Amines

What are Amines?

Here's ammonia. It consists of a nitrogen atom chemically bonded to three hydrogen atoms by single covalent bonds. The nitrogen atom also has a lone pair of electrons.

Fig. 1 - Ammonia

If we replace one or more of the hydrogen atoms with an organic hydrocarbon group, we get an amine. Here's an example. A hydrogen atom has been swapped with a methyl group, forming methylamine. Don't worry – we'll look at how to name amines a little later on.

Fig. 2 - From ammonia to an amine

Amine functional group

The amine functional group is characterized by a nitrogen atom with a lone pair of electrons, covalently bonded to at least one organic hydrocarbon group. We represent the organic group using the letter R. Here's a diagram of the functional group.

Fig. 3 - Amine functional group

Different types of Amines

Above, we mentioned that amines are ammonia derivatives, in which one or more hydrogen atoms have been replaced by an organic hydrocarbon group. We classify amines as either primary, secondary or tertiary, depending on the number of organic groups bonded to the nitrogen. The three types have different general formulae, as shown below.

• Primary amines have one hydrogen atom replaced by an organic group. They have the general formula NH₂R.
• Secondary amines have two hydrogen atoms replaced by organic groups. They have the general formula NHR₂.
• Tertiary amines have all three hydrogen atoms replaced by organic groups. They have the general formula NR₃.

Fig. 4 - Types of amines

Amines can also be aliphatic or aromatic, depending on whether they contain a benzene ring or not.

• In aromatic amines, one or more of the organic groups bonded to the nitrogen atom has a benzene ring.
• In aliphatic amines, all of the organic groups bonded to the nitrogen atom are aliphatic chains. They don't contain any benzene rings.

Fig. 6 - Aromatic and aliphatic amines

Naming Amines

Next, let's move our attention to amine nomenclature. Naming amines can get a little tricky because there is no one universally-accepted way of doing it. We'll focus on just one method today, which we find to be the simplest.

Here are some of the rules you should follow.

• The amine functional group is indicated by the suffix -amine or the prefix amino-.
  • If there is just one amine group present, we use the suffix -amine.
  • If there are multiple amine groups or any other functional groups present, we use the prefix amino-.
• If there is just one amine group present, we show alkyl groups attached to the nitrogen atom using alkyl prefixes, ending in -yl.
• If there are multiple amine groups or any other functional groups present, we show the longest carbon chain using root names and indicate the position of the functional groups using numbers.

Sound confusing? Let's go through some examples.

Examples of amines

First of all, let's find the amine group. We've highlighted it in blue. There's just one, so we use the suffix -amine. Next, identify any alkyl groups attached to the nitrogen atom. Here, there's just one methyl group, which we've highlighted in green. We use the prefix methyl-. This molecule's full name is therefore methylamine.

Fig. 8 - Methylamine

Here's another example.

Once again, this molecule has just one amine group, shown in blue. We therefore use the suffix -amine. But this time, there are three carbon chains attached to the nitrogen atom: two methyl groups, shown in green, and one ethyl group, shown in yellow. We show them using the prefixes methyl- and ethyl-, listing them in alphabetical order. But to show that there are two methyl groups present, we precede methyl- with a quantifier, di-. Putting this all together, we get the name ethyldimethylamine.

Fig. 10 - Ethyldimethylamine

Finally, let's take a look at an amine with multiple functional groups.

You can see that there are not only two amine groups but also a hydroxyl group, shown in pink. We therefore need to use the prefix amino- with the quantifier di-. To show the hydroxyl group, we use the suffix -ol. The carbon chain is three carbon atoms long, shown in green, and so we use the root name -prop-. To show the position of the functional groups, we use numbers. Here, the hydroxyl group takes priority, and so we call the carbon it is attached to ‘carbon 1’. This means that the amine groups are found on carbons 2 and 3. The full name of this molecule is therefore 2,3-diaminopropan-1-ol.

Fig. 12 - 2,3-diaminopropan-1-ol

Properties of Amines

Now that we've learned how to name amines, let's focus on some of their properties.

Polarity

Amines are polar molecules. This is because nitrogen is more electronegative than both carbon and hydrogen. The nitrogen atom becomes partially negatively charged while the carbon and hydrogen atoms become partially positively charged.

Fig. 13 - Amine polarity

Melting and boiling points

Primary and secondary amines have high melting and boiling points compared to similar-sized alkanes. Because the nitrogen atom not only has a lone pair of electrons but is also attached to a hydrogen atom, it can form hydrogen bonds between molecules. These are the strongest type of intermolecular forces and require a fair amount of energy to overcome.

Fig. 14 - Amine hydrogen bonding

Secondary amines have slightly lower boiling points compared to primary amines. This is because the nitrogen atom is found between two organic groups, which decreases the strength of the permanent dipole between nitrogen and hydrogen. A weaker dipole reduces the strength of the hydrogen bonding.

Tertiary amines, on the other hand, have much lower melting and boiling points than both primary and secondary amines. In tertiary amines, all three hydrogen atoms are replaced by an organic hydrocarbon group. There aren't any hydrogen atoms left attached to the nitrogen atom, and thus the molecule can't form hydrogen bonds.

In general, longer-chain amines have higher melting and boiling points than smaller amines. This is because they are larger molecules, which increases the strength of another type of intermolecular force found between molecules – van der Waals attraction. Branched amines have lower melting and boiling points compared to straight-chain amines. This is because they can't pack together as tightly which reduces the strength of the van der Waals attractions.

Solubility

Short-chain amines, including tertiary amines, are soluble in water. This is because they can form hydrogen bonds with water molecules. When it comes to primary and secondary amines, you'll find two lots of hydrogen bonds per molecule: a hydrogen bond between one of the water molecule's hydrogen atoms and the lone pair of electrons found on the amine's nitrogen atom, and also a hydrogen bond between the hydrogen atom attached to the nitrogen and the lone pair of electrons found on the water's oxygen atom. Tertiary amines, on the other hand, can only form one lot of hydrogen bonds, using just the lone pair of electrons found on their nitrogen atom. This means that short-chain tertiary amines are less soluble than short-chain primary and secondary amines.

As the size of the amine increases, the insoluble hydrocarbon chains begin to interfere with and disrupt the hydrogen bonding. This means that longer-chain amines are insoluble.

Fig. 15 - Amine hydrogen bonding with water

Shape

Amines have a trigonal pyramidal shape with nitrogen at the apex. The bond angles between the attached groups are 107°.

Fig. 16 - Amine shape

Basicity

The final property we'll cover is basicity. Amines can act as both weak Bronsted-Lowry bases and Lewis bases. This is thanks to the nitrogen atom's lone pair of electrons.

Primary amines are much better Bronsted-Lowry bases than ammonia. This is because they contain an alkyl group bonded to the nitrogen atom. Alkyl groups are electron-releasing and so increase the availability of the nitrogen atom's lone pair of electrons, making it more appealing to protons. This is known as the inductive effect. Likewise, secondary amines are better bases than primary amines. This is because they have two inductive effects

You might expect tertiary amines to be the best type of base. As a gas, this is true – tertiary amines are better bases than primary and secondary amines. But in aqueous solution, tertiary amines are worse bases than secondary amines. This is because they are less soluble than secondary amines, and so less good at picking up a proton in solution.

Uses of Amines

Before we finish this article, let's briefly explore some of the uses of amines.

• Amines play an important role in your body. All proteins are made up of amines called amino acids, and many neurotransmitters, such as dopamine and serotonin, are amines too.
• A number of painkillers are amines, such as morphine and novocaine.
• Amines play a role in removing carbon dioxide from flue gases.
• Amines are important ingredients in common insecticides, pesticides, and disinfectants.