Amines are found in so many things - tanning products, pesticides, disinfectants, and even medicine. But how are they made? In this article, we'll talk about how amines are prepared. We'll go over a few different methods, like using ammonia or reducing nitriles and amides. Plus, we'll dive into making aromatic amines by reducing nitrobenzene. You'll learn about the conditions needed and some of the steps involved in the process. So, let's get started!
Let's start by talking about how amines are made from ammonia through a process called alkylation. Amines are basically ammonia molecules where some of the hydrogen atoms have been replaced by an organic hydrocarbon group. Alkylation means adding an alkyl group to a molecule.
There are two ways to alkylate ammonia:
For your exams, you only need to know about the first method, which involves using a halogenoalkane. So, we'll focus on that in this article.
To alkylate ammonia using a halogenoalkane, you need to mix a concentrated solution of ammonia with the halogenoalkane in an ethanolic solution and heat it. This results in a nucleophilic substitution reaction, where the ammonia molecule acts as a nucleophile by attacking the partially positively charged carbon atom of the halogenoalkane.
A nucleophile is a negatively charged or partially negatively charged species that attacks electron-deficient species. The nucleophile reacts by donating its lone pair of electrons. Although ammonia isn't negatively charged, it can still act as a nucleophile because its nitrogen atom has a partial negative charge and a lone pair of electrons. On the other hand, the halogenoalkane can be attacked because it has an electron-deficient carbon atom with a partial positive charge.
The overall reaction of alkylation of ammonia with a halogenoalkane produces both a primary amine and an ammonium salt. The reaction uses two ammonia molecules and occurs in two steps. In the first step, one of the ammonia molecules, acting as a nucleophile, attacks the halogenoalkane by replacing the halogen atom with an NH3+ group. This forms a new molecule. In the second step, the second ammonia molecule removes a hydrogen ion from the new molecule formed in the first step, resulting in a primary amine and an ammonium ion. The ammonium ion then reacts with the halogen atom kicked out in the first step to form an ammonium salt.
The general equation for the reaction is:
RNH2 + NH4X ↔ RNH3+X- + NH3
Where RNH2 is the primary amine, NH4X is the ammonium salt, RNH3+X- is the alkylated product, and NH3 is the excess ammonia.
Reacting ammonia with a halogenoalkane is not the only way to prepare amines. Another method, which is widely used in industry, involves reacting ammonia with an alcohol. This reaction also results in the production of a primary amine, but in this case, water is also produced.
For example, when propanol is reacted with ammonia, propylamine and water are produced. The reaction follows a similar mechanism to the reaction between ammonia and a halogenoalkane. In this case, the alcohol molecule acts as a nucleophile and attacks the partially positively charged carbon atom of the halogenoalkane. The reaction proceeds in two steps, with the formation of an intermediate alkoxide ion that reacts with ammonia to produce the primary amine and water.
The general equation for this reaction is:
ROH + NH3 → RNH2 + H2O
Where ROH is the alcohol, RNH2 is the primary amine, and H2O is water.
When an excess of bromoethane is used, the reaction proceeds as follows:
On the other hand, when an excess of ammonia is used, the reaction mainly produces primary amines:
By controlling the ratio of ammonia to halogenoalkane, we can selectively produce primary amines, secondary amines, tertiary amines, or quaternary ammonium salts. This method provides a versatile and efficient way to produce a wide range of different amines with varying properties and functionalities.
Here's what the different molecules look like.
Preparation of amines by reduction of nitriles . Consider the molecule CH3CN. This is ethanenitrile. It is an example of a nitrile, an organic molecule with the -C≡N functional group. Ethanenitrile, an example of a nitrile. Nitriles can be reduced to produce primary amines. This is done using either a strong reducing agent or using hydrogen gas in the presence of a metal catalyst.
One way of reducing nitriles is by using a strong reducing agent, such as lithium aluminium hydride. The correct IPUAC name is lithium tetrahydridoaluminate (III). That's a bit of a mouthful – you'll probably know it instead as LiAlH4. We represent it in equations using [H]. The reaction takes place in a solution of diethyl ether, and a dilute acid is added at the end. Here's the general equation: For example, reacting ethanenitrile with LiAlH4 produces ethylamine.
LiAlH4 is relatively pricey. This means that reducing nitriles using this method is too expensive for use in industry – a different reaction must be used instead. As an alternative, we can reduce nitriles using hydrogen gas in the presence of a metal catalyst such as palladium, platinum, or nickel. This reaction takes place at a high temperature and pressure.
For example, we can achieve the exact same product as in the reaction above by reducing ethanenitrile with hydrogen gas and a nickel catalyst.
We've just learnt how nitriles can be turned into amines by reducing them with LiAlH4. We can do the same thing with amides. Amides are organic molecules with the -CONH2 or -CONHR- functional group. They are similar to amines, except one of the carbon atoms directly attached to the nitrogen atom contains a C=O bond. This means that all amides have an amine group bonded to a carbonyl group. The simplest amide is methanamide, commonly known as formamide.
The reaction between an amide and LiAlH4 also takes place in diethyl ether at room temperature. Once again, a dilute acid is added at the end. Overall, the reaction swaps the carbonyl group's oxygen atom with two hydrogen atoms, resulting in water and an amine. However, the amine varies slightly depending on the type of amide reduced. Reducing primary amides with the formula RCONH2 produces a primary amine.Reducing secondary amides with the formula RCONHR produces a secondary amine.Reducing tertiary amide with the formula RCONR2 produces a tertiary amine.
Here's the general equation.
For example, reacting methanamide with LiAlH4 produces the primary amine methylamine.
Similarly, reacting N-methylmethanamide with LiAlH4 produces the secondary amine dimethylamine.
You can learn more about the organic molecules amides over at the article Amides. Preparation of aromatic amines by reduction of nitro-compounds. All the examples we've looked at before have involved making aliphatic amines. Remember that amines are ammonia derivatives, where one or more hydrogen atoms have been swapped for an organic hydrocarbon group. In aliphatic amines, these organic groups are all open hydrocarbon chains. But in aromatic amines, one or more of these organic groups features an aromatic benzene ring. One example is phenylamine.
When it comes to making aromatic amines, you might be able to guess what we start with: benzene itself. The process involves two stages. We first nitrate benzene into nitrobenzene. We then reduce nitrobenzene into phenylamine. We'll walk you through these stages now.
The production of an aromatic amine, such as phenylamine, typically involves a two-step process. The first step is the nitration of benzene to form nitrobenzene, and the second step is the reduction of nitrobenzene to form phenylamine.
The nitration of benzene is an electrophilic substitution reaction that involves the reaction of benzene with a mixture of concentrated sulfuric and nitric acids at 50°C, using reflux to prevent the escape of any volatile components[^1]. The reaction proceeds as follows:
Overall, the nitration of benzene introduces a nitro group (-NO2) onto the benzene ring, forming nitrobenzene. The nitro group's presence is important for subsequent steps in the synthesis of aromatic amines.
The hydrogen ion reacts with the HSO4- ion, regenerating sulphuric acid. This means that overall, sulphuric acid acts as a catalyst. We're now ready to move on to the next stage: reducing nitrobenzene.
That's a great summary of the key takeaways in the preparation of amines. To add to that, it's worth noting that amines are important organic compounds that find application in a wide range of industries, including pharmaceuticals, agrochemicals, and materials science. The ability to synthesize amines using various methods is critical for the development of new compounds and materials that can improve our lives and the world around us.
What are the preparation methods of amines?
The preparation methods of amines include alkylating ammonia with a halogenoalkane or an alcohol, and also reducing a nitrile or amide.
Which is the best method for preparing a primary amine?
In industry, amines are generally made by alkylating ammonia using an alcohol thanks to the low cost and high availability of ammonia. In the lab, you might instead alkylate ammonia using a halogenoalkane. However, this produces a range of different amines, ranging from primary amines to a quaternary ammonium salt.
Which method is used for preparation of aromatic amines?
Aromatic amines are produced by first nitrating benzene into nitrobenzene, and then reducing nitrobenzene into phenylamine.
Which reaction is used for formation of amines?
You can form amines by alkylating ammonia using a halogenoalkane or ammonia, or reducing a nitrile or amide.
How do you prepare primary amines from nitro-compounds?
To prepare amines from nitro-compounds, you first reduce the nitro-compound using a combination of tin and hydrochloric acid. You then add sodium hydroxide.
