Have you ever wondered what connects soap, biodiesel, plastics, and perfumes? Well, it turns out they all have something in common - esters! Esters are organic molecules that come from carboxylic acids and alcohols. In this article, we'll explain what esters are and how they work. First, we'll define what esters are and how they're named. Then, we'll dive into some of their properties. Finally, we'll briefly touch on the reactions esters are involved in. So, let's get started and learn more about these fascinating molecules! And, if you're interested in organic chemistry or just want to impress your friends, this article is for you.

How do you make esters?

To understand how esters are formed, let's first recall that carboxylic acids have a functional group called the carboxyl group, which is represented by the chemical formula COOH. Carboxylic acids have a general formula of RCOOH, where the R group represents any carbon-based chain. When an alcohol interacts with a carboxylic acid, the hydrogen atom in the carboxyl group is replaced by the R group of the alcohol. The resulting molecule is an ester, with the general formula RCOOR' and the functional group COO. This reaction is known as esterification and is how esters are formed.

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To create esters, we use a process called esterification. This involves reacting a carboxylic acid with an alcohol in the presence of a catalyst, which produces water as a byproduct. In the lab, you can make esters by warming a mixture of carboxylic acid, alcohol, and a few drops of sulfuric acid in a water bath. Once the reaction is complete, pour the mixture into a beaker of water, and you'll see the ester float on top of the water while the remaining acid and alcohol dissolve. Esters often have a pleasant aroma, which is why they are used in perfumes, toiletries, and household cleaners, as well as in food flavorings. For example, ethyl ethanoate has a pear-like scent, which is why it is used in hard-boiled sweets. You'll learn more about how esters react and their properties in the Reactions of Esters section.

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You can see from the molecule above that esters contain one R group with the carbonyl functional group, , and one R group that is simply a hydrocarbon chain. The R group containing the  bond comes from the carboxylic acid whereas the other R group comes from the alcohol.

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Naming esters

As we found out above, esters are made from two different molecules: a carboxylic acid and an alcohol. Their names come in two parts and reflect these two molecules.

The first part of the name comes from the alcohol. You name it just like how you name hydrocarbon side chains in other organic molecules. It uses the alcohol’s root name to show its length and ends in the suffix -yl. The second part of the name comes from the carboxylic acid. It again uses the carboxylic acid’s root name to show its length and ends in the suffix -oate. Even if you don’t know which carboxylic acid or alcohol an ester is made from, you can easily work out its name by counting the carbons in its R group chains. Let’s look at a few examples together.

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Look at the molecule above. The R group on the left contains the carbonyl group. It must come from a carboxylic acid, so it ends in the suffix -oate. If we count the carbons in its chain, we can see that it has two. Therefore we can give it the root name -eth-. If we put the root name and suffix together, we get ethanoate.

However, that is only half of the story! The R group on the right must come from an alcohol. It has just one carbon in its carbon chain, so we call it methyl. Combining the two names gives us methyl ethanoate. Methyl ethanoate. The methyl part of the name comes from the R group circled in green whilst the ethanoate part comes from the R group circled in red.

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Remember to include the carbon that is part of the C=O double bond when counting your carbon chain length.

Here is another example.

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The R group on the left contains the carbonyl functional group, C=O, and so must come from a carboxylic acid. It has three carbon atoms in its carbon chain, so this part of the ester’s name is propanoate. The R group on the right comes from an alcohol and has four carbons in its chain, giving it the name butyl. We call this molecule butyl propanoate.

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When drawing esters, we typically place the R group derived from the carboxylic acid on the left and the R group from the alcohol on the right. However, when naming esters, we always name the R group derived from the alcohol first. To name an ester, we must first number the carbons in the carbon chain backbone, always labeling the carbons that form the ester bond as 1. In the molecule given, the methyl group is attached to carbon 3 in the right-hand R group, and the chlorine atom is attached to carbon 3 in the left-hand R group. Therefore, the name of this molecule would be 3-methylbutyl 3-chloropropanoate. Remember that being able to name and draw organic molecules is essential in organic chemistry, as it allows us to understand their structures and properties, and to predict their behavior in different reactions.

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Instead of saying ethanoate, scientists frequently use the name acetate when talking about esters based on ethanoic acid. For example, methyl ethanoate is commonly called methyl acetate.

Properties of esters

Let’s revisit the general structure of an ester.

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It contains a carbonyl group, the double bond. This group is polar. Oxygen is much more electronegative than carbon and so attracts the shared pairs of electrons in the double bond towards itself, becoming partially negatively charged and leaving carbon partially positively charged. We represent this using the delta symbol, δ. This forms a permanent dipole. Because of this, esters experience permanent dipole-dipole forces between molecules. The single  bond in an ester is also polar. These polar bonds influence the properties of the molecule. Its overall polarity is shown below.

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Melting and boiling points

Esters have intermediate melting points between alkanes and alcohols/carboxylic acids due to their permanent dipole-dipole forces between molecules. Longer chain esters have higher melting and boiling points than shorter chain esters due to stronger van der Waals forces between molecules, while branched esters have lower melting points due to less efficient packing. The boiling points of esters increase with molecular mass. Solubility of esters in water depends on their chain length. Shorter chain esters can form hydrogen bonds with water molecules and are therefore soluble, while longer chain esters are not soluble due to non-polar hydrocarbon chains interfering with hydrogen bonding. It is important to understand the physical properties of esters, as they play a significant role in their uses in industries such as perfumes and flavorings.

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Esters have a wide range of uses, including as perfumes, plastics, flavorings, biodiesel, plasticisers, and solvents. They can undergo acid hydrolysis, base hydrolysis (saponification), and polymerization reactions to form carboxylic acids, alcohols, carboxylic acid salts, and polyesters. Understanding esters and their reactions is important in various industries, including the fragrance, food, and biofuels industries. Key takeaways include that esters have the general formula RCOOR', contain polar groups, and experience permanent dipole-dipole forces between molecules. Short-chain esters are soluble in water, and esters have intermediate melting and boiling points due to their molecular structure and intermolecular forces.

Esters

What is an ester?

An ester is an organic molecule derived from a carboxylic acid and an alcohol. It contains the -COO- functional group.

How do you name esters?

You name esters in two stages according to the alcohol and carboxylic acid they are derived from. The first part of the name comes from the alcohol and always ends in -yl. The second part of the name comes from the carboxylic acid and ends in -oate. For example, the carboxylic acid made from methanol and ethanoic acid has the name methyl ethanoate.

What are esters used for?

Esters are commonly used as flavourings in foods like sweets, or in perfumes and cosmetics. They also react in a polymerisation reaction to make polyester plastics. Hydrolysing an ester using a base produces soap. Biodiesel is a type of ester made by reacting vegetable oils with an alcohol such as methanol, and is a renewable fuel source.

How are esters formed?

Esters are formed in a condensation reaction between an alcohol and a carboxylic acid. This needs a strong acid catalyst and is a reversible reaction.

What type of polymers have the ester functional group?

Polyesters are polymers with the ester functional group.

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