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 Some Other Methods for the Preparation of Aldehydes?
Following are some other methods that are used for the preparation of aldehydes:
From Acyl Chloride:- Acid chlorides are treated with hydrogen gas in the presence of Pd-BaSO4, Sulphur, Xylene which are also known as Lindlar's catalyst. It gives an aldehyde and the reaction is known as Rosenmund reduction.
From Nitriles and Esters:- When acid nitriles are treated with stannous chloride (SnCl2) + HCL followed by hydrolysis it gives an aldehyde. This reaction is also known as Stephen's reduction.
Moreover, nitriles and esters undergo reduction by using DIBAL-H to form amines followed by the formation of aldehydes.
2. How Aromatic Aldehyde can be Prepared by the Oxidation of Methyl Benzene?
Toluene and its derivatives are converted to benzoic acid with the help of oxidizing agents after undergoing oxidation which is possibly stopped with the help of property agents at aldehyde state.
By Using Chromyl Chloride:- When toluene is treated with chromyl chloride (CrO2Cl2) in the presence of CS2 then it gives benzaldehyde which is known as Etard reaction.
By Using Chromic Oxide (CrO3):- Conversation of Toluene or its derivatives into benzylidene diacetate upon treatment of chromic acid (CrO3) in acidic anhydride which on hydrolysis give benzaldehyde.