Step-by-Step Ketone Preparation Techniques and Examples
Ketones can be understood as an organic compound with a carbonyl group, where one carbon atom is bonded to an oxygen atom, whereas the other bonds are hydrocarbon radicals. The compounds have significant physiological properties and are used for medical purposes. Anti-inflammatory agents have ketone groups. Ketones are the building blocks of paints, solvents, lacquers, and textiles. Several manufacturing explosives are made up of ketones. Ketone is also used in the application of the hydraulic agents and preservation and tanning. In no small measure, the ketone is one of the most significant compounds for the growing industrial sector.
Structure of Ketones
A carbonyl group (C=O) has two R groups attached to it, and the R group needs to have at least one molecule of carbon compound. It is the R group that determines the type of ketone. Furthermore, the molecular formula of the same is represented as RCOR.
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Ketone Reactions
Ketones are considered to be one of the most reactive compounds in chemistry. Still, in comparison to the aldehydes, the ketones are less responsive and reactive. Ketones and aldehydes are similar, considering the carbonyl group forming the base of any chemical activity. The ketones have uneven electronic distribution, which is responsible for its polar nature. The unequal distribution is responsible for the carbon atoms having a positive charge.
In the case of the secondary alcohol, ketone preparation is possible with oxidation, resulting in the formation of ketonic compounds. However, in the case of ketones, they do not go through any further process of oxidation similar to that of the aldehydes.
The oxidation of secondary alcohol results in the forming of several oxidizing agents, including chromic acid, potassium permanganate pyridinium chlorochromate, and manganese dioxide.
Identifying Ketones
It is effortless to locate a ketone merely from the chemical word. In case the name of the chemical term ends with a suffice "one," then the compound has the presence of ketone in it.
To identify the ketone from its physical properties, you need to check the solution's water solubility and the boiling point. If it is a keystone, then the water solubility will be very high, and a boiling point will be high. When a solution has the presence of ketone, it can be marked by the charge of the molecules.
In case the solution has a high polarity that is the characteristic of electron hogging, it is considered to be very attractive relatively.
Preparation of Ketones
Several methods of Preparation of Ketones are widely practised in the chemical laboratories and on an industrial scale. Some of the methods have been listed down below:
Preparation of Ketones From Nitriles
Ketones under this process can be obtained by treating the nitriles with the Grignard reagent. After the solution is prepared, it needs to undergo hydrolysis to form the final product, ketone.
For example: When Magnesium is made to react with nitrile with an aqueous acid, then ketone is formed along with Ammonia and Magnesium salt is the residual. Some bonds are formed while some are broken in this chemical process of preparation of ketone from nitrates.
Preparation of Ketones by Dehydrogenation of Alcohols
In case, two hydrogen molecules are removed from the concerned alcohol molecule, then the dehydrogenation process of alcohol occurs. During the oxidation process, the C-O and the O-H strong bonds are broken down to form merely C=O bonds. Thus, to produce ketones, secondary alcohol must go through the process of dehydrogenation.
However, in the case of the tertiary alcohols after the oxidation process, the dehydrogenation must be altered with dehydration. Thus, when it comes to tertiary alcohol, alkalines are used for manufacturing.
Example: When n-Propyl alcohol undergoes an oxidation reaction under copper solution then Propionaldehyde is formed and hydrogen gas is released.
Preparation of Ketones from Acyl Chlorides
In case, the acyl chlorides are treated with as strong metal halide and then the Grignard reagent is made to react, ketones are formed. For example cadmium chloride when made to react with the reactive reagent, then the dialkyl cadmium is a solution obtained. The result formed then is prepared to act with acyl chlorides to create ketones. However, if you consider using Rosenmund's Reaction for the same, then the ketone will not be formed.
Ketones Preparation From Benzene and Substituted Benzenes to Form Aromatic Ketones
Aromatic ketone formation is a straightforward process with the help of benzenes and substituted benzenes. In chemistry, this is supposed to be the best-suited method to prepare aromatic ketone. In the technique, the benzene is treated with acid chloride to obtain ketone. Such a reaction is only possible in the presence of a catalyst such as an anhydrous aluminium chloride, which is a Lewis acid.
FAQs on Ketone Preparation – Methods and Reactions
1. What are the main laboratory methods for the preparation of ketones as per the CBSE syllabus for 2025-26?
According to the CBSE Class 12 curriculum, ketones can be prepared through several key reactions. The most common and important methods include:
- Oxidation of secondary alcohols: Using an oxidising agent to convert a secondary alcohol into a ketone.
- Ozonolysis of alkenes: Cleaving a carbon-carbon double bond in a suitably substituted alkene to form ketones.
- Hydration of alkynes: Adding water to an alkyne (other than ethyne) in the presence of a catalyst.
- Friedel-Crafts acylation: Preparing aromatic ketones by reacting an aromatic compound with an acyl chloride or anhydride.
- From nitriles: Using Grignard reagents to react with nitriles, followed by hydrolysis.
- From acyl chlorides: Reacting acyl chlorides with organometallic compounds like dialkylcadmium.
2. How are ketones prepared by the oxidation of secondary alcohols?
Ketones are prepared by the oxidation of secondary (2°) alcohols. In this reaction, the alcohol is treated with a suitable oxidising agent, such as chromic acid (H₂CrO₄), chromium trioxide (CrO₃), or pyridinium chlorochromate (PCC). The reaction involves the removal of two hydrogen atoms: one from the hydroxyl (-OH) group and another from the carbon atom attached to it. For example, the oxidation of propan-2-ol yields propanone (acetone).
3. Explain how an aromatic ketone like acetophenone is prepared using Friedel-Crafts acylation.
Acetophenone is a classic example of an aromatic ketone prepared via Friedel-Crafts acylation. This reaction involves treating benzene with an acylating agent like acetyl chloride (CH₃COCl) or acetic anhydride ((CH₃CO)₂O). The reaction requires a Lewis acid catalyst, typically anhydrous aluminium chloride (AlCl₃). The catalyst helps generate a highly reactive electrophile, the acylium ion (CH₃C⁺=O), which then attacks the benzene ring to form acetophenone (C₆H₅COCH₃).
4. How can you prepare a ketone from a nitrile using a Grignard reagent?
To prepare a ketone from a nitrile, a Grignard reagent (R-MgX) is used. The process involves two steps. First, the Grignard reagent performs a nucleophilic attack on the carbon atom of the nitrile group (-C≡N). This forms an intermediate imine-magnesium complex. In the second step, this intermediate is subjected to acid hydrolysis, which breaks it down to form a ketone. The R' group of the final ketone (R-CO-R') comes from the nitrile, while the R group comes from the Grignard reagent.
5. What is the mechanism for preparing propanone from propyne?
The preparation of propanone from propyne is an example of the hydration of alkynes. Propyne (CH₃-C≡CH) is treated with water in the presence of an acidic catalyst, typically a mixture of dilute sulphuric acid (H₂SO₄) and mercuric sulphate (HgSO₄). The addition of a water molecule across the triple bond follows Markovnikov's rule, where the -OH group attaches to the more substituted carbon atom. This initially forms an unstable intermediate called an enol (prop-1-en-2-ol). This enol immediately rearranges via a process called tautomerism to form the more stable keto form, which is propanone (CH₃COCH₃).
6. Why can't tertiary alcohols be oxidised to form ketones?
Tertiary (3°) alcohols cannot be oxidised to form ketones because they lack the necessary hydrogen atom on the carbinol carbon (the carbon atom bonded to the -OH group). The mechanism of oxidation requires the removal of a hydrogen atom from this specific carbon. Since a tertiary alcohol has three alkyl groups and no hydrogen atoms attached to its carbinol carbon, this crucial step cannot occur. Therefore, tertiary alcohols are resistant to oxidation under normal conditions.
7. For ketone synthesis, why is dialkylcadmium (R₂Cd) sometimes preferred over a Grignard reagent when reacting with acyl chlorides?
While both can react with acyl chlorides, dialkylcadmium (R₂Cd) is a much less reactive organometallic reagent compared to a Grignard reagent (RMgX). Grignard reagents are so reactive that after forming the ketone, they will immediately attack it in a second step to produce a tertiary alcohol. Dialkylcadmium is reactive enough to react with the highly reactive acyl chloride but is not reactive enough to attack the ketone product. This allows the reaction to stop cleanly at the ketone stage, making it a more controlled and effective method for this specific synthesis.
8. What structural feature of an alkene is necessary for it to produce a ketone upon ozonolysis?
For an alkene to produce a ketone upon ozonolysis followed by a reductive workup (with Zn/H₂O), at least one of the carbon atoms involved in the double bond must be disubstituted. This means the carbon atom must be bonded to two other carbon atoms and have no hydrogen atoms directly attached. When the double bond is cleaved during ozonolysis, this disubstituted carbon atom becomes the carbonyl carbon of a ketone. If the carbon atom has one hydrogen, it forms an aldehyde; if it has two hydrogens, it forms formaldehyde.
