Structure and Nature of Carbonyl Group: Physical Properties, Bonding, Reactivity

Structure and Nature of Carbonyl Group: Aldehydes and ketones are some of the most important organic compounds. But do you know these organic compounds have the same functional group? The functional group in these compounds is the carbonyl group. Due to functional group similarity, aldehydes and ketones share a lot of similarities. These compounds are commonly known as carbonyl compounds. Let’s explore more about the structure and nature of the carbonyl group in this article.

Structure of Carbonyl Group

A carbonyl group is a functional group in which a carbon atom is double-bonded to an oxygen atom \(\left[ {{\rm{ > C = O}}} \right]\). The \(\left[ {{\rm{ > C = O}}} \right]\) is considered as the carbonyl group entity, and the members of the group are known as the carbonyl compounds. Aldehydes and ketones are the simplest carbonyl groups that are usually attached to another carbon compound. 

In the carbonyl group, the carbon atom has an \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridisation and is bonded to three other atoms through a sigma bond. One of the three sigma bonds is formed with an oxygen atom; however, the other two sigma bonds are with hydrogen and/or carbon atoms.

Thus, the carbonyl carbon and the three bonds adopt a trigonal planar geometry with bond angles at nearly \({\rm{12}}{{\rm{0}}^{\rm{o}}}\) However, the fourth valence electron of the carbon atom remains in its unhybridised \({\rm{2p}}\)-orbital. This \({\rm{2p}}\)-orbital develops a \({\rm{\pi }}\) bond by overlapping with the \({\rm{2p}}\)-orbital of the oxygen atom. Besides, the oxygen atom also contains two non-bonded pairs of electrons.

The unhybridised \({\rm{\pi }}\)-electron cloud is perpendicular to the plane of trigonal geometry and forms a ‘side-by-side’ overlap with the \({\rm{2p}}\) orbital of the oxygen atom. The length of the double bond of the carbonyl group is about \(1.2\) angstroms, and it has a strength of about \({\rm{176 – 179Kcal/mol}}\)

Nature of the Carbonyl Group

The carbon-oxygen bond is polarised due to the high electronegativity difference between carbon and oxygen atoms of the \({\rm{ > C = O}}\) bond. Oxygen has a higher electronegativity compared to that of the carbon atom. Thus, the carbonyl carbon demonstrates the property of a Lewis acid, whereas the carbonyl oxygen demonstrates the property of a Lewis base.

Carbonyl compounds contain significant dipole moments. Therefore, it demonstrates more polarity than ethers. The high polarity of the carbonyl group is mainly due to the resonance with respect to neutral and dipolar structures.

The electronegativity difference leads to the development of a slight positive charge over the carbon atom and a slight negative charge over the oxygen atom. Hence, \({\rm{ > C = O}}\) is prone to nucleophilic attack due to carbon’s positive charge and electrophilic attack due to oxygen’s negative charge. The negative charge on the nucleophile attacks the Carbonyl group and becomes a part of the structure, and a positive charge attacks the oxygen. 

There are two resonating structures of the carbon-oxygen dipole. In the major form, carbon has an empty \({\rm{2p}}\) orbital and a full positive charge. In the minor form, both electrons of the pi bond are localised on the oxygen, giving it a full negative charge.

The double bonds of the carbonyl groups are very different from the double bond between the carbon-carbon. 

Bonding and Reactivity

In the carbonyl group, oxygen is more electronegative than carbon and so has a strong tendency to pull electrons in a carbon-oxygen bond towards itself, making the carbon-oxygen bond highly polar. 

In a reaction involving the carbonyl group, the carbon-oxygen double bond gets broken. As a result, the carbonyl group undergoes addition reactions, often followed by the loss of a water molecule known as addition elimination or condensation. 

Some Carbonyl Compounds

Aldehydes

The carbonyl group in aldehydes has a hydrogen atom attached to it together with either a second hydrogen atom or a hydrocarbon group which might be an alkyl group or a benzene ring. For example-

In the aldehyde group, the carbonyl group with the hydrogen atom is always written as -\({\rm{CHO}}\) – never as \({\rm{COH}}\). While naming aldehydes, the total number of carbon atoms in the longest chain, including the one in the carbonyl group, is considered. If side groups are attached to the chain, then the carbon atom in the carbonyl group is always numbered \(1\).

Ketones

The carbonyl group in ketones has two hydrocarbon groups attached. These can either be alkyl groups or benzene rings. For example-

In ketones, the carbonyl group is always present in between the chain and never at the ends.

Where do aldehydes and ketones differ?

In an aldehyde, a hydrogen atom is directly attached to the carbonyl group, whereas in ketones, no hydrogen atom is directly attached to the carbonyl group.

Physical Properties of Carbonyl Compounds

Boiling Points

As the molecules get bigger, the boiling points of the carbonyl compounds rise. The strengths of the intermolecular forces govern the size of the boiling point.

1. Van der Waals dispersion forces: As the molecules get longer, the number of electrons increases and these attractions get stronger. The sizes of the temporary dipoles increase. Hence, the boiling points increase as the number of carbon atoms in the chains increases.
2. Van der Waals dipole-dipole attractions: Along with the dispersion forces, there will also be attractions between the permanent dipoles on nearby molecules. Hence, the boiling points of carbonyl compounds will be higher than those of similarly sized hydrocarbons. 

The carbonyl compounds cannot form a hydrogen bond with each other because they don’t have any hydrogen atoms attached directly to the oxygen atom

Solubility in Water

The lower carbonyl compounds arely soluble in water, but solubility decreases with an increase in the chain length of the organic compound. 

As the length of the chain increases, the hydrocarbon “tails” of the molecules interfere with the solubility in water. This hydrocarbon tail forces itself between the water molecules and breaks the relatively strong hydrogen bonds without replacing them with anything. This makes the process energetically less profitable, and so solubility decreases.

Summary

Aldehydes and ketones are known as carbonyl compounds because they contain the carbonyl functional group. Both of these compounds have the same general formula; hence exhibit similar properties. However, in an aldehyde, a hydrogen atom is directly attached to the carbonyl group, whereas in ketones, no hydrogen atom is directly attached to the carbonyl group. This page explains what a carbonyl group, along with its structure and nature, is. It also looks at the way the carbonyl group affects the bonding and reactivity of carbonyl compounds. It also considers their simple physical properties, such as boiling point and solubility. 

Frequently Asked Questions (FAQs) on Structure and Nature of Carbonyl Group

Q.1. What is the shape of carbonyl compounds?
Ans: In the carbonyl group, the carbon atom has an \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridisation and is bonded to three other atoms through a sigma bond. One of the three-sigma bonds is formed with an oxygen atom; however, the other two sigma bonds are with hydrogen and/or carbon atoms. Thus, the carbonyl carbon and the three bonds adopt a trigonal planar geometry with bond angles at nearly \({\rm{12}}{{\rm{0}}^{\rm{o}}}\) 

Q.2. What is the structural formula of the carbonyl group?
Ans: The structural formula of the carbonyl group is \({\rm{ > C = O}}\), with the carbon atom bonded to two other atoms. Aldehydes and ketones are the simplest carbonyl compounds. Aldehydes have one hydrogen atom and one alkyl or aryl group bonded to the carbonyl carbon atom, while ketones have no hydrogen atom attached to the carbonyl carbon atom.

Q.3. Why is the carbonyl group polar?
Ans: The carbonyl group is polar due to the electronegativity difference between the carbon and oxygen atoms. Oxygen is more electronegative than carbon and so has a strong tendency to pull electrons in a carbon-oxygen bond towards itself, making the carbon-oxygen bond highly polar.

Q.4. Which organic compound contains the carbonyl functional group?
Ans: Aldehydes and ketones contain the carbonyl functional group.

Q.5. What is the difference between aldehyde and ketone?
Ans: In an aldehyde, a hydrogen atom is directly attached to the carbonyl group, whereas in ketones, there is no hydrogen atom directly attached to the carbonyl group.

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