Carbonyl Group
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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.
The carbonyl group structure
The carbonyl group
Why do carbonyl compounds have two R groups? Well, remember that carbon has four electrons in its outer shell, as shown below.
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
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
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
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
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
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
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 polarity of the carbonyl group
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?
If you’re interested in chemistry, you might have heard of the carbonyl group. It’s a special group of atoms that has a carbon atom and an oxygen atom joined together by a double bond, which looks like C=O. This group is important because it can react with other molecules called nucleophiles.
Sometimes, people use the word ‘carbonyl’ to refer to a different type of molecule that has a carbon atom and a single oxygen atom joined to a metal atom. But for now, we’re only talking about the C=O group in organic chemistry. The carbon atom in the carbonyl group is attached to two other groups, which can be different types of atoms and molecules. This affects the way the carbonyl group behaves in chemical reactions. Because oxygen is more electronegative than carbon or hydrogen, the oxygen atom in the carbonyl group pulls electrons away from the carbon atom. This creates a partial positive charge on the carbon atom and a partial negative charge on the oxygen atom, making the carbonyl group an attractive target for nucleophiles.
Aldehydes
The general structure of an aldehyde
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
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.
Ketones
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
An example of a ketone
Propanone, also known as acetone, is the simplest ketone you can find. It’s made up of three carbon atoms and a carbonyl group that sits in the middle of the carbon chain. Ketones are different from aldehydes in that they must have at least three carbon atoms to contain the carbonyl group in the middle of the carbon chain. They are formed when secondary alcohols are oxidised.
Ketones can also react with nucleophiles, just like aldehydes. For example, they can react with the cyanide ion to form hydroxynitriles.
Have you heard of the keto diet? It’s a diet that focuses on fats and proteins instead of carbohydrates. By limiting your intake of sugars, your body goes into a state of ketosis, where it uses fatty acids as fuel instead of glucose. Some people swear by it for weight loss and overall health, but researchers are still unsure about whether it’s good for us or not. Carboxylic acids are organic compounds that contain the carboxyl functional group, -COOH. The name comes from the combination of carbonyl and hydroxyl groups. This functional group contains both a carbonyl group, C=O, and a hydroxyl group, -OH. The general structure of a carboxylic acid contains this functional group, with one of the R groups replaced by a hydroxyl group.
The general structure of a carboxylic acid
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
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
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
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
Esters are a type of organic molecule that are formed through an esterification reaction between a carboxylic acid and an alcohol. The first part of the name of an ester is derived from the alcohol used to make it, while the second part comes from the carboxylic acid used. For example, methyl ethanoate is made from methanol and ethanoic acid. During the esterification reaction, water is also produced as a byproduct. Esters can be hydrolysed back into their original components, a carboxylic acid and an alcohol, using a strong acid catalyst. This process is called ester hydrolysis and is the reverse of esterification. The direction of the reaction can be influenced by changing the concentration of reactants or products, the temperature, or the presence of a catalyst. While esterification and ester hydrolysis are reversible reactions, they are often used for different purposes in chemistry. For example, esterification is commonly used in the production of fragrances, flavors, and plastics, while ester hydrolysis is used to break down fats and oils during digestion.
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
For example, acyl chlorides have a chlorine atom as their Z group. Here’s an example, ethanoyl chloride.
An example of an acid derivative
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.
Properties of carbonyl compounds
The carbonyl group has a significant effect on the properties of carbonyl compounds. One major property is polarity. The carbonyl group is polar due to the difference in electronegativity between the carbon and oxygen atoms. As a result, the carbon atom in the carbonyl group carries a partial positive charge, while the oxygen atom carries a partial negative charge. This polarity affects the physical properties of carbonyl compounds, such as their boiling and melting points, solubility, and reactivity.
Another important property of carbonyl compounds is their ability to form hydrogen bonds. The oxygen atom in the carbonyl group has two lone pairs of electrons, which can form hydrogen bonds with other molecules containing hydrogen atoms. This ability to form hydrogen bonds makes carbonyl compounds more soluble in water than similar compounds that do not contain carbonyl groups. Carbonyl compounds also have unique reactivity due to the presence of the carbonyl group. For example, the carbonyl group can undergo nucleophilic addition reactions, where a nucleophile attacks the carbonyl carbon and forms a new bond. This makes carbonyl compounds useful in many organic synthesis reactions. In summary, the carbonyl group in carbonyl compounds contributes to their polarity, ability to form hydrogen bonds, and unique reactivity. These properties make carbonyl compounds important in many chemical reactions and applications.
Melting and boiling points
Carbonyl compounds also exhibit unique solubility properties. Short-chain carbonyl compounds, such as aldehydes and ketones, are typically soluble in water due to the presence of the carbonyl group, which contains an oxygen atom with two lone pairs of electrons. These lone pairs can form hydrogen bonds with water molecules, allowing the carbonyl compound to dissolve.
However, longer-chain carbonyl compounds, such as carboxylic acids and esters, are generally insoluble in water. This is because their nonpolar hydrocarbon chains interfere with the formation of hydrogen bonds between the carbonyl compound and water molecules. Instead, these compounds tend to dissolve in nonpolar solvents, such as organic solvents like ethanol or acetone. Carboxylic acids are a notable exception to this trend. Despite their longer carbon chains, carboxylic acids are generally soluble in water due to the presence of the hydroxyl (-OH) functional group. This group can form strong hydrogen bonds with water molecules, which outweigh the effects of the nonpolar hydrocarbon chain. In summary, the solubility of carbonyl compounds depends on the length of their carbon chains and the presence of functional groups that can participate in hydrogen bonding. Shorter carbonyl compounds are typically soluble in water, while longer ones tend to be insoluble or only partially soluble. Carboxylic acids are an exception, as they are generally soluble in water due to the presence of the hydroxyl functional group.
Hydrogen bonding between carbonyl compounds and water
Carbonyl compounds have a wide range of uses in various industries. Some of the main uses of carbonyl compounds are:
- Solvents: Carbonyl compounds, such as propanone (acetone), are commonly used as solvents in many industries, including pharmaceuticals, cosmetics, and paints.
- Food and flavorings: The carbonyl group is a common feature in many food and drink compounds, such as carboxylic acids and esters, which are used as flavorings.
- Pharmaceuticals: Many hormones, such as progesterone and testosterone, are ketones. Carbonyl compounds are also used as intermediates in the synthesis of many pharmaceuticals.
- Polymers and resins: Carbonyl compounds, such as formaldehyde, are used in the production of polymers and resins.
- Agriculture: Carbonyl compounds, such as herbicides and insecticides, are used in agriculture to protect crops from pests and diseases.
In conclusion, the carbonyl group and its related compounds have many important applications in various industries. Their unique properties, such as high melting and boiling points and solubility, make them useful in a range of applications from solvents and flavorings to pharmaceuticals and agriculture.
Carbonyl Group
How do you identify a carbonyl group?
You can identify a carbonyl group by drawing out the molecule. The carbonyl group contains an oxygen atom joined by a double bond to a carbon atom. If you see that anywhere in your diagram, you know that you’ve got a carbonyl compound.
What are the properties of the carbonyl group?
The carbonyl group is polar. This means that carbonyl compounds experience permanent dipole-dipole forces between molecules. The oxygen atom in the carbonyl group also has two lone pairs of electrons. This means that it can form hydrogen bonds with water. Because of this, short-chain carbonyl compounds are soluble in water.
What is a carbonyl group?
The carbonyl group consists of an oxygen atom joined to a carbon atom with a double bond. It has the formula C=O.
Which action could produce a carbonyl group?
We can produce the carbonyl group by oxidising alcohols. Oxidising a primary alcohol produces an aldehyde whilst oxidising a secondary alcohol produces a ketone.
