The Triple Bond and its Acidity

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Ethyne is a hydrocarbon, and we never think of a hydrocarbon as an acid, right? Yet ethyne reacts with sodium metal and gives off hydrogen gas, just as an acid does. So, a hydrocarbon behaving like an acid? Strange, isn't it? Questions on this acidity come up often in JEE, so let's find out where it comes from.

0:26Inside the triple bond

So, what is inside the triple bond of an alkyne? The two triply bonded carbons are s p hybridised, which means one s orbital and one p orbital have mixed. Okay? Three bonds join them, but not three of the same kind. There is one sigma bond, the Greek letter sigma, and two pi bonds, the Greek letter pi. Right? The three short lines between the two C symbols mean a triple bond. Its length is one hundred and twenty picometres, written p m. Clear? Now the shape. X and Y stand for any atoms or groups, and each dash is a single bond. All four atoms lie on one straight line. With me? The double bond gave us cis and trans isomers, remember? So, can a triple bond give them too? No, it cannot, and the reason is the shape. On a straight line there is no same side and no opposite side. So there is no geometrical isomerism about a triple bond. Got it?

1:28Why a terminal alkyne is acidic

Now the big idea. One of the two mixed orbitals is s, so an s p carbon has fifty percent s-character; that sign is percent. So this carbon holds a lone pair tightly. Okay? In a terminal alkyne, a hydrogen sits on the triply bonded carbon. When it leaves as a proton, the carbon keeps the electron pair, and we get a carbanion, the acetylide ion. Since that pair is held tightly, the acetylide ion is a fairly stable carbanion. Right? So tell me: ethyne, ethene and ethane, which one is the strongest acid? Yes, exactly, it is ethyne! H C triple bond C H is ethyne, C H two double bond C H two is ethene, and C H three single bond C H three is ethane; the small numbers count hydrogens, and the pointed signs mean greater than. The p K a values, small a below the K, are about twenty five, forty four and fifty; the wavy sign means about. Lower p K a, stronger acid. Clear? But careful: ethyne is acidic only when we compare it with other hydrocarbons. Alkynes are still much weaker acids than water and alcohols. We will need this in a minute. Got it?

2:46Sodium and sodamide replace the acidic hydrogen

Now, R C triple bond C H, where R is an alkyl group, reacts with N a N H two, sodamide. The arrow means gives: R C triple bond C minus, N a plus, and N H three, ammonia; the raised minus and plus are charges. Okay? Sodium metal does the same job. Two R C triple bond C H plus two N a give two R C triple bond C N a, plus H two, hydrogen gas. Notice that two molecules of alkyne give just one molecule of hydrogen. Right? Now notice which hydrogen is replaced. Only a hydrogen on the triply bonded carbon is replaced; the hydrogens on the alkyl group are not touched. With me?

3:33Tests for a terminal alkyne

Now, let us see how to spot a terminal alkyne. First, silver nitrate, A g N O three, dissolved in ammonia, which is what ammoniacal means; the name in brackets is Tollens' reagent. It gives a white precipitate of silver acetylide. Okay? Second, ammoniacal cuprous chloride gives a red precipitate of copper one acetylide; the Roman numeral one means copper in the plus one state. Clear? Alkenes, and non-terminal alkynes, where the triple bond carries no hydrogen, do not react, so there is no precipitate. That is how these tests pick out a terminal alkyne. Got it?

4:12Which base can take the proton?

Now an order of acidity to remember. H two O, water, then ROH, an alcohol, then ethyne, then N H three, ammonia, and last, R H, an alkane. Okay? So here is the trap. O H minus, the hydroxide ion, is a base we use all the time. Surely it can take the proton from ethyne, right? No, no, careful, hydroxide cannot do it! It would turn into water, a much stronger acid than ethyne, so the proton simply goes back. Clear? But N H two minus, the amide ion, can, because it turns into ammonia, below ethyne in the list. Grignard reagents can too; they end up as R H, at the very bottom. So here is a topper's trick: a base works only if its own acid lies below ethyne in this list. With me?

5:05Example 1: H2 volume when 0.1 mol propyne reacts with sodium

Time for the first example. What volume of hydrogen at STP, standard temperature and pressure, is liberated when zero point one mole of propyne, written m o l on screen, reacts completely with sodium? Pause the video and try it first. Done? Okay, let's check. Propyne, C H three C triple bond C H, has one hydrogen on the triple bond. Two propyne plus two N a give two C H three C triple bond C N a, plus H two. So zero point one mole of propyne gives zero point one mole of hydrogen, right? No, no, careful, read the equation again! Two moles of propyne give only one mole of H two. Okay? So zero point one mole of propyne gives half of that, which is zero point zero five mole. Right? At STP, one mole of gas takes up twenty two point four litres. So zero point zero five times twenty two point four gives one point one two litres; capital L is litre, and the cross means times. Forget the two to one ratio, and your answer comes out double. Clear?

6:17Example 2: mass of silver acetylide from 2.6 g of ethyne

Now a harder one. Ethyne is passed into ammoniacal silver nitrate. What is observed? Write the formula of the precipitate, and find the mass obtained from two point six grams of ethyne; g is grams. Pause the video and try it first. Done? Okay, let's check. Ethyne is a terminal alkyne, so we see a white precipitate. Now, ethyne, H C triple bond C H, has two hydrogens. Does silver replace one of them, or both? Yes, both, since both sit on triply bonded carbons! Ethyne plus two of the ion in square brackets, silver with two N H three groups and a plus charge, give A g C triple bond C A g, plus two N H four plus, ammonium ions, plus two N H three. Okay? Now the mass. Ethyne, C two H two, has a molar mass of twenty six grams per mole; g mol to the power minus one means grams per mole. So two point six grams is zero point one mole. Right? The precipitate, in short, is A g two C two. Its molar mass is two times one hundred and eight, plus two times twelve, which gives two hundred and forty grams per mole. Got it? One mole of ethyne gives one mole of precipitate. So the mass is zero point one times two hundred and forty, which gives twenty four grams. The real trap was the formula: put in only one silver, and everything after it goes wrong. With me?

7:52Quick revision: sp carbons, acidity, sodamide and the tests

So, quick revision. The triple bond joins two s p carbons by one sigma and two pi bonds, and it is linear. Fifty percent s-character makes terminal alkynes acidic, but weaker than water. Sodamide or sodium replaces the hydrogen; hydroxide cannot. The tests: white with ammoniacal silver nitrate, red with cuprous chloride. Now go and try the practice problems on this page. All the best!