Cyanides and Isocyanides

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Take an alkyl halide, and treat it with potassium cyanide in alcohol. You get a nitrile, and it has a pleasant smell. Now take the same alkyl halide, and use silver cyanide instead. Same cyanide group in both reagents, right? But this time you get an isocyanide, and its smell is very offensive. Only the metal changed. Today we will find out why, and what each family turns into. This pair comes up often in JEE conversion questions. So, let's begin.

0:34Two isomeric families

So, two families, and the figure shows both. First, the cyanides, also called nitriles. R is an alkyl group, the dash is a single bond, and the three lines are a triple bond. So the alkyl group sits on carbon, and that carbon is triple bonded to nitrogen. Okay? Now turn the group around. In an isocyanide, also called a carbylamine, the alkyl group sits on nitrogen, and the carbon is at the far end. The small plus over the N and the small minus over the C are charges on those atoms. Same atoms, different order, so the two families are isomers. Right? Now let us name them. Nitriles are named as alkanenitriles. Take C H three C N, where the small three written low counts the hydrogen atoms. The methyl group has one carbon, so it is methanenitrile, right? No, it is not, and that is exactly the trap here. We count the carbon of the C N group too, so there are two carbons, and C H three C N is ethanenitrile. Clear? Isocyanides are named alkyl isocyanide, as the brackets say. So C H three N C, the isomer, is methyl isocyanide. Got it?

1:51Making them: the reagent decides

Now, let us make them. Start at the left of the figure, with R X, an alkyl halide, where X is a halogen atom. R X plus potassium cyanide, KCN, in alcohol, as the brackets say, and the arrow means gives. We get the nitrile, RCN, okay? Now we use silver cyanide, A g C N, on the same alkyl halide. This time we get the isocyanide, RNC, our opening puzzle again. We will see the reason in Example one. With me? Two more ways to make nitriles. Amides lose water when heated with phosphorus pentoxide, P two O five, with phosphoryl chloride, POC l three, or with thionyl chloride, SOC l two, and give the nitrile. Clear? Aldoximes also lose water to give nitriles. Next, look at the diazonium salt, A r N two plus, where A r is an aryl group like phenyl, and the raised plus is the charge. With copper cyanide, C u C N, it gives the aryl nitrile, A r C N, right? And on the isocyanide side, primary amines give isocyanides in the carbylamine reaction. The small raised circle after the one means primary. Got it?

3:06What nitriles turn into

Now the nitrile reactions, on the top right of the figure. First, hydrolysis with dilute acid, H three O plus. Through the amide, we get the carboxylic acid, RCOOH, plus the ammonium ion, N H four plus. Look, the C N carbon becomes the COOH carbon, so the acid has the same carbons as the nitrile. Keep this in mind for Example two. Okay? With alkali, O H minus, we get the carboxylate ion, RCOO minus, and ammonia, N H three. Right? Next, reduction, with lithium aluminium hydride, L i A l H four, or hydrogen with nickel, where the slash means with. We get RCH two N H two, a primary amine. Clear? With stannous chloride, S n C l two, and H C l, then water, the nitrile gives an aldehyde, RCHO, in the Stephen reaction. Got it? And with a Grignard reagent, R prime M g X, then dilute acid, we get a ketone, RCOR prime. The prime marks a second alkyl group, which may be different from R, okay?

4:20Isocyanides behave differently

Now the isocyanides, on the lower right of the figure. Again, dilute acid. But now the nitrogen holds the R group, and the carbon is at the far end. So where does that end carbon go? Very good, it leaves as methanoic acid, HCOOH, while the nitrogen stays on the R group. So we also get the primary amine, RNH two. And alkali does not hydrolyse isocyanides at all. Got it? Reduction, with the same reagents, now gives RNHCH three. The end carbon becomes a methyl group on the nitrogen, so this is a secondary amine, and the small two with a circle means secondary. Clear? On heating to about five hundred and twenty kelvin, the capital K on screen, an isocyanide rearranges to the more stable nitrile, RCN, right? And finally, nitriles smell pleasant and boil higher than the isomeric isocyanides, while isocyanides smell very offensive. That smell was our opening puzzle. With me?

5:24Example 1: chloromethane with KCN and with AgCN

Now, Example one, and it settles our opening puzzle. Chloromethane is treated separately with alcoholic potassium cyanide, and with silver cyanide. Name the main products, and explain the difference. Pause the video and try it first. Done? Okay, let's check. Chloromethane has one carbon. With potassium cyanide, we get C H three C N, two carbons now, so ethanenitrile. With silver cyanide, we get its isomer, C H three N C, methyl isocyanide. Okay? Now comes the real question. Both reagents carry the same cyanide group. So why does one attack through carbon, and the other through nitrogen? Here is the reason. Potassium cyanide is ionic, so the cyanide ion, C N minus, is free, and it attacks through carbon, giving the nitrile. Clear? But silver cyanide is covalent, so only the lone pair on nitrogen is free to attack, and we get the isocyanide. So our puzzle is solved: ionic KCN gives the nitrile, and covalent A g C N gives the isocyanide. Got it?

6:35Example 2: ethanol to propanoic acid via a nitrile

Now a harder one, Example two. How is ethanol converted into propanoic acid, which has one carbon more? Pause the video and try it first. Done? Okay, let's check. Ethanol, C two H five O H, has two carbons, and propanoic acid has three. So we must add one carbon. Which reagent from today adds exactly one carbon? Yes, exactly, the cyanide group, C N, adds one carbon. But it needs a halide to replace, so first we turn ethanol into chloroethane. Phosphorus pentachloride, P C l five, or thionyl chloride, written above the arrow, does this. Clear? Next, the cyanide step. Which reagent do we use here, potassium cyanide or silver cyanide? We use potassium cyanide, very good. It gives propanenitrile, C two H five C N, and now we have three carbons. Okay? Silver cyanide would be the trap here. It gives ethyl isocyanide, and dilute acid turns that into ethanamine and methanoic acid, not our acid at all. Right? Finally, hydrolysis with dilute acid turns the C N group into a COOH group. We get propanoic acid, C two H five COOH, with three carbons, exactly as asked. Three steps, and the chain is one carbon longer. With me?

8:07Quick revision: KCN vs AgCN, hydrolysis, reduction

So, quick revision. Potassium cyanide gives the nitrile, with a pleasant smell, and silver cyanide gives the isocyanide, with a very offensive smell. In acid, a nitrile gives a carboxylic acid, while an isocyanide gives an amine and methanoic acid. And on reduction, nitriles give primary amines, and isocyanides give secondary amines. Now go and try the practice problems on this page. All the best!