Aldehydes, ketones, carboxylic acids, and esters. You'll find many of these compounds in things such as perfumes, plants, sweets, your favourite condiments, and even in your body! They have one thing in common - they all contain the carbonyl group.
- This is an introduction to the carbonyl group in organic chemistry.
- We'll start by looking at the carbonyl group, its structure, and its polarity.
- We'll then explore some carbonyl compounds and their properties.
- After that, we'll look at the uses of carbonyl compounds.
What is the carbonyl group?
The carbonyl group is a functional group containing a carbon atom double-bonded to an oxygen atom, C=O. The word 'carbonyl' can also refer to a neutral carbon monoxide ligand bonded to a metal. One example is nickel tetracarbonyl, Ni(CO)4. You'll learn more about ligands in Transition Metals. However, whenever we say 'carbonyl' in the rest of this article, we mean the functional group in organic chemistry: C=O.
Now that we know what the carbonyl group is, let's get straight into its structure and bonding.
The carbonyl group structure
Here is the structure of the carbonyl group:
The carbonyl group. Anna Brewer, Vaia Originals
Let's break this structure down. You'll notice that there is a carbon atom double-bonded to an oxygen atom. You'll also see that there are two R groups. R groups are used to represent the rest of the molecule. For example, they could represent any alkyl or acyl group, or even just a hydrogen atom. The R groups can be the same as each other or completely different.
Why do carbonyl compounds have two R groups? Well, remember that carbon has four electrons in its outer shell, as shown below.
Carbon's outer shell electrons. Anna Brewer, Vaia Originals
To become stable, it wants a full outer shell, which means having eight outer shell electrons. To do this, carbon needs to form four covalent bonds - one bond with each of its outer shell electrons. The C=O double bond takes up two of these electrons. This leaves two electrons, each of which bonds to an R group.
Here's a dot and cross diagram of the covalent bonding in carbonyl compounds. We've shown the carbon atom's outer shell electrons, and the bonded pairs it shares with the oxygen atom and the R groups.
Bonding in the carbonyl group. Anna Brewer, Vaia Originals
Let's look more closely at the C=O double bond. It is made up of one sigma bond and one pi bond.
Sigma bonds are the strongest type of covalent bond, formed by the head-on overlapping of atomic orbitals. These bonds are always the first type of covalent bond found between two atoms.
Pi bonds are another slightly weaker type of covalent bond. They're always the second and third covalent bond found between atoms, formed from the sideways overlap of p-orbitals.
How do sigma and pi bonds form? To understand this, we need to take a deep dive into electron orbitals.
You should know the electron configurations of carbon and oxygen. Carbon has the electron configuration 1s2 2s2 2p2, and oxygen has the electron configuration 1s2 2s2 2p4. These are shown below.
The electron configurations of carbon and oxygen. Anna Brewer, Vaia Originals
To form covalent bonds, carbon and oxygen first need to rearrange their orbitals a little. Carbon first promotes one of the electrons from its 2s orbital into its empty 2pz orbital. It then hybridises its 2s, 2px and 2py orbitals, so that they all have the same energy. These identical hybridised orbitals are known as sp2 orbitals.
Carbon's hybridised orbitals. Anna Brewer, Vaia Originals
The sp2 orbitals arrange themselves at 120° to each other in a trigonal planar shape. The 2pz orbital remains unchanged and positions itself above and below the plane, at a right angle to the sp2 orbitals.
The shape of carbon's orbitals in the carbonyl group. Anna Brewer, Vaia Originals
Oxygen doesn't promote any electrons, but it also hybridises its 2s, 2px and 2py orbitals. Once again, they form sp2 orbitals and the 2pz orbital remains unchanged. But this time, notice that two of oxygen's sp2 orbitals contains two electrons, not just one. These are lone pairs of electrons, which we'll come to later.
Oxygen's hybridised orbitals. Anna Brewer, Vaia Originals
When carbon and oxygen come together to form the carbonyl group, carbon uses its three sp2 orbitals to form single covalent bonds. It forms one covalent bond with each of the two R groups, and one with oxygen's sp2 orbital that contains just one unpaired electron. The orbitals overlap head-on, forming sigma bonds.
To form a double bond, carbon and oxygen now use their 2pz orbitals. Remember that these are found at right angles to sp2 orbitals. The 2pz orbitals overlap sideways, forming another covalent bond above and below the plane. This is a pi bond. We've shown the bonds between oxygen and carbon below.
Sigma and pi bonds between carbon and oxygen in the carbonyl group. Anna Brewer, Vaia Originals
Check out Isomerism for another example of a double bond, this time found between two carbon atoms.
Going back to the carbonyl group structure, we can see that the oxygen atom also has two lone pairs of electrons. These are electron pairs that aren't involved in a covalent bond with another atom. You'll see why they are important later on in the article.
The carbonyl group polarity
You've seen the carbonyl group structure, so we will now explore its polarity.
Carbon and oxygen have different electronegativity values. In fact, oxygen is a lot more electronegative than carbon.
Electronegativity is a measure of an atom's ability to attract a shared pair of electrons.
The difference in each of their electronegativity values creates a partial positive charge in the carbon atom and a partial negative charge in the oxygen atom. This makes the carbonyl group polar. Look at the structure below to see what we mean.
The polarity of the carbonyl group. Anna Brewer, Vaia Originals
The symbol you are seeing, which almost looks like a curly 'S', is thr lowercase Greek letter delta. In this context, δ represents the partial charges of atoms within a molecule. δ+ represents an atom with a partial positive charge, while δ- represents an atom with a partial negative charge.
Because the carbon atom is partially positively charged, it is attracted to negatively charged ions or molecules, such as nucleophiles. Nucleophiles are electron pair donors with a negative or partial-negative charge. This means that many of the reactions involving the carbonyl group are nucleophilic addition reactions. We'll introduce you to some in just a second, but you can also find out more in Reactions of Aldehydes and Ketones.
What are carbonyl compounds?
We've already covered the carbonyl group, its structure, and polarity. So far you have learned that:
The carbonyl group is a functional group with the general formula C=O that is attacked by nucleophiles.
The carbonyl group is composed of a carbon atom double-bonded to an oxygen atom. The oxygen atom forms one sigma bond and one pi bond with the carbon atom. The oxygen atom also has two lone pairs of electrons.
Examples of carbonyl compounds
There are four main examples of carbonyl compounds: aldehydes, ketones, carboxylic acids, and esters.
Aldehydes
What's your favourite perfume brand to wear? Dolce & Gabbana? Coco Chanel? Calvin Klein? Jimmy Choo? Lacoste? Is the list endless? All these fragrant perfumes have one thing in common: they contain compounds called aldehydes.
An aldehyde is an organic compound containing the carbonyl group, with the structure RCHO.
Here's an aldehyde:
The general structure of an aldehyde. Anna Brewer, Vaia Original
If we compare the structure of an aldehyde to the general structure of a carbonyl group compound, we can see that one of the R groups has been replaced by a hydrogen atom. This means that in aldehydes, the carbonyl group is always found at one end of the carbon chain. The other R group can vary.
Examples of aldehydes include methanal. In this aldehyde, the second R group is another hydrogen atom. Another example is benzaldehyde. Here, the second R group is a benzene ring.
Examples of aldehydes. Anna Brewer, Vaia Originals
Aldehydes are formed by the oxidation of a primary alcohol or the reduction of a carboxylic acid. They commonly take part in nucleophilic addition reactions. For example, they react with cyanide ions to form hydroxynitriles and with reducing agents to form primary alcohols. You can find out more about these reactions in Reactions of Aldehydes and Ketones.
Don't know what a primary alcohol is? Check out Alcohols, where all will be explained. You can also find out how primary alcohols are oxidised into aldehydes in Oxidation of Alcohols, and how carboxylic acids are reduced in Reactions of Carboxylic Acids.
We're done with aldehydes for now. Let's move on to some similar molecules, ketones.
Ketones
You can pretty much say that aldehydes and ketones are cousins. The key difference between them is the location of their carbonyl group. In aldehydes, the carbonyl group is found at one end of the carbon chain, giving them the structure RCHO. In ketones, the carbonyl group is found in the middle of the carbon chain, giving them the structure RCOR'.
A ketone is another type of organic compound containing the carbonyl group, with the structure RCOR'.
Here is the general structure of a ketone. Notice how they compare to aldehydes. We already know that in aldehydes, one of the R groups is a hydrogen atom. In ketones, however, both of the R groups are some sort of alkyl or acyl chain.
The general structure of a ketone. Anna Brewer, Vaia Originals
An example of a ketone is propanone. Here, both R groups are a methyl group.
An example of a ketone. Anna Brewer, Vaia Originals
Propanone, CH3COCH3, is the simplest ketone - you can't get any smaller ones. Remember, this is because in ketones, the carbonyl group must be found in the middle of the carbon chain. The molecule must therefore have at least three carbon atoms.
Another key difference between aldehydes and ketones is the way that they are made. Whilst oxidising primary alcohols produces aldehydes, oxidising secondary alcohols produces ketones. Likewise, reducing an aldehyde produces a primary aldehyde, whilst reducing a ketone produces a secondary alcohol. But like aldehydes, ketones also react in nucleophilic reactions. They too react with the cyanide ion to form hydroxynitriles.
Have you ever heard of the keto diet? it involves limiting your intake of carbohydrates, focusing instead on fats and proteins. A lack of sugars in your diet switches your body into a state of ketosis. Instead of burning glucose, your body uses fatty acids as fuel. Some of these fatty acids are switched into ketones, where they circulate in the blood, acting as signalling molecules and sources of energy. The keto diet has been a bit of a craze in the past few years, and some people swear by it for weight loss and overall health. However, researchers are still undecided about whether a state of ketosis is good for us or not.
Carboxylic acid
What do you like to sprinkle your fish and chips with? Some vinegar? A slice of lemon or lime? Ketchup on the side? A dollop of mayonnaise? These condiments all contain carboxylic acids.
A carboxylic acid is an organic compound with the carboxyl functional group, -COOH.
Does the term carboxyl sound familiar? It's a mash-up of the terms carbonyl and hydroxyl. This gives us a clue about the carboxyl functional group: it contains both the carbonyl group, C=O, and the hydroxyl group, -OH. Here's the general structure of a carboxylic acid. Comparing it to the general structure of a carbonyl compound, you can see that one of the R groups has been replaced by a hydroxyl group.
The general structure of a carboxylic acid. Anna Brewer, Vaia Originals
The most common carboxylic acid, found in many of our foods and condiments like ketchup and mayonnaise, is ethanoic acid. Another example is citric acid, found in citrus fruits such as lemons, limes, and oranges. This is a much more complicated carboxylic acid and actually contains three carboxyl groups.
Examples of carboxylic acids. Anna Brewer, Vaia Originals
Carboxylic acids can be produced by oxidising a primary alcohol. For example, if you open a bottle of wine and leave it undisturbed for a while, it will turn sour and acidic. This happens because the alcohol within the wine oxidises into a carboxylic acid.
Like the name suggests, carboxylic acids act like typical acids, although they are only weak ones. They lose hydrogen ions in solution and react with all manner of bases, such as hydroxides and sulphates. They can also be reduced to aldehydes and primary alcohols, and they react with alcohols to form esters. We'll move on to esters next.
Here's a handy diagram showing how you convert between alcohols, aldehydes, ketones, and carboxylic acids.
Converting between alcohols, aldehydes, ketones and carboxylic acids. Anna Brewer, Vaia Originals
You can read more about the reactions that carboxylic acids undergo in Reactions of Carboxylic Acids.
Esters
We mentioned mayonnaise earlier. It is made up of egg yolk, oil, and vinegar. The vinegar contains carboxylic acids, but right now, we're more interested in the oil and egg yolk. They contain triglycerides, which are a type of ester.
An ester is an organic compound with the general formula RCOOR'.
Take a look at the structure of an ester, shown below. Like all of the molecules we have looked at so far, they're a type of carbonyl compound. But notice the position of the carbonyl group. On one side it is bonded to an R group. On the other side, it is bonded to an oxygen atom. This oxygen atom is then bonded to a second R group.
The general structure of an ester. Anna Brewer, Vaia Originals
Some of the most common esters include ethyl ethanoate, ethyl propanoate and propyl methanoate. They typically have fruity smells and are used as flavourings in foods or scents in perfumes.
The structure of ethyl ethanoate. Image credits: commons.wikimedia.org
Don't worry about naming esters for now - Esters has it in much more depth. But if you are interested, the first part of the name is derived from the alcohol used to make the ester, whilst the second part of the name comes from the carboxylic acid. To illustrate, methyl ethanoate is made from methanol and ethanoic acid.
Esters are produced in an esterification reaction between a carboxylic acid and an alcohol. The reaction also produces water. They can be hydrolysed back into a carboxylic acid and an alcohol using a strong acid catalyst.
Esterification and ester hydrolysis are two sides of the same reversible reaction. Head over to Reactions of Esters to find out how we favour one or the other.
Acid derivatives
The final group of compounds we'll look at today are known as acid derivatives. As the name suggests, these are molecules related to carboxylic acids.
Acid derivatives are molecules based on carboxylic acids, where the hydroxyl group has been replaced by another atom or group, Z. They have the formula RCOZ.
Here's their general structure.
The general structure of an acid derivative. Anna Brewer, Vaia Originals
For example, acyl chlorides have a chlorine atom as their Z group. Here's an example, ethanoyl chloride.
An example of an acid derivative. Anna Brewer, Vaia Originals
Acid derivatives are useful because they are much more reactive than carboxylic acids. This is because the hydroxyl group is a poor leaving group - it would much rather stay a part of the carboxylic acid. However, chlorine is a better leaving group. This allows acid derivatives to react with other molecules, and results in adding the acyl group to another compound. This is known as acylation.
The acyl group is a type of carbonyl group, RCO-. It is formed when you remove the hydroxyl group from a carboxylic acid. You can find out more about acylation and acid derivatives in Acylation.
Comparing carbonyl compounds
That's it for the carbonyl compounds! To help you compare them, we've made a handy table summarising their structures and formulae.
Carbonyl compound | General formula | Structure |
Aldehyde | RCHO | 
|
Ketone | RCOR' | 
|
Carboxylic acid | RCOOH | 
|
Ester | RCOOR | 
|
Acid derivative | RCOZ | 
|
Properties of carbonyl compounds
Wondering how the carbonyl group affects the properties of carbonyl compounds? We'll explore that now. Of course, properties vary from compound to compound, but this is a good overview of some of the trends you'll see. But in order to understand the properties of carbonyl compounds, we need to remind ourselves of two important facts about the carbonyl group.
- The carbonyl group is polar. In particular, the carbon atom is partially positively charged and the oxygen atom is partially negatively charged.
- The oxygen atom contains two lone pairs of electrons.
Let's see how that affects the properties of carbonyl compounds.
Melting and boiling points
Carbonyl compounds have higher melting and boiling points than similar alkanes. This is because they are polar molecules and so they all experience permanent dipole-dipole forces. In contrast, alkanes are nonpolar. They only experience van der Waals forces between molecules, which are much weaker than permanent dipole-dipole forces and are easier to overcome.
Carboxylic acids in particular have very high melting and boiling points. This is because they contain the hydroxyl functional group, -OH, so adjacent molecules can form hydrogen bonds. These are the strongest type of intermolecular force and require a lot of energy to overcome.
Solubility
Short-chain carbonyl compounds are soluble in water. This is because the carboxyl group contains an oxygen atom with lone pairs of electrons. These lone pairs of electrons can form hydrogen bonds with water molecules, dissolving the substance. However, longer-chain carbonyl compounds are insoluble in water. Their nonpolar hydrocarbon chains get in the way of the hydrogen bonding, disrupting the attraction, and prevent the molecule from dissolving.
Hydrogen bonding between carbonyl compounds and water. Anna Brewer, Vaia Originals
Uses of carbonyl compounds
Our final topic today will be the uses of carbonyl compounds. We've already mentioned a few, but we'll go over them again and throw in some new ones as well.
- Carbonyl compounds are found in many foods and drinks, from the carboxylic acid in vinegar and the triglycerides in oils to the esters used as flavourings in your favourite sweet treats.
- Propanone is a common solvent and the main ingredient in most nail polish removers and paint thinners.
- Many hormones are ketones, such as progesterone and testerone.
- The aldehyde methanal, also known as formaldehyde, is used as a preservative and to make resins.
By now you should have a good understanding of the carbonyl group and its related compounds, and with any luck, you'll be wanting to learn more. Check out the articles we linked to above to find out more, from esterification and acylation to intermolecular forces and pi and sigma bonds.
Carbonyl Group - Key takeaways
- The carbonyl group is a functional group containing an a carbon atom double-bonded to an oxygen atom, C=O.
- Carbonyl compounds have the structure RCOR'.
- The carbonyl group is polar and the oxygen atom contains two lone pairs of electrons. Because of this, carbonyl compounds can form permanent dipole-dipole forces with each other and hydrogen bond to water.
- Carbonyl compounds often take place in nucleophilic addition reactions.
- Examples of carbonyl compounds include aldehydes, ketones, carboxylic acids, esters, and acid derivatives.
- Carbonyl compounds have high melting and boiling points and short-chain carbonyl compounds are soluble in water.
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