Ester Formation – Alcohols

00:00 Now, I want to briefly touch upon ester formation.

00:03 So, it isn’t just a question that we usually want to convert alcohols into haloalkanes, there are a number of different important steps which proceed via the formation of esters.

00:16 Alcohols react with carboxylic acids and carboxylic acid derivatives, which you will see a little later on, to give esters as products.

00:23 If, for example, you take an alcohol, I’ve shown here ethyl alcohol or ethanol, it’s possible to react it with acetic acid or ethanoic acid to give rise to ethyl ethanoate plus water.

00:37 Note the importance, in this case, of the arrows between the two reactions. This shows that the reaction itself isn’t particularly spontaneous and indeed the reaction between a carboxylic acid and the alcohol takes place as a dynamic equilibrium. And often a number of steps are used in order to facilitate the formation of more product than starting material such the removal of water.

01:03 The equilibrium, in this particular case, is forced by the ester formation using an excess of one reagent, usually the alcohol, which often acts as the solvent, in this particular case, and removing the water as it is formed.

01:17 Now, let’s have a quick look at other things you can do with alcohols.

01:23 So, alcohol oxidation is very important in organic synthesis because it affords us carbonyl groups such as the aldehyde groups shown here in green. It also ultimately forms the carboxylic acid group, which is shown at the other end of this particular diagram, RCOOH.

01:43 And this is important not just from a synthetic perspective because it is this particular function, this oxidation function, which so many enzymes in the body are responsible for.

01:52 For example, if you’re talking about the oxidation of alcohol, ethanol, it is first converted into acetaldehyde by alcohol dehydrogenase. After that, it is then converted into acetic acid where it is passed.

02:09 These are the synthetic steps that correlate to what happens from a biological perspective.

02:15 That is to say you have the same result, but you don’t necessarily use a biological system.

02:19 So, what are the oxidation steps? Primary alcohols can be converted either directly to carboxylic acids in the presence of a strong oxidising agent. Those strong oxidising agents are things like potassium dichromate, remember, of course, the ammonium dichromate from the lecture on ionic chemistry, and also, potassium permanganate, shown there as KMnO4.

02:45 These will actually bypass the formation of the carbonyl and go directly to forming a carboxylic acid. However, that is not always desirable. In fact, sometimes it’s very undesirable. And so, therefore, a more delicate oxidising agent can be used. In this case, I’ve used the example of pyridinium chlorochromate. However, there is a huge range of different oxidising agents and protocols and processes that can actually be used to selectively oxidise a primary alcohol up to the corresponding aldehyde, which is shown in green there.

03:22 So, if we look, however, at secondary alcohols, we’ll see that it’s only actually possible to oxidise it straight up to a particular type of carbonyl compound called a ketone.

03:35 If you think about what that means, it means, in fairness, you only actually have two alcohol groups on either side. So, it’s not possible to lose two protons in the formation of the aldehyde. So, you’re left purely with a ketone. And indeed, if you were to react your ketone any further, all you’d end up doing is breaking it apart and combusting it.

03:56 And finally, tertiary alcohols, this is where we have a carbon to which an OH group is bound with three alcohol substituents, cannot possibly be oxidised up using potassium permanganate or potassium dichromate. This, of course, is because that carbon bears no hydrogens that it can lose in the process of oxidation. And so, therefore, this will give you no reaction.

04:21 As I said before, it is possible to further react these tertiary alcohols like the secondary ones, but would result in combustion products rather than anything remotely useful.

04:33 In acidic conditions, dehydration may happen instead and this is what we’re going to come onto in a moment.