Gattermann Koch Reaction Mechanism & Stepwise Process
Gattermann Koch reaction is essential in chemistry and helps students understand various practical and theoretical applications related to this topic. This reaction is especially important for students preparing different exams as it illustrates a key method for the synthesis of aromatic aldehydes such as benzaldehyde, linking organic name reactions to both industry and research labs.
What is Gattermann Koch Reaction in Chemistry?
A Gattermann Koch reaction refers to the introduction of a formyl group (-CHO) into an aromatic ring such as benzene, using carbon monoxide (CO) and hydrochloric acid (HCl) in the presence of a catalyst like anhydrous aluminium chloride (AlCl3) and cuprous chloride (CuCl).
This concept appears in chapters related to aromatic compounds, aldehydes and ketones, and named organic reactions, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The Gattermann Koch reaction does not have a single molecular formula, but its main example is the formylation of benzene. The general composition is:
C6H6 + CO + HCl → (AlCl3, CuCl) → C6H5CHO + HClHere, benzene (C6H6) reacts with carbon monoxide and hydrochloric acid under the influence of Lewis acid catalysts to form benzaldehyde (C6H5CHO).
Preparation and Synthesis Methods
Industrially and in laboratories, the Gattermann Koch reaction is carried out by passing a mixture of CO and HCl through a solution of benzene in the presence of dry AlCl3 and a small amount of CuCl.
The main purpose is to directly introduce the formyl group into the aromatic ring, thus synthesizing aromatic aldehydes efficiently.
Physical Properties of Benzaldehyde (Key Product)
Benzaldehyde, the primary product, appears as a colorless liquid with a bitter almond odor. It has a boiling point of 179°C, is slightly soluble in water, and is denser than water. These physical attributes make it easy to identify in organic chemistry lab work.
Chemical Properties and Reactions
The Gattermann Koch reaction is an example of an electrophilic aromatic substitution. The formyl cation (HCO+) generated acts as an electrophile and replaces a hydrogen atom on the aromatic ring.
The reaction does not work well on deactivated rings such as nitrobenzene and is generally not applicable for phenol or phenol ethers. Benzaldehyde produced can participate in further reactions like oxidation to benzoic acid or condensation reactions.
Frequent Related Errors
- Confusing Gattermann Koch reaction with the Gattermann reaction (which uses HCN, not CO).
- Assuming the reaction works with phenol or deactivated aromatic rings—it typically does not.
- Forgetting the need for both AlCl3 and CuCl as catalysts for efficiency.
- Ignoring controlled conditions—high pressure and dryness are crucial for safety and product yield.
Uses of Gattermann Koch Reaction in Real Life
The Gattermann Koch reaction is widely used to prepare aromatic aldehydes like benzaldehyde, which are found in flavorings, perfumes, dyes, and pharmaceuticals.
Benzaldehyde serves as an intermediate in the manufacture of drugs and other specialty chemicals. This reaction is also important for organic synthesis research and educational labs.
Relation with Other Chemistry Concepts
Gattermann Koch reaction is closely related to Freidel Crafts Reactions, as both rely on electrophilic aromatic substitution using Lewis acid catalysts. It is also conceptually connected to the Gattermann reaction (uses HCN with ZnCl2), Reimer-Teimann reactions, and Etard reaction for formylation or oxidation of aromatic rings.
Step-by-Step Reaction Example
1. Start with the reaction setup.2. Generation of the formyl cation.
3. Electrophilic attack.
4. Deprotonation and restoration of aromaticity.
5. Final Answer: Aromatic aldehyde (benzaldehyde) is produced as the main product.
Lab or Experimental Tips
Remember the Gattermann Koch reaction by the rule of "CO + HCl + AlCl3/CuCl yields -CHO on benzene." Vedantu educators use this mnemonic to help students recall the required reagents and catalysts.
Always conduct this reaction in a fume hood due to the toxic gases involved and ensure all apparatus is dry to prevent side reactions.
Try This Yourself
- Write the IUPAC name of benzaldehyde.
- Predict the product if toluene is subjected to the Gattermann Koch reaction.
- Explain why phenol does not undergo the Gattermann Koch reaction under normal conditions.
- Compare the main difference between the Gattermann Koch and Gattermann reactions.
Final Wrap-Up
We explored the Gattermann Koch reaction—its mechanism, significance in syntheses of aromatic aldehydes like benzaldehyde, and the catalysts required for efficiency. Understanding this reaction helps you interconnect multiple organic chemistry chapters and prepares you for exam questions.
Related Topics for Deeper Learning: Gattermann Reaction | Aromatic Compounds
FAQs on Gattermann Koch Reaction Explained for Students
1. How can the Gattermann-Koch reaction be explained in simple terms for a Class 12 student?
Think of it as a way to attach a formyl group (–CHO) directly onto an aromatic ring like benzene. Using carbon monoxide (CO) and hydrogen chloride (HCl) gas, this reaction converts a simple aromatic hydrocarbon into an aromatic aldehyde. It's a key method for preparing benzaldehyde, which is an important chemical.
2. What is the main purpose or application of studying the Gattermann-Koch reaction?
The primary importance of this reaction is in the industrial synthesis of aromatic aldehydes. For example, it is a direct method to produce benzaldehyde from benzene. Benzaldehyde is widely used as a precursor for:
- Dyes and pigments
- Perfumes and flavouring agents
- Pharmaceutical compounds
3. What is the step-by-step mechanism of the Gattermann-Koch reaction?
The mechanism is a type of electrophilic aromatic substitution that happens in three main steps:
- Step 1: Generation of the electrophile. Carbon monoxide and HCl react in the presence of the catalyst (AlCl₃/CuCl) to form an unstable formyl cation ([HCO]⁺).
- Step 2: The electron-rich benzene ring attacks the positively charged formyl cation, forming a resonance-stabilised carbocation intermediate.
- Step 3: A base removes a proton (H⁺) from the intermediate, restoring the ring's aromaticity and yielding the final product, benzaldehyde.
4. What is the key difference between the Gattermann reaction and the Gattermann-Koch reaction?
The main difference lies in the reagents used to generate the formyl group. The Gattermann-Koch reaction uses a mixture of carbon monoxide (CO) and HCl, whereas the original Gattermann reaction uses hydrogen cyanide (HCN) and HCl. Because CO is less toxic and more convenient than HCN, the Gattermann-Koch modification is often preferred.
5. Why is a mixture of AlCl₃ and CuCl used as a catalyst, not just one?
Both catalysts play crucial, distinct roles. Aluminium chloride (AlCl₃) is the primary Lewis acid that helps generate the reactive formyl cation electrophile. However, the formyl cation is very unstable. Cuprous chloride (CuCl) helps to stabilise this intermediate, making the reaction more efficient and increasing the overall yield of the aromatic aldehyde.
6. Are there any limitations on which compounds can be used in the Gattermann-Koch reaction?
Yes, the reaction has significant limitations. It works well for benzene and its activated derivatives (like toluene). However, it is not suitable for:
- Phenols and anilines, as the catalyst complexes with the -OH or -NH₂ group.
- Aromatic rings with strong electron-withdrawing groups (like nitrobenzene), as they are too deactivated to undergo electrophilic attack.
- Polycyclic aromatic compounds like naphthalene.
7. How does the Gattermann-Koch reaction relate to other named reactions in the Class 12 syllabus?
This reaction is an important part of the chapter on 'Aldehydes, Ketones, and Carboxylic Acids'. It is one of several key methods for preparing aldehydes. You can compare it with:
- Reimer-Tiemann reaction: Used to formylate phenols, not hydrocarbons.
- Rosenmund reduction: Prepares aldehydes from acyl chlorides.
- Stephen reaction: Prepares aldehydes from nitriles.
Each reaction has a specific starting material and use case.
8. What is the general chemical equation for the Gattermann-Koch reaction?
The general equation shows the formylation of an aromatic ring. For benzene, the specific reaction is:
C₆H₆ (Benzene) + CO + HCl → C₆H₅CHO (Benzaldehyde)
This reaction is carried out under high pressure and in the presence of the AlCl₃/CuCl catalyst.
