8/10/2019 CHEM+2410+-+1st+Exam+_2012_

1/9

1

CHEMISTRY 2410

First ExaminationFall, 2012

Name:

Pledge:

PROBLEM 1.Provide answers for each of the following:

A. The pKas for acetic acid (CH3CO2H) and hydrochloric acid (HCl) are 5 and -7, respectively. Determinethe equilibrium constant (K) for the equation given below showing briefly how you came to yourconclusion.

HCl + CH3CO2 Cl + CH3CO2H

K

1 10

2 15

3 30

4 25

5 20

TOTAL 100

8/10/2019 CHEM+2410+-+1st+Exam+_2012_

2/9

2

B. Correlate the structures with the listed boiling points. Provide a rationalization for the observeddifferences in physical properties.

XA.X = O

B.X =CH2

C. X = NH

:

Observed boiling points: i. 80.7 C

ii. 88 C

iii. 106 C

(5 points each, 10 points total

8/10/2019 CHEM+2410+-+1st+Exam+_2012_

3/9

8/10/2019 CHEM+2410+-+1st+Exam+_2012_

4/9

4

PROBLEM 3.Order the following compounds from most stable to least stable. Give the calculated destabilizingenergy in each case to support your ordering. A table of relevant energies is given at the end of theproblem.

A. Consider the following trimethylcyclohexanes:

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

H3C

IIIIII

B. Examine the conformations depicted:

CH3

CH3

CH3

CH3

H

H

CH3

CH3

CH3

H

H

CH3

CH3

CH3

HH

CH3

CH3

I II III

8/10/2019 CHEM+2410+-+1st+Exam+_2012_

5/9

5

C. Evaluate these substituted decalins:

H

H

CH3

CH3

H

H

CH3

CH3

H

H

CH3

CH3

I II III

H,H eclipsed

CH3,H eclipse

CH3,CH3eclipsed

CH3,CH3gauche

1,3-diaxial CH3/H

1,3-diaxial CH3/CH3

0.9 kcal/mol

0.9 kcal/mol

3.2 kcal/mol

0.1 kcal/mol

1.4 kcal/mol

2.5 kcal/mol

Useful Conformational Energies

(10 points each, 30 points total

8/10/2019 CHEM+2410+-+1st+Exam+_2012_

6/9

6

PROBLEM 4.Consider the substitution reaction below:

A. Supply a clear, stepwise mechanism for this unimolecular reaction (SN1). Show all movement ofelectrons and charges that accompany the transformation of the starting materials into products.

OH

CH3 + HBr

Br

CH3 + H2O

B. Consider subjecting the following compounds to the reaction conditions given in part Aand rankthem from most reactiveto least reactive. Give a reason for your selections.

OH

CH3

CH3

HO

OH

A B C

8/10/2019 CHEM+2410+-+1st+Exam+_2012_

7/9

7

C. Draw an energy diagram (reaction energy versus reaction coordinate) consistent with yoursupplied mechanism (in part A).

(25 points total)

8/10/2019 CHEM+2410+-+1st+Exam+_2012_

8/9

8

PROBLEM 5.Calculate the overall heats of reaction (!H) for each of the transformations given below. Informed bythese calculations, decide the order of reactivity for these reactions frommost likely toleast likely.(NOTE: See useful bond dissociation energies on the next page.)

OH

+ H-Br

Br

+ H2O

Cl

+ H-Cl

OH

+ H2O

H

+ Br2

Br

+ H-Br

+ Cl2

H Cl

+ H-Cl

Reaction 1

Reaction 2

Reaction 3

Reaction 4

8/10/2019 CHEM+2410+-+1st+Exam+_2012_

9/9

9

(20 points)

H-BrH-Cl

Br-Br

Cl-Cl

H-OHPrimary C-H

Primary C-Br

Primary C-Cl

Primary C-OH

Secondary C-H

Secondary C-Br

Secondary C-Cl

Secondary C-OH

Tertiary C-H

Tertiary C-Br

Tertiary C-Cl

Tertiary C-OH

Useful Bond Dissociation Energies

87.5 kcal/mol103 kcal/mol

46 kcal/mol

58 kcal/mol

119 kcal/mol101kcal/mol

71 kcal/mol

84 kcal/mol

95 kcal/mol

98 kcal/mol

72 kcal/mol

84 kcal/mol

96 kcal/mol

96 kcal/mol70 kcal/mol

84 kcal/mol

96 kcal/mol