Part C: Concept Checks

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. In dilute alkali, two molecules of ethanal, the two carbon aldehyde, join up in the aldol reaction. But two molecules of methanal, with just one carbon each, cannot join up this way at all. Instead, in concentrated alkali, one molecule is oxidised and another is reduced. One carbon less, and the whole behaviour changes? That is strange, right? Example one answers it. This is Part C of the unit problem bank: short concept checks, where every answer needs a reason. Questions like these come up often in JEE, so let's begin.

0:42Check 1: why HX cannot replace phenol's OH

Check number one is on screen: H X replaces the O H group of an alcohol, but not that of phenol. In the alcohol reaction, ROH is the alcohol and H X a hydrogen halide, with X standing for the halogen. The arrow means gives: R X, plus H two O, water, with the small two written low. Okay? Phenol has an O H group too. So let me ask you this: what is so different about its carbon oxygen bond? Look where the O H sits: on a ring carbon that is s p two, with the small two written high. And a lone pair of the oxygen spreads into the ring, by resonance. Right? So the carbon oxygen bond, shown with a dash, is shorter: one hundred and thirty six p m, that is picometres, in phenol, against one hundred and forty two picometres in methanol. With me? That is partial double bond character, so the bond is short, strong and hard to break. Oh no, careful here: do not treat phenol like any alcohol just because both have an O H group. Clear? And there is a second reason: breaking this bond would have to form the phenyl cation, C six H five plus, with the plus written high, and that cation is very unstable. Got it?

1:56Check 2: benzoic acid, phenol and NaHCO₃

Now check number two. In the heading, N a HCO three is sodium hydrogencarbonate. Benzoic acid dissolves in its solution, but phenol does not, and we explain it with p K a values, the small a written low. Both are acids, so what decides it? Benzoic acid, four point two; carbonic acid, six point four; phenol, ten point zero. And as the brackets say, a lower p K a means a stronger acid. Okay? Now, the key comparison is with carbonic acid, in the middle. So tell me, which of our two acids is stronger than carbonic acid? Benzoic acid, yes! Its four point two is lower than six point four. So it displaces carbonic acid, carbon dioxide, C O two, is given off, and the acid dissolves. Right? But phenol, at ten point zero, is weaker than carbonic acid, so no carbon dioxide, and phenol does not dissolve. With me? So here is a quick trick: compare with carbonic acid. Only acids stronger than it give carbon dioxide with sodium hydrogencarbonate. Clear? And watch out for this trap: phenol does dissolve in sodium hydroxide, N a O H, just not in sodium hydrogencarbonate. Don't mix them up! Got it?

3:13Check 3: boiling points without an O–H bond

Now check number three, on boiling points. Aldehydes and ketones boil above hydrocarbons of similar mass but below alcohols, and esters boil below acids of the same molar mass. So what does the carbonyl group give these molecules, and what do they lack? First, what it gives. The C double bond O group is polar: delta plus, the Greek letter delta with a small plus written high, is a partial positive charge on carbon, and delta minus a partial negative charge on oxygen. So the molecules attract by dipole dipole attraction. Okay? That extra attraction is why they boil higher than hydrocarbons of similar mass. Right? Now, what they lack: an O H bond. With no hydrogen on oxygen, their molecules cannot hydrogen bond with one another. With me? Alcohols can, so aldehydes and ketones boil lower than alcohols. Esters have no O H bond either, so they boil lower than the acids. Clear? So, for similar molar mass, the greater than signs give the order: acid, then alcohol, then aldehyde or ketone, then ether or alkane. This order comes up often. Got it?

4:26Example 1: why methanal gives Cannizzaro, not aldol

Now the two examples. Example one is our opening puzzle: methanal gives the Cannizzaro reaction, and not the aldol reaction, so tell me why. Here is a hint: think about what the aldol reaction needs. Pause the video and try it first. Done? Okay, let's check. The aldol reaction needs an alpha hydrogen, alpha being the Greek letter on screen, and dilute alkali. An alpha hydrogen sits on the alpha carbon, the carbon next to the carbonyl group. Okay? Now look at methanal, written H, dash, CHO, so the carbon of C double bond O carries only two hydrogens. So tell me, where is its alpha carbon? That is right, there is none at all, so there is no alpha hydrogen, and no enolate ion can form. The aldol reaction needs that enolate, so methanal cannot give an aldol reaction with itself. Right? So methanal takes the other path, the Cannizzaro reaction, which needs no alpha hydrogen and takes place when it is heated with concentrated alkali. With me? Here is the course's equation: two HCHO plus KOH gives C H three O H, methanol, plus HCOOK, potassium methanoate. One molecule is reduced to the alcohol, and the other is oxidised to the salt of the acid. And that solves our puzzle. Got it?

5:51Example 2: why ketone alpha-H is more acidic than ester's

And now Example two, a step harder. The alpha hydrogens of a ketone are more acidic than those of an ester, and you have to explain why. Here is a hint: ask what holds the negative charge once the hydrogen leaves. Pause the video and try it first. Done? Okay, let's check. When an alpha hydrogen leaves as a proton, we get the enolate ion; here is the one from ethanal, and a ketone's enolate works the same way. The minus written high starts on the C H two carbon. The double headed arrow means resonance, and in the second structure the minus is on oxygen, with the equals sign now between the carbons. Okay? So the carbonyl group shares the negative charge, and stabilises the enolate. That is why an alpha hydrogen is acidic at all. Right? Now the ester, RCOOR prime, where the small dash written high is read as prime, and O R prime is the alkoxy group. Its oxygen has a lone pair, right next to the carbonyl group. With me? You might think this extra electronegative oxygen pulls harder, and makes the ester's alpha hydrogens more acidic. So let me ask you this: does that alkoxy oxygen help the enolate, or not? Oh no, careful, it does not help! Its lone pair is shared with the carbonyl group by resonance, so the ester's carbonyl group is already stabilised. Clear? So it is less able to take the enolate's negative charge, while the ketone has no such oxygen. That is why the alpha hydrogens of a ketone are more acidic than those of an ester. Got it?

7:28Quick revision: phenol, NaHCO3, boiling points, alpha-H

So, quick revision. Phenol's carbon oxygen bond has partial double bond character on an s p two carbon, so H X cannot replace its O H group. Sodium hydrogencarbonate gives carbon dioxide only with acids stronger than carbonic acid. No O H bond, no hydrogen bonds: carbonyl compounds boil below alcohols, and esters below acids. No alpha hydrogen, no aldol, but Cannizzaro in concentrated alkali. And the ester's carbonyl group is already stabilised by the alkoxy oxygen, so its alpha hydrogens are less acidic. Now go and try the practice problems on this page. All the best!