For each pair of reactions, predict which will happen more quickly. [For (c) and (d), the more substituted carbocation is more stable due to hyperconjugation.]
(c)

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Step 1: Analyze the two reactions provided. The first reaction involves a tertiary carbocation reacting with chloride ion, while the second reaction involves a methyl carbocation reacting with chloride ion.

Step 2: Recall the concept of carbocation stability. Tertiary carbocations are more stable than methyl carbocations due to hyperconjugation and inductive effects from surrounding alkyl groups.

Step 3: Understand the role of carbocation stability in reaction rates. A more stable carbocation will form faster and react more quickly because it lowers the activation energy of the reaction.

Step 4: Compare the two reactions. The tertiary carbocation in the first reaction is stabilized by three alkyl groups, whereas the methyl carbocation in the second reaction has no alkyl groups to stabilize it.

Step 5: Conclude that the reaction involving the tertiary carbocation will happen more quickly due to the increased stability provided by hyperconjugation and inductive effects.

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Key Concepts

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

Carbocation Stability

Carbocations are positively charged carbon species that can vary in stability based on their substitution. More substituted carbocations are generally more stable due to the inductive effect and hyperconjugation, where adjacent carbon-hydrogen bonds can donate electron density to the positively charged carbon, stabilizing it. Understanding this concept is crucial for predicting reaction rates and mechanisms in organic chemistry.

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 interaction helps to delocalize the positive charge in carbocations, thereby increasing their stability. Recognizing the role of hyperconjugation is essential for evaluating the relative rates of reactions involving carbocations.

Base Stability

The stability of bases is a key factor in determining the rate of reactions, particularly in acid-base chemistry. Stronger bases are typically less stable and more reactive, while weaker bases are more stable and less reactive. In the context of the question, understanding the stability of the bases involved in the reactions will help predict which reaction will proceed more quickly based on the relative strengths of the bases.

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Related Practice

Textbook Question

The following are all substitution reactions, two of which we study in later chapters. With no knowledge of mechanism, what would you expect the ratio of products to be for each reaction, based on a random statistical distribution?

(b) Replacing a hydrogen (H) with bromine (Br):

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Textbook Question

(a) Replacing a hydrogen (H) with deuterium (D):

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Textbook Question

Using bond-dissociation energies, identify the most stable radical. Justify the difference in stability based on the structure.

(d) I• vs •OH

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Textbook Question

For each pair of reactions, predict which will happen more quickly.

[For (a) and (b), think about the stability of the bases involved.]