Amines and Amides: Basicity and the Nitrogen Functional Groups

Amines are organic molecules built around a nitrogen atom. The nitrogen has three single bonds (to carbon or hydrogen atoms) and one lone pair of electrons. That lone pair is the key to everything an amine does. Because it can grab a proton, an amine acts as a base, and because it can attack a positive carbon, it also acts as a nucleophile. Amines are named with the ending -ine, like methylamine or ethylamine. Many smell fishy; the smell of old fish comes from trimethylamine. How available the lone pair is decides how basic the amine is. Alkyl groups push electron density toward nitrogen and make the lone pair easier to share, while a benzene ring pulls the pair into the ring by resonance, so aromatic amines like aniline are much weaker bases. When an amine reacts with an acyl chloride, its nitrogen attacks the carbonyl carbon and an amide forms, with the loss of HCl. In an amide the nitrogen lone pair is shared with the carbonyl group by resonance, so it is no longer free to accept a proton. That is why amides are not basic. This same amide linkage, called a peptide bond in biology, is what holds amino acids together in proteins and what links the units of nylon and many other polymers.

An amine is a molecule with a nitrogen atom bonded to carbon or hydrogen atoms, plus one lone pair of electrons on that nitrogen. Think of ammonia, NH3, with one or more hydrogens swapped for carbon groups. One swap gives a primary amine, two swaps a secondary amine, and three swaps a tertiary amine. The lone pair makes every amine a base: it can accept a proton to form a positively charged ammonium ion. It also makes amines nucleophiles, able to attack electron-poor carbon atoms in reactions. Base strength depends on how free that lone pair is. Alkyl groups donate electron density, so simple alkyl amines in water are a bit stronger bases than ammonia. Attach the nitrogen to a benzene ring instead, and the lone pair spreads into the ring through resonance, so aromatic amines are far weaker bases. Amines react with acyl chlorides to form amides. The nitrogen lone pair attacks the carbonyl carbon, chloride leaves, and HCl is lost. The product amide has its nitrogen lone pair delocalized onto the carbonyl oxygen, which makes the amide bond planar, strong, and not basic at all. Proteins are long chains of amino acids joined by these amide bonds.

Amines and amides are the two big nitrogen-containing families in organic chemistry, and one idea separates them: what is happening to the nitrogen lone pair. In an amine, nitrogen has three single bonds and a lone pair that is free and available. That pair can pick up a proton, so amines are bases, and it can attack a carbon atom, so amines are also nucleophiles. You can make an amine by reacting a haloalkane with ammonia, but watch out: the amine product is itself nucleophilic, so it can attack another haloalkane molecule and react further, giving mixtures of primary, secondary, and tertiary amines. Another route is to reduce a nitrile, which turns the carbon-nitrogen triple bond into a CH2NH2 group. In an amide, the nitrogen sits next to a carbonyl group, and its lone pair is shared with that carbonyl by resonance. The pair is no longer available, so amides do not act as bases, and the C-N bond gains partial double-bond character, making it rigid and hard to break. That stability is exactly why life uses the amide bond, the peptide bond, to build proteins, and why chemists use it in tough polymers like nylon and Kevlar.

To understand amines, picture the nitrogen atom as a small base factory. It carries a lone pair of electrons, and that pair does two jobs: it accepts protons (basic behavior) and it attacks electron-poor carbons (nucleophilic behavior). Not all amines are equally good at these jobs. When alkyl groups surround the nitrogen, they push electron density toward it, so the lone pair is held more loosely and the amine is a stronger base. When the nitrogen is attached to a benzene ring, as in aniline, the lone pair is pulled into the ring by resonance and is much less available, so aromatic amines are weak bases. Ranking amines by base strength is really ranking how free their lone pairs are. Now react an amine with an acyl chloride. The nitrogen lone pair attacks the carbonyl carbon, the chloride leaves, and a molecule of HCl is lost overall. The product is an amide. In the amide, the lone pair is tied up in resonance with the carbonyl group, so the molecule is not basic, and the C-N bond is partly double, flat, and strong. That strong, unreactive link is the peptide bond that joins amino acids into every protein in your body.

This episode of Crash Course Organic Chemistry is all about amines, the nitrogen-containing compounds behind the smell of old fish. The longer a fish sits out, the more trimethylamine builds up, and the fishier it smells. Stinky or not, amines matter a lot in biochemistry, medicine, and farming. The video walks through how amines are formed and why they act as bases, which all comes back to the lone pair of electrons on the nitrogen atom. That lone pair can accept a proton, and how easily it does that sets the base strength of the amine. The episode also revisits two related families you meet in organic chemistry, imines and enamines, which contain carbon-nitrogen double bonds. It is a friendly, fast-paced way to see the big picture of this topic: one small lone pair on nitrogen explains the smell of fish, the basicity of amines, and a whole family of reactions used to build medicines.