Esterification

Esterification Definition

So, esters seem to be the talk of the town, but you're realizing you don't know what everyone is raving about. Well, don't fear because we're here with an ester crash course! Let's jump into it, starting with the basics.

Okay, what? Alkoxy group? Carbonyl functionality? Huh?? Don't worry, we have this handy drawing to simplify things.

To start, a Carbonyl Group is a C=O bond. This group is fairly reactive and can have many different groups attached to it (be sure to check out Nucleophiles and Electrophiles). Since Carbon can have two other bonds, this can result in many different types of organic functional group.

If, for example, there is an O–R group attached to it, that would be an ester! If you're unfamiliar with "R", be sure to take a look at the article R-groups. Essentially, "R" could represent any group. It is just written that way as a placeholder. "R" could be an alkyl group, a phenyl group, a Nitrogen, a halogen, etc. It could be literally anything!

This molecule can be very desirable in certain organic reactions. As such, it is important to understand how to create this molecule through esterification.

The esterification reaction is quite an important reaction which has many benefits. This will be discussed later on in this text. First, perhaps we should provide some relevance.

Ester Examples

Okay, now that we know what an ester is, we can figure out why we care. Well, you may or may not know that esters are all around us. They are in our bodies, in the products we use, and constantly used by nature. For example, any time you eat fatty foods, bananas, or pineapples, you're eating esters. They are used in some brands of nail polish (ethyl acetate), and they're also used in plastics (dialkyl phthalates)!²

One of the most interesting esters is triacylglycerol. This is what makes up fats, which, when consumed, are broken down by the body to form energy. They are also the precursor to soap. The reaction to make soap is an ester hydrolysis reaction, which will be discussed later. Before we go about breaking esters, we should learn how to make them.

Esterification Reaction

Now that we understand what an ester is, we can go about figuring out how to make one. The esterification reaction, however, is a generalization. You will come to understand as you learn more Organic Chemistry that there is more than one way to solve a puzzle. Forming an ester can be done using multiple different methods. We will only look at one in particular, but here is a list of some esterification reactions for your information:

• Fischer-Speier Esterification
• Shiina Esterification
• Steglich Esterification
• Yamaguchi Esterification

There are many other ways to produce esters, most being named after the people who discovered them. They are not super important, so we will just focus on the most common esterification reaction, the Fischer-Speier esterification.

In the reaction, a carboxylic acid is reacted with a primary alcohol. This reaction is done under acidic conditions, usually employing sulfuric acid. It can also be done with some other strong Brønsted acid or Lewis acid. The reaction is commonly referred to as the Fischer reaction, with Speier often being omitted. However, both of them published the reaction, so Speier it seems got the short end of the stick.³

Some reagents do not work with the reaction, which is why some other esterification reactions are employed. What's important to remember is what happens in the reaction. A carbonyl functionality—typically a carboxylic acid or acid chloride—is converted into an ester.

Esterification Catalyst

As is the case with most Chemical Reactions, they will eventually work on their own, but everything goes smoother if you employ a catalyst. The same can be said with the Fischer-Speier reaction, which uses an esterification catalyst. In the reaction shown above, an acid is used as the catalyst. Above the reaction arrow, H⁺ is listed. This is typically the representation of a Brønsted acid being used.

If you see H⁺ in a reaction, it means that there are protons (hydrogen cations) in the solution. The role of a Brønsted acid is to donate protons to the solution, and other reagents. In this reaction, the acidic hydrogens are not being consumed by either reagent, but rather they assist in the reaction to make it happen quicker. At the end of the reaction, they are still in the solution as they were before, ready to assist another reaction.

Esterification Conditions

The conditions for an esterification reaction are as varied as the specific reaction itself. As is the case with most synthetic processes, each reaction requires its own specific conditions to work optimally. What works for one reagent may not work at all for another reagent that is very similar. Organic chemists work tirelessly to explore optimal conditions for each reaction that they discover.

Since each reagent has its own little idiosyncrasies, it would be too cumbersome to list reaction conditions for each one. Instead, some reactions that were previously mentioned will be summarized here to outline some possible reaction conditions which have been shown to work.

* DMAP = 4-dimethylaminopyridine (Brønsted base)

The table provides a very brief look into some reaction conditions of the previously mentioned named reactions. It should be stressed that this is not by any means an all encompassing list. Acid chlorides, for example, are not mentioned in the table. The conversion of an acid chloride to an ester is a very common and effective esterification method. To the best of the author's knowledge, this reaction is not named after anyone, and thus is referred to as alcoholysis of an acid chloride.⁴

Esterification Formula

At this point, we have established esterifications and how to identify them. The last thing that you might need is what these look like in chemical formulas. Unless you are reading papers from the late 19th century, it is unlikely you will see esters written in formula. But who knows, maybe one day you will fall down a rabbit hole of Wikipedia and Google Scholar until you end up reading some old German-written Fischer articles.

Continuing with the previous example we saw of the Fischer-Speier esterification, this reaction can be depicted by chemical formula.

$$ H_3C(CO)OH + HOCH_3 \rightarrow H_3C(CO)OCH_3 + H_2O $$

Typically, this method of depicting reactions is obsolete in academic journals. So, unless you need to learn it for an exam, don't stress too much about learning the exact method to depict things. You will pick up the rest along the way.

Ester Reactions

So far, we've observed that Carboxylic Acids can be converted into esters. However, the reverse reaction can also be done. In fact, esters are converted into Carboxylic Acids in your body every day. Fat molecules are converted into fatty acids and glycerol in an ester hydrolysis reaction.

This reaction is carried out over many steps, using many different enzyme Catalysts. Formally, this reaction is an ester hydrolysis reaction. A hydrolysis reaction is any in which water causes the breaking of bonds. In this reaction, water can be imagined to replace the ester, and then lose one proton to make the carboxylic acid. In reality, it is much more complicated than that, but that's not critical right now.

Reactions using esters represent some very common reactions in Organic Chemistry, and can help achieve many different types of molecules. However, that's boorriinngg. Instead, let's focus on something cool, like making soap!

Saponification

Why soap was originally discovered is something that will never be known. However, it is fairly simple to speculate how it happened. When the ash from burnt wood is mixed with fat molecules, from say an animal, it produces a cleaning agent. Ancient peoples across the globe noticed this at some point and began to utilize this reaction.

From a chemistry perspective, the wood ash contains Lye, which is NaOH, a strong base. When mixed with fat molecules, the hydroxide, ^-OH, can attack the ester molecule in a nucleophilic attack. This is an ester hydrolysis reaction because the ester is being attacked by a mixture of water and hydroxide. The reaction to form soap is very similar to the reaction that occurs in the body. Except, NaOH acts to hydrolyze the ester instead of an enzyme.⁵

We've now taken a very brief, but hopefully, interesting look at esters. The world of esters is a lot larger than what we've seen today. For example, did you know that esters can be used to make more environmentally friendly plastics? Incredible!

Although they didn't know it at the time, the discovery of soap by ancient peoples marked one of mankind's greatest discoveries. Not only has soap saved countless lives, but the discovery of esters has done wonders for our society. Now that you know about esters and esterification, will you try some reactions with esters? Maybe you can try making your own soap!