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Ch.4 - The Study of Chemical Reactions
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Non è quello che usi tu?Cambia libro di testo
Capitolo 4, Problema 49b

The chlorination of pentane gives a mixture of three monochlorinated products.
b. Predict the ratios in which these monochlorination products will be formed, remembering that a chlorine atom abstracts a secondary hydrogen about 4.5 times as fast as it abstracts a primary hydrogen.

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Step 1: Identify the structure of pentane. Pentane is a straight-chain alkane with the molecular formula C5H12. It contains three types of hydrogens: primary hydrogens (attached to terminal carbons), secondary hydrogens (attached to internal carbons), and no tertiary hydrogens.
Step 2: Determine the number of primary and secondary hydrogens in pentane. Each terminal carbon has 3 primary hydrogens, and there are 2 terminal carbons, giving a total of 6 primary hydrogens. Each internal carbon has 2 secondary hydrogens, and there are 3 internal carbons, giving a total of 6 secondary hydrogens.
Step 3: Consider the reactivity of chlorine during the chlorination process. Chlorine abstracts secondary hydrogens 4.5 times faster than primary hydrogens. This means secondary hydrogens are more reactive and will contribute more to the product distribution.
Step 4: Calculate the relative contribution of primary and secondary hydrogens to the product formation. Multiply the number of primary hydrogens by their reactivity factor (1) and the number of secondary hydrogens by their reactivity factor (4.5). This gives the relative contributions: primary hydrogens = 6 × 1 = 6, secondary hydrogens = 6 × 4.5 = 27.
Step 5: Determine the ratio of monochlorinated products. Add the contributions from primary and secondary hydrogens to get the total (6 + 27 = 33). The ratio of products formed from primary hydrogens to secondary hydrogens will be 6:27, which simplifies to 2:9.

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Hydrogen Abstraction

Hydrogen abstraction is a key step in radical halogenation reactions, where a chlorine radical removes a hydrogen atom from an alkane. The rate of abstraction varies depending on the type of hydrogen (primary, secondary, or tertiary). In this case, chlorine abstracts secondary hydrogens more readily than primary ones, influencing the distribution of chlorinated products.
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The definition of hydrogenation.

Radical Stability

The stability of radicals plays a crucial role in determining the products of chlorination. Secondary radicals are more stable than primary radicals due to hyperconjugation and inductive effects. This stability affects the likelihood of forming different monochlorinated products, as more stable radicals are formed preferentially during the reaction.
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The radical stability trend.

Product Distribution Ratios

The product distribution ratios in chlorination reactions can be predicted using the relative rates of hydrogen abstraction. Given that chlorine abstracts secondary hydrogens approximately 4.5 times faster than primary hydrogens, the resulting ratios of monochlorinated products can be calculated based on the number of each type of hydrogen present in pentane, leading to a quantitative understanding of the product mixture.
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Bridged-Products
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The chlorination of pentane gives a mixture of three monochlorinated products.

a. Draw their structures.

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a. Draw the structure of the transition state for the second propagation step in the chlorination of methane.

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When dichloromethane is treated with strong NaOH, an intermediate is generated that reacts like a carbene. Draw the structure of this reactive intermediate, and propose a mechanism for its formation.

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When exactly 1 mole of methane is mixed with exactly 1 mole of chlorine and light is shone on the mixture, a ­chlorination reaction occurs. The products are found to contain substantial amounts of di-, tri-, and tetrachloromethane, as well as ­unreacted methane.

b. How would you run this reaction to get a good conversion of methane to CH3Cl? Of methane to CCl4?

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