Diazotisation

Transcript

0:00Introduction

Hello everyone! Let us start with a strange salt. Chemists make it in the cold and use it at once. They never store it, because when it is dry, it may explode! That salt is benzenediazonium chloride. And here is the puzzle. It can be kept for a short time in cold solution, but its alkyl cousin, ethanediazonium chloride, cannot be obtained at all. So what makes the difference? By the end of this video, you will answer that in one line. Today's topic is diazotisation, and it comes up often in JEE papers. Let's begin.

0:35Diazotisation: the reaction

So, first, the diazonium salt itself. It is a salt of the type Ar N two plus, X minus. Ar is an aryl group, like phenyl, a benzene ring, and X minus is an anion, like chloride. The small raised plus and minus are charges, and the small two written low is a subscript, for two nitrogen atoms. Clear? Now, to make it, we treat a primary aromatic amine with nitrous acid, at two seventy three to two seventy eight kelvin, which is zero to five degrees Celsius; the small circle is the degree sign. This reaction is called diazotisation. Got it? Here is the full equation with aniline, C six H five N H two. Aniline, plus sodium nitrite, N a N O two, plus two H C l. The arrow, with the temperature above it, gives benzenediazonium chloride, the salt from our puzzle, plus sodium chloride and two water molecules. Okay? Now look at the left side of the equation once more: two H C l for just one aniline. Why two? Would one not be enough?

1:40Two HCl, and the real electrophile

Here is the answer: each H C l has its own job. The first one sets free nitrous acid from sodium nitrite. Sodium nitrite plus H C l gives nitrous acid, HNO two, and sodium chloride. Okay? And the second H C l supplies the chloride ion, C l minus, of the salt. So, one to make nitrous acid, and one for the chloride. Right? Now, the species that actually attacks the amine. Nitrous acid takes up a proton, H plus, and loses water. So HNO two plus H plus gives N O plus and water. With me? This N O plus, the nitrosonium ion, is the electrophile. Electrophile means electron loving, so it attacks the nitrogen of the amine. Clear?

2:26Mechanism: amine to diazonium ion in three steps

Now, the amine becomes the diazonium ion in three steps. Step one: the amine nitrogen attacks N O plus, and then loses a proton, H plus, giving the N nitrosoamine; the italic capital N says the N O group sits on the nitrogen. Short lines are single bonds, and the equals sign is a double bond. Okay? Step two: a proton shifts from the nitrogen to the oxygen. Now we have the diazohydroxide: Ar, N double bond N, then O H, right? Step three: the diazohydroxide is protonated, and then it loses a water molecule. What is left is Ar, N plus, triple bond, N, the diazonium ion. The three lines are a triple bond, and the small plus above the first nitrogen is its charge. With me?

3:15Why aromatic? The stability of the salt

Now, back to our puzzle. The only difference between benzenediazonium chloride and ethanediazonium chloride is the group on the nitrogen: a benzene ring, or an ethyl group. So what is the ring doing for the ion? Yes, exactly, the answer is resonance. The positive charge of the arenediazonium ion is delocalised, that is, spread out over the benzene ring, and this stabilises the ion. Okay? The alkanediazonium ion, with an alkyl group in place of the ring, has no such stabilisation. So it loses nitrogen gas, N two, at once. And that is our one line answer to the puzzle. Got it? Now, benzenediazonium chloride itself. It is a colourless solid, soluble in water, and stable only in cold solution. Right? So it is used at once, and never stored, because when it is dry, it may explode. Clear? But change the anion to fluoroborate, B F four minus. This salt, Ar N two plus, B F four minus, is insoluble in water and stable at room temperature, so it can be filtered off and dried. With me? One last idea: other groups on the ring. Take a nitro group, N O two, which withdraws electrons from the ring. Does the diazonium ion become more stable, or less? It becomes less stable, that is correct. Electron withdrawing groups, like nitro, N O two, and cyano, C N, decrease the stability. And electron releasing groups, like methoxy, OCH three, and methyl, C H three, increase it.

4:53Example 1: which amines give a stable diazonium salt

Okay? Time for Example one. Which of these amines give a diazonium salt that is stable at two seventy three kelvin? Aniline, benzylamine, four methylaniline, N methylaniline, and cyclohexylamine. Pause the video and try it first. Done? Okay, let's check. What we need is a primary amino group, N H two, sitting on a benzene ring. Right? Aniline, C six H five N H two, is exactly that, so yes. Four methylaniline is aniline with a methyl group at position four of the ring, so yes again. Now benzylamine. It has a benzene ring and a primary N H two group. So it should work, right? This is exactly the trap, so be careful here. In benzylamine, as the formula shows, the N H two group sits on a C H two group, not on the ring. So the answer is no. Clear? Next, N methylaniline: the italic capital N means the methyl group sits on the nitrogen. So it is a secondary amine, and the answer is no. With me? And cyclohexylamine has a ring, but it is a cyclohexane ring, not a benzene ring. So, no again. Got it? So the answer is aniline and four methylaniline, only these two. Okay?

6:13Example 2: mass of diazonium salt from 4.65 g aniline

Now Example two, with numbers. Find the mass of benzenediazonium chloride formed from four point six five grams of aniline, given the atomic mass of chlorine as thirty five point five. Pause the video and try it first. Done? Okay, let's check. Capital M is the molar mass of aniline: seventy two for six carbons, seven for seven hydrogens, and fourteen for nitrogen. That is ninety three grams per mole; the raised minus one means per mole. Okay? So the moles of aniline, small n, are four point six five over ninety three; the slash means divided by. That gives zero point zero five moles. Right? Our equation says one mole of aniline gives one mole of benzenediazonium chloride, so we get zero point zero five moles of the salt. Some students now use the molar mass of the ion, C six H five N two plus, alone. Is that enough? That is not enough, and this is the trap here, because the salt includes the chloride. Seventy two for the six carbons, five for the five hydrogens, twenty eight for the two nitrogens, and thirty five point five for chlorine. That makes one hundred and forty point five grams per mole. With me? So the mass is zero point zero five times one hundred and forty point five, which gives seven point zero two five. The wavy equals sign means approximately, so the answer is about seven point zero three grams. That's it, really simple. Okay?

7:44Quick revision: diazotisation and diazonium stability

So, quick revision. Primary aromatic amine, sodium nitrite and H C l, at two seventy three to two seventy eight kelvin, give the diazonium salt. The aryl diazonium ion is stabilised by resonance, while alkyl ions lose nitrogen at once. And the chloride is used at once in the cold, while the fluoroborate is a stable solid. Now go and try the practice problems on this page. All the best!