Question for Lab Chem

TWS Chemistry 1013-231 Organic Chemistry I Name___________________________________

2011 Exam#2 - Version 2

MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question. 1) Energy is __________ when bonds are formed and is __________ when bonds are broken; therefore, bond dissociation energies are always __________. A) consumed / released / endothermic B) consumed / released / isothermic C) released / consumed / endothermic D) consumed / released / exothermic E) released / consumed / exothermic 2) Which of the structures below is bicyclo[3.2.1]octane? 1)

2)

A)

B)

C)

D) 3)

3) In the lowest energy chair conformation of cis-1,3-dimethylcyclohexane, how many axial positions are occupied by hydrogen atoms? A) 6 B) 5 C) 4 D) 2 E) 3

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4) 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 -H2 C· + HCl III) Cl· + CH3 -H2 C· → CH3 -CH2 -Cl IV) Cl2 + CH3 -H2 C· → CH3 -CH2 -Cl + Cl· V) Cl2 + UV light → C l· + Cl· A) reactions I and IV B) reaction V C) reactions I and II D) reactions III and IV E) reaction III 5) Which of the following statements is the best statement of the Hammond Postulate? A) Related species that are similar in energy are also similar in structure. B) Transition states are molecular species of finite lifetime whose properties can be probed through free radical reactions. C) In an endothermic reaction, the transition state is closer to the reactants 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) In an exothermic reaction, the transition state is closer in energy to the products. 6) In the first propagation step of the free radical chlorination of methane, which of the following occurs? A) A carbon radical reacts with a chlorine radical B) Cl2 dissociates C) Two chlorine radicals combine D) A chlorine radical abstracts a hydrogen E) A carbon radical reacts with Cl2 7) Given the following heats of combustion per CH2 group within the structure, which cyclic alkane has essentially no angle strain? (Ref: long-chain Alkane = 659 kJ/mol) A) cyclooctane = 663 kJ/mol B) cycloheptane = 662 kJ/mol C) cyclohexane = 659 kJ/mol D) cyclopentane = 664 kJ/mol E) cyclobutane = 686 kJ/mol

4)

5)

6)

7)

2

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

8)

A) 16

B) 2

C) 8

D) 7

E) 6 9)

9) 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 tertiary bromide E) a secondary bromide 10) Among the butane conformers, which occur at energy minima on a graph of potential energy versus dihedral angle? A) eclipsed only B) gauche and anti C) anti only D) gauche only E) eclipsed and totally eclipsed 11) 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) 5.6 times more reactive B) 4.6 times more reactive C) 5.0 times more reactive D) 3.0 times more reactive E) 5.1 times more reactive

10)

11)

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12) Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below. A→B B→C C→D Ea = 15 kcal/mol, H° = 13 kcal/mol Ea = 10 kcal/mol, H° = -6 kcal/mol Ea = 2 kcal/mol, H° = -20 kcal/mol

12)

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) The most exothermic step is rate-limiting. E) All three steps occur at the same rate; there is no rate-limiting step. 13) Which of the statements below correctly describes the chair conformations of trans-1,4-dimethylcyclohexane? A) The lower energy chair conformation contains one axial methyl group and one equatorial methyl group. B) The higher energy chair conformation contains two axial methyl groups. C) The higher energy chair conformation contains one axial methyl group and one equatorial methyl group. D) The two chair conformations are of equal energy. E) The lower energy chair conformation contains two axial methyl groups. 14) When the reaction between methane and chlorine is photochemically initiated, which of the following compounds cannot be formed through a termination reaction? A) Cl2 B) CH3 CH3 C) CH3 Cl D) HCl 15) Identify the correct IUPAC name for the following structure. 13)

14)

15)

A) 1-bromo-5-cyclopentyl-1,4-dimethylpentane B) 2-bromo-6-cyclopentyl-5-methylhexane C) 5-bromo-1-cyclopentyl-2-methylhexane D) 6-bromo-1-cyclopentyl-3,6-dimethylhexane E) none of the above

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16) Given the bond dissociation energies below (in kcal/mol), estimate the step (CH3 )2 CH· + Cl2 → (CH3 )2 CHCl + Cl·. CH3 CH2 CH2 H (CH3 )2 CH H Cl Cl H Cl CH3 CH2 CH2 Cl (CH3 )2 CH Cl A) +22 kcal/mol 98 95 58 103 81 80 B) +45 kcal/mol C) -22 kcal/mol

H° for the propagation

16)

D) -40 kcal/mol 17)

17) Which of the following is a propagation step in the free radical chlorination of dichloromethane? A) · CHCl2 + · CHCl2 → CHCl2 CHCl2 B) Cl2 + UV light → 2 Cl· C) · CHCl2 + Cl· → CHCl3 D) · CHCl2 + Cl2 → CHCl3 + Cl· E) CH2 Cl2 + Cl· → CHCl3 + H· 18) Which of the following correctly lists the conformations of cyclohexane in order of increasing energy? A) chair < boat < twist < half-chair B) chair < twist < half-chair < boat C) half-chair < boat < twist < chair D) half-chair < twist < boat < chair E) chair < twist < boat < half-chair 19) Which of the following statements is a correct description of the most stable conformation of 1,1,3-trimethylcyclohexane? A) Both methyl groups at C-1 are equatorial. B) C-1 is a quaternary carbon and C-3 is a secondary carbon. C) C-1 is a tertiary carbon and C-3 is a secondary carbon. D) The methyl group at C-3 is equatorial. E) C-1 is a tertiary carbon and C-3 is a primary carbon. 20) Consider the bond dissociation energies listed below in kcal/mol. CH3 -Br CH3 CH2 -Br (CH3 )2 CH-Br (CH3 )3 C-Br 70 68 68 65

18)

19)

20)

These data show that the carbon-bromine bond is weakest when bromine is bound to a __________. A) methyl carbon B) secondary carbon C) quaternary carbon D) tertiary carbon E) primary carbon

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

21)

A) Hd

B) He

C) Ha

D) Hb

E) Hc 22)

22) Which of the statements below correctly describes the chair conformations of trans-1,3-diethylcyclohexane. A) The higher energy chair conformation contains two equatorial ethyl groups. B) The lower energy chair conformation contains two axial ethyl groups. C) The two chair conformations are equal in energy. D) The higher energy chair conformation contains two axial ethyl groups. E) The lower energy chair conformation contains two equatorial ethyl groups. 23) Rank the free radicals (I-III) shown below in order of decreasing stability (i.e., from most stable to least stable).

23)

A) I > II > III

B) I > III > II

C) II > I > III

D) II > III > I

E) III > II > I 24)

24) 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 25) The energy barrier for carbon-carbon bond rotation in propane is mainly due to __________. A) torsion strain B) steric strain C) muscle strain D) angle strain E) bond strain

25)

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Answer Key Testname: CHEM 231 EXAM#2 - VERSION 2

1) C 2) C 3) A 4) E 5) A 6) D 7) C 8) E 9) D 10) B 11) A 12) A 13) B 14) D 15) C 16) C 17) D 18) E 19) D 20) D 21) E 22) C 23) B 24) C 25) A

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