Acidity of Phenols

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Phenol and ethanol both carry an O H group. Shake ethanol with sodium hydroxide solution, and nothing useful happens. Shake phenol with it, and the phenol goes into the water as sodium phenoxide. Same O H group, so why the difference? Today we find out what makes phenol an acid, and how groups on the ring change it. Questions on this come up often in JEE, so let's begin.

0:33Phenol versus ethanol

Let us put numbers on it. Phenol is C six H five O H, and ethanol is C two H five O H; the small numbers written low count the atoms. Their p K a values, a small p, a capital K and a small a below, are ten point zero and fifteen point nine. Okay? Now let me ask you this: the smaller p K a, or the larger one, which means the stronger acid? It is the smaller one, yes, exactly! So phenol is the stronger acid, about a million times stronger than ethanol. Now, where does such a big difference come from?

1:11Why phenol is the stronger acid

Reason one is the carbon holding the oxygen. In phenol it is an s p two ring carbon; the small two written high is part of that name. It is electron withdrawing, so the O H bond, the dash on screen, becomes more polar. With me? Now reason two. When phenol loses its hydrogen ion, its oxygen is left with a negative charge. But that oxygen sits right on a benzene ring. So where else can that charge go?

1:39The phenoxide ion: charge spread out

It goes into the ring, very good! This is the phenoxide ion, C six H five O minus; the small minus written high is the negative charge. In the figure, double headed arrows join its resonance structures, and the red minus signs mark the charge on ring carbons. Okay? So the charge is shared by the oxygen, the two ortho carbons on either side of the carbon that carries the oxygen, and the para carbon opposite it. Sharing the charge stabilises the phenoxide ion, so phenol is the stronger acid. Got it? And what about the alcohol? Its alkoxide ion is R O minus, where R is an alkyl group. No such delocalisation is possible, so the charge stays on the oxygen alone. Clear?

2:24Phenol with sodium, NaOH and bicarbonate

Now let us see phenol react as an acid. With sodium: two C six H five O H plus two N a gives two C six H five O N a, sodium phenoxide, plus H two, hydrogen; the arrow means gives. Okay? C six H five O H plus N a O H, sodium hydroxide, gives C six H five O N a plus water. Alcohols do not react with aqueous sodium hydroxide to any useful extent, but phenol does. So our puzzle is solved: phenol is the much stronger acid. Right? Then tell me this: will phenol give carbon dioxide, C O two, with sodium bicarbonate solution, N a HCO three? The answer is no, and this is a common trap. Phenol is weaker than carbonic acid, whose p K a value, in brackets, is six point four. So it does not liberate carbon dioxide from sodium bicarbonate. With me? It works the other way round too: carbon dioxide passed into sodium phenoxide solution sets phenol free. C six H five O N a plus C O two plus H two O gives C six H five O H plus N a HCO three. Got it?

3:35Groups on the ring: pull or push?

Now, groups on the ring, which come up often in JEE questions. Put a nitro group on the ring of phenol; it pulls electrons strongly. Will phenol become a stronger acid, or a weaker one? It becomes stronger, yes, exactly! Electron withdrawing groups, in brackets: N O two, nitro; C N, cyano; CHO, the aldehyde group; and the halogens. They stabilise the phenoxide ion, so the acidity goes up. Okay? The effect is greatest from ortho and para, because the negative charge reaches exactly those carbons, remember? So a nitro group there shares in the delocalisation, taking the charge onto its own oxygen atoms. With me? And what happens from the meta position? The charge never reaches the meta carbon, so only the inductive effect works, pulling electrons through the bonds. Right? Now for the opposite kind of group. Electron releasing groups, like alkyl groups and OCH three, methoxy, destabilise the phenoxide ion, so the acidity goes down. Clear?

4:40pKa values of phenols to remember

Now the p K a values to remember. Picric acid, in brackets two four six trinitrophenol, has just zero point four, and two four dinitrophenol has four point one. So nitro groups at ortho and para make the acid much stronger. Okay? With one nitro group: para is seven point one, ortho seven point two, and meta eight point three; the italic p, o and m stand for para, ortho and meta. Meta is clearly the weakest of the three. Right? And at the bottom: phenol, ten point zero; meta cresol, ten point one; ortho and para cresol, ten point two. Cresols are the methylphenols, and methyl releases electrons, so they are slightly weaker acids than phenol. Got it?

5:24Example 1: acidity order of cresol, phenol, nitrophenols

Now, Example one. Arrange in order of increasing acid strength: phenol, four methylphenol, four nitrophenol, and three nitrophenol. Pause the video and try it first. Done? Okay, let's check. Four means para, opposite the carbon with the O H group, so four methylphenol is para cresol, as the equals sign says. Its p K a value, ten point two, makes it weaker than phenol, at ten point zero. With me? Now three nitrophenol, the meta one. Some students say, no resonance, so no effect, and put it level with phenol. Is that right? No, no, you have to be careful here! The inductive effect still works from meta, so three nitrophenol has a p K a value of eight point three, stronger than phenol. And four nitrophenol, para, gets the full resonance help: seven point one. Okay? The smallest p K a means the strongest acid. So in increasing acid strength: four methylphenol, less than phenol, less than three nitrophenol, less than four nitrophenol. Here each less than sign means a weaker acid than the next one. Very good!

6:37Example 2: which phenols and acids fizz with NaHCO3

Now a harder one. Which of these dissolve in aqueous sodium bicarbonate with effervescence, fizzing with carbon dioxide: phenol, para nitrophenol, picric acid, and benzoic acid? Here is a hint: compare each with carbonic acid. Pause the video and try it first. Done? Okay, let's check. Only acids stronger than carbonic acid, with a p K a value of six point four, give carbon dioxide with sodium bicarbonate. Clear? Now para nitrophenol is much more acidic than phenol, thanks to its nitro group. So surely it fizzes, right? Oh no, careful, check the numbers! Its p K a value is seven point one, and phenol's is ten point zero; the greater than sign says both are larger than six point four. So both are weaker than carbonic acid, and give no carbon dioxide. With me? Now picric acid, at zero point four, is far stronger than carbonic acid. And benzoic acid is a carboxylic acid, and carboxylic acids are stronger than carbonic acid. So these two fizz, and the answer is picric acid and benzoic acid. See how one number, six point four, settles it?

7:53Quick revision: phenoxide, NaHCO3 test and ring substituents

So, quick revision. Phenol is far more acidic than ethanol, because its phenoxide ion is stabilised by resonance. It reacts with sodium and sodium hydroxide, but it is weaker than carbonic acid, so no carbon dioxide from sodium bicarbonate. Withdrawing groups raise the acidity, most from ortho and para; releasing groups lower it. Now try the practice problems on this page. All the best!