The amino group is one of several nitrogen-containing functional groups found in organic molecules. We’ll start with a review of the structure of ammonia and then look at the structure and formulas of different kinds of amines.
Ammonia: A Close Cousin
Before we talk about amines and the amino group, let’s review an inorganic (non-carbon containing) compound: ammonia, NH3. The Lewis electron dot structure of ammonia has one unshared electron pair on nitrogen.
This gives it a tetrahedral electron pair geometry. Because one of the groups is an electron pair, ammonia’s shape is trigonal pyramidal.
Keep this image in your mind as we explore what a difference a carbon atom–or two!–can make.
What’s an Amino Group?
The amino group is one of several nitrogen-containing functional groups found in organic molecules. What distinguishes the amino group is that the nitrogen atom is connected by single bonds to either hydrogen or carbon. Let’s take a look at some examples.
The amino group will always have nitrogen connected to an alkyl group, but that’s not quite enough. If the nitrogen forms a double bond to some other atom, or if the molecule contains anything besides alkyl groups or an aromatic ring, it’s not an amine.
Here’s an example where the molecule contains carbonyl group (C=O) rather than a plain hydrocarbon (alkyl group).
Here’s another example. Even though the molecule contains another nitrogen-carbon single bond, the nitrogen isn’t connected directly to an alkyl group. This molecule isn’t an amine because one of its nitrogen atoms forms a double bond to carbon.
Don’t get too bogged down trying to figure out what isn’t an amine! The important thing is to look for plain old alkyl groups, hydrocarbons, or an aromatic ring.
Then you’ll know you’ve got an amine–helpful when giving it a name or predicting its reactivity.
Amines are organic molecules that contain the amino group. Remember that the nitrogen atom is connected by single bonds to either hydrogen or carbon. Notice the ‘or’–it’s not an ‘and.
‘ This means we don’t have to have a hydrogen, but we could. To be organic, we have to have at least one carbon, but amines could have two or three carbon atoms around the nitrogen.Primary amines are those where the nitrogen atom is connected to just one carbon atom. Ethylamine and aniline are both primary amines. Notice what they have in common; with just one bond to carbon, primary amines will always have two hydrogen atoms. That gives them the condensed formula NH2R, where R is an alkyl group or aromatic ring.
Secondary amines have two carbon-containing groups connected to the nitrogen atom. The groups may be the same or they may be different. You can also mix-and-match alkyl and aromatic groups and still have an amine. Secondary amines have the condensed formula NHR2, where the two R groups can either be the same or different.
As you might guess, amines with three carbon-containing groups on nitrogen are called tertiary amines, just like a tertiary carbon is bonded to three other carbon atoms. With one more alkyl group on nitrogen, the two molecules seen here become tertiary amines.
With three R groups and no hydrogen atoms, tertiary amines have the condensed formula NR3.
Amines and Ammonia
The examples in the last section might make your head spin! Amines can have different numbers of R groups, and the R groups can be any alkyl or aromatic group you can imagine. Even on a given tertiary amine, you can have three different alkyl groups. It’s easy to get caught up in the differences, but it’s also helpful to consider what they have in common. If you remember the similarities, it’s easier to predict the reactivity of molecules that contain the amino group.Here’s a secondary amine, dimethylamine, with its unshared electron pair shown.
Not only is this structurally similar to the ammonia molecule we looked at earlier, but that unshared electron pair means amines react similarly to ammonia.
Let’s review. Amines are organic molecules that are structurally similar to ammonia.
To be organic, a molecule has to contain carbon. For amines, that means at least one alkyl group or aromatic ring. It also means that no other functional groups are present in the molecule. If they are, it’s not an amine. Primary amines have one R group, and the formula NH2R. Secondary amines have two R groups, and their formula is NHR2.
Tertiary amines have three R groups, and their formula is NR3.