Difference Between Carbonyl and Carboxyl

Main Difference – Carbonyl vs Carboxyl

In organic chemistry, a functional group is a chemical group within a molecule that is responsible for the characteristic chemical reactions in the molecule. There are several important functional groups that can be found among organic compounds. Carbonyl group and carboxyl group are two such functional groups. A carbonyl group consists of carbon atoms double bonded to an oxygen atom. Aldehydes and ketones are examples of carbonyl group containing compounds. Carboxyl groups are composed of a carbon atom bonded to an oxygen atom via a double bond and with a hydroxyl group (-OH) via a single bond. The main difference between carbonyl and carboxyl group is that carbonyl group consist of a carbon atom double bonded to an oxygen atom whereas carboxyl group consists of a carbonyl group and a hydroxyl group bonded to each other via the carbon atom of the carbonyl group.

Key Areas Covered

1. What is Carbonyl
     – Definition, Terms, Chemical Structure and Polarity, Examples
2. What is Carboxyl
     – Definition, Carboxylic Acids, Dimer Formation
3. What are the Similarities Between Carbonyl and Carboxyl
     – Outline of Common Features
4. What is the Difference Between Carbonyl and Carboxyl
     – Comparison of Key Differences

Key Terms: Aldehyde, Carbonyl, Carboxyl, Double Bond, Functional Group, Hydroxyl Group, Ketone, Nucleophile, Oxygen, Polarity

What is Carbonyl

A carbonyl group is a chemically organic functional group composed of a carbon atom double-bonded to an oxygen atom. The simplest organic compounds containing carbonyl groups are aldehydes and ketones. Aldehydes are composed of aldehyde groups having the chemical formula –CHO. This group contains a carbonyl group bonded to a hydrogen atom. In ketones, the carbonyl group is attached to two alkyl groups.

Figure 1: Aldehydes and Ketones Contain Carbonyl Groups

Terms

• The functional group containing a carbon atom bonded to an oxygen atom via double bond is known as carbonyl functional group.
• Compounds containing carbonyl groups are known as carbonyl compounds.
• The carbon atom in the carbonyl group is known as carbonyl carbon.

The carbonyl carbon is sp² hybridized. Therefore, it can form two single bonds along with a double bond. A C=O double bond is already present in the carbonyl group. Hence the carbonyl carbon can form two more single bonds. However, the geometry around the carbonyl carbon is trigonal planar.

The C=O bond is polarized due to the difference between electronegativity values of carbon and oxygen. Oxygen is more electronegative and, attracts the bond electron pairs towards itself. Then the oxygen atom gets a partial negative charge whereas the carbon atom gets a partial positive charge. This carbonyl carbon can be attacked by a nucleophile. This polarity of the carbonyl group causes the reactivity of aldehydes and ketones. It also causes the higher boiling points of carbonyl compounds.

Examples of Carbonyl Compounds

• Aldehyde
• Ketones
• Carboxylic acids
• Esters
• Amides
• Enones
• Acyl halides
• Acid anhydrides

What is Carboxyl

The carboxyl group is an organic functional group consisting of a carbon atom double bonded to an oxygen atom and single bonded to a hydroxyl group. Therefore, a carboxyl group is a combination of a carbonyl group and a hydroxyl group. The chemical formula of carboxyl group is –COOH.

Figure 2: Chemical Structure of Carboxyl Group

The major class of compounds containing carboxyl groups is carboxylic acids. Carboxylic acids consist of a carboxyl group bonded to either a hydrogen atom or an alkyl group. Dicarboxylic acids are compounds containing two carboxylic acids.

Carboxyl group can ionize releasing a proton (hydrogen atom) from a hydroxyl group. Since this proton is released as a free proton, carboxylic acids are acids. When the proton is released, the oxygen atom of the hydroxyl group gets a negative charge. This negative charge is stabilized by sharing the electrons of this oxygen atom with the other oxygen atom of the carboxyl group. Therefore, the ionized form is stable.

Compounds containing carboxyl groups can form dimers. A dimer is an oligomer consisting of two structurally similar monomers joined by bonds. Since the hydroxyl group of the carboxyl group is able to form hydrogen bonds, they form hydrogen bonds between carboxyl groups. This causes the formation of dimers.

Similarities Between Carbonyl and Carboxyl

• Both groups contain carbonyl carbon atoms that are bonded to an oxygen atom via a double bond.
• Both are functional groups of organic compounds.
• Both contain sp² hybridized carbonyl carbon atoms.

Difference Between Carbonyl and Carboxyl

Definition

Carbonyl: A carbonyl group is a chemically organic functional group composed of a carbon atom double-bonded to an oxygen atom.

Carboxylic: A carboxyl group is an organic functional group consisting of a carbon atom double bonded to an oxygen atom and single bonded to a hydroxyl group.

Chemical Formula

Carbonyl: The chemical formula of carbonyl group is –C(=O)-.

Carboxylic: The chemical formula of carboxyl group is –COOH.

Substitutes

Carbonyl: Carbonyl group can be attached to two more atoms or group of atoms.

Carboxylic: Carboxyl group can be attached to one more atom or group of atoms.

Polarity

Carbonyl: Carbonyl group has a polarity due to the charge separation between the oxygen atom and the carbon atoms.

Carboxylic: Carboxyl group has a polarity in its carbonyl group.

Proton Releasing

Carbonyl: Carbonyl group cannot release protons.

Carboxylic: Carboxyl group can release a proton.

Dimer Formation

Carbonyl: Carbonyl groups cannot form dimers.

Carboxylic: Carboxyl groups form dimers.

Hydrogen Bonding

Carbonyl: Carbonyl group cannot form hydrogen bonds.

Carboxylic: Carboxyl group can form hydrogen bonds.

Examples

Carbonyl: The simplest carbonyl compounds are aldehydes and ketones.

Carboxylic: The major compounds containing carboxylic groups are carboxylic acids.

Conclusion

Carbonyl group and carboxyl group are two organic functional groups that are responsible for characteristic properties of organic compounds. The main difference between carbonyl and carboxyl group is that carbonyl group consists of a carbon atom double bonded to an oxygen atom whereas carboxyl group consists of a carbonyl group and a hydroxyl group bonded to each other via the carbon atom of the carbonyl group.

Reference:

1. Helmenstine, Anne Marie. “Carboxyl Group Definition and Examples.” ThoughtCo, Sep. 3, 2017, Available here.
2. “The Carbonyl Group.” Chemistry LibreTexts, Libretexts, 10 Sept. 2017, Available here.
3. “Carboxyl group.” The Columbia Encyclopedia, 6th ed, Encyclopedia.com, Available here.

Image Courtesy:

1. “Skeletal formula of a aldehyde group” By Ben Mills – Vector version of this file at Wikimedia Commons (Public Domain) via Commons Wikimedia
2. “Carboxylic Acid General Structure V” By Jü – Own work (Public Domain) via Commons Wikimedia