Aldehydes and Ketones

Aldehydes and Ketones are collectively called as Carbonyl Compounds.

The basic structure of Aldehydes is represented below, in which R is any hydrocarbon chain or could be even hydrogen

Hence the smallest possible aldehyde is Methanal (below).

Whereas, the basic structure of Ketones is represented similarly as Aldehydes, but the Carbonyl Carbon(the carbon to which an Oxygen is double bonded), is surrounded by two hydrocarbon chains instead of one.

Hence the smallest possible ketone is Propanone, or otherwise commonly known as Acetone(below).

A basic Fehling's test and Tollen's test is conducted to differentiate Aldehyde and Ketone.

There are basically two types of aldehydes - Aliphatic and Aromatic aldehydes.

• Aliphatic Aldehydes are nothing but straight chain aldehydes.

• Aromatic Aldehydes are consist of aromatic rings in their structure.

\(\text{Aliphatic Aldehydes}\) Examples:

Ethanal ( commonly known as Acetaldehyde )

Crotonaldehyde

\(\text{Aromatic Aldehydes}\) Examples:

Benzaldehyde

Salicylaldehyde

\(\text{Symmetrical Ketones}\)

In the above basic structure of Ketones, if \(R_1 = R_2\), then the ketone is said to be symmetric.

\(\text{Unsymmetrical Ketones}\)

But if \(R_1 \neq R_2\) , then the ketone is said to be unsymmetric.

A few examples of each is given below:

\(\text{Symmetrical}\)

Acetone

3-Pentanone

\(\text{Unsymmetrical}\)

2-Pentanone

Acetophenone

• Cyclic Ketones

These class of ketones are compounds where, the carbonyl carbon is itself present in a cyclic chain.

Examples:

Cyclohexanone

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Aldehydes and ketones undergo Nucleophilic addition reactions.

Mechanism:

    1)Nucleophilic attack on the electrophilic carbon (c bonded to o).

    2)Hybridization of the carbonyl changes from sp2 to sp3.

    3)Formation of tetrahedahedral intermediate.

    4)proton capture from the medium to form neutral product.

         Nucleophiles : Addition product

1) cyanide : cyanohydrin

2) NaHSO3 : Hydrogen sulphite addition prdt

3) Grignard reagent+H2O : alcohol

4) Alcohol(1 mole) : Hemiacetal

5) NH2-Z (Z=alkyl,aryl,NH2,OH...)