Textbook Question
The chlorination of pentane gives a mixture of three monochlorinated products.
a. Draw their structures.
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Ch.4 - The Study of Chemical Reactions
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 4, Problem 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.
Verified step by step guidance1
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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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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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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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Related Practice
Textbook Question
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.
a. Explain how a mixture is formed from this stoichiometric mixture of reactants, and propose mechanisms for the formation of these compounds from chloromethane.
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Textbook Question
a. Draw the structure of the transition state for the second propagation step in the chlorination of methane.
Show whether the transition state is product-like or reactant-like and which of the two partial bonds is stronger.
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Textbook Question
Peroxides are often added to free-radical reactions as initiators because the oxygen–oxygen bond cleaves homolytically rather easily. For example, the bond-dissociation enthalpy of the O―O bond in hydrogen peroxide (H―O―O―H) is only 213 kJ/mol (51 kcal/mol). Give a mechanism for the hydrogen peroxide-initiated reaction of cyclopentane with chlorine. The BDE for HO―Cl is 210 kJ/mol (50 kcal/mol).
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Textbook Question
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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Textbook Question
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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