Aldehydes

Aldehydes contain the aldehyde group (-CHO), also known as the formyl group. It is a monovalent group, meaning that the carbonyl carbon is attached to only one other group (either a hydrogen atom or a hydrocarbon group). The carbon atom in the aldehyde group is primary (1°) in nature because it is bonded to one alkyl group and one hydrogen atom.

1.0General Structure of Aldehydes

Aldehydes are organic compounds characterized by the presence of a carbonyl group (C=O) attached to at least one hydrogen atom. Their general formula is R−CHO, where R can be a hydrogen atom (in the case of formaldehyde) or a hydrocarbon group (in other aldehydes like acetaldehyde or benzaldehyde). The carbonyl group is bonded to one hydrogen atom and one R-group, making aldehydes highly reactive.

2.0Classification and Nomenclature of Aldehydes

Aldehydes are classified based on the nature of the R-group attached to the carbonyl carbon:

• Aliphatic Aldehydes: These aldehydes contain an alkyl group attached to the carbonyl group. Examples include formaldehyde (HCHO) and acetaldehyde (CH₃CHO).
• Aromatic Aldehydes: In aromatic aldehydes, the carbonyl group is attached to an aromatic ring. An example is benzaldehyde (C₆H₅CHO), where the R-group is a benzene ring.

1. Nomenclature of Aldehydes

Aldehydes are named according to the IUPAC (International Union of Pure and Applied Chemistry) system, as well as their common names. Below are the key rules for naming aldehydes:

IUPAC Naming of Aldehydes:

• Identify the parent chain: The longest carbon chain that includes the carbonyl group (C=O) is considered the parent chain.
• Replace the suffix: The "-e" from the corresponding alkane name is replaced with "-al." For example, methane becomes methanal and ethane becomes ethanal.
• Positioning of the carbonyl group: In aldehydes, the carbonyl group is always at the end of the carbon chain, so it automatically gets the number 1 position. Therefore, the position of the carbonyl group does not need to be mentioned in the name.
• Substituents: Any other groups attached to the parent chain (like methyl, ethyl, or halogens) are treated as substituents and their positions are indicated with numbers.

Examples of IUPAC Naming with Common names:

3.0Preparation of Aldehydes

1. From Acyl Chloride (Rosenmund Reduction): Acyl chlorides are hydrogenated over palladium on barium sulfate to form aldehydes. This is known as the Rosenmund reduction.

2. From Nitriles and Esters: Nitriles are reduced to imines with stannous chloride and hydrochloric acid, which are then hydrolyzed to aldehydes. This is called the Stephen reaction. Alternatively, nitriles and esters can be selectively reduced using diisobutylaluminium hydride (DIBAL-H), followed by hydrolysis to give aldehydes.

$\text{RCN}+{\text{SnCl}}_2+\text{HCl}\to \text{RCH}=\text{NH}\stackrel{{\text{H}}_2{\text{O}}^{+}}{\to }\text{RCHO}$

Similarly, esters are also reduced to aldehydes with DIBAL-H

3. From Hydrocarbons (Aromatic Aldehydes):

• By Oxidation of Methylbenzene: Oxidation of toluene using chromyl chloride (CrO₂Cl₂) yields benzaldehyde through a chromium complex in the Etard reaction. Alternatively, toluene can be oxidized to benzylidene diacetate with chromic oxide (CrO₃), followed by hydrolysis to form benzaldehyde.
• By Side Chain Chlorination: Toluene is chlorinated to benzal chloride, which on hydrolysis forms benzaldehyde. This method is commercially used.
• By Gatterman-Koch Reaction: Benzene or its derivatives are treated with carbon monoxide and hydrogen chloride in the presence of anhydrous AlCl₃ or CuCl to produce benzaldehyde or substituted benzaldehydes.

4.0Physical Properties of Aldehydes

5.0Chemical Properties of Aldehydes

6.0Tests for Aldehydes

1. Tollens' Test

Tollens' Test is a qualitative chemical test used to detect the presence of aldehydes in a compound. The test involves using Tollens' reagent, which is a solution of ammoniacal silver nitrate. Here is the Chemical Reaction-

$\text{RCHO}+2[{\text{Ag(NH}}_3{\text{)}}_2{]}^{+}+3{\text{OH}}^{-}\to {\text{RCOO}}^{-}+2\text{Ag(s)}+2{\text{H}}_2\text{O}+4{\text{NH}}_3$

Note- A positive test shows a shiny silver mirror on the inner surface of the test tube, indicating the presence of an aldehyde. Ketones do not react in this test.

2. Fehling's Test

Fehling's Test is a chemical test used to detect the presence of aldehydes, particularly aliphatic aldehydes. It distinguishes aldehydes from ketones. The test uses Fehling's solution, which is a mixture of two solutions: Fehling's A (a blue solution of copper(II) sulfate) and Fehling's B (a clear solution of sodium potassium tartrate and sodium hydroxide).

General Reaction:

$\text{RCHO}+2C{u}^{2+}+5O{H}^{-}\to {\text{RCOO}}^{-}+C{u}_{2}O(s)+3H_{2}O$

Note: A positive test shows a brick-red precipitate, indicating the presence of an aldehyde. Ketones do not give a positive result.

7.0Uses of Aldehydes

1. Formaldehyde: Used in the production of plastics (e.g., Bakelite), disinfectants, and as a preservative in laboratories.
2. Acetaldehyde: Used in the manufacture of acetic acid, perfumes, and flavoring agents.
3. Benzaldehyde: Used in the food industry for almond flavoring, and in the manufacture of dyes and perfumes.
4. Cinnamaldehyde: Found in cinnamon oil, used as a flavoring agent and in perfumes.
5. Glutaraldehyde: Used as a sterilizing agent and in leather tanning.