Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics

Problem 6a

Chapter 4, Problem 6a

Provide a reasonable arrow-pushing mechanism for the following Lewis acid–Lewis base reactions.
(a)

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Identify the Lewis base and Lewis acid in the reaction. In this case, NH₃ acts as the Lewis base because it has a lone pair of electrons on the nitrogen atom, and BF₃ acts as the Lewis acid because boron is electron-deficient and can accept a pair of electrons.

Recognize the interaction between the Lewis base and Lewis acid. The lone pair of electrons on the nitrogen atom in NH₃ will be donated to the empty p-orbital of boron in BF₃.

Draw the arrow-pushing mechanism. Use a curved arrow to show the movement of the lone pair of electrons from the nitrogen atom in NH₃ to the boron atom in BF₃.

After the electron donation, a coordinate covalent bond is formed between the nitrogen atom and the boron atom. This results in the formation of the complex H₃N⁺—BF₃⁻.

Indicate the charges in the product. The nitrogen atom in NH₃ gains a positive charge due to the donation of its lone pair, and the boron atom in BF₃ gains a negative charge due to the acceptance of electrons.

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Key Concepts

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

Lewis Acids and Bases

Lewis acids are species that can accept an electron pair, while Lewis bases are species that can donate an electron pair. In the reaction shown, BF3 acts as a Lewis acid by accepting an electron pair from the Lewis base, ammonia (NH3), resulting in the formation of a coordinate covalent bond.

Arrow-Pushing Mechanism

The arrow-pushing mechanism is a method used to illustrate the movement of electrons during chemical reactions. In Lewis acid-base reactions, arrows indicate the flow of electron pairs from the Lewis base to the Lewis acid, helping to visualize the formation of new bonds and the changes in charge.

Formation of Adducts

An adduct is a complex formed from the reaction of a Lewis acid and a Lewis base. In this case, the product H3N–BF3 represents an adduct where the nitrogen atom of ammonia donates its lone pair to the boron atom of BF3, resulting in a stable compound that can be analyzed further for its properties.

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Related Practice

Textbook Question

Provide a reasonable arrow-pushing mechanism for the following Lewis acid–Lewis base reactions.

(b)

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Textbook Question

For the following equilibrium processes and the corresponding ∆G°, indicate whether you expect the equilibrium constant to be greater than, equal to, or less than 1. Justify your expectation in words.

(a)

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Textbook Question

Chapter 5 taught us that chemical reactions are random collisions. Experimentally, how can we make molecules collide more often?

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Textbook Question

Which is the most acidic compound in each pair?

(a)

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