What are Aldehydes and Ketones? 

Aldehydes and ketones are organic compounds that incorporate a carbonyl functional group, C=O. The carbonyl carbon atom of this group has two remaining bonds that may be attached with hydrogen or alkyl, or aryl substituents. If at least one of these substituents is hydrogen, the compound is called an aldehyde. If alkyl or aryl groups are present at both ends of carbonyl carbon, the compound is a ketone.

In aldehydes, If the carbonyl carbon has one hydrogen and alkyl group. It is said to be aliphatic aldehydes. If the carbonyl carbon has one hydrogen and aromatic group like benzene. It is said to be aromatic aldehydes.  In ketones, the carbonyl group has 2 hydrocarbon groups attached to it. These can be either the ones containing benzene rings or alkyl groups. Ketone does not have a hydrogen atom attached to the carbonyl group. If carbonyl carbon has two alkyl groups, it is said to be aliphatic ketone.  If carbonyl carbon has one alkyl group and one aryl group, it is said to be aromatic aliphatic ketone.  If carbonyl carbon has two aryl groups, it is said to be aromatic ketones.

What are Carbonyl Compounds?

In inorganic chemistry, 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). In organic chemistry, a carbonyl group can be described as a functional group 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. The compounds containing a carbonyl group ( -C=O) are called carbonyl compounds. Examples of Organic Carbonyl Compounds: These include carbamates, urea, and also the derivatives of phosgene, acyl chlorides chloroformates, carbonate esters, lactones, thioesters, lactams, isocyanates, and hydroxamates.

What is the structure of the Carbonyl Group?

In the carbonyl group, the carbon atom has an sp² hybridisation and is bonded to three other atoms through a sigma bond and adopts a trigonal planar geometry with bond angles at nearly 120^o. 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. The fourth valence electron of the carbon atom remains in its unhybridized 2p-orbital. This 2p-orbital develops a 𝜋 bond by overlapping ‘side-by-side’ with the 2p-orbital of the oxygen atom. Besides, the oxygen atom also contains two non-bonded pairs of electrons. The length of the double bond of the carbonyl group is about 1.2 angstroms, and it has a strength of about 176–179 Kcal/mol. 

The structure of carbonyl group is given below

How to identify Carbonyl Compounds?

To identify the presence of carbonyl compounds such as aldehydes and ketones, the best method is to perform a 2,4-DNP test.The 2,4-DNP reagent is abbreviated as 2,4-dinitrophenyl hydrazine. Aldehydes and ketones react with 2,4-dinitrophenylhydrazine reagents to give a yellow or red-orange precipitate.This test is used for identification of both aldehyde and ketones.

The formation of imines from ketones or aldehydes is exploited through the reagent 2,4-dinitrophenylhydrazine. In this addition-elimination reaction, the primary amino group of the 2,4-DNP attacks the carbonyl of an aldehyde or ketone in an acidic environment. The condensation reaction results in the formation of a hydrazone, which precipitates out of the solution.

Chemical reactions of aldehydes and ketones with 2,4-DNP is given below:

Yellow precipitates indicate non-conjugated ketones or aldehydes, whereas red-orange precipitates indicate conjugated systems. This test is used to differentiate ketones and aldehydes from alcohols and esters with which 2,4-DNP does not react, and thus no precipitate is formed. The ketone or aldehyde derivatives are crystalline materials with very well-defined melting points reported in the literature and can be used to identify specific compounds.

Applications of Carbonyl Compounds

1. Formaldehyde (prepared by the oxidation of methanol) is gas but is generally handled as a 37 percent solution in water, called formalin. It is used in tanning, preserving, and embalming and as a germicide, fungicide, and insecticide for plants and vegetables, but its largest application is in the production of certain polymeric materials. 
2. The plastic bakelite is made by a reaction between formaldehyde and phenol. These polymers are used not only as plastics but even more importantly as adhesives and coatings. Plywood consists of thin sheets of wood glued together by one of these polymers. In addition to Bakelite, the trade names Formica and Melmac are used for some of the polymers made from formaldehyde.
3. The carbonyl compound propanone is used as a solvent since it gets dissolved in water as well as other organic solutions.
4. Butanol is used to provide fragrance for keeping the bread fresh.
5. Acetaldehyde is used as a Synthesizer in many organic reactions.
6. Certain aldehydes occur naturally in flavouring agents. Among these are benzaldehyde, which provides the odour and flavour of fresh almonds; cinnamaldehyde, or oil of cinnamon; and vanillin, the main flavouring agent of vanilla beans.
7. In addition, certain aldehydes perform essential functions in humans and other living organisms. Examples are carbohydrates (including sugars, starch, and cellulose), which are based on compounds with an aldehyde or ketone group and hydroxyl groups.

Carbonyl Compound Detection Experiment:

Experiment Title – 

Tests are given by both aldehydes and ketones, i.e. 2,4-DNP test

Experiment Description – 

2,4-Dinitrophenylhydrazine can be used to analyse aldehydes and ketones by forming yellow-orange precipitates qualitatively. Let’s confirm.

Aim of Experiment – 

To identify the presence of carbonyl compounds using the 2,4-dinitrophenylhydrazine test.

Material Required – • 

Test tube holder: One

2,4-Dinitrophenylhydrazine reagent 

Test tubes: As per need

Compound to be tested

Procedure – 

1. Take 2-3 drops of the liquid compound in a test tube.
2. Add a few drops of an alcoholic solution of 2,4-dinitrophenylhydrazine. 
3. The appearance of a yellow-orange precipitate confirms the presence of a carbonyl group. 
4. If precipitation does not appear at room temperature, warm the mixture in a water bath for a few minutes and cool.

Precautions – 

1. Always use freshly prepared Brady’s reagent to perform the test. 
2. Do not heat the reaction mixture directly on flame if precipitation does not appear.

FAQs on Carbonyl Compounds