Carbonyl Compounds

Carbonyl Compounds definition can be given as the compounds which hold a carbonyl group (which is the C=O group) are known as carbonyl compounds.

The carbonyl group is probably the essential functional group of organic chemistry. These are an integral part of organic chemistry, and their primary members are referred to as ketones, carboxylic acids, and aldehydes.

Carbonyl compounds are further divided into organic and inorganic carbonyl compounds.  This complete article details the carbonyl compounds, which are organic in nature.

What is a Carbonyl Group?

The term carbonyl can also be referred to as carbon monoxide as a ligand, either as a ligand or an organometallic complex (a metal carbonyl, for example, nickel carbonyl).

According to organic chemistry, a carbonyl group can be described as a functional group, which is composed of a double-bonded carbon atom to an oxygen atom: C=O. It is common for many organic compound classes as a part of several larger functional groups. A compound that contains a carbonyl group is often called a carbonyl compound.

Carbonyl Group Structure

The carbonyl structure can be represented as follows:

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Some Organic Carbonyl Compound Examples

A few of the examples of organic compounds include urea, carbamates. The derivatives of phosgene, acyl chlorides chloroformates, carbonate esters, lactones, lactams, isocyanates, hydroxamates.

Aldehydes, Ketones, Carboxylic Acids

When the carbonyl group is linked to either one aryl or an alkyl group and to one hydrogen atom, the resulting carbonyl compound is referred to as an aldehyde. Where the general formula of an aldehyde can be given as R-CHO. Ketones are considered as carbonyl compounds, where the carbonyl carbon is linked to either two aryl or alkyl groups. 

We can generalize their chemical formulae to R-(C=O)-R'. When the carbonyl carbon is bonded to either one alkyl or aryl group, including one OH group, the resultant carbonyl compound is classified as a carboxylic acid. Whereas we can generalize the carbonyl group formula as R-COOH.

Generally, we come to the names aldehydes, ketones when we speak about the Carbonyl compounds. What is the role of these in the carbonyl compounds list? Ketones and Aldehydes are the organic compounds consisting of the functional group same as the carbonyl compounds. The major difference between these compounds is the carbonyl group's position. The carbonyl group in aldehydes is placed at the end of the carbon chain, whereas, coming to ketones, it is located in the middle of the carbon ring. 

Some examples of aldehydes can be given as butanol, propanol, 4-chlorobutanol, and so on and a few examples considering ketones can be given as acetone, propanone, 2-methyl-3-pentanone, and so on.

Properties of Carbonyl Compounds

A few of the carbonyl compound properties are listed below:

• Carbonyl Compounds tend to be polar in nature. They exhibit both the positive and negative charges in slight form. Thus, these are known to be polar molecules.

• Also, these compounds are reported as insoluble in water, but at times, they dissolve other forms of polar molecules.

• These compounds are said to be chemically reactive compounds, which means that they are allowed to control a chemical reaction.

Chemical Reactions of Carbonyl Compounds

The carbonyl group's carbon atom is known to be electrophilic in nature because they tend to attract electron-rich compounds. A few examples of electrophiles can be given as ions, whereas the oxygen atoms are referred to as nucleophiles because they don't contain a rich density of electrons. They are also said to be the nuclei lovers like the bases. 

The reactions of carbonyl compounds are listed below:

• Carbonyl Reduction 

This reaction is a process where the Carbonyl groups are reduced by the hydride reagents like the NaBH₄ and LiAlH₄ with the yeast of the baker or by the catalytic hydrogenation process.

• Carbonyl alkylation 

This is a process where the carbonyl compounds are alkylated with the organometallic compounds use, such as organolithium reagents, Grignard reagents, acetylides, and more.

• Carbonyl Alpha-Substitution Reaction 

This substitution reaction type involves the substitution of the α hydrogen atom using an electrophile.

Carbonyl Compounds Applications

Let us discuss a few applications of carbonyl compounds as listed below:

• The propanone of carbonyl compounds can be used as a solvent because it gets dissolved in water and other organic solutions.

• Formaldehyde can be used in the manufacturing of plastics and in biological laboratories for preservation purposes.

• Butanol is used to provide fragrance for storing the bread fresh.

• Acetaldehyde can be used as a Synthesizer in various organic reactions.

Spectroscopy of the Carbonyl Group

• Infrared Spectroscopy  

The double bond of C=O absorbs the infrared light at wavenumbers approximately between 1600–1900 cm⁻¹ (5263 to 6250 nm). The absorption's exact location is well understood concerning the molecule's geometry. When displayed on an infrared absorption spectrum, this absorption is called "carbonyl stretch". Additionally, the propanone's ultraviolet-visible spectra in water give the carbonyl absorption at 257 nm.

• Nuclear Magnetic Resonance  

The double-bond of C=O exhibits various resonances based on the surrounding atoms, in general, a downfield shift. The carbonyl carbon ¹³C NMR falls in the range of 160–220 ppm.

Carbonyl Group 

The functional groups of aldehydes and ketones are carbonyl groups that form double bonds of oxygen with carbon. In aldehydes, the carbonyl group is at the "end" of the carbon chain, whereas in ketones it is in the "middle" of the carbon chain. The double bond of the carbonyl group is very sensitive to chemical reaction. There are many types of carbonyl compounds  in nature. They all further contain an acyl group (R-C = O) with a substituent attached. The R portion of the structure can have any alkyl, alkenyl,  alkynyl, or other functional group attached to it. 

Either hydrogen, carbon, oxygen, nitrogen, sulfur, or halogen is bonded to the opposite side of the acyl group. Different types of compounds can be formed depending on the various substituents attached to the acyl group.

Structure of Carboxyl Group

A carbonyl group is a functional group in which a carbon atom forms a double bond with oxygen atom (> C = O). (> C = O) is considered a carbonyl group unit and the members of that group are called carbonyl compounds. Aldehydes and ketones are the simplest carbonyl groups and are usually attached to other carbon compounds. 

In the carbonyl group, the carbon atom shows an sp² hybrid orbital and is bonded to three other atoms via sigma bonds. One of the three sigma bonds is formed by an oxygen atom. However, the other two sigma bonds are bonds with hydrogen and / or carbon atoms. 

The difference in the electronegativity values ​​of carbon and oxygen creates a highly polarized double bond. Oxygen atoms have a high electronegativity, so they are more likely to attract electrons, creating a partially negative end. Second, the carbon atom becomes partially positive because the electron is attracted to the oxygen atom.

Carbonyl Compound

1. Aldehydes 

The carbonyl group in aldehydes has a hydrogen atom attached to it together with either aryl or alkyl group, then the compound is known as aldehyde. The carbonyl group is attached to the end carbon. 

For example, methanal, ethanal, propanal etc. 

2. Ketones 

The carbonyl group in Ketones has carbonyl carbon linked with two alkyl or aryl groups or in other words two hydrocarbon groups are attached. Carbonyl group is attached to carbon which is not at the end. 

For example, propanone, butanone, pentan-3-one

Physical Properties of Carbonyl Group 

• These carbonyl groups are polar in nature as they have both positive and negative charges. That's why they are called polar molecules.

• Although these compounds have been reported to be insoluble in water, they may also dissolve other forms of polar molecules. Their solubility decreases with increase in the chain length of organic compounds. 

• These are known  as chemically reactive compounds. This means that they control the reaction of  chemical reactions. 

• As the molecule grows, the boiling point of the carbonyl compound rises. The strength of the intramolecular force determines the magnitude of the boiling point.

Oxidation and Reduction in Carbonyl Group ( Aldehydes and Ketones )

When electrons get transferred, oxidation and reduction reactions occur at the same time. When a molecule is oxidized, electrons are donated to adjacent molecules. Adjacent molecules are reduced when an electron is accepted. 

In general, aldehydes are more reactive than ketones because they have only one H atom instead of an alkyl group as a substituent. Therefore, aldehydes can  participate in other types of reactions, depending on the reaction conditions. 

In the presence of strong oxidizing agents such as potassium permanganate and potassium dichromate, aldehydes can be oxidized to  carboxylic acids. For example, when propanal is oxidized by potassium dichromate, propionic acid is formed. 

On the other hand, both aldehydes and ketones can enter the reduction reaction. Both  react with reducing agents such as sodium borohydride and hydrogen gas in the presence of Lindlar catalysts to produce alcohol. Aldehydes produce primary alcohols by reduction, while ketones produce secondary alcohols.