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Unformatted text preview: CHEMISTRY 322aL/325aL EXAM NO. 3 OCTOBER 30, 2002 Please ___________________ Print Last Name __________________ First Name SSN ________________ (1) (20) _____ (2) (15) _____ (3) (20) _____ (4) (10) _____ (5) (10) _____ (6) (15) _____ (7) (10) _____ (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. 3 -2- Name______________ (1) (20) Complete the chemical reactions below by providing the missing organic product or products within each box. You do not need to show the byproducts of the reactions, or any mechanisms. Be careful to show stereochemical details in the products wherever they are important. CH3 (A) H C C: - Na+ CH2CH 3 (100% ee) C6H5 (B) CH3 I CH2CH 3 (100% ee) C CH3OH C CH3I excess H2 Pt C11H16O CH3 (C) H C 0% ee NaOH H2O. acetone Br CH2CH 2CH2CH 3 (100% ee) C6H14O 100% ee CH3 (D) C6H5 C + C6H5CH 2Br H acetonitrile + CH2CH 2O Na (100% ee) C17H20O 100% ee CH3 Br (E) (CH3)3C H C11H22O as a mixture of diastereomers H2O acetone Chemistry 322a/325a Exam No. 3 (2) (15) -3- Name______________ (A) (10) Circle the compounds in the box below that are chiral.
O CH3CCH2 CH=CH2 Br (CH 3)2CHCHCH3 CH3CH2 NHCHCH3 = CH3CH2 CHClCH2COOH 5-chloro-1-hexanol CH3CH2 CH3 C=C CH2CH2 COOH H
CH3 (CH 3)2CHCH2 CH2OCH 3 H CH3 CH3 H HO CH3 H OH H (B) (5) Identify each pair of structures in the box below as enantiomers, diastereomers, or the same stereoisomer on the line below each pair.
CH3 H Cl Cl H Cl H Cl H HO C H H CH3 OH CH2CH 3 C CH2CH 3 COOH C H Cl CH3 CH3 COOH C Cl H H OH H CH3 OH H CH3 H CH3 H H CH3 Br Br Br Br CH3 H H CH3 Chemistry 322a/325a Exam No. 3 -4- Name______________ (3) (20) Circle the answer that correctly completes each question or statement below. (A Among the three stereostructures below, CH3 H OH CH2 CH3 I C H OH CH3 CH2 CH3 II C CH3 H OH CH2 CH3 III
I and III are the same stereoisomer I, II and III are the same stereoisomer C I and II are the same stereoisomer II and III are the same stereoisomer (B) Which two structures below are chair conformations of trans-1,4-dimethylcyclohexane, and which conformation is more stable?
CH3 CH3 H I H CH3 H II H CH3 H CH3 III CH3 H I and II are conformations of trans -1,4-dimethylcyclohexane, and II is more stable. I and III are conformations of trans -1,4-dimethylcyclohexane, and I is more stable. II and III are conformations of trans -1,4-dimethylcyclohexane, and III is more stable. IIand III are conformations of trans -1,4-dimethylcyclohexane, and II is more stable. (C) Which statement is correct about the four Fischer formulas below?
CH3 H HO CH3 I OH H H H CH3 II CH3 OH OH HO H CH3 III CH3 H OH HO HO CH3 IV CH3 H H I and II are the enantiomers of the chiral diastereomer and III and IV are the enantiomers of the meso diastereomer. I and III are the enantiomers of the chiral diastereomer and II = IV is the meso diastereomer. I and II are the enantiomers of the meso diastereomer and III and IV are the enantiomers of the meso diastereomer. I and III are the enantiomers of the meso diastereomer and II=IV is the chiral diastereomer. Chemistry 322a/325a Exam No. 3 (3) Contd. -5- Name______________ (D) Optically pure (100% ee) (R)-glyceraldehyde has a specific rotation of [a]D = +8.7o. A sample of glyceraldehyde that shows a specific rotation of -4.35 o is a mixture of 50% R and 50% S 40% R and 60% S 25% R and 75% S 100% S (E) In a nucleophilic substitution reaction, R-Nu + R-X + Nu:- X:- the initial rate of the reaction doubles (increases by a factor of 2) when [R-X] is doubled and [Nu:-] is unchanged. However, the initial rate does not change when [Nu:-] is doubled and [RX] is unchanged. The rate expression for this reaction is rate = k[RX]
(circle answer below) k[RX]2 k[RX] [Nu:-] k[RX]2 [Nu:-] (F) In an SN2 reaction, the reactivity order of the alkyl bromides below will be
Br CH3CH 2CH2CHCH3 I CH3Br II CH3CH 2CH2CH 2CH2Br III CH3 (circle answer below) CH3CCH 2Br CH3 II>I>III>IV II>III>IV>I IV II>III>I>IV II>IV>I>III (G) The stability order of the carbocations below is
H + CHCH3 I II H + CH2CH2 III + CH2CH3
(circle answer below) I>II>III III>II>I II>I>III III>I>II (H) The rate of the reaction F- + CH3 CH2 Br CH3 CH2 F + Brwill be greatest in which of the following solvents? (circle your answer) CH3 SOCH3 CH3 CH2 OH 50-50 ethanol-water CH3 OH (I) The reactivity order of the following oxygen nucleophiles in an SN2 reaction is O O (circle answer below) CH3 CH2 S-O CH3 CH2 C-O CH3 CH2 -O I>II>III II>III>I III>II>I III>I>II I III = (J) The most reactive alkyl halide in a nucleophilic substitution reaction is
(circle answer) =
O = II R-F R-Cl R-Br R-I Chemistry 322a/325a Exam No. 3 -6- Name______________ (4) (10) Answer the following questions about a nucleophilic substitution reaction: R-L + Nu:- R-Nu + L:- (A) (2) The mechanism of this reaction involves a single step and the rate expression shows the form rate = k[R-L] [Nu:-]. What notation describes the mechanism of this reaction? (B) (8) This reaction is exergonic by 5 kcal/mol and has a free energy of activation of 10 kcal/mol. Draw below the free energy diagram for this reaction. Be careful to use the energy scale that is referenced to the starting state as a relative free energy of zero kcal/mol. In your diagram: (i) Draw and label the starting and product states, (ii) Draw the change in free energy between the starting and product states and identify the transition state, (iii) Use arrows to show the standard free energy change and the free energy of activation and include the values and proper notations of these parameters.
kcal/mol 10 5 relative free energy 0 -5 -10 progress of reaction Chemistry 322a/325a Exam No. 3 -7- Name______________ (5) (10) The reaction of tert-butyl bromide with potassium hydroxide in a mixed solvent of water and acetone at 25oC produces tert-butyl alcohol and KBr as shown below. This reaction shows first-order kinetics. The rate of the reaction depends only on the concentration of tert-butyl bromide. The rate does not depend on [KOH].
CH3 CH3 C Br CH3 H2O acetone, 25oC CH3 CH3 C OH CH3 + KOH + KBr (A) (4) In the box below, write the mechanism of this reaction as a series of chemical equations that show the important bonding changes. Be careful to identify the key intermediate. (B) (6) The free energy of activation of the rate determining step for this reaction is approximately 20 kcal/mol. Complete the free energy diagram below by showing the change in relative free energy as the reaction proceeds from the reactants to the products. Identify the rate-determining step and the energy change that corresponds to the free energy of activation. Include and identify the key intermediate and its structure in the diagram. relative free energy RBr + KOH ROH + KBr progress of reaction Chemistry 322a/325a Exam No. 3 -8- Name______________ (6) (15) (3R,4S)-3-Bromo-3,4-dimethylhexane reacts in methanol by a methanolysis reaction to give two separable isomeric methyl ether products as indicated in the reaction below. Answer the following questions about this reaction.
CH3 CH2CBrCHCH2 CH3 CH3 CH3 (3R,4S) CH3OH OCH 3 CH3CH2 C CHCH2CH3 + byproducts CH3 CH3 3-methoxy-3,4-dimethylhexane as two separable products (A) (2) The rate expression for this reaction has the form rate = k[3-bromo-3,4-dimethylhexane]. What is the notation that describes the mechanism of this reaction? (B) (10) Sketch a plausible mechanism for this reaction by completing the scheme below. Be sure to draw a stereochemically correct structure of the key intermediate inside the box. Your mechanism must show how the key intermediate leads to the formation of the two separable products. Complete the detailed stereostructures of the products inside the boxes below.
CH2CH3 Br CH3 C
3 CH3 CH3 OH H CH2CH3
(3R,4S) C4 key intermediate CH3 OH CH2CH3 C
3 CH2CH3 C
3 C4 CH2CH3
two separable products C4 CH2CH3 (C) (3) What is the relationship of the two products? Will each of the products be optically active? Briefly explain. Chemistry 322a/325a Exam No. 3 -9- Name______________ (7) (10) The rate of an SN1 reaction generally increases with a change from a less polar to a more polar solvent. For example, the rate of the solvolysis reaction of tert-butyl bromide increases with increasing polarity of the ROH (such as a change from ethanol to a mixture of ethanol-water). (CH3 )3COR + HBr (CH3 )3CBr ROH Draw free energy diagrams below that include the starting state, transition state, and key intermediate state for this reaction in both a less polar and more polar solvent. Make sure all the above features are clearly labeled. Illustrate in the diagram and explain below why the reaction rate is faster in the more polar solvent. Relative Free Energy Progress of Reaction ...
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