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Unformatted text preview: CHEMISTRY 322aL/325aL EXAM NO. 2 OCTOBER 9, 2002 Please ___________________ Print Last Name __________________ First Name SSN ________________ (1) (15) _____ (2) (20) _____ (3) (10) _____ (4) (20) _____ (5) (15) _____ (6) (15) _____ (7) (05) _____ (100) Grader ______ ______ ______ ______ ______ ______ ______ TA's Name______________ Lab Day & Time__________ first letter of last name I will observe all the rules of Academic Integrity while taking this exam. _______________________________ Chemistry 322a/325a Exam No. 2 -2- Name______________ (1) (15) (A) (12) Supply the information requested for each of the following reactions. (See information in the Appendix, as needed). Write yes or no in the box above the arrow to indicate if reactions (i) - (iv) are spontaneous in the direction shown. In (v) identify the Lewis acid and Lewis base for the reaction shown.
Spontaneous? (i) CH3 CH2 - Li+ + CH3 OH conjugate acid
Spontaneous? + conjugate base (ii) CH3 C CH + NaH conjugate acid
Spontaneous? + conjugate base (iii) C 6H5CH 2COO -Na+ + CH3 OH conjugate acid
Spontaneous? + conjugate base (iv) O H+ CH3 CCH3 + CH3OH +O H = Identify the Lewis acid and Lewis base in reaction (v). (v) CH3 CH2 OH + (B) (3) From information in the Appendix, calculate the pKb value of ammonia, NH3. Show your simple calculation and place your answer in the box below. = CH3 CCH3 + CH3OH ZnCl2 + CH3 CH2 -O-ZnCl2 H pKb of NH3 Chemistry 322a/325a Exam No. 2 (2) (20) -3- Name______________ (A) (12) Provide systematic or IUPAC names for the structures, or provide structures inside the boxes for the names, as requested below.
CH2 CH3 CH3 CHCH2CH=CH 2 CH3 CH3 CH2 CHCH2CHCH3 Cl cis-2-methyl-3-heptene 5-methyl-4-hexen-1-ol (B) (8) Provide structures for the following constitutional isomers of C4H10O that are identified by their common or alkyl group names. isobutyl alcohol sec-butyl alcohol butyl alcohol tert-butyl alcohol Chemistry 322a/325a Exam No. 2 -4- Name______________ (3) (10) Answer the questions below about the following reaction of acetone and ethyllithium. :O: CH3 -C-CH3 + CH3 CH2 : Li+ - diethyl ether :O: - Li+ CH3 -C-CH3 CH2 CH3
: (A) (4) In the above reaction, the nucleophile is ___________, and the electrophile is __________ In the above reaction, the Lewis base is ____________, and the Lewis acid is ____________. (B) (2) Show the movements of all electron pairs in the above reaction using the curved arrow formalism. Add the curved arrows to the equation in the above box. (C) (4) This reaction is run in diethyl ether, a nonaqueous solvent. Clearly explain (using information in the Appendix) why this reaction cannot be run in water. Write a chemical equation in the box below that describes what happens in water. = reaction in water Chemistry 322a/325a Exam No. 2 -5- Name______________ (4) (20) Circle the correct answer for the following statements or questions. (A) The intensity of an infrared band is often given in absorbance units (A) where A = Log (Io/I). When an ir band shows an absorbance of A = 1 at a specified wavelength, the sample has absorbed approximately what percentage of the incident light at that wavelength? 1% 10% 50% 90% (B) Potential Lewis acids among the structures below are _ + CH3 -N-CH3 Cl-B-Cl CH3 -C-CH3 :Br: Cl CH3 CH3
(circle answer) (C) According to the pKa values, the order of acid strength (strongest to weakest) of the compounds below is
HI pKa -10 I H2 O 15.7 II CH3 NH3+ 10.6 III CH 3COOH 4.7 IV
(circle answer) (D) The order of base strength (strongest to weakest) of the chemical species below is (Hint: Refer to the information in the Appendix).
NH3 I (E) The changes in standard free energy at 25 C for the acid dissociation of three o carboxylic acids are given below. According to the DG values, the order of acid strength (strongest to weakest) is
COOH COOH COOH
(circle answer) DGo (kcal/mol) :
I : : only III I, II and III I and IV II and III II III IV I>II>III>IV IV>III>II>I II>III>IV>I I>IV>III>II CH CH O
3 2 CH3 OH III NH (circle answer) 2 II>IV>III>I IV>II>I>III IV>I>II>III II>III>IV>I II IV
0 I>II>III Cl 5.4 I CH3 5.9 II OH 6.2 III II>I>III III>II>I I>III>II Chemistry 322a/325a Exam No. 2 (4) Contd. -6- Name______________ (F) Among the structural representations for C6H14 below, the two structural formulas that are actually the same constitutional isomer are
CH3CH 2CH2CH 2CH2CH3 I CH3 CH3CHCHCH 3 CH3 IV CH3CH 2CH2CHCH3 CH3 II CH3CH 2CHCH3 CH2CH 3 V CH3CH 2CHCH2CH3 CH3 III CH3 CH3CH 2CCH3 CH3 VI (circle answer) II and III III and V IV and V IV and VI (G) What different types of H are in the structure below? CH3 (circle answer below) 1o, 2o and 3o H. 1oand 2o H. 1o and 3o H. 2o and 3o H. CH3 (H) The constitutional isomer of C6H12 that has no ring strain is
(circle structure below) (I) The number of different groups of equivalent H in the alkane below is
CH3 CH3 CCH2CH 2CH 3 CH3
(circle answer below) two four five six (J) In the alkane below, the C-C bond with the largest barrier to rotation is
1 2 CH3 CH2 CH CH3 3 CH2 CH3
(circle answer below) 1 2 3 Chemistry 322a/325a Exam No. 2 -7- Name______________ (5) (15) The heats of combustion of three constitutional isomers of methylcyclopentene have been measured in the liquid phase: 1-methylcyclopentene, DHC= -896.85 kcal/mol 3-methylcyclopentene, DHC= -900.22 kcal/mol. 4-methylcyclopentene, DHC= -901.68 kcal/mol (A) (2) In the box below, write a balanced general equation for the combustion reaction of these isomers.
balanced equation for the combustion reaction C 6H10 + (B) (6) Provide the structures for the isomers inside the proper boxes, as requested below. combustion products 1-methylcyclopentene 4-methylcyclopentene 3-methylcyclopentene (B) (7) Show how the heats of combustion provide a measure of the relative stabilities of the three isomers by drawing an energy state diagram below. The relative energy scale on the Y-axis is broken for convenience. Draw and label the starting and product states for the combustion reactions of the three methylcyclopentenes. Use an arrow to connect each starting state with the product state and indicate the enthalpy change. Clearly indicate the isomer associated with each starting state, and show the energy differences that measure the relative stabilities of the isomers.
relative energy in kcal/mol 900 890 enthalpy 0 The most stable isomer is ____________________ Chemistry 322a/325a Exam No. 2 -8- Name______________ (6) (15) Rotation around the C-C bond in a disubstituted ethane of general structure XCH2-CH2Y gives rise to different conformations with different energies. (A) (4) For example, during a complete 360o rotation around the C -C bond in butane, the potential energy changes as shown below. The diagram shows that conformations A and B differ by 0.9 kcal/mol. Draw Newman Projection formulas for conformations A and B in the boxes. Relative Potential Energy 4.5 kcal/mol A B
0.9 kcal/mol 3.8 kcal/mol 0 60 120 180 Rotation 240 300 360 degrees (B) (2) Briefly, why is conformation A more stable than conformation B? (C) (4) In 1,2-ethanediol (HOCH2CH2OH), rotation around the C-C bond yields anti and gauche conformations where the gauche conformation is more stable than the anti conformation by 2.7 kcal/mol. Draw the Newman projection formulas of the anti and a gauche conformation for 1,2-ethanediol in the correct boxes below.
Newman projection formulas of HOCH2CH 2OH
1,2-ethanediol anti gauche (D) (5) Studies on 1,2-dimethoxyethane (CH3OCH2CH2OCH3) indicate the anti conformation is more stable than the gauche. What factor could explain the energetic preference for the gauche conformation in 1,2-ethanediol? Draw a Newman projection formula in the box below illustrating your explanation. Why is this factor not possible in the anti conformation? Chemistry 322a/325a Exam No. 2 -9- Name______________ (7) (5) I am a hydrocarbon with the formula C5H8. In my infrared spectrum there are characteristic absorptions near 2900 cm-1, 3300 cm-1, and 2200 cm-1. I react with hydrogen gas over a platinum catalyst to produce another hydrocarbon with the formula C5H12. This hydrocarbon has all three types of hydrogens: primary, secondary and tertiary. Draw my structure in the box below and write my IUPAC name on the line below the box. ...
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