Amides and Urea

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Ethanamine and ethanamide both have an N H two group at the end of a short carbon chain. Dip red litmus into ethanamine solution, and it turns blue, because amines are bases. Now try the same with ethanamide. Same N H two group, so blue again, right? Well, the litmus stays red, because ethanamide is practically neutral. So where did the basic nitrogen go, and how do we still detect an amide? These ideas are useful for JEE Main and your Class twelve boards, so let's begin.

0:38Amides: neutral, and held by hydrogen bonds

So, what is an amide? On screen, RCONH two. R is an alkyl or aryl group, C O is the carbonyl group, and the N H two group sits right on the carbonyl carbon. The small numbers written low are subscripts. Okay? Now, amines are bases, because the lone pair on nitrogen can take up a proton. An amide has a nitrogen with a lone pair too. So let me ask you this: is an amide a base? The answer is hardly at all, because an amide is practically neutral. The lone pair on nitrogen is delocalised onto the carbonyl oxygen, so it is much less available. That solves the first half of our puzzle. Right? Next, the dash between N and H is a bond, and the three small dots between H and O stand for a hydrogen bond. Amide molecules hold one another through these hydrogen bonds, so amides are high-melting solids. So every amide is a solid, right? Not quite, and this is a small trap. Methanamide, HCONH two, is a liquid. Remember that one exception. Clear?

1:51Hydrolysis of amides, and a simple test

Now, hydrolysis, which means splitting the amide with water. With acid, we get the carboxylic acid, RCOOH, plus the ammonium ion, N H four plus. The arrow means gives, and the plus written high is a positive charge. So what changes if we use an alkali instead? Good thinking, only the forms change. In alkali, the acid comes out as the carboxylate ion, RCOO minus, and the nitrogen comes out as free ammonia, N H three. The minus written high is a negative charge. With me? And that gives us a simple test. Warm an amide with sodium hydroxide, N a O H, and you smell ammonia. That smell is a test for amides.

2:38Amides: four reagents, four products

Got it? Now four reagents, and four products. Phosphorus pentoxide, P two O five, or phosphorus oxychloride, POCL three, pulls water out of the amide and gives a nitrile, RCN, okay? Lithium aluminium hydride, L i A l H four, reduces the amide to an amine with the same number of carbons, RCH two N H two. Now, bromine with sodium hydroxide, where the slash means together with, also gives an amine. So tell me, same number of carbons again? This time the amine has one carbon fewer, RNH two. This is the Hofmann bromamide reaction. Look, this is a classic trap: lithium aluminium hydride keeps every carbon, while Hofmann loses one. So always count the carbons. Right? And nitrous acid, HNO two, gives the carboxylic acid and nitrogen gas, N two. Remember that nitrogen gas; we will measure it with urea in Example two. With me?

3:44Urea: the diamide of carbonic acid

Now let's meet urea. On screen, N H two in brackets with a small two outside means two N H two groups, both on one carbonyl carbon. Carbonic acid has two O H groups there; replace both by N H two, and you get its diamide, urea. Clear? Here is a quick one for you. Urea's molar mass is sixty grams per mole. What percentage of it is nitrogen? Two nitrogen atoms, at fourteen each, make twenty eight. So percent N, the percentage of nitrogen, is twenty eight over sixty, times one hundred, where the cross means times, and that gives forty six point seven percent. Nearly half of urea is nitrogen. Okay? Urea is made from carbon dioxide and ammonia under pressure, through ammonium carbamate. Right? And it was first obtained by Wöhler, by heating ammonium cyanate. Got it?

4:41Urea: biuret, nitrous acid, hydrolysis

Now three reactions of urea. Heat it gently, and two molecules, shown by the big two in front, join together and lose one ammonia. The product is biuret. With me? Biuret, with the formula on screen, gives a violet colour with alkaline copper sulphate, C u S O four. Okay? This is the biuret test, and it is also given by proteins, and by peptides with two or more peptide bonds. So a dipeptide, with just one peptide bond, does not give it. Now, amides give nitrogen gas with nitrous acid. So what does urea give? Yes, nitrogen again, but there is more. One urea, with two nitrous acid, gives one carbon dioxide, two nitrogen, and three water. So there are two gases this time. Keep that in mind for Example two. Right? And plain hydrolysis of urea gives carbon dioxide and ammonia. Clear? One last fact, on a serious note. Methyl isocyanate, made from methylamine, C H three N H two, and phosgene, was the gas of the Bhopal disaster. On screen, the dashes and double lines are single and double bonds. Got it?

5:54Example 1: ethanamide warmed with aqueous NaOH

Time for Example one, the easier one. What is observed when ethanamide is warmed with aqueous sodium hydroxide? Pause the video and try it first. Done? Okay, let's check. Ethanamide is C H three CONH two. Warm alkali means alkaline hydrolysis, which gives sodium ethanoate, C H three COON a, and ammonia. With me? So ammonia gas is evolved. You can smell it, and moist red litmus turns blue. See, there is our puzzle again: ethanamide alone kept the litmus red, but the ammonia it releases turns it blue. Clear? And sodium ethanoate, being a salt, remains in solution. So the full answer is that ammonia is evolved, and sodium ethanoate remains in solution. Got it?

6:46Example 2: total gas at STP from 0.60 g urea and HNO2

Now Example two, a little harder. Find the total volume of gas at STP, standard temperature and pressure, evolved when zero point six zero grams of urea reacts completely with nitrous acid. Pause the video and try it first. Hint: write the equation first. Done? Okay, let's check. The molar mass of urea is sixty grams per mole. So n, the moles of urea, is zero point six zero over sixty, which gives zero point zero one mole. Okay? From the equation, one urea gives two nitrogen, so we get zero point zero two mole of nitrogen. One mole of gas at STP takes twenty two thousand four hundred millilitres, so the answer is four hundred and forty eight millilitres, right? Oh no, careful, that is the trap, because the question asks for the total gas, and carbon dioxide is a gas too. Water is a liquid, so we leave it out. Zero point zero one mole of carbon dioxide plus zero point zero two mole of nitrogen makes zero point zero three mole. With me? So V, the volume, is zero point zero three times twenty two thousand four hundred, which gives six hundred and seventy two millilitres, written m L on screen. See how the trap hid in one word, total? Got it?

8:13Quick revision: amides, LiAlH4 vs Br2/NaOH, urea tests

So, quick revision. Amides are practically neutral, and warming one with sodium hydroxide gives the smell of ammonia. Lithium aluminium hydride keeps every carbon, while bromine with sodium hydroxide loses one. And for urea, the biuret test is violet, and with nitrous acid, urea gives carbon dioxide plus two nitrogen. Now go and try the practice problems on this page. You will find them easy. All the best!