Phenols, Alcohols and Carbon Acids

Transcript

0:00Introduction

Hello everyone! First, here is a small puzzle. Phenol is a weak acid, with a p K a of ten. Add three nitro groups to its ring, and you get picric acid, with a p K a of just zero point four! That is nearly ten units, and each unit is a factor of ten. So how do three small groups make phenol as strong as a carboxylic acid, or stronger? And a second surprise: even a C H bond can act as an acid. This comes up often in JEE, so let's begin.

0:33Why phenol is an acid: the phenoxide ion

So, why is phenol an acid at all? Phenol, C six H five O H, gives up H plus and leaves the phenoxide ion, C six H five O minus. The small numbers sit low, the charges high, and the two half arrows mean equilibrium. Okay? In the figure, the double-headed arrows mean resonance. The negative charge starts on oxygen, then moves, as the small red minus sign, to an ortho carbon, the para carbon and the other ortho carbon. Spread out like this, the anion is more stable, so the acid is stronger. Now tell me, which ring carbons never get the charge? The meta carbons, yes, exactly! The small italic o, p and m mean ortho, para and meta, and on the ring the charge reaches only ortho and para, never meta. Remember this, right?

1:29Groups on the ring: pull or push

Now let us put groups on the ring. An electron-withdrawing group stabilises the phenoxide ion and makes a stronger acid, and an electron-releasing group does the opposite. Clear? But the position matters. At ortho or para, a withdrawing group acts by minus R, the resonance effect, and by minus I, the inductive effect. At meta, only minus I acts, since the charge never reaches meta. With me? Now the cresols, the methyl phenols. Methyl releases electrons, so every cresol is weaker than phenol: phenol, greater than meta cresol, greater than para cresol, about equal to ortho cresol. Greater than means more acidic, and the wavy equals sign means approximately. Got it? Now the methoxy group, OCH three, in the badge. Its oxygen pushes electrons into the ring by plus R, so it always gives a weaker acid, right? No, no, careful, the position matters again! At meta, plus R cannot act, so only the minus I pull works, and the acid is stronger than phenol. Okay? But at para, the plus R effect wins, and the acid is weaker than phenol. So the same group gives opposite results. Right?

2:46The nitrophenols: nitro group at ortho, meta or para

Now come the three nitrophenols. N O two, the nitro group in the heading, pulls by minus I from every position, and by minus R from ortho and para. Ortho is the closest to the O H group, and the inductive pull falls with distance. So which one is the strongest acid? If you said ortho, oh no, careful, that is the trap! Para is the strongest, at seven point one, with ortho just behind, at seven point two, then meta, at eight point three, and phenol, at ten point zero. These are p K a values, with the small a written low, and lower means stronger. Okay? So why does ortho lose to para? Its O H hydrogen is held in an intramolecular H bond, a hydrogen bond inside the same molecule, so it is harder to lose. Clear?

3:38Alcohols and alkoxide ions

Now let us take alcohols. In solution, methanol, C H three O H, is the most acidic, then primary, secondary and tertiary; the small circle up high means degree. Each step adds an alkyl group on the carbon carrying the O H group. Why does that weaken the acid? Alkyl groups are plus I groups, very good! They push electrons onto the oxygen of the alkoxide, R O minus, with R the alkyl group, and destabilise it. And a bulky alkoxide is poorly solvated. Clear? Now flip it. The weaker the acid, the stronger its conjugate base, so alkoxide basicity runs in the opposite order. So tell me, which alkoxide is the strongest base? It is tert butoxide, that is correct! That is C H three in brackets, three of them, then C O minus, from the weakest acid, the tertiary alcohol.

4:38Carbon acids: when is a C–H bond acidic?

Okay? Now comes the second surprise. The C H bond, the dash being the bond, is acidic when the carbanion left behind is stabilised. First, an s p carbon holds the lone pair better, so alk one ynes, the terminal alkynes, have a p K a of twenty five. Okay? The second way is a carbonyl group, C double bond O, the equals sign being the double bond. The charge spreads onto its oxygen, so ketones have a p K a of about twenty. And what if a C H two, with the small two written low, sits between two carbonyls? Yes, the charge spreads onto both oxygens, and the p K a falls to nine in pentane two four dione, C H three, C O, C H two, C O, C H three. Two groups beat one! With me? The third way is a nitro group: nitromethane has a p K a of ten. The fourth is aromaticity: cyclopentadiene has a p K a of sixteen, because its anion is aromatic. Right? And a C H two group between two electron-withdrawing groups is called an active methylene group. It comes up often. Got it?

5:53Example 1: acidity order of nitrophenols and picric acid

Now example one, and our puzzle comes back. Arrange in order of decreasing acidity: phenol, two four dinitrophenol, four nitrophenol, and two four six trinitrophenol. Here is a hint: the numbers are positions, with the O H carbon as carbon one. Pause the video and try it first. Done? Okay, let's check. Carbons two and six are ortho, and carbon four is para. So four nitro is para; two four dinitro, ortho and para; and two four six trinitro, ortho, ortho and para. Okay? From our rule, each nitro group at ortho or para adds to the stabilisation of the phenoxide ion, by minus R and minus I together. Right? So the more such groups, the stronger the acid: two four six trinitrophenol, greater than two four dinitrophenol, greater than four nitrophenol, greater than phenol. Clear? And the p K a values agree: zero point four, four point one, seven point one and ten point zero. So our puzzle is solved: in picric acid, all three nitro groups sit at ortho or para. Got it?

7:09Example 2: C-H acidity of diketone, keto ester, malonate

Now example two, a harder one. Arrange in order of decreasing acidity of the C H bond: propanone, pentane two four dione, ethyl three oxobutanoate, and diethyl propanedioate. Here is a hint: what kind of carbonyl sits next to the acidic carbon? Pause the video and try it first. Done? Okay, let's check. Propanone has one ketone carbonyl, and pentane two four dione has two. Two groups beat one, so the dione beats propanone. Right? Ethyl three oxobutanoate has a ketone plus an ester carbonyl, and diethyl propanedioate, also called diethyl malonate, has two esters. Both have two carbonyls, just like the dione. So are they just as acidic? No, they are not, and this is the trap! The ester oxygen also pushes electrons into its carbonyl by plus R, which competes with the carbanion's charge for that carbonyl. So an ester carbonyl helps less. With me? So the order is pentane two four dione, greater than ethyl three oxobutanoate, greater than diethyl propanedioate, and propanone, with one group, comes last. Clear? And the p K a values agree: nine, eleven, thirteen, and about twenty. See how two simple rules give the whole order?

8:28Quick revision: phenols, alcohols and C-H acids

So, quick revision. Phenols: withdrawing groups help most at ortho and para, and at meta only minus I acts. Alcohols: methanol, then primary, secondary and tertiary, with alkoxide basicity in reverse. And C H acids need a stabilised carbanion: two carbonyls beat one, and a ketone beats an ester. Now go and try the practice problems on this page. All the best!