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Ch. 8 - Alkenes 1: Properties and Electrophilic Additions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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Tutti i libri di testoMullins 1st EditionCh. 8 - Alkenes 1: Properties and Electrophilic AdditionsProblema 23
Capitolo 7, Problema 23

A wayward chemist proposed the following mechanism for the addition of HBr to an alkene.

(a) Why is this mechanism unlikely?
(b) Compare the reaction coordinate diagrams for the actual mechanism studied in Section 8.3.1 and this alternate mechanism on the same graph.

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1
Identify the proposed mechanism: The image shows a two-step mechanism where the alkene first reacts with HBr to form a carbocation intermediate, followed by the attack of a hydride ion (H-) on the carbocation.
Explain why the mechanism is unlikely: In typical electrophilic addition reactions of alkenes with HBr, the alkene acts as a nucleophile and attacks the proton (H+) from HBr, forming a carbocation. The bromide ion (Br-) then attacks the carbocation. The presence of a hydride ion (H-) as a nucleophile is highly unusual and unlikely under normal conditions.
Discuss the stability of intermediates: Carbocations are generally stabilized by hyperconjugation and inductive effects. The proposed mechanism suggests a hydride ion attacking a carbocation, which is not a common or favorable reaction pathway.
Compare with the actual mechanism: In the actual mechanism, the alkene first forms a carbocation by protonation, and then the bromide ion attacks the carbocation to form the final product. This is a more straightforward and energetically favorable pathway.
Sketch the reaction coordinate diagram: For the actual mechanism, the diagram would show an initial energy increase to form the carbocation, followed by a decrease as the bromide ion attacks. The proposed mechanism would have an additional energy barrier for the unlikely hydride attack, making it less favorable overall.

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Electrophilic Addition Mechanism

The electrophilic addition mechanism is a fundamental reaction pathway for alkenes, where an electrophile (like HBr) attacks the electron-rich double bond of the alkene. This process typically involves the formation of a carbocation intermediate, which is then attacked by a nucleophile. Understanding this mechanism is crucial for analyzing the proposed mechanism and identifying its flaws.
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Features of Addition Mechanisms.

Carbocation Stability

Carbocation stability is a key concept in organic chemistry, as the stability of the carbocation intermediate significantly influences the reaction pathway. Carbocations can be classified as primary, secondary, or tertiary, with tertiary carbocations being the most stable due to hyperconjugation and inductive effects. Evaluating the stability of the proposed carbocation in the mechanism helps determine the likelihood of the reaction occurring as suggested.
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Determining Carbocation Stability

Reaction Coordinate Diagrams

Reaction coordinate diagrams visually represent the energy changes during a chemical reaction, illustrating the transition states and intermediates involved. These diagrams allow for a comparison of different mechanisms by showing the energy barriers and the relative stability of intermediates. Analyzing the reaction coordinate diagrams for both the proposed and actual mechanisms will highlight differences in energy profiles and support conclusions about their feasibility.
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Coordination Complexes
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