ESSAY. Write your answer in the space provided or on a...

QOI 0809

Name___________________________________

ESSAY. Write your answer in the space provided or on a separate sheet of paper.

1) What is meant by the mechanism of a chemical reaction?

SHORT ANSWER. Write the word or phrase that best completes each statement or answers the question.

2) __________ is the study of reaction rates. 2)


3) The chlorination of methane is characterized by a high quantum yield. Explain what this means.

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

5) When light is shined on a mixture of chlorine and chloromethane, carbon tetrachloride is one of the components

of the final reaction mixture. Propose a series of mechanistic steps which explain this observation.


6) Species with unpaired electrons are called __________. 6)

MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question.

7) Which of the following is not a possible termination step in the free radical chlorination of

methane?

A) ·CH3 + Cl2 → CH3Cl + Cl·

B) ·CH3 + Cl· → CH3Cl

C) ·CH3 + ·CH3 → CH3CH3

D) ·CH3 + wall → CH3-wall

E) Cl· + wall → Cl-wall

7)


8) Within the visible spectrum, it has been experimentally determined that blue light is the most effective in

initiating the chlorination of methane. What is the mechanistic significance of this observation?

9) 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?

10) Write a detailed, stepwise mechanism for the following reaction.

1


11) For a given reaction, if △G° is greater than zero, then:

A) Keq < 0

B) Keq = 0

C) 0 < Keq < 1

D) Keq = 1

E) Keq > 1

11)


12) For the reaction A + B → C + D, △G° = -5.00 kcal/mol. What is the corresponding equilibrium constant at 25°C?

R = 1.987 cal/mol·K.


13) Assume the reaction A + B → C + D proceeds to equilibrium. Calculate the equilibrium

concentration of D at 25°C, given that the starting concentrations of A and B are 2M and that △G°

for the reaction is 1.0 kcal/mol. R = 1.987 cal/mol·K.

A) 0.40M B) 0.60M C) 1.00M D) 1.40M E) 1.60M

13)


14) Consider the transformation of A to B (i.e., A → B). If at equilibrium at 25°C the concentration of A is 20% of the

initial concentration of A, determine the value of △G° (in kcal/mol) for this reaction. R = 1.987 cal/mol·K.


15) If △G° for a given reaction at 25°C is less than zero, which of the following statements also correctly

describes this reaction at this temperature?

A) The reaction must be exothermic.

B) The reaction must be endothermic.

C) Keq is greater than zero.

D) Both A and C are true.

E) Both B and C are true.

15)

16) Which of the following statements correctly describes the contribution of △S° to △G°?

A) The entropy term makes a greater contribution to △G° at low temperatures.

B) The entropy term makes a greater contribution to △G° at high temperatures.

C) The entropy term makes a greater contribution to △G° in exothermic reactions.

D) The entropy term makes a greater contribution to △G° in endothermic reactions.

E) The entropy term always makes a more significant contribution to △G° than does the

enthalpy term.

16)

17) Which is a measure of the randomness of a system?

A) entropy

B) enthalpy

C) free energy

D) halogenation

E) stoichiometry

17)

2

18) If a reaction is exothermic, then:

A) △S° < 0

B) △S° > 0

C) △H° < 0

D) △H° > 0

E) both B and D

18)


19) If stronger bonds are formed and weaker bonds are broken, then the reaction is

__________.

19)


20) The hydrogenation of acetylene to produce ethane is shown below. Is △S° for this reaction positive, negative, or

impossible to predict? Explain your reasoning.

C2H2 (g) + 2H2 (g) → C2H6 (g)


21) The bond dissociation energy is the amount of energy required to break a bond __________.

A) homolytically

B) heterolytically

C) so as to produce the more stable pair of ions

D) via hydrogenation

E) none of the above

21)

22) Energy is __________ when bonds are formed and is __________ when bonds are broken; therefore,

bond dissociation energies are always __________.

A) released / consumed / exothermic

B) released / consumed / endothermic

C) consumed / released / exothermic

D) consumed / released / endothermic

E) consumed / released / isothermic

22)

23) 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 carbon-bromine bond is weakest when bromine is bound to a

__________.

A) methyl carbon

B) primary carbon

C) secondary carbon

D) tertiary carbon

E) quaternary carbon

23)

3

24) Given the bond dissociation energies below (in kcal/mol), estimate the △H° for the propagation

step (CH3)2CH· + Cl2 → (CH3)2CHCl + Cl·.

CH3CH2CH2H 98

(CH3)2CHH 95

ClCl 58

HCl 103

CH3CH2CH2Cl 81

(CH3)2CHCl 80

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

24)


25) Given the bond dissociation energies below (in kcal/mol), calculate the overall △H° for the following reaction:

(CH3)3CH + Br2 → (CH3)3CBr + HBr

(CH3)3CH 91

(CH3)3CBr 65

BrBr 46

HBr 88

CH3Br 70


26) Given the chlorination of acetone shown below, choose the correct rate law.

CH3COCH3 + Cl2 → CH3COCH2Cl + HCl

A) rate = [CH3COCH3]

B) rate = [Cl2]

C) rate = [CH3COCH3][Cl2]

D) rate = [CH3COCH3][Cl2]1/2

E) cannot be determined from stoichiometry; must be determined experimentally

26)


27) The following reaction occurs readily: CH3Br + I- → CH3I + Br-.

Experimentally one finds that if the concentration of I- is doubled, the rate doubles. Also if the concentration of

CH3Br is halved, the rate is halved. What is the rate equation for this reaction?

4


28) Consider the reaction (CH3)3CBr + CH3CH2OH → (CH3)3COCH2CH3 + HBr.

Experimentally one finds that if the concentration of (CH3)3CBr is tripled, the rate of the reaction

triples. One also finds that if the concentration of CH3CH2OH is doubled, the rate of the reaction is

unchanged. Which of the following correctly describes the kinetics of this reaction?

A) The reaction is third order in (CH3)3CBr.

B) The reaction is first order in CH3CH2OH.

C) The reaction is second order overall.

D) The reaction is first order overall.


28)

29) In the hydrocarbon shown below, how many tertiary hydrogens are present?

A) 0 B) 1 C) 2 D) 3 E) 4

29)


30) __________ is the minimum kinetic energy reacting molecules must possess to overcome

the repulsions between their electron clouds when they collide.

30)


31) The rate of a reaction typically increases as the temperature increases because:

A) the A term in the Arrhenius equation increases.

B) the fraction of molecules with kinetic energy greater than Ea increases.

C) the activation energy decreases.

D) the activation energy increases.

E) the molecules make more collisions with the wall of the reaction vessel.

31)


32) Given an activation energy of 15 kcal/mol, use the Arrhenius equation to estimate how much faster the reaction

will occur if the temperature is increased from 100°C to 120°C. R = 1.987 cal/mol·K.

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


34) The difference in energy between reactants and the transition state is known as

__________.

34)

5


35) 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 △H°

for the reaction C → D.

36) Consider the reaction: CH3CH2· + Br2 → CH3CH2Br + Br· .

Given that this reaction has an activation energy of +6 kcal/mol and a △H° of -22 kcal/mol, sketch a

reaction-energy diagram for this reaction. Label the axes and show Ea and △H° on your drawing.

37) Consider the conversion of X to Z through the sole intermediate Y. Given the reaction-energy diagram shown

below, which step is the rate-limiting step? Explain your reasoning.

38) 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.


39) Consider the three-step mechanism for the reaction of A through intermediates B and C to produce

D shown below.

A → B Ea = 15 kcal/mol, △H° = 13 kcal/mol

B → C Ea = 10 kcal/mol, △H° = -6 kcal/mol

C → D Ea = 2 kcal/mol, △H° = -20 kcal/mol

Which of the three steps is rate-limiting?

A) The reaction of A to B.

B) The reaction of B to C.

C) The reaction of C to D.

D) All three steps occur at the same rate; there is no rate-limiting step.

E) The most exothermic step is rate-limiting.

39)

6


40) Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown

below.

A → B Ea = 15 kcal/mol, △H° = 13 kcal/mol

B → C Ea = 10 kcal/mol, △H° = -6 kcal/mol

C → D Ea = 2 kcal/mol, △H° = -20 kcal/mol

What's the enthalpy difference between reactant A and intermediate C?


41) The major monobrominated product which results when ethylcyclohexane is subjected to free

radical bromination is:

A) a primary bromide

B) a secondary bromide

C) a tertiary bromide

D) a quaternary bromide

E) bromomethane

41)

42) Which of the halogens below undergoes free radical halogenation with ethane most rapidly?

A) fluorine B) chlorine C) iodine D) bromine E) pyridine

42)

43) How many distinct dichlorination products can result when isobutane is subjected to free radical

chlorination?

A) 1 B) 2 C) 3 D) 4 E) 6

43)

44) Rank the free radicals (I-III) shown below in order of decreasing stability (i.e., from most stable to

least stable).

A) I > III > II B) II > III > I C) I > II > III D) II > I > III E) III > II > I

44)


45) Write the structures of all of the monobromination products of 1,1,3,3-tetramethylcyclobutane.

46) When 1,1,3,3-tetramethylcyclobutane is brominated at 125°C, the relative reactivity of the 1°: 2°: 3° hydrogens is

approximately 1:82:1600. Estimate the amount of each monobromination product.

47) The relative reactivity of the 1°: 2°: 3° hydrogens of (CH3)3CCH2CH3 in free radical chlorination is 1: 3.8: 5.0.

Provide the structure of each monochlorination product, and estimate the relative amount of each in the

mixture of monochlorinated products.

7

48) What is the relative reactivity of 2° vs 1° hydrogens in the free radical bromination of n-butane if the ratio of

1-bromobutane to 2-bromobutane formed is 7:93?

49) Predict the major monobromination product in the following reaction.

(CH3)3CCH2CH3 + Br2 hν

50) Predict the major monobromination product in the following reaction.

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


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

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

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

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

D) The structure of the transition state in an organic reaction is always modeled on the structure

of the reactants leading to that transition state.

E) Transition states are molecular species of finite lifetime whose properties can be probed

through free radical reactions.

52)


53) Use the Hammond Postulate to explain why free radical brominations are more selective than free radical

chlorinations.

54) List the following radicals in order of increasing stability (i.e., from least stable to most stable).

(CH3)3C·, CH2CHCH2·, CH3CH2·, CH3·, (CH3)2CH·

55) When Br radical 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.

56) When acetaldehyde (CH3CHO) is deprotonated, the resulting anion is stabilized by resonance. Draw the major

resonance contributing forms of this anion.

8


57) Which of the following is a carbene?

A) CH2CHO-

B) CH3CH2·

C) :CCl2

D) CH3CH2+

E) NCO-

57)

58) How do alkyl substituents stabilize a carbocationic center to which they are attached?

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

58)


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

60) 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.


61) Which of the following correctly expresses the standard Gibbs free energy change of a reaction in

terms of the reaction temperature (T) and equilibrium constant (K)?

A) △G° = e-K/RT

B) △G° = eK/RT

C) △G° = RTlnK

D) △G° = -RTlnK


61)

62) Which of the following correctly expresses the standard Gibbs free energy change of a reaction in

terms of the changes in enthalpy and entropy?

A) △G° = △H° - T△S°

B) △G° = △H° + T△S°

C) △G° = △S° - T△H°

D) △G° = △S° + T△H°


62)

63) Which of the following is true for the initiation step of a free radical chlorination reaction?

A) △H° > 0 and △S° > 0

B) △H° > 0 and △S° < 0

C) △H° < 0 and △S° > 0

D) △H° < 0 and △S° < 0

E) △H° = 0 and △S° = 0

63)

9

64) Which of the following is true for the termination step of a free radical chlorination reaction?

A) △H° > 0 and △S° > 0

B) △H° > 0 and △S° < 0

C) △H° < 0 and △S° > 0

D) △H° < 0 and △S° < 0

E) △H° = 0 and △S° = 0

64)


65) What is the name of the major monobrominated product which results when 3-methylpentane is subjected to

Br2/hν conditions?

66) What is the name of the major monobrominated product which results when methylcyclohexane is subjected to

Br2/hν conditions?


67) How many distinct monochlorinated products can result when isobutane is subjected to free radical

chlorination?

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

67)

68) How many distinct monochlorinated products can result when cyclopentane is subjected to free

radical chlorination?

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

68)

69) Which of the following reactive intermediates can best be described as both nucleophilic and

strongly basic?

A) carbanions

B) carbocations

C) carbenes

D) free radicals

E) alkanes

69)


70) Provide the two propagation steps in the free-radical chlorination of ethane. 70)

71) Explain how the termination step in a free-radical chain reaction stops the chain. 71)


72) Consider the reaction of A being converted into B at 25°C. If the ΔG° of this reaction is

+1.0 kcal/mol, the Keq is __________ and the % conversion is __________.

A) 0.18, 15% B) 0.43, 30% C) 1.0, 50% D) 2.3, 70% E) 5.4, 84%

72)


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

weaker bonds broken and stronger bonds formed?

73)

10

74) Does one expect ΔS° in a propagation step of the free-radical chlorination of methane to be

greater than zero, less than zero, or approximately equal to zero? Briefly explain your

choice.

74)

75) Explain the significance of the frequency factor A in the Arrhenius equation. 75)

76) Explain the significance of the exponential factor e-Ea/RT in the Arrhenius equation. 76)


77) Which H atom in the molecule shown will be most readily abstracted by a bromine radical?

A) Ha B) Hb C) Hc D) Hd E) He

77)

78) When two carbenes collide, they immediately dimerize to give __________.

A) an alkane

B) an alkene

C) an alkyne

D) an aromatic ring

E) a carbanion

78)

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

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

79)

80) In the first propagation step of the free radical chlorination of methane, which of the following

occurs?

A) Cl2 dissociates

B) A chlorine radical abstracts a hydrogen

C) A carbon radical reacts with Cl2

D) A carbon radical reacts with a chlorine radical

E) Two chlorine radicals combine

80)

11


81) Given a reaction in which reactant A is converted only to product B at 25°C, what Keq

results if at equilibrium 80% of A has become B?

81)

82) Predict the signs of DH° and DS° in the reaction of cyclohexene with H2 to form

cyclohexane.

82)

83) Predict the sign of DS° in the combustion of propane. 83)

84) Which compound has the smaller bond dissociation energy for its carbon-chlorine bond,

CH3Cl or (CH3)3CCl? Explain your reasoning.

84)



+0.5 kcal/mol, the Keq is __________ and the % conversion is __________.

A) 0.18, 15% B) 0.43, 30% C) 1.0, 50% D) 2.3, 70% E) 5.4, 84%

85)


-0.5 kcal/mol, the Keq is __________ and the % conversion is __________.

A) 0.18, 15% B) 0.43, 30% C) 1.0, 50% D) 2.3, 70% E) 5.4, 84%

86)


87) Provide the major organic product that results when 2,2,4-trimethylpentane is subjected to

free radical bromination.

87)

88) The hydrogen atom abstraction step in the free radical bromination of methane is

endothermic. Use the Hammond Postulate to speculate on the extent of bond formation

and bond cleavage in the transition state.

88)

89) The hydrogen atom abstraction step in the free radical chlorine of methane is exothermic.

Use the Hammond Postulate to speculate on the extent of bond formation and bond

cleavage in the transition state.

89)

12


90) In the reaction of Cl2 with ethane and UV light, which of the following reactions would be a chain

termination event(s)?

I) Cl· + CH3-CH3 → CH3-CH2-Cl + H·

II) Cl· + CH3-CH3 → CH3-H2C· + HCl

III) Cl· + CH3-H2C· → CH3-CH2-Cl

IV) Cl2 + CH3-H2C· → CH3-CH2-Cl + Cl·

V) Cl2 + UV light → C l· + Cl·

A) reaction V

B) reactions I and IV

C) reactions III and IV

D) reactions I and II

E) reaction III

90)

91) In the reaction of Cl2 with ethane and UV light, which of the following reactions would be a

propagation event(s)?

I) Cl· + CH3-CH3 → CH3-CH2-Cl + H·

II) Cl· + CH3-CH3 → CH3-H2C· + HCl

III) Cl· + CH3-H2C· → CH3-CH2-Cl

IV) Cl2 + CH3-H2C· → CH3-CH2-Cl + Cl·

V) Cl2 + UV light → C l· + Cl·

A) reactions I and V

B) reactions II, III and IV

C) reactions I and IV

D) reactions II and IV

E) reactions I, II and IV

91)

92) Which of the following is a propagation step in the free radical chlorination of dichloromethane?

A) · CHCl2 + Cl2 → CHCl3 + Cl·

B) · CHCl2 + Cl· → CHCl3

C) CH2Cl2 + Cl· → CHCl3 + H·

D) Cl2 + UV light → 2 Cl·

E) · CHCl2 + · CHCl2 → CHCl2CHCl2

92)

13

93) Which of the presented mechanisms would be the most energetically favorable and thus the most

likely mechanism to actually occur for the following free radical chain reaction ? (bond dissociation

energies -- H-H = 104 kcal/mol; Cl-Cl = 58 kcal/mol; H-Cl = 103 kcal/mol)

H2 + Cl2 hv light

2 HCl

A) H2 hv light

H· + H·

H· + Cl2 → Cl· + HCl

H· + Cl· → HCl

B) Cl2 hv light

Cl· + Cl·

Cl· + H2 → H· + HCl

H· + Cl2 → HCl + Cl·

C) H2 hv light

H· + H·

H· + Cl2 → Cl· + HCl

Cl· + H2 → HCl + H·

D) Cl2 hv light

Cl· + Cl·

Cl· + H2 → H· + HCl

H· + Cl· → HCl

93)

94) Consider the following substitution reaction with a ΔG∘ value of -91.1 kJ/mole.

HO- + CH3Cl ↔ CH3OH + Cl-

Given this information which of the following statements must be true?

(R = 8.315 J/mole K)

A) The Keq at 25∘C for this reaction is very large, in other words this reaction proceeds to near

completion as written, left to right under standard conditions

B) The Keq at 25∘C for this reaction is very small (<1), in other words this reaction does not

proceed from left to right but rather is favored from right to left under standard conditions

C) At 250∘C the equilibrium concentration is shifted right in favor of the products (CH3OH and

Cl-). In other words there is more product than at 25∘C

D) At 250∘C the equilibrium concentration of products and reactants is nearly the same

E) Both A and C are correct.

94)

95) A certain free radical chlorination reaction of 2,3-dimethylbutane leads to the production of two

major monochlorinated products 1-chloro-2,3-dimethylbutane and 2-chloro-2,3-dimethylbutane.

When run through a Gas chromatograph, it was determined that the product distribution was

51.7% 1-chloro-2,3-dimethylbutane and 48.3% 2-chloro-2,3-dimethylbutane. If we assign the

primary H's a reactivity of 1.0, what is the relative reactivity of the tertiary H's.

A) 5.0 times more reactive

B) 5.1 times more reactive

C) 4.6 times more reactive

D) 5.6 times more reactive

E) 3.0 times more reactive

95)

14

96) Which of the following reactive intermediate species maintains sp3 hybridization?

A) methyl carbanion

B) dibromocarbene

C) tertiary carbocation

D) secondary alkyl radical

E) both B and C

96)

97) The Arrhenius equation mathematically models which of the following statements?

A) The rate of a chemical reaction increases exponentially with increasing concentration of

reactants.

B) The rate of a chemical reaction is directly related to the Ea and that increasing the temperature

will alter the Ea for that reaction.

C) Increasing the temperature of a chemical reaction increases the number of particles in the

reaction that have the minimum energy required to meet the Ea.

D) The rate of a chemical reaction is exponentially related to the Ea and relatively small

differences in the Ea can dramatically affect the reaction rates of similar reactions at the same

temperature.

E) both C and D

97)

15

Answer KeyTestname: UNTITLED1

1) The mechanism of a reaction is the complete, step-by-step description of exactly which bonds break, which bonds

form, and the order in which these events occur in the transformation of reactants into products.ID: oc6w 4-1Diff: 2Skill:

2) KineticsID: oc6w 4-2Diff: 1Skill:

3) Many molecules of product are formed for every photon of light which is absorbed by the reaction mixture.ID: oc6w 4-3Diff: 3Skill:

4) Cl-Cl + photon (hν) → 2 Cl·ID: oc6w 4-4Diff: 2Skill:

5) CH3Cl + Cl· → · CH2Cl + HCl

· CH2Cl + Cl-Cl → CH2Cl2 + Cl·

CH2Cl2 + Cl· → · CHCl2 + HCl

· CHCl2 + Cl-Cl → CHCl3 + Cl·

CHCl3 + Cl· → · CCl3 + HCl

· CCl3 + Cl-Cl → CCl4 + Cl·

ID: oc6w 4-5Diff: 3Skill:

6) radicals or free radicalsID: oc6w 4-6Diff: 1Skill:

7) AID: oc6w 4-7Diff: 2Skill:

8) Blue light is effectively absorbed by chlorine but not by methane. Thus the initiating step is most likely the production

of two chlorine atoms from molecular chlorine.ID: oc6w 4-8Diff: 3Skill:

9) Make sure the molar ratio of methane to chlorine is relatively large.ID: oc6w 4-9Diff: 2Skill:

16


10)


11) CID: oc6w 4-11Diff: 1Skill:

12) 4648ID: oc6w 4-12Diff: 3Skill:

13) BID: oc6w 4-13Diff: 3Skill:

14) -0.82 kcal/molID: oc6w 4-14Diff: 2Skill:





19) ExothermicID: oc6w 4-19Diff: 2Skill:

17


20) △S° < 0. The three reactant molecules have significantly more freedom of motion, and randomness, than does the

single molecule of product into which they are converted.ID: oc6w 4-20Diff: 2Skill:



23) DID: oc6w 4-23Diff: 1Skill:


25) -16 kcal/molID: oc6w 4-25Diff: 2Skill:

26) EID: oc6w 4-26Diff: 2Skill:

27) rate = k[CH3Br][I-]




30) The activation energy or EaID: oc6w 4-30Diff: 1Skill:


18


32) The reaction will occur about 2.8 times faster.ID: oc6w 4-32Diff: 3Skill:

33) transition stateID: oc6w 4-33Diff: 1Skill:

34) the activation energy or EaID: oc6w 4-34Diff: 1Skill:

35) -4 kcal/molID: oc6w 4-35Diff: 2Skill:

36)


37) The conversion of Y to Z has a higher Ea than does the conversion of X to Y. The conversion of Y to Z is therefore

rate-limiting.ID: oc6w 4-37Diff: 2Skill:

38)


19



40) +7 kcal/molID: oc6w 4-40Diff: 2Skill:





45)


46) primary bromide, 3.5%; secondary bromide, 96.5%ID: oc6w 4-46Diff: 3Skill:

47) ClCH2C(CH3)2CH2CH3, 45.9%; (CH3)3CCHClCH3, 38.8%; (CH3)3CCH2CH2Cl, 15.3%


48) The 2° hydrogens are 20 times more reactive than the 1° ones.ID: oc6w 4-48Diff: 3Skill:

49) (CH3)3CCHBrCH3ID: oc6w 4-49Diff: 2Skill:

20


50)


51) (CH3)4C or neopentane or 2,2-dimethylpropane



53) The first propagation step in free radical bromination is endothermic while the analogous step in free radical

chlorination is exothermic. From the Hammond Postulate, this means that the transition state for the bromination is

product-like (ie, radical-like) while the transition state for the chlorination is reactant-like. The product-like transition

state for bromination has the C-H bond nearly broken and a great deal of radical character on the carbon atom. The

energy of this transition state reflects most of the energy difference of the radical products. This is not true in the

chlorination case where the transition state possesses little radical character.ID: oc6w 4-53Diff: 2Skill:

54) CH3· < CH3CH2· < (CH3)2CH· < (CH3)3C· < CH2CHCH2·


55) CH3

·

C HCHCH2 ↔ CH3CHCH·

C H2ID: oc6w 4-55Diff: 2Skill:

56)




21


59) Cationic center is sp2 hybridized, therefore the CCC bond angle is 120°.ID: oc6w 4-59Diff: 3Skill:

60) In the bromination of pentane, the lowest energy reaction pathways go through secondary free radical intermediates to

produce secondary alkyl bromides (2-bromopentane and 3-bromopentane). Thus 1-bromopentane is a very minor

product. All of the hydrogen atom abstractions of cyclopentane lead to the same secondary radical which eventually

leads to bromocyclopentane.ID: oc6w 4-60Diff: 2Skill:





65) 3-bromo-3-methylpentaneID: oc6w 4-65Diff: 2Skill:

66) 1-bromo-1-methylcyclohexaneID: oc6w 4-66Diff: 2Skill:




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70) CH3CH3 + Cl· → CH3CH2· + HCl

CH3CH2· + ClCl → CH3CH2Cl + Cl·


71) In order for a free-radical chain to propagate, each mechanistic step must yield a

free-radical species as one of its products. A mechanistic step that does not yield a free-radical stops the chain. Such a

step is known as a termination step.ID: oc6w 4-71Diff: 1Skill:


73) In an exothermic reaction, weaker bonds are broken and stronger bonds are

formed.ID: oc6w 4-73Diff: 1Skill:

74) The propagation steps of the free-radical chlorination of methane are shown

below.

CH4 + Cl· → CH3· + HCl

CH3· + ClCl → CH3Cl + Cl·

In each of the steps, two reactant molecules generate two product molecules. This similarity in the number of

molecular species means that the disorder in the reaction is neither greatly increased nor diminished. Therefore, one

expects ΔS° in either propagation step to be approximately equal to zero.ID: oc6w 4-74Diff: 2Skill:

75) The frequency factor accounts for the number of collisions between reacting

molecules and the fraction of these collisions with the proper orientation for the reaction to happen. There is a linear

relationship between the number of these collisions and the rate constant of the reaction. For a given reaction A is a

constant.ID: oc6w 4-75Diff: 2Skill:

76) The exponential factor e-Ea/RT in the Arrhenius equation corresponds to the

fraction of collisions in which the reacting molecules have the appropriate activation energy to react at a given

temperature.ID: oc6w 4-76Diff: 2Skill:

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81) 4ID: oc6w 4-81Diff: 2Skill:

82) DH° < 0 and DS° < 0ID: oc6w 4-82Diff: 2Skill:

83) DS°>0ID: oc6w 4-83Diff: 2Skill:

84) The carbon-chlorine bond of (CH3)3CCl has the smaller bond dissociation energy. The carbon radical that results

upon bond cleavage is a tertiary radical. Tertiary alkyl radicals are more stable than primary due to hyerconjugative

or inductive stabilization.ID: oc6w 4-84Diff: 2Skill:



87)


24


88) The endothermic nature of this step means that the energy of the transition state is closer to that of the step's products

than to its reactants. Using the Hammond Postulate this means the transition state more closely resembles a carbon

radical than a bromine radical. The carbon-hydrogen bond is almost completely broken and the hydrogen-bromine

bond completely formed in the transition state.ID: oc6w 4-88Diff: 3Skill:

89) The exothermic nature of this step means that the energy of the transition state is closer to that of the step's reactants

than to its products. Using the Hammond Postulate this means the transition state more closely resembles a bromine

radical than a carbon radical. The carbon-hydrogen bond has broken very little and the hydrogen-chlorine bond has

formed very little in the transition state.ID: oc6w 4-89Diff: 3Skill:









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