Stephen reaction is also known as Stephen aldehyde synthesis as the end product of this reaction is aldehydes. It is another important named reaction of organic chemistry. It was discovered by Henry Stephen that’s why it is known as Stephen Reaction or Stephen Aldehyde Synthesis. Henry Stephen is an English chemist. Stephen reaction is an organic redox reaction. It is also called Stephen Reduction Reaction.
Stephen Reaction
In Stephen reaction nitrile reacts with tin chloride and hydrochloric acid and forms iminium salt then quenching of iminium salt with water gives aldehyde. We get ammonium chloride or ammonia also as a byproduct. Reaction can be written as follows-
Examples of Stephen Reaction
Following are the examples of Stephen reaction –
Conversion of acetonitrile into acetaldehyde.
Conversion of benzonitrile into benzaldehyde.
Stephen Reaction Mechanism
Tin(II) chloride reacts with hydrochloric acid and forms tin tetra chloride and two H+ ions. As Sn+2 changes into Sn+4 two electrons are released. Reaction is given below –
We are using methyl cyanide (as alkyl cyanide) to show the mechanism. In cyanide group nitrogen is more electronegative than carbon. So, its partially negative and carbon is partially positive. One pie bond breaks between carbon and nitrogen of cyanide and electrons move to nitrogen atom simultaneously two electrons released in the above reaction accepted by carbon atom of cyanide. Thus, both carbon and nitrogen atoms get negative charge on them. Reaction is given below –
Now it reacts with two H+ ions and form iminium. Then iminium reacts with water and gives desired product aldehyde. In the process ammonia is released as by product. Reaction is given below –
Stephen reaction is used for production of aldehydes by using alkyl cyanides.
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FAQs on Stephen Reaction Mechanism
1. What is the Stephen reaction and what is its primary use in organic chemistry?
The Stephen reaction, also known as the Stephen aldehyde synthesis, is a chemical reaction that converts a nitrile (R-C≡N) into an aldehyde (R-CHO). Its primary use is as a specific method for preparing aldehydes from nitriles using stannous chloride (SnCl₂) and hydrochloric acid (HCl) as reagents, followed by hydrolysis.
2. What is the step-by-step mechanism of the Stephen reaction?
The mechanism of the Stephen reaction occurs in two main stages:
- Step 1: Reduction to Iminium Salt: The nitrile is first treated with stannous chloride (SnCl₂) in the presence of hydrochloric acid (HCl). The SnCl₂ acts as a reducing agent, reducing the nitrile to a stable intermediate called an aldimine hydrochloride salt (an iminium salt).
- Step 2: Hydrolysis: The resulting iminium salt is then subjected to hydrolysis (reaction with water, often with heating). This step breaks the carbon-nitrogen double bond of the imine and replaces it with a carbon-oxygen double bond, yielding the final aldehyde product and ammonium chloride.
3. Can you provide a common example of the Stephen reaction as per the Class 12 syllabus?
A classic example is the preparation of ethanal (acetaldehyde) from ethanenitrile (acetonitrile). The reaction is as follows:
CH₃C≡N (Ethanenitrile) + 2[H] ---[SnCl₂/HCl]--> CH₃CH=NH (Ethanimine) ---[H₂O, Δ]--> CH₃CHO (Ethanal) + NH₃
This reaction demonstrates the conversion of a nitrile group into an aldehyde group, which is a key preparation method in the 'Aldehydes, Ketones, and Carboxylic Acids' chapter.
4. Why are stannous chloride (SnCl₂) and hydrochloric acid (HCl) specifically used in the Stephen reaction?
Both reagents play crucial and distinct roles. Stannous chloride (SnCl₂) is the primary reducing agent; it donates electrons to the nitrile's carbon atom. Hydrochloric acid (HCl) provides the acidic medium necessary for the reaction and protonates the nitrogen atom, making the nitrile more susceptible to reduction. It also forms the stable iminium salt intermediate, preventing over-reduction of the nitrile all the way to a primary amine.
5. How does the Stephen reaction compare to using DIBAL-H for preparing aldehydes from nitriles?
Both methods convert nitriles to aldehydes, but they differ in their reagents and conditions.
- The Stephen reaction uses stannous chloride in a strong acidic medium (HCl) and is a classic, named reaction.
- Reduction with DIBAL-H (Diisobutylaluminium hydride) is a more modern, versatile, and often higher-yielding method. It is a powerful but selective reducing agent that works at low temperatures (e.g., -78°C) to form an imine intermediate, which is then hydrolysed to an aldehyde. Unlike the Stephen reaction, DIBAL-H can also reduce esters to aldehydes, making it more broadly applicable.
6. What happens to the imine intermediate formed during the reaction, and why is hydrolysis essential?
The intermediate formed is an iminium salt (R-CH=N⁺H₂ Cl⁻), which is relatively stable in the acidic, non-aqueous environment. This intermediate contains a carbon-nitrogen double bond. Hydrolysis (the addition of water) is the essential final step because it is what converts this imine into the desired aldehyde. Water attacks the carbon of the C=N bond, leading to the formation of the C=O bond of the aldehyde and the release of an ammonium salt, effectively completing the synthesis.
7. What are the major limitations or disadvantages of the Stephen reaction?
The main limitation of the Stephen reaction is its sensitivity to other functional groups. The strong acidic conditions (HCl) can react with acid-sensitive groups present in the starting material. Furthermore, the reaction is primarily used for the synthesis of aldehydes only and cannot be used to prepare ketones. Another potential issue is that the stannic chloride (SnCl₄) byproduct can be difficult to remove from the reaction mixture, sometimes complicating the purification process.
