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Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 5 - Chemical Reaction Analysis: Thermodynamics and KineticsProblem 31b
Chapter 4, Problem 31b

We discuss the following reactions in subsequent chapters. Given the mechanisms shown, draw the mechanism of the reverse reaction.
(b)

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Analyze the forward reaction mechanism provided in the image. The reaction involves the nucleophilic attack of the cyanide ion (C≡N⁻) on the carbon atom bonded to bromine, leading to the substitution of bromine with the cyanide group.
To reverse the reaction, consider the reverse substitution mechanism. Bromide ion (Br⁻) will act as the nucleophile and attack the carbon atom bonded to the cyanide group.
Identify the leaving group in the reverse reaction. In this case, the cyanide group (C≡N⁻) will act as the leaving group when bromide ion attacks the carbon atom.
Draw the curved arrows to represent the reverse mechanism. The lone pair of electrons on Br⁻ will attack the carbon atom, forming a bond between Br and the carbon. Simultaneously, the bond between the carbon and the cyanide group will break, resulting in the release of C≡N⁻.
Ensure the products of the reverse reaction are correctly represented. The final products will be the original alkyl bromide and the cyanide ion (C≡N⁻), which were the reactants in the forward reaction.

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Key Concepts

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

Reaction Mechanisms

A reaction mechanism is a step-by-step description of how a chemical reaction occurs at the molecular level. It outlines the sequence of bond-breaking and bond-forming events, including the formation of intermediates and transition states. Understanding the mechanism is crucial for predicting the products of a reaction and for drawing reverse reactions accurately.
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Nucleophiles and Electrophiles

Nucleophiles are species that donate an electron pair to form a chemical bond, while electrophiles are electron-deficient species that accept an electron pair. In the provided reaction, the carbon atom in the alkyne acts as a nucleophile, attacking the electrophilic bromine atom. Recognizing these roles is essential for understanding how to reverse the reaction and identify the correct products.
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Curved Arrow Notation

Curved arrow notation is a visual representation used in organic chemistry to illustrate the movement of electrons during chemical reactions. Arrows indicate the direction of electron flow, helping to clarify how bonds are formed or broken. Mastery of this notation is vital for accurately depicting reaction mechanisms, including the reverse reaction in the question.
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Related Practice
Textbook Question

For the following acid–base reactions studied in Assessment 5.25, draw a likely transition state. Be sure to indicate in your drawing the degree to which bonds are broken or formed.

(b)

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Textbook Question

We discuss the following reactions in subsequent chapters. Given the mechanisms shown, draw the mechanism of the reverse reaction.

(a)

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Textbook Question

Write the rate law for the following reaction and identify which molecules are present in the rate-determining step. Draw a possible transition state and propose a mechanism.


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Textbook Question

Assuming that ∆H° = -15kcal/mol for the reaction in Assessment 5.31(b), show the transition state for the forward and reverse reactions.

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Textbook Question

For the following acid–base reactions studied in Assessment 5.25, draw a likely transition state. Be sure to indicate in your drawing the degree to which bonds are broken or formed.

(c) H3O+ + Br– ⇌ H2O + HBr

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Textbook Question

All things being equal, would you expect a first-order reaction to be faster or slower than a second-order reaction?

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