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1.5: Practice Questions for Chapter 4

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  • 1) Write an equation to describe the initiation step in the chlorination of methane.

    2) Reaction intermediates that have unpaired electrons are called __________.

    3) When light is shined on a mixture of chlorine and chloromethane, methylene chloride (dichloromethane) is one of the products. Write the propagation steps that explain the formation of dichloromethane from chloromethane under these conditions.

    4) Which of the following is NOT a possible termination step in the free radical chlorination of methane? Hint: see textbook, p. 128.

    A) \(\cdot CH_{3} + Cl_{2} \rightarrow CH_{3}Cl + Cl \cdot\)

    B) \(\cdot CH_{3} + \textrm{wall of container} \rightarrow CH_{3}-wall\)

    C) \(\cdot CH_{3} + \cdot CH_{3} \rightarrow CH_{3}CH_{3}\)

    D) \(\cdot CH_{3} + Cl \cdot \rightarrow CH_{3}Cl\)

    E) \(Cl \cdot + \textrm{wall of container} \rightarrow Cl-wall\)

    5) Chlorination of methane can result in a mixture of chlorinated products. What experimental conditions should be used to favor the production of chloromethane over the other chlorinated products?

    6) Write a detailed, stepwise mechanism (i.e. initiation and propagation steps) for the following reaction.

    7) Provide a definition of bond dissociation energy.

    8) Energy is __________ when bonds are formed and is __________ when bonds are broken; therefore, bond dissociation energies are always __________.

    A) released / consumed / endothermic

    B) consumed / released / exothermic

    C) consumed / released / endothermic

    D) consumed / released / isothermic

    E) released / consumed / exothermic

    9) Consider the bond dissociation energies listed below in kcal/mol.

    CH3-Br = 70             CH3CH2-Br = 68            (CH3)2CH-Br = 68             (CH3)3C-Br = 65

    These data show that the C-Br bond is weakest when bromine is bound to a ______.

    A) methyl carbon B) quaternary carbon C) primary carbon D) 3o carbon E) 2o carbon

    10) If stronger bonds are formed and weaker bonds are broken in a chemical reaction, then the reaction is

    A) exothermic

    B) an equilibrium reaction

    C) endothermic

    D) all of the above

    E) none of the above

    11) Given the bond dissociation energies below (in kcal/mol), estimate the ΔHo for the propagation step \((CH_{3})_{2}CH \cdot + Cl_{2} \rightarrow (CH_{3})_{2}CHCl + Cl \cdot\)

    CH3CH2CH2-H 98

    (CH3)2CH-H 95

    Cl-Cl 58

    H-Cl 103

    CH3CH2CH2-Cl 81

    (CH3)2CH-Cl 80

    A) -40 kcal/mol B) +22 kcal/mol C) -22 kcal/mol D) +45 kcal/mol

    12) Using your table of bond dissociation energies (in kcal/mol), calculate the overall ΔHo for the following reaction: (CH3)3CH + Br2 ---------> (CH3)3CBr + HBr

    13) What term describes the highest-energy state in a molecular collision which leads to reaction?

    A) exothermic

    B) endothermic

    C) reaction intermediate

    D) transition state

    E) free radical

    14) The difference in energy between reactants and the transition state is known as __________.

    15) Consider the conversion of C to D via a one-step mechanism. The activation energy of this conversion is 3 kcal/mol. The energy difference between D and the transition state of the reaction is 7 kcal/mol. Estimate ΔHo for the reaction C ----> D.

    6) Consider the reaction: \(CH_{3}CH_{2} \cdot + Br_{2} \rightarrow CH_{3}CH_{2}Br + Br \cdot\)

    If this reaction has an activation energy of +6 kcal/mol and a ΔHo of -22 kcal/mol, sketch a reaction-energy diagram for this reaction, label the axes, and show Ea and ΔHo on your drawing.

    17) Consider the one-step conversion of F to G. Given that the reaction is endothermic by 5 kcal/mol and that the energy difference between G and the transition state for the process is 15 kcal/mol, sketch a reaction-energy diagram for this reaction. Make sure to show how the given energy differences are consistent with your sketch.

    18) The major monobrominated product which results when ethylcyclohexane is subjected to free radical bromination is:

    A) bromomethane

    B) a primary bromide

    C) a quaternary bromide

    D) a secondary bromide

    E) a tertiary bromide

    19) How many distinct dichlorination products can result when isobutane is subjected to free radical chlorination?

    20) What is the relative reactivity of secondary vs. primary hydrogens in the free radical bromination of n-butane if the ratio of 1-bromobutane to 2-bromobutane formed is 7:93?

    21) Predict the major monobromination product in the following reaction, and name it by IUPAC rules.

    \((CH_{3})_{3}CCH_{2}CH_{3} + Br_{2} \rightarrow \)

    22) Predict the major monobromination product in the following reaction, and name it by IUPAC rules

    23) What C5H12 isomer will give only a single monochlorination product?

    24) Which of the following statements is the best statement of the Hammond Postulate?

    A) In an exothermic reaction, the transition state is closer in energy to the products.

    B) The structure of the transition state in an organic reaction is always modeled on the structure of the reactants leading to that transition state.

    C) In an endothermic reaction, the transition state is closer to the reactants in structure.

    D) Transition states are molecular species of finite lifetime whose properties can be probed through free radical reactions.

    E) Related species that are similar in energy are also similar in structure.

    25) Use the Hammond Postulate to explain why free radical brominations are more selective than free radical chlorinations.

    26) List the following radicals in order of increasing stability

    \((CH_{3})_{3}C \cdot\)    \(CH_{2}=CHCH_{2} \cdot\)    \(CH_{3}CH_{2} \cdot\)  \(CH_{3} \cdot\)   \((CH_{3})_{2}CH \cdot\)

    27) When Bromine reacts with 1-butene (CH3CH2CH=CH2), the hydrogen atom which is preferentially abstracted is the one which produces a resonance stabilized radical. Draw the major resonance contributing forms of this radical.

    28) How do alkyl substituents stabilize a carbocation?

    A) Through an inductive donation of electron density to the cationic center.

    B) Through an inductive removal of electron density from the cationic center.

    C) Through hyperconjugation.

    D) both A and C

    E) both B and C

    29) Describe the hybridization of the cationic center and predict the CCC bond angle in (CH3)3C.

    30) Free radical bromination of pentane results in poor yields of 1-bromopentane, while cyclopentane can be readily brominated under similar conditions to yield bromocyclopentane. Offer an explanation.

    31) What is the name of the major monobrominated product which results when 3-methylpentane is subjected to Br2/hν conditions?

    32) What is the name of the major monobrominated product which results when methylcyclohexane is subjected to Br2/hν conditions?

    33) Provide the two propagation steps in the free-radical chlorination of ethane.

    34) In an exothermic reaction, are stronger bonds broken and weaker bonds formed or are weaker bonds broken and stronger bonds formed?

    35) How many secondary hydrogens are present in the hydrocarbon below?

    A) 16 B) 7 C) 8 D) 2 E) 6

    36) How many tertiary hydrogens are present in the hydrocarbon below?

    A) 2 B) 3 C) 4 D) 1 E) 5

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