Ch. 14 - Structural Identification 1: Infrared Spectroscopy and Mass Spectrometry

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

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Mullins 1st Edition

Ch. 14 - Structural Identification 1: Infrared Spectroscopy and Mass Spectrometry

Problema 6a

Capitolo 13, Problema 6a

Identify the more stable carbocation in each pair.
(a)

Guida verificata passo dopo passo

Step 1: Analyze the structure of the first carbocation. It is a secondary carbocation attached to a cyclohexane ring. Secondary carbocations are moderately stable due to the inductive effect of adjacent alkyl groups, which can donate electron density to stabilize the positive charge.

Step 2: Analyze the structure of the second carbocation. It is a benzylic carbocation, where the positive charge is adjacent to a benzene ring. Benzylic carbocations are highly stable due to resonance stabilization. The positive charge can delocalize over the aromatic ring, spreading the charge across multiple atoms.

Step 3: Compare the stability of the two carbocations. Resonance stabilization in the benzylic carbocation provides significantly more stability than the inductive effect in the secondary carbocation.

Step 4: Recall the general rule: carbocations stabilized by resonance are more stable than those stabilized by inductive effects alone. This makes the benzylic carbocation the more stable option.

Step 5: Conclude that the benzylic carbocation is more stable due to resonance stabilization, which allows the positive charge to be delocalized over the aromatic ring.

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Carbocation Stability

Carbocations are positively charged carbon species that can vary in stability based on their structure. Stability increases with the degree of substitution; tertiary (3°) carbocations are more stable than secondary (2°), which are more stable than primary (1°). This is due to hyperconjugation and the inductive effect from surrounding alkyl groups that help to disperse the positive charge.

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05:58

Determining Carbocation Stability

Hyperconjugation

Hyperconjugation is a stabilizing interaction that occurs when the electrons in a sigma bond (usually C-H or C-C) interact with an adjacent empty p-orbital or a positively charged carbon. This delocalization of electrons helps to stabilize the carbocation by spreading out the positive charge over a larger area, making the carbocation less reactive and more stable.

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04:28

Understanding trends of alkene stability.

Inductive Effect

The inductive effect refers to the electron-withdrawing or electron-donating effects transmitted through sigma bonds in a molecule. Alkyl groups are electron-donating, which can stabilize a carbocation by reducing the positive charge's density. The greater the number of alkyl groups attached to the carbocation, the more stable it becomes due to this inductive effect.

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