which of the following shows a valid curved-arrow mechanism?

Chemistry
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Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
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which of the following shows a valid curved-arrow mechanism?

The image displays a series of organic reaction mechanisms involving the movement of hydride ions in benzyl carbocations. Each structure shows a benzene ring attached to a carbon with a positive charge, which is subject to a hydride shift indicated by a red arrow.

**Details of Each Structure:**

1. **First Structure**:
   - A benzene ring is bonded to a carbon with a methyl group. The central carbon is positively charged.
   - The red arrow indicates the movement of a nearby hydrogen atom (with its bonding electron pair) towards the positively charged carbon, signifying a hydride shift.

2. **Second Structure**:
   - Similar to the first structure, but the position of the positive charge has shifted due to the movement of the hydride from the adjacent carbon, leading to a more stable carbocation.

3. **Third Structure**:
   - The benzene ring remains constant while the rearrangement continues. Another hydride shift is indicated by the red arrow, showing the transfer of another hydrogen atom to the positively charged carbon.

4. **Fourth Structure**:
   - This final structure follows further rearrangement, producing a more stabilized carbocation. The hydride shifts result in the optimal positioning of positive charges, maximizing resonance stabilization from the benzene ring.

**Educational Context:**

These schematic diagrams illustrate the concept of carbocation rearrangements in organic chemistry. The hydride shifts are crucial mechanisms in many reactions, such as the rearrangement of carbocations during alkyl shifts or in electrophilic aromatic substitutions, contributing to increased stability due to resonance and hyperconjugation effects. This understanding is important in predicting reaction pathways and the formation of major organic products.
Transcribed Image Text:The image displays a series of organic reaction mechanisms involving the movement of hydride ions in benzyl carbocations. Each structure shows a benzene ring attached to a carbon with a positive charge, which is subject to a hydride shift indicated by a red arrow. **Details of Each Structure:** 1. **First Structure**: - A benzene ring is bonded to a carbon with a methyl group. The central carbon is positively charged. - The red arrow indicates the movement of a nearby hydrogen atom (with its bonding electron pair) towards the positively charged carbon, signifying a hydride shift. 2. **Second Structure**: - Similar to the first structure, but the position of the positive charge has shifted due to the movement of the hydride from the adjacent carbon, leading to a more stable carbocation. 3. **Third Structure**: - The benzene ring remains constant while the rearrangement continues. Another hydride shift is indicated by the red arrow, showing the transfer of another hydrogen atom to the positively charged carbon. 4. **Fourth Structure**: - This final structure follows further rearrangement, producing a more stabilized carbocation. The hydride shifts result in the optimal positioning of positive charges, maximizing resonance stabilization from the benzene ring. **Educational Context:** These schematic diagrams illustrate the concept of carbocation rearrangements in organic chemistry. The hydride shifts are crucial mechanisms in many reactions, such as the rearrangement of carbocations during alkyl shifts or in electrophilic aromatic substitutions, contributing to increased stability due to resonance and hyperconjugation effects. This understanding is important in predicting reaction pathways and the formation of major organic products.
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Arrows are drawn from high electron density to lower electron density.

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