Acidity

Transcript

0:00Introduction

Hello everyone! Here is a puzzle. Ethanoic acid, the acid in vinegar, has an O H group, and so does phenol. Drop each one into baking soda solution, sodium hydrogen carbonate. Ethanoic acid fizzes with bubbles of carbon dioxide, but phenol gives no bubbles at all. Same O H group, so why the difference? Today we find out why, and what makes one acid stronger than another. Ordering acids by strength comes up often in JEE, so let's begin.

0:36Why a carboxylic acid gives up H⁺

RCOOH is a carboxylic acid: R is the rest of the molecule, and COOH is the carboxyl group. The small numbers written low count atoms. The two half arrows mean an equilibrium: the acid gives its proton to water, forming RCOO minus, the carboxylate ion, and H three O plus. The signs written high are the charges. Okay? Now, why does it give up its proton so easily? Let me ask you this: what makes the carboxylate ion so stable? Resonance, yes, exactly! The dashes are single bonds, and the equals sign is a double bond. In the first structure, the oxygen in brackets is double bonded and the last oxygen carries the minus charge; in the second, they swap. The double-headed arrow links resonance structures. With me? The two structures are equivalent; only the oxygens trade roles. So the negative charge is shared equally by two oxygen atoms, and both C O bonds have the same length. Clear? So carboxylic acids have a p K a of about four to five: a small p, then a capital K with a small a written low. And the rule: the smaller the p K a, the stronger the acid. Got it?

1:50Where carboxylic acids stand

Now, where do carboxylic acids stand? The greater than sign means is a stronger acid than. Mineral acids beat carboxylic acids, which beat phenols, which beat alcohols. Right? So, back to our puzzle. Ethanoic acid is a carboxylic acid, and RCOOH with N a HCO three, sodium hydrogen carbonate, releases C O two, carbon dioxide. That was our fizz. Okay? And phenols give no carbon dioxide. Recall from the phenols chapter that in the phenoxide ion the charge also spreads onto ring carbons, while the carboxylate ion keeps it on two oxygens, in equivalent structures. Clear? Now, one exception: phenols with several nitro groups, N O two, do release carbon dioxide. With me?

2:36Groups that pull and groups that push

Now, what makes one carboxylic acid stronger than another? Suppose a group pulls electrons away from the carboxyl group. Will the acid be stronger, or weaker? Stronger, very good! Pulling electrons away stabilises the carboxylate ion, so the proton leaves more easily. Such a group is called electron-withdrawing. Right? And an electron-releasing group, like methyl, C H three, pushes electrons in and makes the acid weaker. Clear? Now two more rules. The effect grows with the number of such groups, and it falls as the group moves away from the carboxyl group. Example two uses both. Okay?

3:20The pKa ladder: halogens pull hard

Trifluoroethanoic acid, C F three COOH, has a p K a of just zero point two three. With three chlorines, C C l three COOH, it is zero point six five; with two, CHC l two COOH, it is one point four eight. Fewer halogens, weaker acid. See the pattern? Fluoroethanoic acid has only one fluorine, but fluorine is the most electronegative element. So, will it beat trichloroethanoic acid, with three chlorines? No, it does not; many students slip here! FCH two COOH has a p K a of two point five nine, and C l C H two COOH has two point eight seven. Both are far weaker, so the number of groups really matters. Got it? And with no halogen, ethanoic acid, C H three COOH, has four point seven six, and propanoic acid, C H three C H two COOH, has four point eight seven. Alkyl groups release electrons, so these are the weakest. Right?

4:27Phenyl, vinyl and substituted benzoic acids

Now benzoic acid, C six H five COOH, with a benzene ring, the phenyl group, on the carboxyl group. Many students guess the ring pushes electrons like an alkyl group, making it weaker than ethanoic acid. What do you think? That guess is wrong, benzoic acid is stronger! Its p K a, four point one nine, is below the four point seven six of ethanoic acid. But it is weaker than methanoic acid, HCOOH, at three point seven five. Okay? The reason: the ring carbon joined to the carboxyl group is s p two; the small two written high is part of the name. Such a carbon is more electronegative than an alkyl carbon, so it pulls electrons, and a vinyl group does the same. Clear? On the ring, say at the para position, opposite the carboxyl group, the nitro group, N O two, raises the acidity, while releasing groups like methoxy, OCH three, and methyl, C H three, lower it. With me? Now the ortho effect, a special case. Ortho means the group sits on the ring carbon right next to the carboxyl group. Nearly every ortho-substituted benzoic acid is stronger than benzoic acid, even two methylbenzoic acid, with its electron-releasing methyl group! Got it?

5:46Example 1: which acid is stronger in each of three pairs

Now, Example one. Which is the stronger acid in each pair? Part A, methanoic or ethanoic acid. Part B, ethanoic or chloroethanoic acid. And part C, benzoic acid or phenol. Pause the video and try it first. Done? Okay, let's check. In part A, methanoic acid has just a hydrogen on the carboxyl group, while ethanoic acid has a methyl group, C H three, which releases electrons. So methanoic acid is stronger. Right? In part B, chlorine, C l, withdraws electrons, so chloroethanoic acid is stronger: a p K a of two point eight seven, against four point seven six. Clear? In part C, both have a benzene ring, but benzoic acid is a carboxylic acid, and carboxylic acids are stronger than phenols. So only benzoic acid releases carbon dioxide from sodium hydrogen carbonate. Okay?

6:46Example 2: order of bromopropanoic acids by acid strength

Now a harder one, Example two. Arrange in order of increasing acid strength: propanoic acid, two bromopropanoic acid, three bromopropanoic acid, and two two dibromopropanoic acid. Pause the video and try it first. Done? Okay, let's check. The small numbers above the carbons number the chain: the carboxyl carbon is carbon one, then come carbons two and three. Propanoic acid has no bromine, B r, so it is the weakest. With me? Three bromopropanoic acid has one bromine, on carbon three, farther from the carboxyl group, so a small pull. Two bromopropanoic acid also has one bromine, but on carbon two. So, are these two acids equally strong? No, not at all! This pull through the bonds is the inductive effect, and it dies away with distance. On carbon two, right next to the carboxyl group, bromine pulls hard; on carbon three it pulls less. So two bromo is stronger. Got it? And two two dibromopropanoic acid has two bromines, both on carbon two. More groups, bigger effect, so it is the strongest. Right? So the order of increasing acid strength is propanoic, then three bromo, then two bromo, then two two dibromo; the less than sign means weaker than. You used both rules, number and distance, in one question.

8:12Quick revision: resonance, inductive effect and ortho effect

So, quick revision. Two equivalent resonance structures stabilise the carboxylate ion, so carboxylic acids release carbon dioxide from sodium hydrogen carbonate. Withdrawing groups strengthen the acid, releasing groups weaken it, and more groups, or nearer ones, have a bigger effect. An s p two carbon on the carboxyl group strengthens the acid, and nearly all ortho-substituted benzoic acids are stronger. Now try the practice problems on this page. All the best!