Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds

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

Wade 9th Edition

Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds

Problem 18

Chapter 22, Problem 18

Propose a mechanism for the aldol condensation of cyclohexanone. Do you expect the equilibrium to favor the reactant or the product?

Verified step by step guidance

Step 1: Identify the reactants and the reaction conditions. Cyclohexanone is a ketone, and aldol condensation typically occurs under basic or acidic conditions. For this mechanism, assume basic conditions with a strong base like hydroxide (OH⁻).

Step 2: Deprotonation of the alpha hydrogen. The base abstracts an alpha hydrogen from cyclohexanone, forming an enolate ion. The enolate ion is stabilized by resonance, where the negative charge is delocalized between the oxygen atom and the alpha carbon.

Step 3: Nucleophilic attack by the enolate ion. The enolate ion acts as a nucleophile and attacks the carbonyl carbon of another cyclohexanone molecule. This forms a beta-hydroxyketone intermediate after protonation of the oxygen atom.

Step 4: Dehydration to form the alpha, beta-unsaturated ketone. Under the reaction conditions, the beta-hydroxyketone undergoes elimination of water (E1cB mechanism) to form the final product, an alpha, beta-unsaturated ketone (in this case, 2-cyclohexenone).

Step 5: Analyze the equilibrium. The equilibrium of aldol condensation reactions depends on the stability of the product. In this case, the product (2-cyclohexenone) is stabilized by conjugation of the double bond with the carbonyl group, so the equilibrium is expected to favor the product over the reactant.

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Key Concepts

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

Aldol Condensation

Aldol condensation is a reaction between aldehydes or ketones that contain alpha-hydrogens, leading to the formation of β-hydroxy carbonyl compounds (aldols) followed by dehydration to yield α,β-unsaturated carbonyl compounds. This reaction involves nucleophilic addition of an enolate ion to a carbonyl carbon, followed by elimination of water. Understanding this mechanism is crucial for predicting the products and the conditions under which the reaction occurs.

Enolate Ion Formation

Enolate ions are formed when a base abstracts an alpha-hydrogen from a carbonyl compound, resulting in a resonance-stabilized anion. This species is a key intermediate in aldol condensation, as it acts as a nucleophile that attacks another carbonyl compound. The stability of the enolate ion influences the reaction's rate and the equilibrium position, making it essential to grasp for predicting the outcome of the reaction.

Equilibrium in Reactions

In chemical reactions, equilibrium refers to the state where the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. For aldol condensation, the position of equilibrium can be influenced by factors such as temperature, concentration, and the nature of the reactants. Understanding whether the equilibrium favors reactants or products helps in predicting the feasibility and extent of the reaction.

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Textbook Question

Propose a mechanism for the acid-catalyzed bromination of pentan-3-one.

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Textbook Question

Give the expected products for the aldol condensations of (a) propanal.

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Textbook Question

Show the products of the reactions of these carboxylic acids with PBr₃/Br₂ before and after hydrolysis.

(a) pentanoic acid

(b) phenylacetic acid

(d) oxalic acid

Textbook Question

Give the expected products for the aldol condensations of (c) pentan-3-one.

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Textbook Question

Give the expected products for the aldol condensations of (b) phenylacetaldehyde.

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Textbook Question

Acid-catalyzed halogenation is synthetically useful for converting ketones to α,β-unsaturated ketones, which are useful in Michael reactions (Section 22-18). Propose a method for converting cyclohexanone to cyclohex-2-en-1-one, an important synthetic starting material.

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