Methods for Ketones Only

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. You have an acyl chloride, and you want a ketone, by swapping its chlorine for a carbon group. You already know the Grignard reagent, the usual tool for adding a carbon group to a carbonyl compound. So, you add it and collect your ketone, right? Not so fast, because the Grignard reagent does not stop at the ketone. It goes on, and you end up with a tertiary alcohol. So how do we stop exactly at the ketone? Today we learn three methods that make ketones only, and JEE does ask about them, so let's begin.

0:36Method 1: acyl chlorides and a dialkylcadmium

So, method number one starts with R M g X, a Grignard reagent. R is an alkyl group, M g is magnesium, and X stands for a halogen, okay? Two R M g X, plus cadmium chloride, C d C l two, and the arrow means gives. We get R two C d, a dialkylcadmium, plus two M g, X in round brackets, C l. The small two written low says that two R groups sit on the cadmium. Right? Now this dialkylcadmium meets R prime COC l, an acyl chloride. Prime, the little dash on the R, marks a second group that may be different. Two R prime COC l, plus R two C d, gives two R prime COR, our ketone, plus C d C l two. Okay? Now look closely at R prime COR, the ketone. Which group came from the cadmium reagent, and which part came from the acyl chloride? Yes, exactly! The R group comes from the cadmium reagent, and the R prime C O part comes from the acyl chloride. And here is the key point. The cadmium reagent is mild, so it does not attack the ketone once it is formed. Clear? But wait. The Grignard reagent already carries the same R group. So why make the cadmium reagent at all? Why not use R M g X directly? Because the Grignard reagent does not stop there. It would add to the ketone as well, and go on to the tertiary alcohol. That is exactly our puzzle from the start, and the mild cadmium reagent is the answer. Got it?

2:12Method 2: nitriles with a Grignard reagent

Now method number two starts from RCN, a nitrile. Add a Grignard reagent, R prime M g X, and we get R C, then in round brackets, equals N M g X, then R prime. That equals sign inside the brackets is a double bond from this carbon to the nitrogen, and the nitrogen carries the M g X part. With me? This is not a ketone yet. We follow it with hydrolysis, a reaction with water, usually with dilute acid added. So what does hydrolysis turn this into? Into the ketone, RCOR prime, very good! Hydrolysis replaces the whole equals N M g X part by an oxygen. So one group, R, comes from the nitrile, and the other group, R prime, comes from the Grignard reagent. Right?

2:59Method 3: Friedel–Crafts acylation

Method number three is Friedel Crafts acylation. Here, A r H is an aromatic ring, like benzene, with A r standing for aryl. Add an acyl chloride, RCOC l, with anhydrous aluminium chloride, A l C l three. Anhydrous means completely dry. We get A r COR, an aryl ketone, plus H C l. Okay? So, what actually attacks the ring? The electrophile is the acylium ion. It is R, a short dash for a single bond, then C, then three lines for a triple bond to O, and the small plus sign up top is a positive charge. Remember this name; it comes up often. Got it? And there is a big advantage here: no rearrangement occurs. The acyl group enters the ring just as it is. Clear? Now tell me this. Once one acyl group is in the ring, will a second acyl group also enter? No, it stops after just one, and here is why. The acyl group withdraws electrons from the ring, so the product is deactivated, and only one acyl group enters. Students often forget this, so remember it. With me?

4:09Example 1: acetophenone from benzene and ethanoyl chloride

Time for Example one, an easy one. Name the ketone formed from benzene and ethanoyl chloride, with anhydrous A l C l three. Pause the video and try it first. Done? Okay, let's check. First of all, which of our three methods is this? Benzene, an acyl chloride and anhydrous aluminium chloride together mean Friedel Crafts acylation, method three. Right? Ethanoyl chloride is C H three COC l, so it brings C H three C O, the acyl group. Okay? This acyl group takes the place of one hydrogen on the ring. So C six H six, benzene, plus C H three COC l, gives C six H five COCH three, plus H C l. Clear? And C six H five COCH three is acetophenone; in brackets you see its systematic name, one phenylethanone. Easy, isn't it?

5:06Example 2: benzoyl chloride with dimethylcadmium vs CH3MgBr

Now Example two, a little harder, and it brings back our puzzle. Name the ketone formed from benzoyl chloride and dimethylcadmium. And why is methylmagnesium bromide not used instead? Pause the video and try it first. Done? Okay, let's check. This is method one. Benzoyl chloride, C six H five COC l, is our acyl chloride, with phenyl as R prime. Dimethylcadmium is C H three in round brackets, with a small two outside, then C d: two methyl groups on the cadmium. So two C six H five COC l, plus dimethylcadmium, gives two C six H five COCH three, plus C d C l two. Okay? Now look at the product, C six H five COCH three. Have you seen this ketone before? Yes, it is acetophenone again, the same ketone as in Example one! So two different methods give the same product. Clear? Now the second part. Methylmagnesium bromide carries the same methyl group. So why not use it instead? Because this Grignard reagent, C H three M g B r, would not stop at the ketone. It would add to the acetophenone as well. Right? And after hydrolysis, we get a tertiary alcohol, two phenylpropan two ol. That is C six H five, then C, then O H in round brackets, then C H three in brackets with a small two: one carbon carrying a phenyl, an O H and two methyl groups. So there is our puzzle, solved: the mild cadmium reagent stops at the ketone, while the Grignard reagent goes on to the alcohol. Got it?

6:48Quick revision: dialkylcadmium, nitrile, Friedel-Crafts

So, quick revision. Method one: an acyl chloride with a dialkylcadmium gives a ketone, while a Grignard reagent would go on to the tertiary alcohol. Method two: a nitrile with a Grignard reagent, then hydrolysis, gives a ketone. And method three: Friedel Crafts acylation, with anhydrous aluminium chloride, gives an aryl ketone, with just one acyl group on the ring. Now go and try the practice problems on this page. You will find them easy. All the best!