Ch. 18 - Ketones and Aldehydes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 18 - Ketones and Aldehydes

Problem 15

Chapter 18, Problem 15

Propose a mechanism for each cyanohydrin synthesis just shown.

Verified step by step guidance

Step 1: Identify the starting material and the reagent. In each case, the starting material is either an aldehyde or a ketone, and the reagent is hydrogen cyanide (HCN). The reaction is catalyzed by cyanide ions (CN⁻).

Step 2: Understand the mechanism. The cyanohydrin synthesis involves nucleophilic addition of the cyanide ion (CN⁻) to the carbonyl group (C=O) of the aldehyde or ketone. This is followed by protonation of the resulting alkoxide intermediate.

Step 3: Initiate the reaction. The cyanide ion (CN⁻) attacks the electrophilic carbon atom of the carbonyl group, breaking the π bond and forming a tetrahedral intermediate. Represent this step as:

C_{1} (C = O) + {CN}_{-} \to C_{1} (C - O - {CN}_{-})

Step 4: Protonation of the intermediate. The negatively charged oxygen atom in the tetrahedral intermediate is protonated by HCN, forming the cyanohydrin product. Represent this step as:

C_{1} (C - O - {CN}_{-}) + HCN \to C_{1} (C - OH - {CN}_{-})

Step 5: Analyze the steric and electronic effects. The yield of cyanohydrin depends on the steric hindrance around the carbonyl group. For propanal, the reaction proceeds efficiently due to minimal steric hindrance. For butan-2-one, the yield is slightly lower due to increased steric hindrance. For di-tert-butylketone, the reaction is slow and yields are poor due to significant steric hindrance around the carbonyl group.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Cyanohydrin Formation

Cyanohydrins are formed through the nucleophilic addition of hydrogen cyanide (HCN) to carbonyl compounds, such as aldehydes or ketones. In this reaction, the nucleophile (cyanide ion) attacks the electrophilic carbon of the carbonyl group, leading to the formation of a hydroxyl group and a cyano group on the same carbon atom.

Nucleophilic Addition Mechanism

The nucleophilic addition mechanism involves the attack of a nucleophile on an electrophile, resulting in the formation of a new bond. In the case of cyanohydrin synthesis, the cyanide ion acts as the nucleophile, while the carbonyl carbon serves as the electrophile. This mechanism typically includes a tetrahedral intermediate before the final product is formed.

Stereochemistry of Cyanohydrins

Cyanohydrins can exhibit stereochemistry due to the presence of a chiral center formed during their synthesis. The addition of HCN to the carbonyl carbon can lead to the formation of two enantiomers, depending on the orientation of the nucleophile's attack. Understanding the stereochemical implications is crucial for predicting the properties and reactivity of the resulting cyanohydrins.

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Related Practice

Textbook Question

Show how you would accomplish the following syntheses.

c. hexan-1-ol → 2-hydroxyheptanoic acid

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Textbook Question

Sodium triacetoxyborohydride, NaBH(OAc)₃, is a mild reducing agent that reduces aldehydes much more quickly than ketones. It can be used to reduce aldehydes in the presence of ketones, such as in the following reaction:

(b) Propose a mechanism for the reduction of an aldehyde by sodium triacetoxyborohydride.

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Textbook Question

Show how you would accomplish the following syntheses.

(a) acetophenone → acetophenone cyanohydrin

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Textbook Question

Propose mechanisms for

(b) the base-catalyzed hydration of acetone to form acetone hydrate.

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Textbook Question

Show how you would accomplish the following syntheses.