Chapter 19: Problem 21
Treatment of 2 -cyclohexenone with HCN/KCN yields a saturated keto nitrile rather than an unsaturated cyanohydrin. Show the structure of the product, and propose a mechanism for the reaction.
Short Answer
Expert verified
The product is a saturated keto nitrile, formed via conjugate addition of CN⁻.
Step by step solution
01
Identify the Structure of 2-cyclohexenone
2-cyclohexenone is a cyclic compound with a six-membered ring structure that contains a double bond between carbon 2 and carbon 3, and a ketone group (C=O) attached to carbon 1. It is typically drawn as a cyclohexane ring with a carbonyl group and an adjacent carbon-carbon double bond.
02
Determine the Reagents and Initial Reaction Step
The reaction involves treating 2-cyclohexenone with hydrogen cyanide (HCN) in the presence of potassium cyanide (KCN). Initially, the cyanide ion (CN⁻) acts as a nucleophile and attacks the carbon atom of the carbonyl group in 2-cyclohexenone, resulting in the formation of a cyanohydrin intermediate.
03
Conjugate Addition and Tautomerization
Instead of forming an unsaturated cyanohydrin, the intermediate undergoes a conjugate addition. The nucleophilic cyanide ion adds across the unsaturated bond in 2-cyclohexenone (1,4-addition), which eventually leads to rearrangement and formation of a saturated keto nitrile structure after protonation and isomerization steps.
04
Structure of the Product
The product is a saturated keto nitrile. This means the ring no longer contains a double bond and now has a nitrile group (C≡N) attached to the carbon that previously formed the double bond, while the carbonyl group remains intact.
05
Final Product Confirmation
Draw the final product structure: a cyclohexane ring with a ketone group at the original position of the carbonyl, and a nitrile group attached to the previously double-bonded carbon, effectively saturating the ring.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Cyano Group Addition
In the process of forming a saturated keto nitrile from 2-cyclohexenone, the key reaction step involves the addition of a cyano group (–C≡N) to the molecule. This group comes from the cyanide ion, CN⁻, which is a potent nucleophile. The cyano group addition is crucial because it introduces a nitrile functionality into the compound, modifying its structure and properties.
The cyanide ion targets the carbonyl carbon (C=O) of 2-cyclohexenone originally. This is a well-known point of attack in organic chemistry because the carbonyl carbon is partially positively charged due to the electronegativity of the oxygen atom. However, the key difference in this reaction is that the cyano group addition ultimately contributes to the formation of a saturated keto nitrile rather than an unsaturated intermediate, which involves a further rearrangement process.
The cyanide ion targets the carbonyl carbon (C=O) of 2-cyclohexenone originally. This is a well-known point of attack in organic chemistry because the carbonyl carbon is partially positively charged due to the electronegativity of the oxygen atom. However, the key difference in this reaction is that the cyano group addition ultimately contributes to the formation of a saturated keto nitrile rather than an unsaturated intermediate, which involves a further rearrangement process.
2-Cyclohexenone
2-Cyclohexenone is an organic compound belonging to the family of cyclic enones. It consists of a six-membered cyclohexene ring with a carbonyl group (C=O) at position one, and a double bond between the second and third carbon atoms. This structure makes it particularly sensitive to nucleophilic attack.
Its dual functional groups are key to its reactivity: the carbonyl oxygen is electron-withdrawing, making the adjacent carbon atom susceptible to nucleophilic attack, while the double bond presents opportunities for 1,4-conjugate addition. Understanding the dual nature of 2-cyclohexenone is essential for predicting the outcome of reactions it undergoes.
Its dual functional groups are key to its reactivity: the carbonyl oxygen is electron-withdrawing, making the adjacent carbon atom susceptible to nucleophilic attack, while the double bond presents opportunities for 1,4-conjugate addition. Understanding the dual nature of 2-cyclohexenone is essential for predicting the outcome of reactions it undergoes.
Nucleophilic Addition
Nucleophilic addition is a core concept in organic chemistry that defines the interaction between a nucleophile and an electrophile. In this context, the electrophilic part of 2-cyclohexenone is its carbonyl carbon, which is relatively electron-poor. The nucleophile is the cyanide ion, CN⁻, which is rich in electrons and therefore eager to donate electrons to electron-poor sites.
In this reaction, the cyanide ion first adds to the carbonyl carbon of 2-cyclohexenone. This is a classic nucleophilic addition where the pi bond of the oxygen-carbon breaks, allowing the formation of a cyanohydrin intermediate. However, the reaction does not stop here; it continues with further transformations that rearrange the molecule into the desired product.
In this reaction, the cyanide ion first adds to the carbonyl carbon of 2-cyclohexenone. This is a classic nucleophilic addition where the pi bond of the oxygen-carbon breaks, allowing the formation of a cyanohydrin intermediate. However, the reaction does not stop here; it continues with further transformations that rearrange the molecule into the desired product.
Tautomerization
Tautomerization is a chemical process that involves the migration of a proton (and accompanying shift of electron density) within the molecule, resulting in an isomeric change. In the keto nitrile formation from 2-cyclohexenone, tautomerization plays a subtle but essential role.
Once the cyano group adds to 2-cyclohexenone, the product undergoes further transformations that lead to tautomerization. This step involves the rearrangement of the atoms within the molecule to yield a more stable structure, where the cyanide ion has moved across the double bond (1,4-addition). This rearrangement, followed by protonation, ultimately culminates in the saturated keto nitrile, demonstrating the importance of tautomerization in achieving the final structure.
Once the cyano group adds to 2-cyclohexenone, the product undergoes further transformations that lead to tautomerization. This step involves the rearrangement of the atoms within the molecule to yield a more stable structure, where the cyanide ion has moved across the double bond (1,4-addition). This rearrangement, followed by protonation, ultimately culminates in the saturated keto nitrile, demonstrating the importance of tautomerization in achieving the final structure.