Show the resonance forms for the enolate ions that result when the following compounds are treated with a strong base.
(c) ethyl α-cyanoacetate

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Identify the acidic hydrogen in ethyl α-cyanoacetate. The most acidic hydrogen is the one on the α-carbon (the carbon adjacent to both the ester group and the nitrile group) because it is flanked by two electron-withdrawing groups, which stabilize the resulting enolate ion.

Treat the compound with a strong base (e.g., hydroxide or alkoxide). The base will abstract the acidic hydrogen from the α-carbon, forming the enolate ion. This results in a negative charge on the α-carbon.

Draw the first resonance form of the enolate ion. In this form, the negative charge resides on the α-carbon, and the double bond remains between the α-carbon and the carbonyl carbon of the ester group.

Draw the second resonance form by delocalizing the negative charge. Move the lone pair of electrons from the α-carbon to form a double bond with the carbonyl carbon, and push the π-electrons of the carbonyl group onto the oxygen atom. This places the negative charge on the oxygen atom of the ester group.

Draw the third resonance form by delocalizing the negative charge further. Move the lone pair of electrons from the α-carbon to form a double bond with the nitrile carbon, and push the π-electrons of the nitrile group onto the nitrogen atom. This places the negative charge on the nitrogen atom of the nitrile group.

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Key Concepts

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

Enolate Ion Formation

Enolate ions are formed when a strong base abstracts a proton from the alpha carbon of a carbonyl compound, resulting in a resonance-stabilized anion. This process is crucial in reactions involving carbonyl compounds, as it allows for nucleophilic attack on electrophiles. In the case of ethyl α-cyanoacetate, the presence of the cyano group influences the stability and reactivity of the enolate formed.

Resonance Structures

Resonance structures are different Lewis structures for the same molecule that illustrate the delocalization of electrons. In the context of enolate ions, resonance allows for the distribution of negative charge across multiple atoms, enhancing stability. Understanding how to draw these structures is essential for predicting the behavior of enolate ions in chemical reactions.

Alpha-Substituted Carbonyl Compounds

Alpha-substituted carbonyl compounds, like ethyl α-cyanoacetate, have functional groups attached to the carbon adjacent to the carbonyl group. These substituents can significantly affect the acidity of the alpha hydrogen and the stability of the resulting enolate ion. Recognizing the influence of these substituents is key to understanding the reactivity and resonance forms of the enolate ions generated.

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