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Amines

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Chemistry

What do chemical dyes, proteins and the hormone serotonin have in common? They are all types of amines.

  • This article is about amines in organic chemistry.
  • We'll start by looking at the amine functional group.
  • We'll then learn how to classify amines and learn their formulae.
  • Then we'll explore amine nomenclature and you'll be able to have a go at naming different amines.
  • Finally, we'll look at properties and uses of amines.

What are Amines?

An amine is an organic derivative of ammonia, in which one or more of the hydrogen atoms has been replaced by a hydrocarbon group.

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.

Amines, ammonia, StudySmarterAmmonia. Anna Brewer, StudySmarter Original

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.

Amines, ammonia amine, StudySmarterFrom ammonia to an amine. Anna Brewer, StudySmarter Original

Amine functional group

The functional group of an amine 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.

Amines, amine functional group, StudySmarterAmine functional group. Anna Brewer, StudySmarter Original

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Above, we mentioned that amines are ammonia derivatives, in which one or more hydrogen atoms have been replaced by an organic hydrocarbon group.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.

Amines, types of amines, StudySmarterTypes of amines. Anna Brewer, StudySmarter Original

Remember how we said that the nitrogen atom in an amine has a lone pair of electrons? Amines can use this pair to form a fourth bond with another organic group. This turns the amine into a quaternary ammonium ion, which has the affectionate nickname of 'quat'. Quaternary ammonium ions have a permanent positive charge and are readily turned into quaternary ammonium salts, which have a variety of uses. For example, they're found in products ranging from disinfectants to hair softeners.

Amines, quaternary ammonium cation, StudySmarterA quaternary ammonium cation. Anna Brewer, StudySmarter Original

You'll discover more uses of amines in the article Uses 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.

Amines, aromatic and aliphatic amines, StudySmarterAromatic and aliphatic amines. Anna Brewer, StudySmarter Original

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.

Learn how your exam board wants you to name amines and practice applying that system to the examples below.

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

Name this amine.

Amines, example methylamine, StudySmarterAn example of an amine. Image credits: commons.wikimedia.org

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.

Amines, example methylamine, StudySmarterMethylamine. Created using images from commons.wikimedia.org

Here's another example.

Name this amine.

Amines, example ethyldimethylamine, StudySmarter

An example of an amine. Image credits: commons.wikimedia.org

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.

Amines, example ethyldimethylamine, StudySmarter

Ethyldimethylamine. Created using images from commons.wikimedia.org

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

Name this amine.

Amines, example amine with multiple functional groups, StudySmarterAn example of an amine with multiple functional groups. Anna Brewer, StudySmarter Original

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.

Amines, example 2,3-diaminopropan-1-ol,  StudySmarter2,3-diaminopropan-1-ol. Anna Brewer, StudySmarter Original

If another functional group is present, you always use the prefix -amino alongside the suffix of the other functional group. The only exception to this rule is if the other functional group is a halogen. In this case, you carry on using the suffix -amine and instead use the prefix for the halogen.

For more on nomenclature, check out Naming Conventions.

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.

Amines polarity StudySmarterAmine polarity. Anna Brewer, StudySmarter Original

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.

Amines hydrogen bonding StudySmarterAmine hydrogen bonding. Anna Brewer, StudySmarter Originals

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.

Check out Intermolecular Forces for more on hydrogen bonding and van der Waals forces.

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.

Amines, hydrogen bonding water, StudySmarterAmine hydrogen bonding with water. Anna Brewer, StudySmarter Original

Shape

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

Amines, shape, StudySmarterAmine shape. Created using images from commons.wikimedia.org

Not sure where this shape and bond angle have come from? Shapes of Molecules has got you covered.

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.

Bronsted-Lowry bases are proton acceptors, whilst Lewis bases are lone pairs of electron donors.

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.

The strength of different amines as bases is something we look at in much more detail in the article Amines Basicity.

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.

Check out Uses of Amines for more.

Amines - Key takeaways

  • An amine is an organic derivative of ammonia, in which one or more of the hydrogen atoms has been replaced by a hydrocarbon group.
  • Amines can be classified as primary, secondary, or tertiary.
    • 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.

  • An amine bonded to aliphatic organic groups is an aliphatic amine, whilst an amine bonded to a benzene ring is an aromatic amine.
  • Amines are named using the suffix -amine or the prefix amino-.
  • Amines are polar molecules. They have relatively high melting and boiling points and short-chain amines are soluble in water.
  • Amines act as weak bases. The relative strength of an amine as a base depends on the availability of the nitrogen atom's lone pair of electrons.
  • Amines play important roles in the body, in drugs, and as disinfectants.

Amines

An amine is an organic derivative of ammonia, meaning one or more of ammonia's hydrogen atoms is replaced with an organic group.

You typically name amines using the suffix -amine. However, if there is more than one amine group or any other functional group present, you use the prefix -amino

The formula for an amine depends on the type of amine you are dealing with. Primary amines have the general formula NH2R, secondary amines have the general formula NHR2 and tertiary amines have the general formula NHR3.

Examples of amines include amino acids, which make up all of the proteins in our body.

Amines can be identified by their functional group, which contains a nitrogen atom with a lone pair of electrons bonded to at least one organic group.

Final Amines Quiz

Question

Which of the following is the correct IUPAC name for the amine?


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Answer

Ethylmethylamine

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Question

What is an aliphatic amine?

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Answer

An amine that has no aromatic rings bonded to the nitrogen atom.

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Question

What is the definition of an amine?

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Answer

An organic derivative of ammonia.

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Question

What is an amine?

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Answer

An amine is an organic derivative of ammonia, in which one or more of the hydrogen atoms has been replaced by a hydrocarbon group.

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Question

Amines are derivatives of _____.

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Answer

Ammonia

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Question

What is the amine functional group?

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Answer

A nitrogen atom with a lone pair of electrons, covalently bonded to at least one organic hydrocarbon group.

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Question

Primary amines have ____ organic groups attached to the nitrogen atom.

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Answer

1

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Question

Tertiary amines have _____ hydrogen atoms attached to the nitrogen atom.

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Answer

0

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Question

Is this a primary, secondary or tertiary amine?

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Answer

Tertiary

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Question

Compare and contrast aliphatic and aromatic amines.

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Answer

Both are ammonia derivatives, containing nitrogen atoms bonded to one or more organic groups. Aliphatic amines are bonded to just open chain organic groups whereas aromatic amines are bonded to one or more benzene rings.

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Question

Which of the following prefixes and suffixes can be used to name amines?

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Answer

-amine

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Question

True or false? Amines are polar.

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Answer

True

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Question

Draw a general example of an amine with the correct partial charges.


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Answer

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Question

Amines have _____ melting and boiling points compared to similar-sized alkanes.

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Answer

Higher

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Question

State and explain the trend in melting and boiling points seen in primary, secondary, and tertiary amines.

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Answer

  • Secondary amines have lower melting and boiling points than primary amines. This is because they contain an extra organic group bonded to the nitrogen atom, which reduces the strength of the nitrogen's dipole. 
  • Tertiary amines have much lower melting and boiling points than primary and secondary amines. This is because they don't contain a hydrogen atom bonded to the nitrogen and so can't form hydrogen bonds.

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Question

Short-chain amines are _____ in water.

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Answer

Soluble

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Question

As chain length increases, the solubility of amines _____.

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Answer

Decreases

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Question

What is the shape of an amine?

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Answer

Trigonal pyramidal

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Question

What is the bond angle found in amines?

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Answer

107° 

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Question

True or false? Amines act as acids.

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Answer

False

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Question

True or false? Tertiary amines are stronger bases than secondary amines.

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Answer

It depends

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Question

Why are tertiary amines soluble, despite not containing any N-H bonds?

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Answer

Tertiary amines can form hydrogen bonds with water using the lone pair of electrons found on the nitrogen atom. This forms hydrogen bonds with the hydrogen atoms in water molecules.

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