HS Chemistry - Carbonyl Functional Groups

Esters

Overview of The Page

This page will cover:

What are esters?
How are esters formed?
How are esters named?
What reactions do esters participate in?

Carbonyl functional groups are functional groups that contain a C=O bond $i.e. there is at least one instance of a Carbon atom double-bonded to an Oxygen atom$ . This does not mean, however, that all C=O bonds are carbonyl bonds. CO₂, for example, has the structural formula O=C=O, but the C=O bonds are not considered carbonyl because it is not an organic compound. Esters are a type of carbonyl functional group, and this subpage will look at them.

Esters, which have sweet smells and are found in fruits, are not too different from carboxylic acids. In carboxylic acids, the C=O is single-bonded to one non-Hydrogen atom and single-bonded to a hydroxyl group $OH$ , and is found at the end of the organic molecule's main chain. In esters, the C=O is single-bonded to a non-Hydrogen atom before the C=O group, and single-bonded to another Oxygen atom, which is then bonded to the rest of the organic molecule. Therefore, ester functional groups are not found at the end of the organic molecule's main chain.

The formula for the ester functional group is R₁COOR₂, where R₁ represents the rest of the organic molecule before the ester group, and R₂ represents the rest of the organic molecule after the ester group.

Esters are named differently, and to understand how they're named, it's helpful to look at how they're made.

Esterification

An ester is usually only formed in one way: by reacting a carboxylic acid with an alcohol. This reaction requires an acid as a catalyst to initiate the reaction - concentrated H₂SO₄ is commonly used for this purpose. As the reactants must be heated for a while in order for the reaction to proceed, an esterification reaction is carried out in a reflux apparatus to prevent the reactants from evaporating.

An alcohol contains a hydroxyl $OH$ group, while a carboxylic acid contains a COOH group. This is crucial to the reaction mechanism for esterification, which is known as the Fisher esterification mechanism:

A dissociated H⁺ ion in solution attacks the COOH group's C=O bond. The structural formula of the carboxylic acid is R₁COOH, where R₁ represents the rest of the carboxylic acid molecule.
The double bond is broken as the Hydrogen ion forms a covalent bond with the double-bonded Oxygen atom, forming a hydroxyl group. This leaves the Carbon atom as a carbocation. There are now two hydroxyl groups bonded to the Carbon atom. The structural formula at this point is R₁COHOH⁺.
Oxygen is more electronegative than Carbon or Hydrogen, so the Oxygen atom in the alcohol's hydroxyl group has a slight negative charge. This is attracted to the carbocation, and the alcohol forms a covalent bond with the Carbon atom. There are now three hydroxyl groups bonded to the Carbon atom. The structural formula of the organic molecule at this point is R₁COHOHOHR₂⁺, where R₂ represents the rest of the alcohol molecule.

The Oxygen atom in the newly attached hydroxyl group is now bonded to three atoms - one too many. The positive charge of the Carbon atom is transferred to the least electronegative atom, Hydrogen, in the newly attached alcohol, and the Hydrogen atom, now an H⁺ ion, splits off the hydroxyl group. There are now two hydroxyl groups bonded to the Carbon atom. The structural formula at this point is R₁COHOHOR₂.
The H⁺ ion that split off attaches to the other hydroxyl group, forming a covalent bond with it and transferring its positive charge to the Oxygen. The structural formula at this point is R₁COH₂OHOR₂⁺.
The Oxygen atom transfers its positive charge to the Carbon atom and splits off with two Hydrogen atoms bonded to it. Thus, H₂O is formed. There is now one hydroxyl group bonded to the Carbon atom. The structural formula of the organic molecule at this point is R₁COHOR₂⁺, but we also have H₂O as a product now.
The Carbon atom forms a double-bond with the Oxygen atom in the hydroxyl group, and the Hydrogen atom leaves as an H⁺ ion. There are now no hydroxyl groups bonded to the Carbon atom. The structural formula of the organic molecule is now R₁COOR₂, which is the structural formula of an ester. However, we also have a free H⁺ ion, to account for the H⁺ ion that initiated the reaction. Therefore, the H⁺ ion from the acidic solution was a catalyst.

Since the reaction of an alcohol and a carboxylic acid yielded the products of an ester and water, esterification is a condensation reaction.

The structural formula of the ester $R~1~COOR~2~$ is not too different from the structural formula of the carboxylic acid $R~1~COOH$ . Both have the carboxylate section $R~1~COO$ . Carboxylates have the suffix "-oate", and the parent part of the name indicates the number of Carbon atoms present in the main chain of the carboxylate. Since esters contain a carboxylate section, they too follow this naming convention. Esters have the suffix "-oate", and the parent part of the name indicates the number of Carbon atoms present in the main chain of the carboxylate.

The carboxylic acid group takes precedence over the alcohol group, so even if the alcohol has a longer main chain, the parent part of the ester's name will still indicate the number of Carbon atoms that were present in the main chain of the carboxylic acid reactant. For example, if we reacted ethanol and butanoic acid together to form an ester, the parent portion of our ester's name would come from the carboxylic acid, and the ester's name's parent portion would be "but-". If we reacted 1-butanol and ethanoic acid together to form an ester, the parent portion of our ester's name would be "eth-", as it would still come from the carboxylic acid, even if the alcohol had a longer main chain.

Therefore, the part of the ester that came from the alcohol must be noted in the prefix. To name this part, we name it as though it were a side chain, based on how many Carbon atoms are in this part $methyl\-, ethyl\-, etc.$ . The name is determined based on how many Carbon atoms are in this part of the ester $the part after the Oxygen atoms$ , not the whole ester. For example, if we had two Carbon atoms after the Oxygen atoms, this part would be named ethyl-. If there are any other groups attached to this part of the molecule, they are also named $and numbered$ in the prefix, but their numbering starts from the Carbon atoms after the Oxygen atoms, not from the beginning of the ester.

Thus, if we reacted propan-1-ol with propanoic acid:

CH₃CH₂COOH + CH₃CH₂CH₂OH → CH₃CH₂COOCH₂CH₂CH₃ + H₂O

Reacting propan-1-ol with propanoic acid

The products would be propyl propanoate and H₂O.

The alcohol used doesn't need to contain only one functional group. For example, if we reacted 3-bromopropan-1-ol with butanoic acid:

CH₃CH₂CH₂COOH + CH₂BrCH₂CH₂OH → CH₃CH₂CH₂COOCH₂CH₂CH₂Br + H₂O

Reacting 3-bromopropan-1-ol with butanoic acid

The products would be 3-bromopropyl butanoate and H₂O.

We don't have to use primary alcohols in esterification either. For example, if we reacted the secondary alcohol propan-2-ol with butanoic acid:

Reacting propan-2-ol with butanoic acid

The products would be isopropyl butanoate and H₂O. It's necessary to name the prefix isopropyl because of the naming conventions for alkyl side chains covered in Alkanes. Notice how the Oxygen atom bonds to the Carbon atom not in a straight line, but where the hydroxyl group previously was. This pattern that holds true for all secondary and tertiary alcohols. It also holds true for primary alcohols, but since in primary alcohols the hydroxyl group is at the end of the chain, we simply see the rest of the alcohol added in a straight line.

As an example of what the skeletal formula of an ester would be, here's the skeletal formula for propyl propanoate:

Skeletal formula for propyl propanoate

Other Reactions

Esters can be formed through the condensation reaction of esterification, where an alcohol and a carboxylic acid react to produce an ester and water. It follows that if we react an ester with water, we would be able to reverse the reaction. This is the hydrolysis reaction for esters.

In order to undergo hydrolysis, esters must be heated with an acidic or alkaline solution in reflux. Heating esters with water in an acidic solution in reflux yields an alcohol and carboxylic acid, as the acid acts as a catalyst. But if the mixture continues to be heated, the alcohol and carboxylic acid undergo esterification, yielding an ester and H₂O. Thus, when an acid is used as a catalyst, the reaction is reversible.

However, when a base is used, the reaction is irreversible - the products after the ester undergoes hydrolysis cannot be reacted with each other to yield an ester. The reason for this is because when a base is used, a carboxylate salt and an alcohol are produced, rather than a carboxylic acid and an alcohol. Since the base contains a hydroxide $OH^\-^$ ion, the OH^- ion for the hydrolysis reaction comes from the base rather than from the water. Then, the base's cation reacts with the carboxylate ion to produce a carboxylate salt instead of carboxylic acid. Since a carboxylic acid is not produced in the reaction, it cannot occur in reverse.

The hydrolysis of esters using a base is also referred to as saponification, which was used to make soap molecules.