Textbook Question
Which of the carbocations in part a is most stable?
1. the isobutyl cation?
2. the n-butyl cation?
3. the sec-butyl cation?
6. Thermodynamics and Kinetics
Carbocation Stability
3:42 minutesProblem 2b
Textbook Question
Which is more stable: a methyl cation or an ethyl cation? Why?
Verified step by step guidance1
Step 1: Understand the concept of carbocation stability. Carbocations are positively charged carbon species, and their stability is influenced by factors such as hyperconjugation, inductive effects, and resonance.
Step 2: Analyze the structure of a methyl cation (CH₃⁺). A methyl cation has no alkyl groups attached to the positively charged carbon, meaning it lacks hyperconjugation and inductive stabilization. It is highly unstable.
Step 3: Analyze the structure of an ethyl cation (CH₃CH₂⁺). The ethyl cation has one alkyl group (a methyl group) attached to the positively charged carbon. This methyl group provides hyperconjugation and inductive effects, which help stabilize the positive charge.
Step 4: Compare the two carbocations. The ethyl cation is more stable than the methyl cation because the methyl group in the ethyl cation donates electron density through hyperconjugation and inductive effects, reducing the electron deficiency of the positively charged carbon.
Step 5: Conclude that the ethyl cation is more stable than the methyl cation due to the stabilizing effects of the attached alkyl group, which are absent in the methyl cation.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Carbocation Stability
Carbocation stability is influenced by the degree of alkyl substitution. Tertiary carbocations are more stable than secondary, which are more stable than primary, due to hyperconjugation and inductive effects. The presence of alkyl groups can donate electron density to the positively charged carbon, stabilizing the cation.
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Hyperconjugation
Hyperconjugation is a stabilizing interaction that occurs when the electrons in a sigma bond (C-H or C-C) interact with an adjacent empty p-orbital of a carbocation. This delocalization of electrons helps to spread out the positive charge, thereby increasing the stability of the carbocation. More alkyl groups lead to more hyperconjugative interactions.
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Inductive Effect
The inductive effect refers to the electron-withdrawing or electron-donating effects of substituents through sigma bonds. Alkyl groups are electron-donating, which can help stabilize a positive charge in a carbocation. The more alkyl groups attached to the cation, the greater the inductive stabilization, making the cation more stable.
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