Classification of Aldehydes and Ketones

Classification of Aldehydes and Ketones: Aldehydes and ketones belong to the group of compounds collectively called the ‘carbonyl compounds.’ The carbonyl compounds are those compounds that contain a carbonyl group or a group which comprises a group that has a double bond between a carbon atom and an oxygen atom. Since the carbonyl group has several atoms or groups of atoms attached on either side of it, the carbonyl compounds are segregated into three main classes of compounds, as given below:

• Aldehydes with a structure of: \({\rm{R – CHO}}\) (Hydrogen and alkyl group attached to it)
• Ketones with a structure of: \({\rm{R}} – {\rm{CO}} – {{\rm{R}}^\prime }\)
• Carboxylic acids with structure: \({\rm{R}} – {\rm{CO}} – {\rm{OH}}\)

Each of these classes of compounds has unique properties and reactions. Since they all contain the carbonyl group \({\rm{(C = O)}}\), they are classified as carbonyl compounds.

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Occurrence of Aldehydes and Ketones

The Carbonyl group is one of the most significant functional groups in organic chemistry. There are several biological pathways, such as the role of \(\beta \)-carotene in promoting healthy vision, flavours and odours like those of vanillin, and so on, which are all due to the carbonyl groups. For example, Cinnamaldehyde is present in cinnamon bark, vanilla in vanilla bean, camphor in camphor trees, carvone in spearmint, and so on. The present article will cover the general classifications of aldehydes and ketones and their properties.

Structure of Aldehydes and Ketones

The difference between aldehydes and ketones is that, in aldehydes, the carbonyl group is bonded to carbon and hydrogen; in ketones, the carbonyl group is bonded to two carbon atoms from an alkyl and aryl group.

The structure of aldehydes and ketones are as follows:

Nomenclature of Aldehydes and Ketones

Ketones and aldehydes are named after two systems of nomenclature, the common and the IUPAC nomenclature.

Common Nomenclature

Aldehydes
The common names for aldehydes are derived from those of the carboxylic acids \(( – {\rm{COOH}})\), by replacing the \({\rm{‘ic’}}\) from the end with an aldehyde. The substituents and their locations are denoted by Greek letters of \(\alpha ,\beta ,\) etc.
Example: \({\rm{C}}{{\rm{H}}_3}{\rm{COOH}}\) – Acetaldehyde

Ketones
The common names for ketones are given by naming the alkyl or aryl (R) groups attached to either side of the carbonyl group. Additionally, the alkyl phenyl ketones are named by adding an ‘Acyl’ before the phenone group. Again here, the locations where the substituents are attached are indicated by Greek letters, \(\alpha {\alpha ^\prime },\beta {\beta ^\prime }\), etc.
Example:

IUPAC Nomenclature

The IUPAC names of both aliphatic ketones and aldehydes are derived from their corresponding alkanes. The ending \(^\prime {{\rm{e}}^\prime }\) of alkanes is replaced with an \(^\prime – {\rm{a}}{{\rm{l}}^\prime }\) in aldehydes and \(^\prime {\rm{on}}{{\rm{e}}^\prime }\) in ketones.

For naming and numbering carbon atoms, in aldehydes, the longest carbon chain is taken as the primary chain. The numbering starts from the carbon nearer to the aldehyde group. In ketones, the numbering starts at the end, which is nearer to the carbonyl group. Prefixes are added to the substituents in alphabetical order, with numbers added to indicate their position in the carbon chain.

In cyclic ketones, the carbonyl group is number one, and the other rules apply as in other ketones. For aldehydes, the cyclic members have the suffix ‘carbaldehyde’ added to the name after the cycloalkane. Again, the numbering starts with the carbon atom, which is attached to the aldehyde group. So, the simplest member, with benzene attached to the aldehyde group, is called benzenecarbaldehyde or, simply, benzaldehyde.

Some examples and the naming of ketones and aldehydes are as follows:

Ketones	Common Name	IUPAC Name
\({\text{C}}{{\text{H}}_3} – {\text{CO}} – {\text{C}}{{\text{H}}_3}\)	Acetone	Propanone
\({\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{CH}}\left( {{\text{C}}{{\text{H}}_3}} \right) – {\text{CO}}\) \( – {\text{CH}}\left( {{\text{C}}{{\text{H}}_3}} \right) – {\text{C}}{{\text{H}}_2} – {\text{C}}{{\text{H}}_3}\)	Di-sec-butylketone	\(3,\,5\)-dimethyl heptan – \(4\)-one
	\(1\)-(\(4\)-fluorophenyl) ethan- \(1\)- one	\(4\)- fluoroacetophenone

Structure of Carbonyl Group and Resonance

The carbon in the carbonyl group is \({\rm{s}}{{\rm{p}}^2}\) hybridised and can form \(3\) sigma bonds. The fourth valence electron that is there in carbon stays in its \(^\prime {{\rm{p}}^\prime }\) orbital and forms a \(\pi \)- bond by overlapping with the \(^\prime {{\rm{p}}^\prime }\) orbital present in the oxygen atom. The oxygen atom also possesses two non-bonding electron pairs. Therefore, the carbon in the carbonyl carbon and the three atoms attached to it, lie in one plane. The \(\pi \)-electron cloud lies above and below the plane formed by the carbonyl carbon and the three atoms. This arrangement renders a trigonal coplanar structure to the molecule, with a bond angle of \({120^ \circ }\).

When compared to carbon, oxygen is extremely electronegative. Thus, it makes the carbon-oxygen double bond in the carbonyl group polarized. The electron cloud is near the oxygen atom due to its electronegative nature. This gives a partial positive charge on the carbon and a partial negative charge on oxygen.

Thus, the carbonyl carbon is an electrophilic centre (Lewis acid), while the carbonyl-oxygen is a nucleophilic centre (or Lewis base). The carbonyl compounds have good dipole moments and are polar than ethers. The resonance in the carbonyl group can also explain their high polar nature. The basis of resonance in the carbonyl group involves a neutral and a dipolar structure, as given:

Physical Properties of Aldehydes and Ketones

Some of the most significant physical properties of aldehydes and ketones are as follows:

1. Methanal, the first member of the series in aldehyde, is a gas at room temperature, while ethanal is a volatile liquid. The ketones are mostly liquids or solid at room temperature.
2. Boiling points of aldehydes and ketones are more than the hydrocarbons and ethers of comparable molecular masses. The reason is because of the weak intermolecular association in them due to the dipole-dipole interactions. However, the boiling and melting points of aldehydes and ketones are lesser than alcohols due to the absence of intermolecular hydrogen bonding.
3. Lower members of aldehydes and ketones, such as ethanal and propanone, are soluble in water in all proportions. This is because they form a hydrogen bond with water. However, with the increase in the length of the alkyl chains, the solubility of aldehydes decreases. Most aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, etc.
4. While lower aldehydes have a sharp, unpleasant odour, the odour becomes less sharp as the molecular size increases. Several naturally occurring aldehydes and ketones are used in the blending of perfumes and other flavouring agents.

Summary

Aldehydes and ketones belong to a class of organic compounds called carbonyl compounds. The carbonyl compounds are classified into three different classes depending upon the atoms or groups of atoms surrounding the carbonyl (double bond between carbon and oxygen) group, like carboxylic acids, aldehydes and ketones. The ketones and aldehydes are named according to the common system and IUPAC system of nomenclature. The carbonyl group in ketones and aldehydes have a unique structure and exhibit resonance due to the difference in electronegativity between carbon and oxygen atom. The aldehydes and ketones exhibit unique physical properties due to the presence of the carbonyl group in them, such as high melting and boiling point, unique odour and so on.

FAQs

Q.1. What is the importance of aldehydes in our everyday life?
Ans: Aldehydes and ketones are present in nature in several forms, such as cinnamaldehyde lending its unique fragrance to cinnamon bark, and the presence of vanillin provides a unique fragrance to vanilla, which is the basis of vanilla essence.

Q.2. How are aldehydes and ketones different?
Ans: Aldehydes have one alkyl or aryl group and one hydrogen attached to the carbonyl group, while ketones have two aryl or alkyl groups attached on two sides of the carbonyl group. Aldehydes have a functional group of \( – {\rm{CHO}}\), while ketones have the functional group: \({\rm{ – C = O}}\).

Q.3. Why are the boiling points of Ketones and Aldehydes lower than alcohols?
Ans: Aldehydes and ketones do not exhibit intermolecular hydrogen bonding like alcohols. Hence, they have a lower boiling point and melting point than alcohols.

Q.4. Why are aldehydes and ketones classified differently?
Ans: The aldehydes and ketones both have a carbonyl group, but they are classed differently because the groups of atoms surrounding the carbonyl group are different from ketones. Thus, while aldehydes have one alkyl or aryl group and one hydrogen attached to the carbonyl group, while ketones have two aryl or alkyl groups attached on two sides of the carbonyl group.

Q.5. What are the IUPAC names for Valeraldehyde and Acrolein?
Ans: IUPAC names are as follows:
a. Valeraldehyde – Pentanal
b. Acrolein – Prop-\(2\)-en-\(1\)-al

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