What are Aldehydes and Ketones?
Aldehydes and Ketones are both simple organic compounds that contain a carbonyl group. A carbonyl group contains a carbon-oxygen double bond. The aldehydes and ketones organic compounds are quite simple due to the carbon atom present in the carbonyl group lacking reactive groups like Cl or OH.
Both organic compounds incorporate a carbonyl functional group, as C=O. These are the organic compounds, having the structures RC(=O)R’ and -CHO, where R and R’ represents the carbon-containing substituents, respectively.
What is Aldehyde?
Aldehydes contain the carbonyl group, having one hydrogen atom attached to it together with either a hydrogen group or a 2nd hydrogen atom, which can be the one containing a benzene ring or an alkyl group.
Aldehyde examples can be given as follows.
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We can notice that all the compounds given above have the exact same end to the molecule. The one and only difference is the complexity of the other attached group.
What are Ketones?
Ketones contain the carbonyl with 2 hydrocarbon groups attached to it. These are either the ones containing either the alkyl groups or the benzene rings. They do not have any hydrogen atom attached to the carbonyl group.
Ketone examples can be given as follows:
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In general, propane is written as CH3COCH3. The carbonyl group in pentanone could be in the middle of the chain or next to the end by giving either pentane-2-one or pentane-3-one.
Occurrence of Aldehydes and Ketones
Combined with the other functional group, aldehydes and ketones are widespread in nature. The compounds such as vanillin (vanilla bean), Citra (lemongrass), cinnamaldehyde (cinnamon bark), helminthosporal (a fungal toxin), camphor (camphor trees), and carvone (spearmint and caraway) are found chiefly in plants or microorganisms. Whereas the compounds such as testosterone (male sex hormone), progesterone (female sex hormone), cortisone (adrenal hormone), and muscone (musk deer) have a human and animal origin.
Preparation of Aldehydes and Ketones
Aldehydes and Ketone compounds can be prepared by various methods. Let us discuss those below:
Formation by Oxidation of Alcohols
The primary and secondary oxidation of alcohol leads to the formation of aldehydes and ketones. Oxidation becomes possible, using the common oxidizing agents such as K2Cr2O7, KMnO4, and CrO3. The strong oxidizing agents help in the oxidation of primary alcohol to aldehyde and then to a carboxylic acid.
Primary alcohols that have low molecular weight can undergo oxidation and form aldehydes. The reaction mixture after the formation of aldehyde can avoid further oxidation if the temperature of the reaction is modulated so that the aldehyde’s boiling point is lower than the alcohol, which helps in the aldehyde distillation from the reaction mixture soon after the formation of aldehyde. Thus, it is essential to maintain a reaction temperature of more than 349K slightly. Let us look at the reaction given below.
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The preparation of Aldehyde and Ketone is possibly done by the oxidation of primary and secondary alcohol by agents like Collins reagents (Chromium trioxide-pyridine complex), PCC (pyridinium chlorochromate), and Cu at 573 K.
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Collin’s Reagents (Chromium Trioxide-Pyridine Complex)
Collin’s reagent, which is also called the chromium trioxide-pyridine complex, is a good oxidizing reagent for the conversion of a primary alcohol to aldehydes. In addition, Collin’s reagent has an advantage, which is, it helps to cease the further oxidation of aldehydes to carboxylic acids. However, the reaction with Collin’s reagent becomes possible in a non-aqueous medium like CH2Cl2.
PCC (Pyridinium Chlorochromate)
The pyridine mixture, along with the HCl and CrO3 in dichloromethane, leads to the formation of PCC (C5H5NH+CrO3 Cl–) or Pyridine chlorochromate.
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We can prepare the ketones by using similar oxidizing agents from the secondary alcohols.
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Formation by Alcohols Dehydrogenation
This method of preparation applies in case of the conversion of volatile alcohols to aldehydes. In general, it is used in industrial applications. The alcohol vapors are passed through the heavy metal catalysts like Ag or Cu in this respective technique. Besides, primary alcohol produces aldehyde, and secondary alcohol produces ketones, respectively.
As an example, alcohols undergo dehydrogenation when vapors of either primary alcohol or secondary alcohol pass through the copper gauze at a temperature of 573 K. The example given below represents how n-propyl alcohol leads to the propionaldehyde formation in the process of dehydrogenation.
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It is also possible to use various metal catalysts like silver or copper under heating conditions during the dehydrogenation of alcohol. However, this particular technique is apt for the conversion of valuable alcohols into aldehydes. Furthermore, it is much useful in industrial applications.
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Also, this is one of the better methods for the preparation of aldehydes and ketones because further oxidation is not possible for aldehydes. Therefore, there is no conversion risk of aldehydes to carboxylic acids.
This is one of the better methods used for the preparation of Aldehyde and Ketone. In addition, there are also other methods used based on the requirements.
FAQs on Aldehydes and Ketones
1. What are aldehydes and ketones, and what is their common functional group?
Aldehydes and ketones are organic compounds that both contain the carbonyl group (C=O) as their functional group. An aldehyde has at least one hydrogen atom attached to the carbonyl carbon, giving it the structure R-CHO. A ketone has two hydrocarbon groups (alkyl or aryl) attached to the carbonyl carbon, with a general structure of R-CO-R'.
2. How do the structures of aldehydes and ketones primarily differ?
The main structural difference lies in the atoms bonded to the carbonyl carbon (C=O).
- In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom. This structure makes the aldehyde group terminal, meaning it is always at the end of a carbon chain.
- In a ketone, the carbonyl carbon is bonded to two carbon-containing groups (alkyl or aryl). This makes the ketone group non-terminal, as it is always located within a carbon chain.
3. How can you chemically distinguish between an aldehyde and a ketone?
You can distinguish between aldehydes and ketones using mild oxidising agents, as aldehydes are easily oxidised while ketones are not. The two most common laboratory tests are:
- Tollens' Test (Silver Mirror Test): Aldehydes react with Tollens' reagent (ammoniacal silver nitrate) to form a distinct silver mirror on the inner surface of the test tube. Ketones do not give this test.
- Fehling's Test: When heated with Fehling's solution, aliphatic aldehydes form a reddish-brown precipitate of copper(I) oxide. Ketones do not react.
4. What are some common methods for preparing aldehydes and ketones as per the CBSE syllabus?
As per the CBSE 2025-26 syllabus, common preparation methods include:
- Oxidation of Alcohols: Controlled oxidation of primary alcohols yields aldehydes, whereas the oxidation of secondary alcohols produces ketones.
- Dehydrogenation of Alcohols: Passing the vapours of primary or secondary alcohols over heated copper (at 573 K) gives aldehydes and ketones, respectively.
- Ozonolysis of Alkenes: The cleavage of alkenes using ozone, followed by a reaction with zinc dust and water, yields aldehydes, ketones, or a mix of both.
- Friedel-Crafts Acylation: Aromatic ketones are prepared by treating an aromatic hydrocarbon with an acid chloride in the presence of a Lewis acid catalyst like anhydrous AlCl₃.
5. Why are aldehydes generally more reactive than ketones in nucleophilic addition reactions?
Aldehydes are more reactive than ketones towards nucleophiles for two main reasons:
- Electronic Factors: The two alkyl groups in a ketone are electron-donating, which reduces the positive charge on the carbonyl carbon and makes it less electrophilic (less attractive to nucleophiles). Aldehydes only have one such group.
- Steric Factors: A nucleophile can easily attack the carbonyl carbon in an aldehyde because it is only blocked by a small hydrogen atom. In contrast, the two bulkier alkyl/aryl groups in a ketone create steric hindrance, making it more difficult for the nucleophile to approach.
6. What are some examples of aldehydes and ketones found in everyday life?
These compounds are widespread and have important applications. For instance:
- Formaldehyde (an aldehyde) is used as a biological preservative and in manufacturing resins like Bakelite.
- Acetone (a ketone) is a common industrial solvent and the main ingredient in most nail polish removers.
- Cinnamaldehyde (an aldehyde) is the compound responsible for the distinct flavour and aroma of cinnamon.
- Vanillin (an aldehyde) is the primary flavouring agent extracted from vanilla beans.
7. Why do aldehydes and ketones have higher boiling points than alkanes but lower than alcohols of similar molecular mass?
This trend is explained by their intermolecular forces:
- Their boiling points are higher than nonpolar alkanes because the polar carbonyl group (C=O) leads to strong dipole-dipole interactions between molecules.
- Their boiling points are lower than alcohols because they cannot form intermolecular hydrogen bonds with each other, as they lack a hydrogen atom bonded to an oxygen atom (-OH group). Alcohols have strong hydrogen bonding, which requires more energy to break.
8. Can a ketone be easily oxidised like an aldehyde? Explain why or why not.
No, a ketone cannot be easily oxidised. The oxidation of an aldehyde involves breaking the relatively weak carbon-hydrogen (C-H) bond on the carbonyl group. In a ketone, there is no such C-H bond. Oxidising a ketone requires breaking a much stronger carbon-carbon (C-C) bond, which needs vigorous conditions (strong oxidising agents and high temperatures) and results in the cleavage of the molecule into smaller carboxylic acids.
9. How are aldehydes and ketones named using the IUPAC system?
The IUPAC nomenclature rules for these compounds are as follows:
- For aldehydes, the '-e' from the name of the parent alkane is replaced with the suffix '-al'. For example, a two-carbon aldehyde is named ethanal. The aldehyde group is always considered carbon-1.
- For ketones, the '-e' from the parent alkane's name is replaced with '-one'. The position of the carbonyl group must be indicated with a number in the carbon chain. For example, CH₃COCH₂CH₃ is named butan-2-one.
10. What is the general mechanism of nucleophilic addition to the carbonyl group?
The nucleophilic addition reaction to the C=O group typically follows a two-step mechanism:
- Step 1: Attack of the Nucleophile: A nucleophile (Nu⁻) attacks the partially positive (electrophilic) carbonyl carbon. This breaks the weaker pi (π) bond, and its electrons shift onto the oxygen atom, forming a tetrahedral alkoxide intermediate.
- Step 2: Protonation: The negatively charged alkoxide intermediate is unstable and quickly accepts a proton (H⁺) from the reaction medium (e.g., water or acid) to form the final, neutral addition product.
