Introduction Of Aldehyde, Structure, Classification And Nomenclature, JEE Chemistry 2024

Introduction Of Aldehyde :

• An aldehyde is an organic compound with the structural formula R−CH=O in organic chemistry. Aldehydes have two hydrogen atoms or one hydrogen atom and one carbon-containing group, such as an alkyl, aryl, or aralkyl group, attached to the carbonyl group. As an example,

• The carbonyl group in ketones is bonded to two carbon-containing groups, which could be different or identical alkyl or aryl groups. A ketone is referred to as simple or symmetrical if both R and R' groups are the same; if R and R' are different, the ketone is referred to as mixed or unsymmetrical.

Structure Of Aldehyde And Ketone

Structure Of Aldehyde And Ketone : The carbonyl carbon in aldehydes and ketones is sp ² hybridized. Depending on the structure of the aldehyde or ketone, one of the three sp ² hybridized orbitals forms a σ-bond with a half-filled p-orbital of oxygen atom, while the other two are consumed in σ-bond formation with hydrogen and carbon.

The unhybridized p-orbital of carbonyl carbon forms a π-bond by sideways overlapping with another half-filled p-orbital of oxygen atom.

Classification Of Aldehyde

Classification Of Aldehyde : Aldehydes are classified as aliphatic and aromatic aldehydes.

Aliphatic Aldehydes

Aliphatic Aldehydes : Aliphatic aldehydes are substances in which the sp ³ hybridized carbon atom, which is a saturated carbon atom, has the −CHO group (formyl group) directly attached to it.

e.g. CH ₃ CHO (Acetaldehyde)

Aromatic Aldehydes : Aromatic aldehydes: These are substances that have an aromatic ring directly attached to the —CHO group.

E.g. Benzaldehyde

Classification Of Ketones

Classification Of Ketones : Ketones are classified based on their substituents. Ketones are classified broadly into symmetrical and unsymmetrical derivatives based on the equivalence of the two organic substituents attached to the carbonyl center.

A. Aliphatic ketones:

Aliphatic ketones are compounds that have two alkyl groups attached to a >C=O group. Aliphatic ketones are further divided into simple and mixed ketones according to the kinds of alkyl groups that are bound to the carbonyl carbon.

(i) Simple or symmetrical ketones: Simple or symmetrical ketones are those in which the two alkyl groups bound to the carbonyl carbon are the same. As an example,

(ii) Mixed or unsymmetrical ketones: Unsymmetrical or mixed ketones are those in which the carbonyl carbon is bonded to two distinct alkyl groups. As an example:

B. Aromatic ketones:

Aromatic ketones are substances in which a >C=O group is joined to one aryl group and one alkyl group, or to two aryl groups.

Nomenclature of Aldehydes

Nomenclature of Aldehydes: There are two methods for aldehyde and ketone nomenclature.

Common or Trivial Names: The following are the common names for the carbonyl compounds.

Aldehydes undergo oxidation to produce carboxylic acids. The majority of aldehydes get their common names from the common names of corresponding carboxylic acids by substituting the word "aldehyde" for the ending "-ic acid."

Example:

HCHO- formaldehyde from formic acid (HCOOH)

CH ₃ CHO−acetaldehyde from acetic acid (CH ₃ COOH)

Greek letters α, β, γ, δ, etc., represent the positions of the substituents in the carbon chain.

The carbon α is directly associated with the aldehyde group, followed by the carbon β, and so on.

Ketones

Ketones: The names of two alkyl or aryl groups can be entered in alphabetical order to obtain the common names of ketones, which are then followed by the word "ketone." Greek letters are used to indicate the locations of substituents; the carbon closest to the carbonyl group is first. Certain ketones have historical common names; dimethyl ketone, for example, is referred to as acetone. A-phenone is the name given to a ketone in which the carbonyl group is joined to a benzene ring. Therefore, acyl groups are typically appended to phenone as a prefix to name alkyl phenyl ketones. Only a few ketones are permitted to keep their common names by IUPAC.

Iupac System For Nomenclature Of Aldehyde And Ketones

Nomenclature Of Aldehyde

● The IUPAC names of aliphatic aldehydes are obtained by substituting the suffix "al" for the "e" in the names of corresponding alkanes. As a result, an aldehyde is called a "alkanal."

● For instance, formaldehyde, or methanal, is what the IUPAC system names H ₂ C=O.

● Since the aldehyde group is always connected at the end of the main carbon chain, it is always given the first position in the numbering system. Numbering is not always required to be included in the naming.

● Greek characters like α, β, and so on are used to indicate where the substituents are in the carbon chain. The aldehyde group is directly bonded to the α-carbon, the carbon next to the aldehyde group is bonded to the  β-carbon, and so on.

Examples:

Nomenclature Of Ketone

● The IUPAC guidelines identify the ketones with the suffix -one.

● The position of the carbonyl group within the main chain is determined by its location number, which can be found anywhere.

● Typically, the carbonyl carbon is assigned the lowest number when the chain is numbered starting from the end.