Stephen Reaction Mechanism - Imine, Benzaldehyde, DIBAL-H, FAQs

Q: What is the density of Benzaldehyde?

The density of Benzaldehyde is 1.04g/cm 3 .

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What is Stephen Reaction:

Defining Stephen reaction, Stephen reaction is also called as Stephen aldehyde synthesis or Stephen Reduction Reaction. It was first discovered by a chemist Henry Stephen. This reaction is organic redox reaction, whose end product is an aldehyde. The reaction is also named as Stephen reduction reaction.

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What is Stephen Reaction:
Stephen Reaction Mechanism:
Imine:
Benzaldehyde:
Mendius Reaction Mechanism:
DIBAL-H:

Stephen Reduction Reaction:

In Stephen reduction reaction when methyl nitrile reacts with Hydrochloric acid and tin chloride it produces the iminium salt. This iminium salt further quench with water to give aldehyde as product. We also get some by product in this reaction such as ammonium chloride or ammonia. The aldehydes produced in the reaction is aromatic or aliphatic in nature. The iminium salt produced as intermediate products is not stable so further quenching is required in Stephen reduction reaction.

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Explain Stephen Reaction:

In Stephen reaction mechanism when Tin(II) chloride with hydrochloric acid than it produces tin tetra chloride and two hydrogen ions. As in this case of Stephen reaction the Tin(II) stage gets converted to form Tin(IV) by the removal of two electrons from it. Here to show the Stephen reduction reaction we take methyl cyanide or phenyl cyanide as alky nitrile or aryl nitrile in which nitrogen is more electronegative compare to carbon.

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So it is found that nitrogen is partially negative and carbon is partially positive. Pie bond present in cyanide, in which one is broke down and due to that electrons move to nitrogen atom and simultaneously two electrons which were released in the reaction will accepted by carbon of cyanide atom. Therefore both atom carries negative charge on it.

Nitrile to Aldehyde Mechanism:

Here we are showing an example of Stephen reaction. In Stephen reaction acetaldehyde is being produces by the action of methyl cyanide. The action of nitrile to aldehyde is shown below in the equation.

Nitrile to aldehyde mechanism

In the above Stephen reaction mechanism the intermediate product was formed and this product is imine, which is formed by the action of nitrile when undergoers reduction with stannous chloride and hydrogen chloride in ethyl acetate solvent. This intermediate product of Stephen mechanism will further hydrolyse to yield the required aromatic or aliphatic aldehyde.

Stephen Reaction Mechanism:

Steps followed by the Stephen reaction mechanism:

Step1. In the first step of Stephen reaction the gaseous hydrogen chloride is being added to the nitrile, which reacts to give its correlated salt as shown below in the reaction.

Stephen reaction mechanism

Step2. In the next second step of the Stephen reaction single electron transfer occur in Stannous(II) chloride to form its salt. The reaction is:

Stephen reaction mechanism

Step3. In the third step of Stephen reaction the obtained salt gets precipitates. The salt is aldimine tin chloride shown in the reaction.

Stephen reaction mechanism

Step4. In this step hydrolysis of the obtained salt is done which gives us amide. This end product is the r5equired aldehyde as shown below:

Stephen reaction mechanism

By the above steps we get the end product as aldehyde in Stephen reaction mechanism. One important key point is to note that instead of using aliphatic nitrile, aromatic nitrile is more efficient to be used. Substitutes that are added which improves the electron density also improves the formation of salt of tin chloride. Amide chloride formation can also be promoted by electron withdrawing substitutes.

NCERT Chemistry Notes:

Imine:

An Imine is defined as a chemical compound that contains the atom of carbon and nitrogen with double bond. Imine is the functional group used in many reactions of organic chemistry. The Nitrogen atom of Imine can be attached to any organic group or with hydrogen easily. The term Imine first used in 1883 by German Chemist named Albert Ladenburg.

Imine Group:

Imines will easily undergo to hydrolysis with their correlated compounds of amine or carbonyl. Imines may get precipitated when reacted under aldehydes and ketones. In the reaction mechanism of heterocycles imines are widely used as the intermediate product. Formation Imines ca be possible by the condensation of primary amines and aldehydes. Synthesis of Imines can be predicted as shown below.

Synthesis of imines

The important point is Iminium cation is functional group in which nitrogen has fourth bond, which gives iminium cation to positive charge.

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Benzaldehyde:

Benzaldehyde is the common organic compound which is consisted of benzene ring with formyl substituent in it. It is mostly used as for industrial purposes and is the simplest aromatic aldehyde. Benzaldehyde formula is . Benzaldehyde density is 1.04 g/. According to IUPAC nomenclature it is named as Benzenecarbaldehyde. Benzaldehyde is found as colourless liquid. Its smells like almond. It is the primary component of bitter almon oil.

Extraction of Benzaldehyde is possible from number of other natural resources. For the other bakery products or flavoured cakes it is used as flavoring agent. It can also be used in the cosmetic products, pharmaceuticals to plastic additives etc. Benzaldehyde can also be used as bee repellent. Benzaldehyde is safe to use as it does not produces non-carcinogenic compounds which is used in foods and cosmetics. Structure of Benzaldehyde is shown below:

Benzaldehyde structure

Mendius Reaction Mechanism:

In the Mendius reaction mechanism organic compounds are involved which contains the cyanide group. Formation of ammines occur by the action of alcohol and alkali metals.In the Mendius reduction reaction ethanolic sodium is used as reagent in it.Basically we can define the Mendius reaction as the reduction of alkyl and aryl cyanides to produce amines in the presence of nascent hydrogen is Mendius reduction recation. When nitrogen atom are attached to carbon atom such compounds are called Nitriles compound. In this triple bond is present in between carbon and nitrogen.

The formation of nitriles occur by dehydration of amides which means when amides are heated, it gets dehydrated to form nitriles. Dehydration process occur in the presence of phosphorous oxide.The reaction of converting to nitriles by the process of dehydration is as follows:

Amide to nitrile convertion

The obtained nitrile is further reacted with sodium atom, and it undergoes to reduce in the presence of alcohol. Such procedure yields the primary amine. And this reduction reaction of nitriles to ammines is so called Mendius Reduction reaction mechanism.

The reduction will proceeded as follows:

Nitrile to amine

DIBAL-H:

Diisobutylaluminium hydride is so called DIBAL-H which is a reducing agent.This compound is found originally as co-catalyst and are known as organoaluminium compound.The compound is used as polymerization of alkenes.The DIBAL can be prepared by the use of heat, which means heating up triisobutylaluminium to get beta-hydride elimination.The DIBAL compound is basically used in reduction for converting the carboxylic acids.The DIBAL is electrophilic reducing agent so it quickly reacts with electron-poor compound.DIBAL. It is found Colourless. It is more efficiently used in place of lithium hydride and it also helps in reducing nitrioles to aldehyde so DIBAL reduces the acids as well.

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Reaction of DIBAL-H With Cyanide:

For the formation of aldehyde and ketone, it requires strong reducing agent. Electrophillic reducing agent works well in such situation where we convert cyanide to aldehyde and ketone. The reaction of such reducing agent with cyanide is as follows:

Cyanide to aldehyde

The DIBAL-H works good as Lewis acid and has a aluminium atom in it. The electron pair donated by the nitrile group and can be accepted readily by this Lewis acid. Formation of Imine occurs in this reaction when the hydrogen ion of DIBAL-H is attached with carbon atom of cyanide. Now after the formation of imine as intermediate hydrolysis is done to form aldehyde. For the conversion of cyanide to ketone we use different reagent such as Grignard reagent. This is how we convert Cyanide to aldehyde.

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Frequently Asked Questions (FAQs)

1. What is ethyl cyanide?

Ethyl cyanide is also called as propionitrile or propanenitrile. It is the organic compound and simply a aliphatic nitrile. The compound is found as colourless liquid and are water soluble. It is as similar to acetonitrile but have slightly higher boiling point in comparison.

2. What do you mean by quenching chemistry?

Quenching chemistry is rapid cooling of any substance or any compound. Cooling of compound can be done with the process known as hydrolysis.In very short time of interval it freezes the reaction and prevents it from decomposition.

3. What is methyl aldehyde?

Methyl aldehyde is also known as formaldehyde which is a naturally occurring oragic compound with pungent odour. The compound polymerises to form paraformaldehyde, therefore it is stored as in aqueous solution. This is the simplest form of aldehyde with relation to formic acid.

4. What is the density of Benzaldehyde?

The density of Benzaldehyde is 1.04g/cm³.

5. What is the role of benzaldehyde in the Stephen reaction?

Benzaldehyde is not typically used in the Stephen reaction. The reaction converts nitriles to aldehydes, so benzaldehyde would be a product, not a reactant. However, benzaldehyde can be used as a model compound to study aldehyde reactions or as a starting material in related reactions.

6. Can the Stephen reaction be monitored by IR spectroscopy?

Yes, IR spectroscopy can be used to monitor the Stephen reaction. The disappearance of the nitrile peak (around 2250 cm^-1) and the appearance of the aldehyde peak (around 1720 cm^-1) can be observed.

7. How does the workup procedure affect the yield of the Stephen reaction?

The workup procedure is critical in the Stephen reaction. Careful control of pH during hydrolysis is necessary to maximize aldehyde yield and minimize side reactions. Too acidic conditions can lead to aldehyde loss, while basic conditions can cause aldol condensations.

8. What's the role of temperature control in the Stephen reaction?

Temperature control is crucial in the Stephen reaction. Low temperatures (-78°C) are typically used to control the reactivity of DIBAL-H and prevent over-reduction. Careful warming is then required for the hydrolysis step to form the aldehyde.

9. Can the Stephen reaction be used in flow chemistry?

While challenging due to the low temperature requirements, the Stephen reaction can be adapted for flow chemistry. This could potentially improve scalability and control over reaction conditions, but would require careful engineering of the flow system.

10. Why is DIBAL-H used in the Stephen reaction?

DIBAL-H (diisobutylaluminum hydride) is used in the Stephen reaction because it's a strong reducing agent that can selectively reduce nitriles to imines without further reduction to amines. Its bulky structure also helps control the reaction and prevent over-reduction.

11. How does DIBAL-H compare to other reducing agents for nitrile reduction?

DIBAL-H is more selective than many other reducing agents. It can stop at the aldehyde stage, whereas stronger reducing agents like LiAlH4 would reduce the nitrile all the way to a primary amine.

12. What's the significance of using low temperatures in the Stephen reaction?

Low temperatures (around -78°C) help control the reactivity of DIBAL-H and prevent over-reduction. This allows the reaction to stop at the imine intermediate stage rather than continuing to the amine.

13. Can the Stephen reaction be used in industrial processes?

While the Stephen reaction is valuable in laboratory settings, its use in large-scale industrial processes is limited due to the need for low temperatures and the cost of DIBAL-H. Alternative methods are often preferred for industrial aldehyde production.

14. What are some common side reactions in the Stephen reaction?

Common side reactions include over-reduction to amines (if excess DIBAL-H is used), formation of dimers or trimers of the imine intermediate, and reactions with other functional groups present in complex molecules.

15. What's the stereochemistry of the imine intermediate in the Stephen reaction?

The imine intermediate typically forms as a mixture of E and Z isomers. However, this stereochemistry doesn't affect the final product, as both isomers hydrolyze to the same aldehyde.

16. What's the mechanism of the hydrolysis step in the Stephen reaction?

The hydrolysis step involves protonation of the imine nitrogen, followed by nucleophilic attack of water on the imine carbon. This forms a tetrahedral intermediate, which then collapses to give the aldehyde, eliminating ammonia or an amine.

17. What's the role of the aluminum in DIBAL-H during the Stephen reaction?

The aluminum in DIBAL-H acts as a Lewis acid, coordinating with the nitrogen of the nitrile and making it more electrophilic. It also stabilizes the imine intermediate by forming an Al-N bond.

18. Can the Stephen reaction be used in total synthesis of natural products?

Yes, the Stephen reaction can be a valuable tool in total synthesis, especially when a nitrile needs to be converted to an aldehyde at a specific stage. Its selectivity makes it useful in complex molecule synthesis where other functional groups are present.

19. How does the Stephen reaction compare to the Luche reduction?

These reactions serve different purposes. The Stephen reaction reduces nitriles to aldehydes, while the Luche reduction selectively reduces α,β-unsaturated ketones to allylic alcohols. They use different reagents and have different applications in organic synthesis.

20. How does the Stephen reaction compare to the Rosenmund reduction?

Both reactions produce aldehydes, but from different starting materials. The Stephen reaction reduces nitriles, while the Rosenmund reduction reduces acyl chlorides. The Stephen reaction uses DIBAL-H, while Rosenmund uses H2 gas and a poisoned palladium catalyst.

21. How does the Stephen reaction fit into the broader context of organic synthesis?

The Stephen reaction is a valuable tool in organic synthesis for converting nitriles to aldehydes. This transformation is important because aldehydes are versatile intermediates that can undergo many further reactions, such as oxidations, reductions, and carbon-carbon bond formations.

22. How does the Stephen reaction compare to ozonolysis for aldehyde synthesis?

Both can produce aldehydes, but from different starting materials. The Stephen reaction reduces nitriles, while ozonolysis cleaves alkenes. Ozonolysis can produce aldehydes or ketones, depending on the alkene, while the Stephen reaction only produces aldehydes.

23. What are some alternatives to the Stephen reaction for nitrile reduction?

Alternatives include the use of other reducing agents like DIBAL-H at higher temperatures (for full reduction to amines), or partial hydrogenation using catalysts like Raney nickel. The choice depends on the desired product and the presence of other functional groups.

24. How does the presence of other functional groups affect the Stephen reaction?

Other functional groups can complicate the Stephen reaction. Groups that are sensitive to reducing conditions (like esters or ketones) may also react with DIBAL-H. Protecting group strategies may be necessary for complex molecules.

25. What is the Stephen reaction mechanism?

The Stephen reaction mechanism is a method for converting nitriles (R-CN) into aldehydes (R-CHO) using a strong reducing agent like DIBAL-H (diisobutylaluminum hydride). It involves the formation of an imine intermediate, which is then hydrolyzed to form the aldehyde.

26. How does the imine intermediate form in the Stephen reaction?

The imine intermediate forms when DIBAL-H reduces the nitrile (R-CN) group. One hydride from DIBAL-H attacks the carbon of the nitrile, forming a C=N double bond and an Al-N bond. This results in an aluminoimine intermediate, which is the precursor to the imine.

27. What conditions are necessary for the Stephen reaction to occur?

The Stephen reaction typically requires anhydrous conditions, low temperatures (around -78°C), and an inert atmosphere. These conditions help control the reactivity of DIBAL-H and prevent side reactions.

28. How does the hydrolysis step in the Stephen reaction work?

After forming the imine intermediate, the reaction mixture is treated with a mild acid (like dilute HCl) to hydrolyze the imine. This breaks the C=N bond, replacing it with a C=O bond to form the aldehyde product.

29. Can the Stephen reaction be used with all types of nitriles?

The Stephen reaction works well with most aliphatic and aromatic nitriles. However, nitriles with sensitive functional groups or those prone to side reactions may require modified conditions or alternative methods.

30. What's the difference between the Stephen reaction and the Stephen reduction?

There is no difference

31. Can the Stephen reaction produce ketones?

No, the Stephen reaction specifically produces aldehydes. It cannot produce ketones because the reaction starts with a nitrile (R-CN), which has only one carbon attached to the CN group.

32. Why is the Stephen reaction considered a partial reduction?

The Stephen reaction is considered a partial reduction because it reduces the nitrile (R-CN) to an aldehyde (R-CHO) without fully reducing it to a primary amine (R-CH2-NH2). It stops at the aldehyde oxidation state.

33. What's the overall change in oxidation state during the Stephen reaction?

In the Stephen reaction, the carbon atom of the nitrile group (oxidation state +3) is reduced to the oxidation state of an aldehyde (+1). This represents a decrease in oxidation state by 2.

34. Can the Stephen reaction be used to synthesize chiral aldehydes?

The Stephen reaction itself doesn't create a new chiral center. However, if the starting nitrile is chiral, the chirality is retained in the product aldehyde, making it useful in chiral synthesis.

35. How does the Stephen reaction compare to the Lindlar reduction?

While both are reduction reactions, they serve different purposes. The Stephen reaction reduces nitriles to aldehydes, while the Lindlar reduction selectively reduces alkynes to cis-alkenes. They use different catalysts and have different applications in organic synthesis.

36. How does the electronic nature of the nitrile affect the Stephen reaction?

Electron-withdrawing groups attached to the nitrile make it more electrophilic and thus more reactive towards DIBAL-H. Electron-donating groups have the opposite effect, potentially slowing the reaction.

37. How does solvent choice affect the Stephen reaction?

Non-polar, aprotic solvents like toluene or dichloromethane are typically used. These solvents don't interfere with the DIBAL-H and allow it to react selectively with the nitrile. Polar solvents could potentially react with DIBAL-H or affect its reactivity.

38. What's the importance of anhydrous conditions in the Stephen reaction?

Anhydrous conditions are crucial because DIBAL-H is highly reactive with water. The presence of water would consume the DIBAL-H, reducing its effectiveness and potentially leading to side reactions.

39. Can the Stephen reaction be performed on a large scale?

While possible, scaling up the Stephen reaction presents challenges due to the need for low temperatures and anhydrous conditions. Heat transfer and maintaining uniform reaction conditions become more difficult on larger scales.

40. What's the environmental impact of the Stephen reaction?

The Stephen reaction uses stoichiometric amounts of aluminum-containing reagents, which can have environmental implications. Proper disposal of aluminum waste is necessary. The use of organic solvents also requires consideration in terms of environmental impact.

41. Can the Stephen reaction be performed asymmetrically?

The Stephen reaction itself doesn't create a new stereocenter, so asymmetric versions are not typically relevant. However, chiral DIBAL-H analogues have been developed for other reactions, which could potentially be applied to nitrile reductions.

42. How does the electronic structure of DIBAL-H contribute to its reactivity in the Stephen reaction?

DIBAL-H's reactivity stems from its electron-deficient aluminum center and the hydride's reducing power. The aluminum acts as a Lewis acid, activating the nitrile, while the hydride acts as a nucleophile, attacking the activated nitrile carbon.

43. What's the importance of the Stephen reaction in pharmaceutical synthesis?

The Stephen reaction is valuable in pharmaceutical synthesis for converting nitrile-containing intermediates to aldehydes. This transformation can be key in synthesizing complex drug molecules, especially when selective reduction is needed in the presence of other functional groups.

44. How does the Stephen reaction compare to the Nef reaction?

While both can produce aldehydes, they start from different compounds. The Stephen reaction reduces nitriles, while the Nef reaction converts nitro compounds to aldehydes or ketones. They involve different mechanisms and reagents.

45. Can the Stephen reaction be used in polymer chemistry?

Yes, the Stephen reaction can be used in polymer chemistry, particularly in the modification of nitrile-containing polymers. It could potentially be used to introduce aldehyde groups into polymer chains, altering their properties or allowing for further functionalization.

46. What's the role of steric hindrance in the Stephen reaction?

Steric hindrance can affect the Stephen reaction. Bulky groups near the nitrile can slow down the reaction by making it harder for the DIBAL-H to approach. This can sometimes be advantageous for selectivity in complex molecules.

47. How does the Stephen reaction compare to the Bouveault aldehyde synthesis?

Both reactions produce aldehydes but from different starting materials. The Stephen reaction reduces nitriles, while the Bouveault aldehyde synthesis reduces esters. The Stephen reaction uses DIBAL-H, while Bouveault uses sodium and ethanol.

48. Can the Stephen reaction be used in cross-coupling chemistry?

While not directly involved in cross-coupling, the Stephen reaction can be used to prepare aldehyde-containing substrates for subsequent cross-coupling reactions. Aldehydes can participate in various C-C bond forming reactions.

49. What's the significance of the imine intermediate in the Stephen reaction?

The imine intermediate is crucial as it represents the partially reduced state between the nitrile and the aldehyde. Its formation and subsequent hydrolysis allow for the controlled, selective reduction to the aldehyde without further reduction to the amine.

50. How does the Stephen reaction compare to the Fukuyama reduction?

Both can produce aldehydes, but from different starting materials and using different mechanisms. The Stephen reaction reduces nitriles using DIBAL-H, while the Fukuyama reduction converts thioesters to aldehydes using palladium catalysis and silane reducing agents.

51. Can the Stephen reaction be used in natural product modification?

Yes, the Stephen reaction can be valuable in natural product modification, especially for converting nitrile-containing natural products to aldehydes. This can allow for further functionalization or for studying structure-activity relationships.

52. What's the role of Lewis acid catalysis in the Stephen reaction?

While not typically considered a catalytic process, the aluminum in DIBAL-H acts as a Lewis acid, activating the nitrile for nucleophilic attack. This Lewis acid behavior is integral to the mechanism of the Stephen reaction.

53. How does the Stephen reaction compare to the Soai autocatalytic reaction?

These reactions serve very different purposes. The Stephen reaction is a method for reducing nitriles to aldehydes, while the Soai reaction is an autocatalytic asymmetric addition of organozinc reagents to aldehydes. They have different mechanisms, reagents, and applications.

54. Can the Stephen reaction be combined with other reactions in one-pot procedures?

Yes, the Stephen reaction can potentially be combined with other reactions in one-pot procedures. For example, it could be followed by a reductive amination or an aldol condensation. However, careful control of conditions would be necessary to avoid unwanted side reactions.