AIEEE and Earlier JEE Main Questions: Acids

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Methanoic acid, HCOOH, has a p K a of three point seven five. Replace the hydrogen on its carboxyl carbon with a methyl group, and you get ethanoic acid, C H three COOH, with a p K a of four point seven six. One small methyl group, and the acid becomes about ten times weaker. Why should it matter so much? Interesting question, right? Today we answer that with two real A I triple E questions. Acid strength questions like these come up often in JEE, so let's begin.

0:40The carboxylate ion, and the pKₐ ladder

So, what makes a carboxylic acid an acid? RCOOH, where R is the alkyl group, gives its proton to water, H two O, forming the carboxylate ion, RCOO minus, and H three O plus. The pair of half arrows means an equilibrium, the small numbers written low count atoms, and the raised minus and plus are charges. Okay? Now, an acid gives up its proton easily only when the ion left behind is stable. So tell me, what makes RCOO minus stable? Resonance, yes, exactly! The negative charge is shared by the two oxygen atoms, through two equivalent resonance structures. And a shared charge means a stable ion. Clear? Now, we measure acid strength by p K a: a small p, a capital K, and a small letter a written low. On the ladder at the right, p K a runs from zero to fifty, and ethanoic acid sits at about four point eight. So tell me, does a smaller p K a mean a stronger acid or a weaker one? It means a stronger acid, exactly! The red arrow below the axis, marked stronger acid, points left, towards the smaller p K a. Phenol, at ten, and water, at fifteen point seven, sit further right, so they are much weaker acids. Never mix up this direction. Got it? Now let us read the course's own values. Methanoic acid, three point seven five. Ethanoic acid, four point seven six, which the ladder rounds to four point eight. Propanoic acid, C H three C H two COOH, four point eight seven. So from a hydrogen, to a methyl, to an ethyl group, the p K a goes up and the acid gets weaker. With me?

2:30Groups that pull, groups that push

Now, let us see what other groups do to the carboxylate ion. Electron-withdrawing groups, like fluorine, F, and chlorine, C l, pull electrons away from the COO minus. The charge is spread out, the ion is stabilised, and the acid is stronger. Okay? Now, an alkyl group, like methyl, does the opposite: it releases electrons. So what does that do to the carboxylate ion? It pushes even more negative charge onto the ion, yes! In the course's words, the electron-releasing methyl group intensifies the negative charge on the ethanoate ion and makes it less stable. So ethanoic acid is weaker than methanoic acid, and our opening puzzle is solved. Right? Now there are two more rules to remember. The effect grows with the number of such groups, and dies away with distance from COOH, the carboxyl group. This is the inductive effect, a pull or push passed along the bonds, and it fades quickly. Clear? Now let us look at the numbers. Chloroethanoic acid, C l C H two COOH, with one chlorine, has a p K a of two point eight seven. Trifluoroethanoic acid, C F three COOH, with three fluorines, has zero point two three, the strongest on the course's list. More pulling groups, a much stronger acid. Got it?

3:51Example 1: lowest pKa among four carboxylic acids

Time for Example one, from A I triple E two thousand and five. Which has the lowest p K a: propanoic acid, two methylpropanoic acid, methanoic acid, or ethanoic acid? Pause the video and try it first. Done? Okay, read the question carefully first. Many students see the word lowest and hunt for the weakest acid. So which is it, the strongest acid or the weakest? We want the strongest one, of course! Remember the red arrow on our ladder: smaller p K a, stronger acid. Okay? Methanoic acid, HCOOH, has only a hydrogen on its carboxyl carbon, and no alkyl group at all. Nothing pushes extra charge onto its ion. Right? The other three all carry alkyl groups: a methyl in ethanoic acid, an ethyl in propanoic acid, and in two methylpropanoic acid, a carbon carrying two methyl groups. Each releases electrons and weakens the acid. With me? So the answer is methanoic acid, with the lowest p K a, three point seven five. That is our opening puzzle, asked as an exam question, and the only trap was the word lowest. Clear?

5:09Example 2: increasing acid strength, the methoxy trap

Now a harder one, Example two, from A I triple E two thousand and six. Arrange them by increasing acid strength. Acid A is C H three C O two H, ethanoic acid. Acid B is C H three OCH two C O two H, which has a methoxy group. Acid C is C F three C O two H, with three fluorines. And acid D is C H three in brackets, with a small two after the bracket for two methyl groups, then CHCO two H, okay? Increasing means the weakest comes first. Pause the video and try it first. Done? Okay, let's check, starting with the easy one. Acid C has three fluorine atoms on the carbon next to the carboxyl group, three strong pulls, very close by. Its p K a is zero point two three, so C is the strongest of all four. Right? Next, compare acid D with acid A, the plain ethanoic acid. The dash on screen marks the bond to the rest of the molecule. Acid A has one methyl group on the carboxyl carbon, but acid D has a carbon carrying two methyl groups, so it releases more electrons. More release, a less stable ion, so D is weaker than A, and the less than sign means weaker. With me? Now the interesting one: acid B, and its methoxy group, C H three O, on screen. At the para position of a benzene ring, a methoxy group releases electrons and makes benzoic acid weaker. So B should be weaker than A, right? No, no, careful, because that is exactly the trap! On the ring, the oxygen pushes its lone pair into the ring by resonance. But acid B has no ring. Its oxygen sits on a C H two, and oxygen is more electronegative than carbon, so it can only pull electrons along the bonds. That pull stabilises the ion, so B is stronger than A after all. Got it? Now set acid B against acid C, the fluorine acid. Acid B has one oxygen on the carbon next to the carboxyl group, while acid C has three fluorine atoms there. One pull against three strong pulls, so C stays the strongest. Clear? So, weakest first: D, less than A, less than B, less than acid C, the strongest. And remember the trap: the same methoxy group pushes from the para position of a ring, but only pulls through a C H two. So always check where a group sits, okay?

7:41Quick revision: pKa, inductive effect, acid strength

So, quick revision. Lower p K a means a stronger acid, so methanoic acid beats ethanoic acid, which beats propanoic acid; here the greater than sign means stronger. Clear? Electron-withdrawing groups strengthen the acid, while electron-releasing alkyl groups weaken it. Right? And the effect grows with more such groups, and with groups closer to the carboxyl group. Now go and try the practice problems on this page. All the best!