Carbonyl Functional Group

Introduction to the Carbonyl Group

The carbonyl functional group is a carbon atom double-bonded to an oxygen atom. The image below highlights (in red) the carbonyl group structure.

Carbonyl functional group highlighted in red

The other two bonds coming off carbon (label R and R’ in this case), can be any atom. The carbonyl part is just the carbon double bonded to oxygen (seen in red). Depending on what the -R groups are, it may be another functional group that contains a carbonyl within it. These functional groups are listed in a section below.  

Carbonyl Functional Group Details

The oxygen atom in the structure has two lone pairs. This makes the group more reactive than a regular carbon-carbon double bond.

The electrons in the double bond tend to be towards the oxygen due to oxygen’s higher electron affinity. Therefore, the structure is sometimes drawn as having the following resonance structures.

Resonance structures of carbonyl

In the above case, the carbon has a partial positive charge, while the oxygen carries a partial negative charge.

The average double bond length in the carbonyl is about 1.2 angstroms. The bond will get longer as polarity decreases. The polarity changes based on the other bonds the carbon has.

In an IR spectrum, carbonyl groups stand out. The carbonyl functional group peak is around 1900-1600cm-1. This section of the spectrum does not have many other peaks. The carbonyl also tends to have a very strong peak. Therefore, IR spectroscopy readily identifies and quantifies carbonyl peaks. It can also help to identify carbonyl peaks within other functional groups.

Carbonyl vs Ketone vs Aldehyde vs Others

Aldehydes and ketones are functional groups that contain a carbonyl. The main difference is carbon has to be bonded to certain groups besides double bonded to the oxygen to be one of these functional groups.

To be a ketone, the carbonyl carbon is bonded to two carbon-containing substituents. These can be the same or different. Below you see an example of a ketone.  R and R’ are carbon-containing and maybe the same or different from each other. The simplest ketone is acetone. In the case of acetone, both R and R’ are methyl (CH3) groups.

Ketone carbonyl containing

To be an aldehyde, the carbonyl carbon is bonded to a hydrogen and another group. In the molecule shown below, R can be anything.

carbonyl containing aldehyde

Ketones and aldehydes will react similarly to each other due to having similar structures.

Carbonyl Group Reactions

The carbonyl functional group can undergo many reactions. Generally, nucleophilic reagents are attracted to the carbon atom. The electrophilic reagents are attracted to the oxygen atom.

Often acids will react with the carbonyl oxygen atom to stabilize the partial negative charge that is present on it.

When a nucleophile reacts with the carbon in the carbonyl functional group, there is often no leaving group. Instead, electrons in the double bond (pi bond) push up to the oxygen atom. This is called a nucleophilic addition reaction.

The exact reactions that occur will depend on what other R groups are attached to the carbonyl carbon. The larger the R group, the more steric hindrance will slow down or prevent a reaction. For example, there is more steric hindrance in a ketone than an aldehyde.

The R group present will also affect the polarity of the bond. The polarity will also influence the reactivity of the molecule.

Many named reactions involve a carbonyl group. A select few are listed below.

  • Aldol Reaction: The Aldol reaction creates carbon-carbon bonds. A type of this is the Michael Addition.
  • Grignard Reaction: Forms a secondary or tertiary alcohol from a reaction with a ketone or aldehyde
  • Nozaki-Hiyama Coupling: Couples an aldehyde to a halide-containing compound. Chromium salts catalyze the reaction.
  • Wittig Reaction: A aldehyde or ketone reacts with a ylide to form an alkene

Carbonyl Compounds

Here are some of the other functional groups that include a carbonyl group within them

  • Ketone (R and R’ = carbon-based substituents)
  • Aldehyde
Carboxylic acid

Listed below are some carbonyl examples in common molecules.

  • Formaldehyde
  • Acetone
  • Carbon Dioxide
Carbon dioxide contains two carbonyls
  • Acetic Acid (Vinegar)
Acetic acid or vinegar
  • Capsaicin (Compound that causes spiciness in hot peppers)
Capsaisin, hot pepper hotness contains a carbonyl