The Carboxyl Group

Transcript

0:00Introduction

Hello everyone! Here is a small lab puzzle. Two bottles of white solid have lost their labels. We know that they hold benzoic acid and phenol, but not which is which. Both turn blue litmus red, so litmus cannot tell them apart. Can one simple solution sort them out in seconds? It can, and by the end you will know two ways to do it. Today we learn the tests for the carboxyl group. They come up often in JEE practical chemistry, so let's begin.

0:31Test 1: litmus, and why it is not enough

So, first meet our group. This is the carboxyl group, COOH, and the little dash in front is its bond to the rest of the molecule. We write the acid as RCOOH, where capital R stands for the rest of the molecule. Okay? Test number one is the simplest: the acid turns blue litmus red. Now let me ask you this: does red litmus prove that we have a carboxylic acid? Not by itself, and that is the first trap. Phenols also turn blue litmus red, like the phenol in our puzzle. So litmus says acidic, but not which kind. We need a sharper test. Right?

1:11Test 2: sodium hydrogencarbonate, NaHCO₃

Now test number two, the most important one: sodium hydrogencarbonate, N a HCO three, where the small three written low counts the oxygen atoms. The arrow means gives: RCOOH plus sodium hydrogencarbonate gives the sodium salt, RCOON a, plus carbon dioxide, C O two, plus water. One product is a gas, so what will you see in the test tube? Yes, exactly, you will see bubbles, and the course calls it brisk effervescence: quick, lively fizzing. But how do you confirm that the gas is carbon dioxide? That is right if you said lime water. Carbon dioxide is colourless and odourless, and it turns lime water milky. Got it? Now, back to our puzzle. Ordinary phenols, like plain phenol, do not react with sodium hydrogencarbonate, so there is no effervescence. The exception to remember is picric acid, which has three nitro groups and is strong enough to fizz. Benzoic acid fizzes, phenol stays quiet, and the bottles are sorted! Clear?

2:22Why only the acid? And where does the gas come from?

But wait a moment: both of them turn blue litmus red, so both are acidic. Then why can phenol not release carbon dioxide from bicarbonate? Look at the ion the acid leaves behind: RCOO minus, the carboxylate ion, and the little minus written high is its charge. It has two equivalent resonance structures, and they stabilise it. Okay? So a carboxylic acid is stronger than carbonic acid and decomposes the bicarbonate, while phenol is weaker, so it cannot. Now one more question from the course. Does the carbon of the carbon dioxide come from the acid, or from the bicarbonate? Look at the ionic form: HCO three minus, the bicarbonate ion, plus H plus, the hydrogen ion with its positive charge written high, gives water and carbon dioxide. So the carbon comes from the bicarbonate; the acid only hands over a hydrogen ion. Right? So here is a topper's trick for numericals. With excess sodium hydrogencarbonate, each COOH group gives one carbon dioxide. So count the groups, not the molecules: a dibasic acid, with two such groups, gives two moles of carbon dioxide per mole. Example two depends on this. With me?

3:42Test 3: the ester test

Test number three is the ester test. Warm the acid with ethanol and concentrated sulphuric acid, H two S O four; conc on screen is short for concentrated. There is no colour and no gas, so how will we know it worked? We know it by the smell. The acid and the alcohol form an ester, and the ester smells fruity. Clear? For example, acetic acid gives ethyl acetate, with a pleasant fruity smell; the arrow means gives. Got it?

4:13Test 4: neutral ferric chloride, FeCl₃

Test number four uses neutral ferric chloride, F e C l three. First neutralise the acid, then add neutral ferric chloride; the word neutral appears twice, so do not skip it. Okay? Acetate, from acetic acid, gives a deep red colour. Now take benzoate, from benzoic acid. Many students guess violet here, like phenol. Is that right? That guess is wrong, so be careful. Benzoate gives a buff precipitate, a solid, not a violet colour. Remember that word, buff. Got it? The violet colour belongs to phenol. So neutral ferric chloride is our second way to sort the bottles: a buff precipitate for benzoic acid, and violet for phenol. Right?

5:01Example 1: CO2 volume at STP from 0.60 g acetic acid

Time for Example one. What volume of carbon dioxide at STP, standard temperature and pressure, is given by zero point six zero grams of acetic acid with excess sodium hydrogencarbonate? Pause the video and try it first. Done? Okay, let's check. Acetic acid, C H three COOH, has two carbons, four hydrogens and two oxygens. So its molar mass, capital M, is twenty four plus four plus thirty two, which gives sixty grams per mole; g mol to the power minus one means grams per mole. Okay? So the moles, small n, are zero point six zero over sixty, which gives zero point zero one zero moles; m o l means mole. Right? Excess bicarbonate means all the acid reacts, and with one COOH group, each mole of acid gives one mole of carbon dioxide. So we get zero point zero one zero moles of carbon dioxide. Clear? At STP, one mole of gas occupies twenty two thousand four hundred millilitres, m L on screen. So the volume, capital V, is zero point zero one zero times twenty two thousand four hundred, which gives two hundred and twenty four millilitres. Remember this number. Got it?

6:21Example 2: CO2 from oxalic acid dihydrate, the dibasic trap

Now a slightly harder one. Take zero point six three grams of oxalic acid dihydrate, whose molar mass is one hundred and twenty six, with excess sodium hydrogencarbonate. At STP, what volume of carbon dioxide do we get? Pause the video and try it first. Here is a hint: count the COOH groups. Done? Okay, let's check. Oxalic acid is two COOH groups joined together, and the small two outside the bracket counts them. The dot, then two H two O, means two molecules of water of crystallisation, which is what dihydrate means. Okay? The molar mass, one hundred and twenty six, already includes this water. So the moles are zero point six three over one hundred and twenty six, which gives zero point zero zero five moles. And zero point zero zero five times twenty two thousand four hundred gives one hundred and twelve millilitres. So the answer is one hundred and twelve millilitres, right? That answer is wrong, and this is the trap! Oxalic acid is dibasic, with two COOH groups, so each mole of it gives two moles of carbon dioxide. Two times zero point zero zero five gives zero point zero one zero moles. With me? So the volume is zero point zero one zero times twenty two thousand four hundred, which gives two hundred and twenty four millilitres. Look, it is the same answer as Example one: half the moles of acid, but twice the gas from each mole. Nice, isn't it?

8:02Quick revision: litmus, NaHCO3, ester, neutral FeCl3

So, quick revision. Blue litmus turns red with acids and with phenols, so litmus alone is not enough. Sodium hydrogencarbonate gives brisk effervescence of carbon dioxide with acids only. The ester test gives a fruity smell, and with neutral ferric chloride, acetate gives a deep red colour and benzoate a buff precipitate. In numericals, each COOH group gives one carbon dioxide. Now go and try the practice problems on this page. All the best!