An amine is a derivative of ammonia (NH3). Where ammonia is composed of a nitrogen with three hydrogens attached, at least one hydrogen is replaced by a carbon chain.
Structure of an amine:
An amine functional group is a nitrogen attached to at least one hydrocarbon. Like alkyl halides and alcohols, amines get degrees—the only difference is that amines get their degrees based on how many carbons are directly attached to the nitrogen. Assuming “R” stands for a carbon, let’s see what those structures look like:
4 degrees of amine RNH2, R2NH, R3NH, R4N+
From left to right: primary amine (1º), secondary amine (2º), tertiary amine 3º, quaternary amine (4º). Quaternary amines will always have a positive formal charge on the nitrogen.
Hyrbidization and geometry:
Are those structures drawn above the whole story? Nope! They don’t account for VSEPR! Recall that “Bond sites will repel each other as much as possible.” Let’s figure out the hybridization and geometry of the following amine:
Trimethylamine hybridization and geometry answer
A non-quaternary amine has three R-groups or hydrogens and one lone pair. Since it’s got four groups, it’s sp3-hybridized. When an atom has three atoms attached and one lone pair, that atom’s geometry will be trigonal pyramidal. This is different from an atom that has four atoms attached and no lone pair; that atom would be tetrahedral.
Amine Chemistry:
Acid-Base reactions:
Do you expect amines to be acidic or basic? Let’s see what it looks like when ammonia reacts with hydrochloric acid:
NH3 reacting with HX to form NH4-X
Since reactions always move in the direction of the weaker acid, we can see that this reaction should proceed to the right. The basic nitrogen will remove the acidic hydrogen from the chlorine, and an ammonium salt (ammonium ion) will form. Remember that the direction of equilibrium will be from more acidic to less acidic.
Substitution:
What does it look like when an amine acts as a nucleophile? An amine can perform an SN2 reaction when a molecule has a good-enough leaving group. Let’s look at ethylamine (a primary amine) reacting with chloromethane in an amine alkylation reaction:
Ethylamine and chloromethane substitution reaction
Types of amines:
Amines are extremely common in Organic Chemistry. Let’s look at two examples of types of amines:
Aniline and butylamine
The molecule on the left is called aniline, and it has an amino group on the aromatic molecule benzene ("amino" is the name that we give to an amine that is a substituent). The reactivity and chemical properties of aromatic amines are a bit different from those of an aliphatic amine (aka alkyl amine) like the n-butylamine on the right. Amines can act as activating groups on benzene, but we’ll talk more about that in Orgo 2 in EAS.
Where can amines be found in nature? You can find amines in amino acids, amine hormones, alkaloids produced by plants, and tons more places. Are you familiar with the smell of decaying fish? You can thank amines like trimethylamine for that one.
To learn about other nitrogen-containing organic compounds, like those with carbonyls, check out my videos on functional groups. Good luck studying!
