Show how each of the following compounds could be converted...

Key Concepts

Benzylic Oxidation

This concept involves the selective oxidation of the benzylic position—the carbon directly attached to an aromatic ring. Compounds with a benzylic hydrogen, whether they are alkylbenzenes or substituted benzylic derivatives (like alcohols or halides), can be oxidized by strong oxidants to form a carboxyl group. This transformation is key in converting many aromatic side chains into benzoic acid, as the oxidation process typically proceeds regardless of the additional group attached to the benzylic carbon.

Use of Strong Oxidizing Agents

Oxidation reactions in organic chemistry often rely on powerful oxidizing agents. Reagents like potassium permanganate (KMnO4) are instrumental in carrying out benzylic oxidations. These oxidants are capable of breaking C–H bonds at the benzylic position and converting alkyl, alcohol, or other functionalized side chains on aromatic rings into carboxylic acids, even amidst diverse functional groups.

Reaction Conditions and Mechanism

Understanding the reaction conditions—such as heat, pH (often alkaline), and solvent—is essential when converting various compounds to benzoic acid. These conditions affect the rate and outcome of oxidation reactions. A clear grasp of the underlying mechanism, including the formation of intermediates like radicals or cationic species at the benzylic position, aids in predicting and optimizing the conversion process.

Functional Group Interconversion

This concept underscores the transformation of one functional group into another, a fundamental tool in organic synthesis. Converting a methyl or other side-chain substituent on an aromatic ring into a carboxyl group exemplifies a broader strategy where careful choice of reagents and conditions enables the systematic modification of molecular frameworks. Such transformations are pivotal in designing synthetic routes toward target molecules like benzoic acid.

Transcript

00:03 Suppose you want to make different molecules into carboxylic acids.

00:08 So let's take an example of making ethylbenzine into benzoic acid.

00:14 Now, what would be the best way to convert this aromatic molecule into an aromatic acid? well, one of the best ways to do this synthesis is to use potassium permanganate.

00:35 And the key rule for potassium permanganate is you need to have a benzylic hydrogen.

00:42 And a benzylic hydrogen is this hydrogen right here and this one as well.

00:51 There are two hydrogens there.

00:52 Hydrogens that are on the carbon directly adjacent to a benzene ring, kind of like allelic hydrogens when we were talking about alchemes.

01:03 In fact, this is a double bond.

01:08 And these hydrogens are right next to it, making it that benzylic hydrogens are, in fact, allelic hydrogens as well.

01:17 They just have a special name because of benzene.

01:25 And you need to have hydrogens there for potassium permanganate to work.

01:31 Otherwise, it would not.

01:35 But it would turn every benzillic carbon into a carboxylic acid.

01:41 Every single benzodia carbon.

01:44 And how this works is we would use potassium permanganate, kmn -o -4, with water and heat.

02:03 And this is a basic salt.

02:08 And what i mean by this is potassium has a neutral charge.

02:12 Or potassium salts have a neutral charge typically.

02:16 For example, like if we had kcl, this would have a neutral charge because k -o -h, potassium hydroxide, is a strong acid, strong base, my bad, strong base.

02:34 And if it is a strong base, it will have a neutral salt.

02:41 However, hmno4, per manganic acid, is a weak acid.

02:52 Therefore, this would make basic salts.

02:56 And because this is a basic salt overall, because neutral plus a basic equals basic, it's kind of like taking a zero and adding a positive number to it, you end up getting a positive ph.

03:11 And it's kind of a basic because of that.

03:15 And since it's basic salt, we are dealing in a basic medium.

03:20 And whenever we're dealing in a basic medium and we want to make an acid, we would have to do an acid.

03:27 Workup.

03:31 And that is our best synthesis.

03:35 Keep in mind that if there were multiple benzillic carbons with hydrogens, say this molecule, and we were to apply the same synthesis, it would make every single benzolic carbon that has hydrogens into a carboxylic acid.

04:05 And you may wonder what what happens to these carbons and what happens to that carbon up above? well, it ends up becoming part of our complex with permanganate, so we would not worry about it.

04:19 But this is an important trick for polysynthesis.

04:23 Let's try another example.

04:26 How would we make this into a benzoic acid? well, there really is only one way to do this, and it's by making it into a greenyard reagent.

04:39 And what we do is first we'd react magnesium magnesium with diethyl ether you can also write et2o really any ether works so we could use thf, which is tetrahidrofurin, which looks something like this but anything can work on this molecule or anything can be used as a proper solvent, as long as it's an ether, that is.

05:24 And we'd follow it up with co2.

05:30 And previously, when you've done greenyard reagents, you've acted it on a, when you had a greenyard reagent, you would react it with a carbonyl, and you would get an aldehyde complex, or you'd react it to an aldehyde, and you'll end up with an alcohol, right? and well, co2 looks something like this...

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