CHEMISTRY 2311B
EXAM 3
Bunz/Marder
October 20, 2004
Name:__________________________
ID#:____________________________
Break each problem down into parts if you can.
GOOD LUCK!
1._____
(25)
2._____
(20)
3._____
(10)
4._____
(14)
5._____
(15)
6._____
(16)
XC_____
(3)
Total______
___________________________
1. Short answer questions (25 points total)
(a) What is the order of acidity (most acidic first) for the following compounds?(CIRCLE
THE CORRECT ANSWER, from the choices given below the compounds) (5 points)
CH4
CF3COOH
I
CH3COOH
II
(1) IV, III, II, I
CH3COH
III
IV
(2) II, III, IV, I
(2) I, II, III, IV
(3) III, IV, I, II
(b) Indicate if the following statements are true or false: (5 points)
(1) The most stable product will always form most quickly in a reaction. F
(2) Heating a reaction can change the free energy of a reaction. T
(3) The reaction pathway with the lowest energy barrier will always lead to the major product of a
reaction. F
(4) The kinetic product formed is always higher in energy than the starting materials. F
(5) The thermodynamic product is always the most stable product of the reaction. T
(c) When racemic 2-iodoheptane is stirred with ethanol for an extended period, the
product will be (CIRCLE THE CORRECT ANSWER) (5 points).
(i)
optically pure (R)-2-ethoxyheptane
(ii)
optically pure (S) -2-ethoxyheptane
(iii)
optically pure (S)-2-heptanol
(iv)
racemic -2-ethoxyheptane
(e) Which of the following compounds would react faster with NaSCH3 in an SN2
reaction? (CIRCLE THE CORRECT ANSWER)(5 points)
(1) Bromocyclopentane.
(2) 1-bromopentane.
(3) 2-bromopentane.
(4) 3-bromopentane.
(5) Neopentyl bromide.
(f) Which of the following is NOT a nucleophile? (CIRCLE THE CORRECT ANSWER)(5points)
(1) BH3
(2) CH3O(3) CH3MgBr
(4) All are nucleophiles.
(5) None is a nucleophile.
3. Acidity (20 points)
(a)Which species is the stronger Lewis Acid? (CIRCLE THE CORRECT ANSWER)
Why? (Use resonance forms to explain your answer.) (8 points)
O
H
H
B
H
H
O
B
H
O
H
____________
BH3 is the stronger Lewis acid; in B(OH)3 the oxygens can donote electron density to the empty boron
orbital stabilizing it.
(b) Which is a stronger acid HOCH3 or HSeCH3? (CIRCLE THE CORRECT
ANSWER) Why? (8 points)
HSeCH3 Because the electronegativity is not that different, but the bond strength
between Se and H is much lower than between O and H
(c)
What is the pKa of methane? (1 point) 50
(d)
What is the pKa of acetylene (HCCH)? (1 point) 25
(e)
Which is a stronger acid? (1 point) acetylene
(f)
Why is one a stronger acid than the other? (2 points)
There is more s character in the orbital accomodating the negative charge in HCC– (sp)
rel to that in CH3– (sp3). The s orbital “sees” more of the nucleus on the average thereby
than a p-orbital, and so an electron in an s orbital is stabilized by the extra interaction
with the nucleus.
3. For each of the following reactions, indicate whether ΔG is greater than or less than
zero. Indicate your choice over to each equilibrium pair of arrows. (10 points)
ΔG < 0
NaOH + H2
(a)
H2O + NaH
ΔG > 0
(b)
C H3C H C H
C H3C H CNa
+ NaOH
+ H2O
4. Consider the reaction A goes to B or C (i.e., B←A→C) for which a reaction
coordinate diagram is shown below. (14 points)
A
B
ΔG‡B→A =
10 Kcal/mol
ΔGB C =
-11 Kcal/mol
ΔGBA =
1 Kcal/mol
ΔG‡A→C =
10 Kcal/mol
C
(a) What is ΔGC→B? (3 points) +11kcal/mol
(b) What is ΔGC→A? (3 points) +12 kcal/mol
(c) Which rate constant is larger that for the formation of C from A or that for the
formation of B from A, or are they the same. (3 points) A–>B has a larger rate constant
(the barrier is only 9 kcal/mol vs. 10 kcal/mol for A–>C).
(d) If the reaction is heated at a high temperature for a long time what product would you
expect to see (assuming this product is stable under the reaction conditions) (5 points)
C, as it is the most stable
5. Draw the principal (major) organic product expected from each of the following
reactions. Clearly indicate stereochemistry where appropriate.
(a) (4 points)
CH3
D
NaSCH3
Br
H
H
acetone
SCH3
CH2CH2CH3
CH2CH2CH3
(b) (5 points)
C CNa
CH3OCH2CH2Br
DMSO
C CCH2CH2OCH3
(c) (7 points)
(S)-2-Butanol
1) Tosylchloride,
pyridine 0°C
2) Na I
I
3. (a) Give the mechanistic symbols (SN1, SN2) that are most consistent with the
following statements. Some of these statements are consistent with more that one
symbol! Indicate so! (16 points)
SN1 , SN2
(i) These reactions require good leaving groups.
__SN1___
(ii) t-butyl iodide reacts with methanol only by this mechanism.
_SN2 _
(iii) These reaction mechanisms have second-order kinetics.
_SN1 , SN2_
(iv) Benzyl bromide can react with nucleophiles by this(these)
mechanism(s).
_SN2 , _
(v) These reactions are favored in polar aprotic solvents.
_SN2 _
(vi) Inversion of configuration typically occurs when optically active
molecules with good leaving groups proceed by this reaction.
Extra Credit: Provide a mechanism using the curved arrow formalism and resonance
structures to explain how both compounds B and C can be formed from compound A,
given the knowledge that the first step involves protonation of the OH group to give the
intermediate “D”. You only need to provide a mechanism from intermediate “D” on to
the products. (3 points)
OH
Cl
Cl
HCl
+
B
A
Cl
"D"
C
Cl