Deck 12: Radicals

Question 1

During the free radical chlorination of methane,which of the following reactions has the lowest collision frequency?

A)Cl ∙ + ∙ CH₃ → CH₃Cl
B)Cl ∙ + Cl₂ → Cl₂ + Cl ∙
C)∙ CH₃ + CH₄ → CH₄ + ∙ CH₃
D)∙ CH₃ + Cl₂ → CH₃Cl + Cl ∙

Answer 1

Cl ∙ + ∙ CH₃ → CH₃Cl

Question 2

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.

Answer 2

tertiary carbon.

Question 3

If cyclohexane reacts with excess Cl₂ at high temperature,how many distinct dichlorocyclohexane products are possible? Include all stereoisomers.

A)5
B)6
C)7
D)8
E)9

Answer 3

9

Question 4

Calculate the overall ΔH° for the reaction shown given the bond dissociation energies (in kcal/mol)below: (CH₃)₃C-H + Cl-Cl → (CH₃)₃C_-Cl + H-Cl
91 58 78.5 103

A)+181.5 kcal/mole
B)+58.0 kcal/mole
C)+32.5 kcal/mole
D)-32.5 kcal/mole
E)-57.5 kcal/mole

Question 5

Which of the following is a chain propagation step in the free radical chlorination of methane?

A)CH₄ + Cl ∙ → ∙ CH₃ + HCl
B)Cl₂ → 2 Cl ∙
C)Cl ∙ + ∙ CH₃ → CH₃Cl
D)∙ CH₃ + CH₄ → CH₄ + ∙CH₃

Question 6

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

Question 7

The reaction Br₂ + CH₃Br → CH₂Br₂ + HBr was carried out.Which of the following mechanism steps is productive,but relatively unlikely to occur?

A)Br ∙ + CH₃Br → HBr + ∙ CH₂Br
B)Br ∙ + ∙ CH₂Br → CH₂Br₂
C)Br ∙ + Br₂ → Br₂ + Br ∙
D)Br ∙ + ∙ CH₃ → CH₃Br

Question 8

Which of the following is the initiation step for the monobromination of cyclohexane?

A)I
B)II
C)III
D)IV
E)V

Question 9

Which of the following products result from the disproportionation reaction between two propyl radicals?

A)
B)CH₃CH₂CH₃ + CH₃CHCH₃
C)CH₃CH₂CH₂CH₂CH₂CH₃
D)CH₃CH₂CH₃ + CH₂ CHCH₃
E)CH₂ C CH₂ + CH₃CH₂CH₃

Question 10

The major type of reactions that alkanes undergo is

A)electrophilic substitution reactions.
B)electrophilic addition reactions.
C)free radical substitution reactions.
D)free radical addition reactions.
E)nucleophilic substitution reactions.

Question 11

Which of the following is the rate-determining step for the monobromination of cyclohexane?

A)I
B)II
C)III
D)IV
E)V

Question 12

Which of the following does not occur in a propagation step in the free radical bromination of ethane to form bromoethane?

A)a C-H bond breaks
B)a Br-H bond forms
C)a C-Br bond forms
D)a Br-Br bond breaks
E)a C-C bond breaks

Question 13

The reaction Br₂ + CH₃Br → CH₂Br₂ + HBr was carried out.Which of the following mechanism steps is both productive and relatively likely to occur?

A)Br ∙ + ∙ CH₂Br → CH₂Br₂
B)Br ∙ + ∙ CH₃ → CH₃Br
C)Br ∙ + Br₂ → Br₂ + Br ∙
D)Br ∙ + CH₃Br → HBr + ∙ CH₂Br

Question 14

Given the bond dissociation energies below (in kcal/mol),estimate the ΔH° for the propagation step (CH₃)₂CH∙ + Cl₂ → (CH₃)₂CHCl + Cl∙.
CH₃CH₂CH₂-H 98
(CH₃)₂CH-H 95
Cl-Cl 58
H-Cl 103
CH₃CH₂CH₂-Cl 81
(CH₃)₂CH-Cl 80

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

Question 15

How many products are formed from the monochlorination of ethylcyclohexane? Ignore stereoisomers.

A)6
B)8
C)5
D)9
E)11

Question 16

Which of the following reactions is a termination step in the free radical chlorination of methane?

A)Cl₂ + Cl ∙ → Cl ∙ + Cl₂
B)Cl₂ → 2 Cl ∙
C)∙ CH₃ + Cl ∙ → CH₃Cl
D)CH₄ + Cl ∙ → HCl + ∙ CH₃

Question 17

Explain why alkanes are generally considered unreactive compounds.

Question 18

Why are alkanes so unreactive?

Question 19

What is the major product of the following reaction?

A)I
B)II
C)III
D)IV
E)V

Question 20

How many dichlorinated products,including stereoisomers,can be isolated when (S)-2-chlorobutane reacts with Cl₂/hv?

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

Question 21

What C₅H₁₂ isomer will give only a single monochlorination product?

Question 22

Name the two brominated products which result when 2,3-dimethyl-2-butene reacts with NBS.

Question 23

Species with unpaired electrons are called ________.

Question 24

Which of the following is not an intermediate or product in the reaction of

A)I
B)II
C)III
D)IV
E)V

Question 25

Given the bond dissociation energies below (in kcal/mol),calculate the overall ΔH° for the following reaction:
(CH₃)₃CH + Br₂ → (CH₃)₃CBr + HBr
(CH₃)₃C-H 91
(CH₃)₃C-Br 65
Br-Br 46
H-Br 88
CH₃-Br 70

Question 26

How many monochlorinated products would be obtained from 2-methylbutane? Show the structures and give their IUPAC names.

Question 27

How many electrons are contained in the p orbital of the methyl radical?

A)zero
B)one
C)two
D)three

Question 28

Write a detailed,stepwise mechanism for the following reaction.

Question 29

When light is shown 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.

Question 30

What is the major product of the following reaction?

A)I
B)II
C)III
D)IV
E)V

Question 31

Which of the following most nearly describes the geometry of the methyl radical?

A)trigonal pyramid,bond angle 109.5°
B)trigonal pyramid,bond angle 120°
C)trigonal planar,bond angle 109.5°
D)trigonal planar,bond angle 120°

Question 32

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?

Question 33

Identify as initiation,propagation,or termination.

A)initiation = b; propagation = a,c,g; termination = d,e,f
B)initiation = c,d,e; propagation = b; termination = a,f
C)initiation = b; propagation = c,d,e; termination = a,f
D)initiation = b; propagation = a,f; termination = c,d,e
E)initiation = a,f; propagation = c,d,e; termination = b

Question 34

Consider the following monobromination reaction,then answer the following questions. a)Give the structures and the IUPAC names for the products.
b)Give the common names for the products.
c)Calculate ΔH° for the overall reaction using the following data for the indicated bond dissociation energies: d)Calculate the percent yield for each product.(relative rate of abstraction of 3° hydrogen is 1600; 2° is 82; and 1° is 1.)
e)Propose a step-by-step mechanism for the major product only.
f)Draw a schematic potential energy diagram for the rate-determining step (RDS)only.
g)Does the transition state of the RDS resemble more closely the reactants or the products?
h)Would the value of the activation energy be different for different alkanes? Explain.
i)Would the reaction slow down or speed up if I₂ is used instead of Br₂? Explain.

Question 35

Consider the reaction: CH₃CH₂ ∙ + Br₂ → CH₃CH₂Br + Br ∙.Given that this reaction has an activation energy of +6 kcal/mol and a ΔH° of -22 kcal/mol,sketch a reaction-energy profile for this reaction.Label the axes and show E_a and ΔH° on your drawing.

Question 36

A hydrocarbon with molecular formula C₅H₁₂ is subjected to free radical chlorination conditions and only one monochlorinated product resulted.Provide the structure of this product.

Question 37

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

Question 38

An alkane with the molecular formula C₅H₁₀ forms only one monochlorinated product when heated with Cl₂/hv.Give the structure and the IUPAC name for this alkane.

Question 39

Which of the following is the most stable radical?

A)I
B)II
C)III
D)IV
E)V

Question 40

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

Question 41

Calculate the percentage of 1-chloro-2-methylbutane in the following reaction.

A)11.56%
B)27.77%
C)23.14%
D)35.18%
E)13.88%

Question 42

How many distinct alkyl chlorides can be obtained from monochlorination of 2,3-dimethylpentane?

A)5
B)8
C)10
D)12
E)16

Question 43

Which H in the molecule below is removed to generate the most stable carbon radical? Show the structure of this radical.

Question 44

Name each distinct alkyl chloride (including stereoisomers)that can be generated from monochlorination of 2,2-dimethylbutane.

Question 45

Calculate the theoretical percent yields of the four monochlorinated products that result when 2-methylbutane is subjected to free radical chlorination.Assume that the relative ease of hydrogen abstraction in the chlorination process is 5 for 3°; 3.8 for 2°; and 1 for 1° hydrogens.

Question 46

When Br ∙ 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.

Question 47

Give the structure of the free radical intermediate for each product in a previous problem.

Question 48

Calculate the percentage of 2-chloro-3-methylbutane in the following reaction.

A)11.56%
B)27.77%
C)23.14%
D)35.18%
E)13.88%

Question 49

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.

Question 50

When butane undergoes free radical bromination,the product mixture contains 98% 2-bromobutane and 2% 1-bromobutane.How many times more susceptible to hydrogen atom abstraction is a secondary hydrogen in butane than is a primary hydrogen?

A)100
B)73.5
C)50
D)8)7
E)1)5

Question 51

An unknown sample is suspected of being either ethane or isobutane.How would you distinguish between the two alkanes?

Question 52

Calculate the percentage of 2-chloro-3,4-dimethylheptane formed in the following reaction.

A)6)70
B)11.16
C)16.96
D)22.32
E)26.80

Question 53

Calculate the percentage of 1-chloro-3-methylbutane in the following reaction.

A)11.56%
B)27.77%
C)23.14%
D)35.18%
E)13.88%

Question 54

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?

Question 55

List the following radicals in order of increasing stability (ie,from least stable to most stable).
(CH₃)₃C ∙,CH₂
CHCH₂ ∙,CH₃CH₂ ∙,CH₃ ∙,(CH₃)₂CH ∙

Question 56

Calculate the percentage of 3-chloro-3,4-dimethylheptane formed in the following reaction.

A)6)70
B)11.16
C)16.96
D)22.32
E)26.80

Question 57

Calculate the percentage of 1-chloro-3,4-dimethylheptane formed in the following reaction.

A)6)70
B)11.16
C)16.96
D)22.32
E)26.80

Question 58

Calculate the percentage of 2-chloro-2-methyl butane in the following reaction.

A)11.56%
B)27.77%
C)23.14%
D)35.18%
E)13.88%

Question 59

Identify the hydrogen that will react the fastest in a radical halogenation reaction.

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

Question 60

The following molecule contains how many 1°,2°,and 3° hydrogens?

Question 61

The major monobrominated product which results when methylcyclohexane 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.

Question 62

Provide the major organic product of the reaction below.

Question 63

Predict the major monobromination product in the following reaction.
(CH₃)₃CCH₂CH₃ + Br₂

Question 64

Which of the following is true about the halogenation of methane,CH₄?

A)The activation energy for F ∙ + CH₄ → ∙ CH₃ + HF is larger than any other halogen.
B)O₂ slows these reactions by inhibiting the termination step.
C)The activation energy for I ∙ + CH₄ → ∙ CH₃ + HI is relatively small.
D)The reaction must take place in the liquid state.
E)I₂ is unreactive because the activation energy for I ∙ + CH₄ → ∙ CH₃ + HI is relatively large.

Question 65

Explain the regioselectivity observed in the radical addition of HBr to 2-methylpropene.

Question 66

Which of the following is the best synthesis of 1,1-dibromopropane?

A)
B)
C)
D)
E)

Question 67

Provide the major organic product of the reaction shown below.

Question 68

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

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

Question 69

What is the major product of the following reaction?

A)I
B)II
C)III
D)IV
E)V

Question 70

Calculate the percentages of each product in the following reaction.

Question 71

A radical mechanism is postulated to occur when cyclohexene reacts under which of the following conditions?

A)Br₂,CCl4
B)H⁺,H₂O
C)BH₃∙THF
D)HBr,peroxide
E)Hg(OAc)₂,H₂O

Question 72

In the free radical chlorination of ethane,the step in which the Cl radical abstracts a H atom from ethane is ________ and the transition state most closely resembles ________.

A)exothermic,the reactants
B)endothermic,the reactants
C)exothermic,the products
D)endothermic,the products
E)endothermic,a carbocation

Question 73

Would chlorination or bromination of 2,5-dimethylhexane produce a greater yield of 1-halo-2,5-dimethylhexane?

Question 74

Provide the structure of the major organic product of the following reaction.

Question 75

Which of the following is a step in the mechanism of the reaction shown?

A)
B)
C)
D)
E)

Question 76

Give the best product for the following reaction.

A)
B)
C)
D)
E)

Question 77

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

Question 78

What is the major product of the following reaction?

A)I
B)II
C)III
D)IV
E)V

Question 79

Show the propagating steps in the addition of HBr to 1-pentene in the presence of peroxide.

Question 80

Provide the major organic product of the reaction below.