Nitrous Acid & Aliphatic Amines: Reactions & Physical Characteristics

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The goal of this lesson will be to learn about how aliphatic amines react with nitrous acid. Our primary points of discussion will include the reactions that the amines undergo and the physical characteristics that follow.

Fingerprints and Molecules

Have you ever watched one of those crime scene sitcoms on tv? When a crime happens and the detectives arrive at the crime scene, they start to look around very carefully to find any evidence that may help them identify who committed the crime, right? One of the most important things the detectives can recover from the scene are fingerprints left behind by the perpetrator. Fingerprints are extremely helpful because they are unique to every person, and they serve as one of the primary ways someone can be identified.

We can kind of use a similar analogy in the context of our lesson for today. Sometimes there are tests we can run on organic compounds to help us qualitatively provide diagnostic or identification information about them. Since certain types of atoms respond differently to the test, we can essentially think about that response being a 'fingerprint' of the molecule. Today we are going to be talking about the way in which what are called aliphatic amines react with nitrous acid. Our goals will be to understand the specific reactions they undergo and how we can use the physical characteristics of those reactions as diagnostic information regarding the amine. Let's play detective!

What are Aliphatic Amines?

It might be logical to briefly define aliphatic amines first and foremost. Aliphatic amines are organic compounds (meaning they're carbon-based) that contain at least one nitrogen atom connected to a saturated carbon-hydrogen backbone. The word saturated here simply means that there are no double or triple bonds present, only single bonds between carbon atoms. An example of an aliphatic amine would be butylamine.

Butylamine is an example of an aliphatic amine that contains a nitrogen atom connected to a carbon-based network with all single bonds

Notice how there's a nitrogen present at the very end of the molecule, and also how there are only single bonds that connect the carbon atoms to one another.

Reactions of Aliphatic Amines with Nitrous Acid

What happens when an aliphatic amine reacts with nitrous acid? That's the question we want to try and address next in our journey. It turns out that it depends on the type of amine we are dealing with in terms of its substitution pattern on the nitrogen atom itself. Amines are derived from ammonia, and can be classified as either primary, secondary, or tertiary. How we classify them depends on the number of hydrogen atoms bonded to nitrogen. If one hydrogen on the nitrogen atom has been replaced with a carbon-based group and there are two hydrogens left, it's primary. If it has two carbon-based groups and one hydrogen, then it's secondary, and if it has three carbon-based groups and no hydrogens, it's tertiary. Let's dissect each of these cases more carefully.

Primary Amines

When a primary amine reacts with nitrous acid, the product is either an alcohol or an alkene, but the more interesting part about the reaction is that nitrogen gas is liberated as well. Thus, once the reaction happens the solution will start to bubble, kind of like a soda, as the nitrogen gas is evolved. This is a fingerprint for primary amines!

Primary amines react with nitrous acid to give an alcohol plus nitrogen gas

Secondary Amines

When a secondary amine (one hydrogen bonded to the nitrogen atom) reacts with nitrous acid, what's called a nitrosamine is formed. The exact definition of a nitrosamine really isn't important here, the key is that when the nitrosamine is formed, it results in an oil being given off in the solution. The oil sometimes is colored, and kind of looks like olive oil floating in a pan of water when you're cooking pasta on the stove at home. The presence of this oil is a diagnostic tool, and represents the fingerprint of a secondary amine.

Tertiary Amines

Unfortunately for tertiary amines, no gases or oils are evolved when they react with nitrous acid. However, that doesn't mean there isn't any diagnostic information we can get from the reaction. When tertiary amines react with nitrous acid, usually a white solid will immediately precipitate (form an insoluble solid) out from the reaction solution. This particular solid is called an ammonium salt, and anytime we see it formed from the reaction we can begin to hypothesize the presence of a tertiary amine.

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