Carbylamine Reaction and Nitrous Acid

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. You have two bottles that look exactly alike: one holds a primary amine, the other a secondary amine. You are given only chloroform and alcoholic potassium hydroxide. So how will you tell which bottle is which? Believe it or not, your nose will give you the answer! And later, we will see how the volume of a gas can reveal the formula of an unknown amine. Questions on these reactions come up often in JEE, so let's begin.

0:33Carbylamine (isocyanide) test

First, the carbylamine test. We heat the amine with chloroform, CHC l three, and alcoholic KOH, that is, potassium hydroxide dissolved in alcohol. The small numbers written low are subscripts; they count the atoms. Okay? Here is the equation for a primary amine, RNH two, where R is an alkyl or aryl group. It reacts with chloroform and three KOH, and the arrow means gives. We get RNC, with three K C l and three water molecules. Right? This RNC is an isocyanide, with a very offensive smell, so the test is also called the isocyanide test. And there is the answer to our puzzle: the bottle that gives this awful smell holds the primary amine! Got it? Now let me ask you this: does a secondary amine give this test too? The answer is no. Only primary amines respond, aliphatic or aromatic, and secondary and tertiary amines give no smell. One way to remember it: in the equation, RNH two loses both hydrogens on its nitrogen to become RNC, the isocyanide. Secondary amines have only one such hydrogen, and tertiary amines have none. Clear? And the reactive species here is dichlorocarbene, C C l two; the two dots before the carbon are its lone pair of electrons. Just remember this name for now. Okay?

2:01Nitrous acid: primary aliphatic amines

Now our second reagent, nitrous acid, HNO two. For this, we take sodium nitrite, N a N O two, with hydrochloric acid, H C l. Right? With a primary aliphatic amine, RNH two plus HNO two gives ROH, the alcohol, plus N two, which is nitrogen gas, plus water. The alcohol is the main product. Okay? The nitrogen comes from a diazonium salt that forms first. From an aliphatic amine, this salt is unstable, so it loses nitrogen at once. Got it? Look at the equation again: one mole of amine gives one mole of nitrogen gas. So measuring the nitrogen tells us the moles of amine, and that is how amines are estimated. We will use this in example two. With me? One last point: a carbocation forms on the way, a carbon carrying a positive charge, and it can rearrange. So in harder problems, the alcohol may not be the one you expect. Clear?

3:02Nitrous acid with other amines

Now, aniline is also a primary amine. So with nitrous acid, it should also lose nitrogen at once and give an alcohol, right? Not in the cold, and this is the trap. At two seventy three to two seventy eight kelvin, a primary aromatic amine gives a stable diazonium salt, A r N two plus, C l minus. Here A r is an aryl group, capital K is kelvin, and the raised plus and minus are the charges. Only on warming does it lose nitrogen, and then it gives phenol, not an alcohol. So remember both conditions, aromatic and cold. Okay? Now a secondary amine gives a yellow oily N nitrosoamine: R two N joined by a single bond to another nitrogen, which has a double bond to oxygen. The italic capital N in the name means the nitroso group, N O, sits on the nitrogen. Right? Tertiary aliphatic amines only form a soluble nitrite salt, and nothing more. Got it? But a tertiary aromatic amine is nitrosated on the ring, at the para position. So N N dimethylaniline gives green para nitroso N N dimethylaniline; the italic p means para. One reagent, and a different result for each class of amine. With me?

4:24Example 1: which amines give the carbylamine test

Now, our first example. Which of these give the carbylamine test: ethanamine, N methyl methanamine, aniline, N N dimethylaniline, and benzylamine? Pause the video and try it first, and here is a hint: classify each amine. Done? Okay, let's check. Ethanamine, C two H five N H two, has one carbon group on the nitrogen, so it is primary, and it gives the test. Right? Next, N methyl methanamine. The brackets with the small two mean two C H three groups, both on the nitrogen. So it is secondary, and no isocyanide forms. Okay? Now, aniline is aromatic. So does it still give the test? Yes, it does, because the test needs only a primary amine, aliphatic or aromatic. Aniline, C six H five N H two, is primary, so it gives the smell. Got it? Now, N N dimethylaniline. Many students see the word aniline and tick it, so careful, this is the trap! The ring and both methyl groups sit on the nitrogen, so it is tertiary, and it does not respond. Clear? And benzylamine, C six H five C H two N H two. Its N H two group sits on a C H two carbon, so it is primary, and it gives the test. With me? So the answer is ethanamine, aniline and benzylamine, the three primary amines. Aromatic or not did not matter; only the class of the amine did. Okay?

6:03Example 2: molar mass of an amine from N2 volume at STP

Now a harder one, and this is the second idea from the start: a volume of gas will give us a formula. Zero point seven three grams of a primary aliphatic amine gives two hundred and twenty four millilitres of nitrogen at STP, standard temperature and pressure, with nitrous acid. Find its molar mass and molecular formula. Pause the video and try it first, and here is a hint: count the moles of nitrogen. Done? Okay, let's check. At STP, one mole of a gas takes up twenty two thousand four hundred millilitres. So n of N two, the moles of nitrogen, is two hundred and twenty four over twenty two thousand four hundred, which gives zero point zero one mole. Right? Now tell me, how many moles of the amine reacted? Yes, exactly, n of amine is also zero point zero one mole, because one mole of amine gives one mole of nitrogen. Got it? So the molar mass, capital M, is the mass over the moles, and zero point seven three over zero point zero one gives seventy three grams per mole. The raised minus one on mol means per mole. Clear? Now the formula. The amine is RNH two, and N H two weighs fourteen plus two, which is sixteen. So R is seventy three minus sixteen, which gives fifty seven. With carbon as twelve and hydrogen as one, C four H nine is forty eight plus nine, also fifty seven, so R is C four H nine. Okay? So the molecular formula is C four H nine N, right? Oh no, careful, that is the trap, because we forgot the two hydrogens on the nitrogen. The amine is C four H nine N H two, so the molecular formula is C four H eleven N, and forty eight plus eleven plus fourteen gives seventy three. See, a volume of gas gave us the formula! With me?

8:07Quick revision: carbylamine test and amines with HNO2

So, quick revision. The carbylamine test picks out primary amines by the offensive smell of an isocyanide. With nitrous acid, primary aliphatic amines give nitrogen gas and an alcohol, while primary aromatic amines, at two seventy three to two seventy eight kelvin, give a diazonium salt. Secondary amines give a yellow N nitrosoamine, and tertiary amines give a nitrite salt, or para nitrosation if aromatic. Now go and try the practice problems on this page. All the best!