleophilic substitution.

Because the mechanism involves collision between two species in the slow step (in this case, the only step) of the reaction, it is known as an SN2 reaction.

Note:  Unless your syllabus specifically mentions SN2 by name, you can just call it nucleophilic substitution.

If your examiners want you to show the transition state, draw the mechanism like this:

The reaction of tertiary halogenoalkanes with cyanide ions

The facts

The facts of the reaction are exactly the same as with primary halogenoalkanes. If the halogenoalkane is heated under reflux with a solution of sodium or potassium cyanide in ethanol, the halogen is replaced by -CN, and a nitrile is produced.

For example:

Or if you want the full equation rather than the ionic one:

The mechanism

This mechanism involves an initial ionisation of the halogenoalkane:

followed by a very rapid attack by the cyanide ion on the carbocation (carbonium ion) formed:

This is again an example of nucleophilic substitution.

Explanation: