Organic Synthesis Reactions: Worked Example 2

In a two-step synthesis, propylamine can be formed from bromoethane.

Draw the structure of compound A…

And suggest reagents and conditions for each step.

Now, to answer this question, we should first look at the functional groups of the compounds we’re given

So, here we have a…

This compound is a halogenoalkane

And this compound is an amine.

Next, we look at the number of carbons in each compound

So, this compound contains…

Our halogenoalkane contains two carbons

But our amine contains one extra carbon. 

So, we know that to make this amine, we need to:

Remove this bromine atom… 

Add an amine group

And add an extra carbon.

Now, we’ve seen that an easy way to add one carbon to an organic compound is to form a nitrile

So, for this step of the reaction, we could add…

To produce a nitrile, we can add either sodium cyanide or potassium cyanide to a compound

And this works, because we’ve seen that we can form nitriles from halogenoalkanes!

So, let’s say we react our halogenoalkane with sodium cyanide.

The structure of compound A must be…

When we convert a halogenoalkane to a nitrile, the halogen is substituted for a nitrile group

So, compound A must be propanenitrile!

So now we’ve got the structure of compound A…and a reagent in step 1

All we need to do now is fill in the remaining conditions for step 1…

And give reagents and conditions for step 2!

So, to carry out step 1, we also need…

To convert a halogenoalkane into a nitrile, we need our cyanide to be dissolved in ethanol.

And we need to heat the mixture.

In step 2, we’re reducing the nitrile group to an amine 

So, for this reaction, we can use hydrogen gas and a nickel catalyst

And now, this is our complete synthesis!