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Esterification

What do you think is the most important chemical reaction utilized by humans? The fermentation process to make certain foods and Alcohols? The production of ammonia fertilizers which support our current population? How about the reaction which makes soap? It is hard to say how many lives the use of soap has saved, but it is estimated to be in the hundreds of millions. Like many groundbreaking chemical discoveries, soap was discovered accidentally by ancient civilizations thousands of years ago.1 But, how does this reaction work, and why was it discovered? This reaction is known as saponification, and it is a reaction which deals with Esters. Here you will learn all about esters and esterification. So, stick around, and we'll "clean up" any unawareness you have about esterification!

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Esterification

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What do you think is the most important chemical reaction utilized by humans? The fermentation process to make certain foods and Alcohols? The production of ammonia fertilizers which support our current population? How about the reaction which makes soap? It is hard to say how many lives the use of soap has saved, but it is estimated to be in the hundreds of millions. Like many groundbreaking chemical discoveries, soap was discovered accidentally by ancient civilizations thousands of years ago.1 But, how does this reaction work, and why was it discovered? This reaction is known as saponification, and it is a reaction which deals with Esters. Here you will learn all about esters and esterification. So, stick around, and we'll "clean up" any unawareness you have about esterification!

  • Today we will start by discussing what Esters are and what an esterification is.
  • Then, we will look at the specifics of some esterification reactions.
  • Finally, we will assess some other reactions with esters; namely the making of soap.

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.

An ester is an organic molecule which has an alkoxy group bonded to a carbonyl functionality.

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.

Esterification is a reaction which results in the formation of an ester.

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.

An acid chloride is similar to a carboxylic acid, but instead of an –OH group bonded to the carbonyl, a –Cl group is attached. Acid chlorides are typically more reactive than Carboxylic Acids. This is because chloride is a much better leaving group than hydroxide.

In an esterification reaction, a carbonyl Carbon is attacked by a nucleophile, causing one of the other substituents to trade places with it. A group which is more stable on its own will leave faster. Chloride anions are more stable in solution than hydroxide anions are, which makes the acid chloride more reactive than the acid. This is due to the size of the leaving groups, and their ability to balance the negative charge.

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.

In some iterations of esterification reactions, it was mentioned that Lewis acids can be used as Catalysts, instead of Brønsted acids. Lewis acids increase the reactivity of the carbonyl moiety by lowering the energy of the LUMO (lowest unoccupied molecular orbital). This makes them more susceptible to attack.

Since carbonyls are electrophiles, a nucleophile will donate electron density into the lowest energy orbital. If the orbital has its energy lowered more by the Lewis acid, it makes it more reactive. This is because the orbital gap between the nucleophile and the electrophile are narrowed. For a brief look at these orbitals, take a look at Molecular Orbital Theory.

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.

Reaction
Starting Material
Catalyst
Product
Fischer-Speier Esterification
Carboxylic Acid
H+ or Lewis Acid
Ester
Shiina Esterification
Carboxylic Acid
Acid or Base
Ester
Steglich Esterification
Carboxylic Acid
DMAP*
Ester
Yamaguchi Esterification
Carboxylic Acid
DMAP*
Ester

* 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

The resulting molecule acts as soap, and is very efficient at interacting with both polar and non-polar molecules. This particular soap molecule has a 9-carbon chain. However, typical consumer soaps have 12 - 18 carbons in their non-polar chain.

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!

Esterification - Key takeaways

  • Today we learned that esters consist of a carbonyl group, C=O, with an alkoxy group O–R, bonded to it.
  • The esterification reaction involves formation of an ester from an alcohol and a carbonyl.
  • Typical esterificationcatalysts are acids or bases, depending on the reagents.
  • Esters can go through reactions such as ester hydrolysis, which is the splitting of an ester molecule.
  • Saponification is the formation of soap by splitting an ester molecule.

References
  1. Cody Cassidy, Who Discovered Soap? What to Know About the Origins of the Life-Saving Substance, Time, 2020.
  2. John McMurry, Organic Chemistry, 9th ed., 2016.
  3. Emil Fischer & Arthur Speier, Darstellung der Ester, Untersuchungen aus Verschiedenen Gebieten, pp. 285 - 291.
  4. Johnathan Clayden; Nick Greeves; Stuart Warren, Organic Chemistry, 2nd ed., 2012.
  5. David Klein, Organic Chemistry, 3rd ed., 2017.

Frequently Asked Questions about Esterification

Esterification is a reaction which forms an ester, typically from a primary alcohol and a carboxylic acid. This can be catalyzed with acids or bases. A common example is the Fischer-Speier esterification.

An esterification reaction is one where an ester is formed. This typically forms from an alcohol and a carboxylic acid or acid chloride.

Esterification can be catalyzed by strong Brønsted acids or bases. There are also some methods which use Lewis acids to catalyze this reaction.

To form an ester, a carbonyl must already be present. This typically includes a carboxylic acid or an acid chloride. If the carboxylic acid is acting as a nucleophile, then a carbon electrophile is required. If the carbonyl is acting as the electrophile, a primary alcohol is required.

The purpose of esterification is to form ester molecules, which have many applications. They can be used to form plastics, soaps, or pharmaceutical drugs.

Test your knowledge with multiple choice flashcards

Which description describes an ester?

What is an esterification reaction?

What happens to triacylglycerols in the body?

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