Textbook Question
b. Using arrows, show the electron rearrangement that takes place in each reaction.
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16. Conjugated Systems
Sigmatropic Rearrangement
14:02 minutesProblem 36
Textbook Question
Propose a mechanism for the following reaction:
Verified step by step guidance1
Step 1: Analyze the reactants and products. The reactants include a nitro-substituted pyridine and an ester with two carbonyl groups. The products are a substituted pyridine derivative (A) and a carboxylic acid (B). This suggests a nucleophilic acyl substitution mechanism.
Step 2: Identify the reactive sites. The nitro group on the pyridine ring is electron-withdrawing, making the pyridine ring more electrophilic. The ester contains two carbonyl groups, which are susceptible to nucleophilic attack.
Step 3: Initiate the reaction mechanism. Heat (Δ) facilitates the reaction. The pyridine nitrogen acts as a nucleophile and attacks the carbonyl carbon of the ester, forming a tetrahedral intermediate.
Step 4: Collapse of the tetrahedral intermediate. The intermediate collapses, expelling the ethoxide group (CH₂CH₃O⁻) and forming a new bond between the pyridine nitrogen and the carbonyl carbon. This results in the formation of product A.
Step 5: Formation of product B. The expelled ethoxide group reacts with a proton to form ethanol (CH₃CH₂OH), which subsequently oxidizes to form the carboxylic acid product B.
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14mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electrocyclic Reactions
Electrocyclic reactions are a class of pericyclic reactions where a conjugated system undergoes a concerted transformation to form a cyclic compound. These reactions are characterized by the rotation of a sigma bond and the formation or breaking of pi bonds, often influenced by thermal or photochemical conditions. Understanding the stereochemistry and the rules governing these reactions, such as the Woodward-Hoffmann rules, is crucial for predicting the products.
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Mechanism of Reaction
The mechanism of a reaction describes the step-by-step process by which reactants are converted into products. It includes the identification of intermediates, transition states, and the sequence of bond-breaking and bond-forming events. Proposing a mechanism requires knowledge of reaction conditions, the stability of intermediates, and the energy changes involved, which are essential for understanding how the reaction proceeds.
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Thermal Conditions in Organic Reactions
Thermal conditions refer to the influence of heat on the rate and outcome of chemical reactions. In organic chemistry, heating can provide the necessary energy to overcome activation barriers, facilitating reactions such as electrocyclic transformations. The temperature can also affect the selectivity and stereochemistry of the products, making it important to consider when proposing mechanisms for reactions that involve heat.
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