Introduction to Aldehydes
Carbonyl compounds are the organic compounds in which carbon-oxygen double bonds are present. In organic chemistry, one of the most essential functional groups is carbonyl carbon. These compounds are widely used in the synthesis of solvents and also in the manufacturing of several reagents and chemicals in the industries. Carbonyl compounds are mainly of two types- Aldehydes, and ketones. Aldehydes and ketones contain a carbonyl group which is referred to as a simple organic compound. These functional groups contain a carbon-oxygen double bond. In the carbonyl group, the carbon present lacks reactive groups such as Cl or OH which make these organic compounds simple.
Aldehydes
Aldehydes are the compounds in which hydrogen and carbon are attached to carbonyl groups whereas ketones are the compounds in which two carbons are attached to the carbonyl group.
Aldehydes are the organic compounds and classes of carbonyl group which is surrounded by hydrogen and Alkyl group (R).
In Aldehyde, the Carbonyl group contains one alkyl group on one end and hydrogen on the other end. The aldehyde in the condensed form is written as -CHO. Here, Ar and R denote the aryl and alkyl members respectively.
Physical Properties and Characterization of Aldehydes
Aldehydes have very diverse properties. These diverse properties depend on the remainder of the molecule. Aldehydes that are more soluble in water are generally smaller in size. Acetaldehyde and formaldehyde and acetaldehyde are completely soluble in water. Pungent odours are associated with more volatile aldehydes. Other properties of aldehydes are unique to each type of aldehyde.
Spectroscopic methods are generally used to identify aldehydes. A strong νCO band near 1700 cm−1 is displayed with the help of IR spectroscopy. The formyl hydrogen center absorbs near δH 9.5 to 10 in their 1H NMR spectra. This constitutes a distinctive part of the spectrum.
The General Method of Preparation of Aldehydes
Depending upon the requirements and types of aldehydes (Aliphatic, Aromatic, and Cyclic), there are several methods that can be used for the preparation of aldehydes.
Functional Group Transformations
In functional group transformation conversion of any functional group into aldehydes, the functional group takes place. Aldehydes can be formed by oxidation of alcohols or by the reduction of acid nitriles, chlorides or esters.
C-C Bond Cleavage
Aldehydes can also be prepared from suitably substituted alkenes upon their ozonolysis.
Methods of Preparation of Aldehydes
Aldehydes can be prepared by several methods but one of the best ways for preparing the aldehydes includes by oxidation of primary alcohols. For the successful oxidation of primary alcohol, these mild oxidizing agents like DMP, PCC, and Swern are pretty much important. It can also be prepared by using some reducing agents such as DIBAL during the reduction of some carbonyl compounds.
Preparation of Aldehydes from Alcohols
Aldehydes and ketones can be formed by the oxidation of primary and secondary alcohols respectively. KMnO4, CrO3, and K2Cr2O7 are some of the oxidizing agents which help in the oxidation of primary and secondary alcohols.
Aldehydes can also be prepared by the oxidation of primary alcohols or by some other reagents such as Collin's reagent (chromium trioxide pyridine complex, CrO3.2C5H5N), Pyridinium chlorochromate (PCC) and by Copper (Cu) at 573K.
By Collin's Reagent
It is the 1:2 mixture of chromium trioxide and pyridine in dichloromethane. Collins reagent is considered as a good oxidizing reagent for the preparation of aldehydes by the oxidation of primary alcohols because it helps to prevent further oxidation to a carboxylic acid. This action is only possible in a non-aqueous medium like CH2Cl2.
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PCC ( Pyridinium Chlorochromate)
It is the mixture of Cr2O3, HCl, and pyridine in 1:1:1. This leads to the formation of PCC (C5H5N+HCrO3Cl-).
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Dehydrogenation of Alcohols
This method is applied for volatile alcohols to convert them into aldehydes. It is widely used in industrial applications. In this technique, Primary alcohols are passed over heavy metal catalysts like Cu to obtain a product that is an aldehyde.
For example, when primary alcohols undergo dehydrogenation then its vapors pass over copper gauze at 573K temperature.
Here, n-Propyl alcohol undergoes dehydrogenation after passing over copper at 573K temperature to form a proportion aldehyde.
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During the dehydrogenation of alcohols, various metal catalysts use silver under certain heating conditions. However, this method is considered as one of the best methods for the aldehydes preparation from alcohol because aldehydes cannot further oxidize and there is no risk of the formation of carboxylic acid from the conversion of aldehyde.
Moreover, it is also considered a helpful conversation between aldehydes and valuable alcohol.
Preparation from Hydrocarbons
The aldehyde can be prepared from hydrocarbons depending upon their structures.
Ozonolysis of Alkenes
When Ozone is added to an alkene in the presence of chloroform it gives an additional product known as Ozonide. Upon reduction of ozonide which when heated with zinc dust it gives an aldehyde or Ketone depending upon the structure of alkene. This reaction is known as the analysis of alkenes.
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Hydration of Alkynes
When alkynes are hydrolyzed in the presence of H2SO4 at HgSO4 at 333 K, it gives an enol i.e (alkane OH) group which undergoes tautomerism.
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Preparation of Aromatic Aldehyde
Gattermann-koch Reaction
When benzene is treated with the mixture of carbon monoxide and HCL in presence of anhydrous AlCl3, CuCl gives benzaldehyde.
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Side-Chain Halogenation
The aromatic aldehyde can also be prepared by side-chain halogenation most preferably side-chain chlorination followed by hydrolysis.
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FAQs on Preparation of Aldehydes
1. What are the two main ways to prepare aldehydes from alcohols?
Aldehydes can be prepared from primary alcohols using two primary methods as per the CBSE Class 12 syllabus for the academic year 2025-26:
- Controlled Oxidation: This involves using a mild oxidising agent like Pyridinium Chlorochromate (PCC). PCC selectively oxidises primary alcohols to aldehydes and prevents their further oxidation into carboxylic acids.
- Catalytic Dehydrogenation: In this industrial method, the vapours of a primary alcohol are passed over a heated heavy metal catalyst, such as copper (Cu) at 573 K. This process removes hydrogen to form an aldehyde.
2. How can acetaldehyde be prepared from an alkyne?
Acetaldehyde (ethanal) is prepared by the hydration of ethyne (acetylene). This reaction involves passing ethyne gas through a solution of dilute sulphuric acid (H₂SO₄) and mercuric sulphate (HgSO₄) at about 333 K. An unstable intermediate, vinyl alcohol, is formed, which then undergoes tautomerism to yield the more stable acetaldehyde.
3. Explain the Etard reaction for preparing benzaldehyde.
The Etard reaction is a specific method for synthesising benzaldehyde from toluene (methylbenzene). In this reaction, toluene is treated with chromyl chloride (CrO₂Cl₂) in a non-polar solvent like carbon disulphide (CS₂). This forms a brown chromium complex, which upon subsequent hydrolysis yields benzaldehyde. This method is crucial as it stops the oxidation at the aldehyde stage.
4. Why is a mild oxidising agent like PCC essential for preparing aldehydes from primary alcohols?
Using a mild oxidising agent like Pyridinium Chlorochromate (PCC) is critical because aldehydes themselves are easily oxidised. Strong oxidising agents, such as potassium permanganate (KMnO₄), would not stop at the aldehyde stage. They would immediately continue the oxidation process, converting the newly formed aldehyde into a carboxylic acid. PCC is selective and lacks the water needed for over-oxidation, ensuring the reaction halts at the desired aldehyde product.
5. What is the fundamental difference between the Rosenmund reduction and the Stephen reaction?
Both the Rosenmund reduction and the Stephen reaction produce aldehydes, but they differ fundamentally in their starting materials and reagents:
- Starting Material: The Rosenmund reduction starts with an acyl chloride (acid chloride), R-COCl. The Stephen reaction begins with a nitrile (cyanide), R-CN.
- Reagents: Rosenmund reduction uses catalytic hydrogenation with a poisoned catalyst (H₂ with Pd/BaSO₄) to prevent further reduction to an alcohol. The Stephen reaction uses a two-step process involving reduction with stannous chloride and HCl (SnCl₂/HCl) to form an imine, followed by hydrolysis to get the aldehyde.
6. How does the Gattermann-Koch reaction introduce a formyl group onto a benzene ring?
The Gattermann-Koch reaction is a method of formylation used to prepare benzaldehyde directly from benzene. It works by treating benzene with a mixture of carbon monoxide (CO) and hydrogen chloride (HCl) gas. The reaction requires a catalyst, typically a mixture of anhydrous aluminium chloride (AlCl₃) and cuprous chloride (CuCl), under high pressure. This process effectively attaches a formyl group (-CHO) to the benzene ring, converting it into benzaldehyde.
7. Can ozonolysis of an alkene produce only an aldehyde? Explain with an example.
Yes, ozonolysis can produce only an aldehyde if the starting alkene is symmetrical and each carbon in the double bond is attached to at least one hydrogen atom. For example, the ozonolysis of But-2-ene (CH₃-CH=CH-CH₃) followed by a reductive workup with zinc dust and water cleaves the double bond. This breakage results in the formation of two molecules of the same aldehyde, acetaldehyde (ethanal), as each of the original doubly-bonded carbons becomes a carbonyl carbon of an aldehyde.
