Chem1b Problem Set Two

Name
Section
TA
Ch 1b, Problem Set 2
Due Friday Jan 23, 2015
at 4 PM in the Ch1b Box
Chem1b Problem Set Two Ch1b, Problem Set One Due Friday, Jan. 23, 2015 at 4 PM in the Ch1b Box Problems 6 and 7 are designated  as no collaboration problems. These problems will be considered as quiz-­‐like problems. There are a total of five pages to this problem set. Problem One (21 points). Organic acids and bases. a) (8 points) For the molecules below, which of the labeled protons, HA or HB, is more acidic? Rationalize your answer for each molecule in one sentence or with an appropriate drawing (a picture is worth a thousand words). (a)
(b)
HA
H2C
H3C
H3C
HA
(c)
(d)
CH2
S
HB
O
HA
HB
HB
O
H3C
HA
CH3
HB
O
b) (4 points) Rank the following molecules from least acidic to most acidic. Rationalize your answer with no more than two sentences. O
Cl
Cl
O
O
H
Cl
Cl
O
O
H
F
F
Cl
(i)
O
H
F
(ii)
(iii)
c) (4 points) Rank the following molecules from least basic to most basic. Rationalize your answer with no more than two sentences. NH
C 4H 9
N
C 4H 9
C 4H 9
H
(i)
NH 2
(iii)
d) (5 points) Rank the following compounds from least acidic to most acidic. Rationalize your answer with no more than two sentences or with appropriate drawings. (i)
(ii)
H
H
H
H
NH
CH2
O
CH2
(ii)
(iii)
(iv)
Name
Section
TA
Ch 1b, Problem Set 2
Due Friday Jan 23, 2015
at 4 PM in the Ch1b Box
Problem Two (17 points). Nucleophiles and Electrophiles (i) (8 points) Some molecules can act as both nucleophiles and electrophiles. For the following molecules, identify the nucleophilic site with a square, and the electrophilic site with a circle. (a)
(b)
(c)
O
H 3C
(d)
OH
CH 3
N
H 3C
H
H 3C
C
N
N
CH 3
CH 3
(ii) (8 points) The amino acid arginine is shown below. Identify the three most nucleophilic sites. Identify the most electrophilic site. NH
H 2N
N
H
O
OH
NH 2
Problem Three (12 points). Reactions between Nucleophiles and Electrophiles Draw curved arrow mechanisms for the following reactions and provide the organic product. a) (4 points) H3C
H3C
CH3
+
O
Br
H3C
b) (4 points) MgBr
+
CO2
c) Hint: write out resonance structures for the starting materials. (4 points) O
H3C
N
CH3
O
H
+
Cl
P
Cl
Cl
Name
Section
TA
Ch 1b, Problem Set 2
Due Friday Jan 23, 2015
at 4 PM in the Ch1b Box
Problem Four (10 points). Acids and conjugate bases. Determine which acid in each pair yields the stronger conjugate base. Justify each answer with 1-­‐2 sentences or with the appropriate drawing. a) HCl, HBr b) HNO2, HNO3 c) OH
N
O
OH
N
vs.
H
d) HF (Ka = 6.6*10–4), HCN (Ka = 6.2 * 10–10) e) H3C
NH3
H3C
OH2
vs.
Problem Five (16 points) .Resonance Structures and Charge Delocalization. Provide resonance structures as requested for the following compounds. Use curved arrows to demonstrate the delocalization of charge. a) (4 pts) Provide two additional resonance structures to show delocalization of the negative charge. O
O
CH 3
b) (4 pts) Provide three additional resonance structures to show delocalization of the negative charge. O
c) (8 pts) Consider the two molecules shown below. For each molecule, use resonance structures to show delocalization of the phenoxide anion. Which molecule (a) or (b) would you predict to have a more acidic conjugate acid? Name
Section
TA
Ch 1b, Problem Set 2
Due Friday Jan 23, 2015
at 4 PM in the Ch1b Box
O
O
N
N
O
O
O
(a)
(b)
O
Problem Six (12 points). Dipole Moments of Methyl Halides. Consider the following data for methyl halides, ordered in increasing value of the molecular dipole moment (in units of D, Debye): Molecule Molecular Dipole C-­‐X bond length Moment (D) (Å) CH3F 1.858 1.39 CH3Cl 1.896 1.78 CH3Br 1.820 1.93 CH3I 1.641 2.14 Here is some context to get a sense for how similar these values really are. CH4 is symmetric, and therefore has no dipole moment. (This has consequences for the spectroscopy of CH4, as you will see later in this course.) CH3CN has dipole moment of 4.0 D, which is one of the larger values. The range for dipole moments (in Debyes) is approximately equal to the range of values for the Pauling electronegativity. (a) (4 pts) Based on your knowledge of periodic trends and electronegativities, will the carbon atoms in methyl halides have a positive or negative partial charge? Briefly explain your answer (no credit for an answer longer than three sentences). (b) (4 pts) Based on your knowledge of periodic trends and electronegativities, for which of the three molecules does the carbon atom have the largest magnitude partial charge? For which molecule does the carbon atom have the smallest magnitude partial charge? (No need to explain your answer) (c) (4 pts) Briefly explain why these molecules can have very different values for partial charge on the carbon atom yet have nearly the same dipole moments (no credit for an answer longer than three sentences). Name
Ch 1b, Problem Set 2
Section
Due Friday Jan 23, 2015
TA
at 4 PM in the Ch1b Box
 Problem Seven (12 points). Phenolphthalein is commonly used as an indicator for acid-­‐base titrations. Reaction of phenolphthalein with NaOH results in deprotonation to give A, which then leads to formation of compound B. (i) Used curved arrows to show how A converts B (how do the electrons flow?). (ii) Phenolphthalein is effective as an indicator to titrate solutions of weak acids in what pH range? (The pKa values for a series of common acids are provided for reference). (iii) Would you use bromocresol green as an indicator for a solution of higher or lower pH (relative to the solution in part ii)? HO
O
O
OH
OH
OH
aq. NaOH
O
O
O
O
O
O
A
phenolphthalein
B
acid
pK a
–7
HCl
HO
Br
Br
OH
S
Br
–2.8
O O
OH
OH
H 3C
4.20
Br
O
S
O
CH 3
O
O O
H 3C
bromocresol green
OH
4.75
9.95
OH
CH 3OH
15.5
O
H 3C
CH 3
19.3