Textbook Question
Show the resonance forms for the enolate ions that result when the following compounds are treated with a strong base.
(c) ethyl α-cyanoacetate
24. Enolate Chemistry: Reactions at the Alpha-Carbon
Enolate
1:47 minutesProblem 3c
Textbook Question
Give the important resonance forms for the possible enolate ions of the following:
(c) pentane-2,4-dione
Verified step by step guidance1
Identify the structure of pentane-2,4-dione: This compound contains two carbonyl groups (C=O) at positions 2 and 4 of the pentane chain. The alpha-hydrogens (hydrogens on the carbon atoms adjacent to the carbonyl groups) are acidic and can be deprotonated to form enolate ions.
Determine the possible sites of deprotonation: The alpha-hydrogens are located on the carbon atom between the two carbonyl groups (position 3). Deprotonation at this position leads to the formation of an enolate ion.
Draw the enolate ion: After deprotonation, the resulting enolate ion will have a negative charge on the alpha-carbon (position 3). This negative charge can delocalize onto the oxygen atoms of the carbonyl groups through resonance.
Illustrate the resonance forms: The negative charge on the alpha-carbon can shift to one of the carbonyl oxygens, forming a double bond between the alpha-carbon and the carbonyl carbon. This creates a resonance structure where the oxygen carries a negative charge. Similarly, the negative charge can delocalize to the other carbonyl group, forming another resonance structure.
Summarize the important resonance forms: The enolate ion of pentane-2,4-dione has three major resonance forms: (1) the structure with the negative charge on the alpha-carbon, (2) the structure with the negative charge on the oxygen of the carbonyl group at position 2, and (3) the structure with the negative charge on the oxygen of the carbonyl group at position 4. These resonance forms contribute to the stability of the enolate ion.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enolate Ions
Enolate ions are negatively charged species formed from the deprotonation of a carbon adjacent to a carbonyl group. They play a crucial role in various organic reactions, particularly in nucleophilic addition and condensation reactions. The stability of enolate ions is influenced by resonance, where the negative charge can be delocalized between the carbon and the adjacent carbonyl oxygen.
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Resonance Structures
Resonance structures are different Lewis structures for the same molecule that illustrate the delocalization of electrons. In the case of enolate ions, resonance allows the negative charge to be shared between the carbon atom and the carbonyl oxygen, stabilizing the ion. Understanding resonance is essential for predicting the reactivity and stability of enolate ions in organic reactions.
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Pentane-2,4-dione
Pentane-2,4-dione is a diketone with two carbonyl groups located at the second and fourth carbon atoms of a five-carbon chain. This structure allows for the formation of multiple enolate ions, as either carbonyl can be deprotonated. The presence of two carbonyl groups also enhances the resonance stabilization of the resulting enolate ions, making it a key compound in organic synthesis.
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