KEY Exams … · para directing, activating the benzene ring towards electrophilic aromatic substitution (EAS, chapter 16). However, when in the presence of the common catalyst for
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Chemistry 261
Exam 2 Practice
Fall 2017
The following practice examination contains 30 questions valued at 3 point/question
unless otherwise noted. Wednesday’s exam will also contain 30 questions, with an
additional 9 points available as bonus/challenge questions
Name: KEY
ELECTRON DEFICIENT COMPOUNDS AND LEWIS ACID-BASE DEFINITIONS
1. Which of the following is not a Lewis acid?
a. AlCl3
b. H3O+
c. FeCl3
d. SO3
e. N/A; all of the above will act as Lewis acids
By definition, a Lewis acid is an electron pair acceptor. G.N. Lewis’s development in 1906
(the same year Brønsted and Lowry developed the proton transfer concept) focuses on
acceptors of an electron pair, is the broadest definition of acidity, and as such Bronsted
acids are a subset of Lewis acids1. Notice that SO3 reacts with H2O to generate H2SO4,
a principal component of acid rain
1 For the interested student, the Lewis acid Wiki is pretty good
ELECTRON DEFICIENT COMPOUNDS AND LEWIS ACID-BASE REACTIONS
2. The amino group on the weak base aniline (anilinium ion pKa = 4.87) is strongly ortho and
para directing, activating the benzene ring towards electrophilic aromatic substitution
(EAS, chapter 16). However, when in the presence of the common catalyst for
electrophilic aromatic bromination of the benzene ring, FeBr3, the reaction is strongly
inhibited, with complex I being formed. The aniline amino group has acted as
Complex I
a. A Lewis acid
b. A Lewis base
c. A Brønsted acid
d. A Brønsted acid
e. An electrophile
Sorry for the wordiness on this one, but it shows that identifying the potential for Lewis
adduct formation can be critical to assessing reaction potential. One way to make better
sense of FeBr3 as a Lewis acid is to name it – Iron(III) bromide…iron is in the +3
oxidation state and has available d orbitals for coordinate covalent bond formation
NUCLEOPHILES, ELECTROPHILES, AND LEAVING GROUPS
3. In the reaction between NaCN and propyl chloride to generate cyanopropane (formally
butanenitrile) the chloro group acts as
a. A nucleophile
b. An electrophile
c. A Lewis acid
d. A Lewis base
e. A leaving group
Nucleophiles, electrophiles, and leaving groups, oh my! Most of the reactions you will see
in this class are 2 electron processes involving a nucleophilic attacking species (I suppose it
“attacks” based on curved arrow conventions), an electrophilic species being attacked, and
a leaving group to maintain the requisite number of bonds for a noble gas electronic
configuration
NUCLEOPHILES, ELECTROPHILES, AND LEAVING GROUPS
4. Which of the following is the strongest electrophile?
a. CH3CO2CH3
b. CH3CO2H
c. CH3OCH3
d. CH3COCH3
e. CH3CCNa+
Clearly, you must be able to translate condensed structural formulas into full Lewis
structures: (a) is an ester (b) is a carboxylic acid (c) is an ether, and (d) is a ketone. As
a [poorly] stabilized carbanion, (e) is a strong base and nucleophile. The carbonyl is the
most polarizing, but the extra oxygen in options (a) and (b) make them more electron rich
BRØNSTED-LOWRY ACIDS AND BASES – CONJUGATE ACID AND BASE DEFINITION
5. Which of the following is the conjugate base of ammonia?
a. NH4+
b. NH3
c. NH2
d. N2H4
e. N/A; none of the above is the conjugate base of ammonia
Critical to understand this, and to understand the weaker the acid (in this case the weak
base ammonia has somehow been forced to act as an acid) the stronger the conjugate base
– the amide ion is a very strong base
BRØNSTED-LOWRY ACIDS AND BASES – CONJUGATE ACID AND BASE DEFINITION
6. Which of the following are Brønsted bases in the following equilibrium?
H2O + H2O OH + H3O+
a. H2O and OH
b. H3O+ and OH
c. H2O and H2O
d. H3O+ and H2O
e. Only OH as water cannot simultaneously be an acid and a base
Water in the forward direction, hydroxide in the reverse direction. Of course water is
amphiprotic, accepting or donating H+ depending on circumstance – this is fundamentally
the basis for the pH scale
FUN WITH pKa’s
7. What is the approximate pKa of hexanoic acid (commonly caproic acid)?
a. 2
b. 3
c. 4
d. 5
e. 6
Actual pKa = 4.88. Acetic acid pKa = 4.76. Without anything to distort the electron
distribution in relation to acetic acid, the pKa will not change much
FUN WITH pKa’s
8. Which of the following compounds would be deprotonated by potassium tert-butoxide?
a. Hexane
b. 1-Hexene
c. 1-Hexyne
d. (b) & (c)
e. None of the above
This question really isn’t that difficult given tBuOK is the conjugate base of an alcohol (an
alkoxide, with the negative charge on an electronegative oxygen) while the answer options
would leave a carbon anion. Options (1) or (2) sinply are not happening regardless of the
base employed, while (c) has an approximate pKa = 25 while tBuOH has a pKa = 18. A 7
unit difference is too large to affect any significant deprotonation
FUN WITH pKa’s
9. Which of the following compounds would be deprotonated by sodium hydride?
a. Hexane
b. 1-Hexene
c. 1-Hexyne
d. (b) & (c)
e. None of the above
With a pKa ≈ 35, NaH easily deprotonates terminal alkynes of pKa = 25. I suppose by
this point you have rightly concluded the free energy of activation for proton transfer
reactions is sufficiently low that if there is a favorable pKa difference, reaction will occur.
In fact, some of the very strong base reactions must be run at reduced temperature so
that things don’t get out of hand
FREE ENERGY AND CHEMICAL EQUILIBRIUM
10. Which of the following correctly expresses the relationship between pKa and the standard
free energy of dissociation?
a. Gao = 2.3RTpKa
b. Gao = -2.3RTpKa
c. Gao = 10pKa/2.3RT
d. Gao = 10pKa/2.3RT
e. pKa = 2.3RTGao
Math! Note the change from the more usual Ka form given the definition of p as –log; i.e.
Gao = -2.3RTlogKa. Rearranging and taking the antilog places -Ga
o/2.3RT as an
exponent. Since 2.3RT translates to 1.4 kcal/mol under standard conditions, slight
changes in Gao correspond to large scale changes in equilibrium ratios. Spending some
time with table 3.2 gives you a very nice feel for this fact
STRUCTURE-ACIDITY RELATIONSHIPS
11. Rank the bold-faced hydrogens in the following compounds from most acidic to least
acidic.
a. I > II > III > IV > V
b. III > V > II > I > IV
c. V > II > IV > III > I
d. III > I > V > II > IV
e. V > III > I > II > IV
Protonated ether (-2.5) benzoic acid with electron withdrawing group (≈3) phenol (10)
vinylic (44) and straight up alkane (50)
STRUCTURE-ACIDITY RELATIONSHIPS
I II III IV V
CF3
COOH
H
O
H H
OH
CH3
H3C
H3C
12. Rank the identified hydrogens from the most acidic to least acidic in the compound
shown below
a. Ha > Hb > Hc > Hd
b. Ha > Hc > Hd > Hb
c. Hc > Hd > Ha > Hb
d. Hd > Ha > Hc > Hb
e. Hc > Ha > Hd > Hb
I really like this question – imagine you were starting at pH = -1 and then the pH was
slowly raised…can you see the protons being sequentially removed? The ring structure
corresponds to the non-nuclephilic (too bulky and inductive removal of lone pair electron
density) weak base pyridine (pyridinium pKa = 5.2) which is a very useful “proton sink” for
reactions that generate acid by-products
STRUCTURE-ACIDITY RELATIONSHIPS
13. For the simple hydrides listed below, which is the correct order of decreasing pKa
values?
a. CH4 > NH3 > H2O > H2S > HBr
b. HBr > H2S > H2O > NH3 > CH4
c. HBr > H2O > NH3 > H2S > CH4
d. NH3 > H2S > CH4 > H2O > HBr
e. H2S > H2O > HBr > NH3 > CH4
Decreasing? With methane on the list? Game over! Notice H2S has a pKa = 7 (thiols
similar to phenol @ 10-12), while water pKa = 15.7 just as HCl is a much stronger acid
than HF
ACID-BASE REACTION PRODUCT PREDICTION – CURVED ARROW APPROACH
14. What is/are the products of the following acid-base mechanism?
a.
b.
c.
d.
e. None of the above
Pretty straightforward – nowhere else for the carbanion to resonate to, and having an
electron deficient C next to a carbanion simply isn’t tenable (option (d))
H
BrMg
+ BrMg H
MgBr
+ CH3CH3
H
MgBr
+ CH3CH3
ACID-BASE REACTION PRODUCT PREDICTION – CURVED ARROW APPROACH (IMPLIED)
15. What is/are the products of the following acid-base reaction?
a.
b.
c.
d.
e. N/A; There is no acid-base reaction between the reactants shown
Option (a) seems pretty tempting, but comparing the pKa values shows the alkyl amide ion
(yes, I don’t like the naming any more than you do) competes much more fiercely than the
alkynide ion for the hydrogen in question
N
H
+Na
N +
Na
H
NH
+
Na
H
H
N
NH +
Na
H
STRUCTURE AND BONDING IN ALKENES
16. Which of the following unknown acyclic molecules contains no rings or double bonds
a. C5H13N
b. C5H10O
c. C6H10Cl2O
d. C7H12IBr
e. More than one of the above
CnH2n+2, adding 1 for N, ignoring O, and counting halogens as hydrogen
STRUCTURE AND BONDING IN ALKENES
17. For which of the following molecules can resonance give a completely equivalent, non-
charge separated form?
a. Benzene
b. 1,3,5-Hexatriene
c. 1,3-Cyclobutadiene
d. 1,3-Butadiene
e. More than 1 of the above
Benzene is pretty obvious, 1,3-cyclobutadiene less so. If the molecule is not cyclic, then
a charge separated form must be the end result. 1,3-cyclobutadiene is the poster child
for anti-aromatic compounds, and has a very short half-life, since electrons need to be
placed in non-bonding MO’s2
DOUBLE BOND STEREOISOMERS/CIS-TRANS & E,Z NOMENCLATURE
18. Please draw the structure for (Z)-2-vinyl-2-pentenoic acid
Z for zusammen or together. Phantom rule on carbonyl oxygen gives higher priority than
phantom rule on ethenyl group
2 Much more on this later – the interested student should see Hückel’s Rule
DOUBLE BOND STEREOISOMERS/CIS-TRANS & E,Z NOMENCLATURE
19. Please name the following compound, adhering to IUPAC conventions
Name: 2,5-cyclohexadien-1-ol
The higher oxidation state of the alcohol dictates numbering. Alkenes are within a 6
membered ring – on need to indicate Z/cis or E/trans. Indicating the alcohol as the “1”
position is overkill in this case
UNSATURATION NUMBER OR INDEX OF HYDROGEN DEFICIENCY
20. What is the unsaturation number for the compound immediately below
a. 2
b. 3
c. 4
d. 5
e. 6
5 bonds and 1 ring
PHYSICAL PROPERTIES OF ALKENES
21. Which of the following will have the highest boiling point
a. 1-pentene
b. 1-hexene
c. cyclopentene
d. cyclohexene
e. All of the above boil within 5 oC of one another
Some questions I like so much I may just revisit them (wink). See practice quiz 2 for
further details
RELATIVE STABILITIES OF ALKENE ISOMERS
22. Which alkene would you expect to be most reactive toward acid-catalyzed hydration?
Hint: resonance possibilities may surprise you
a. Styrene (vinyl benzene)
b. Toluene (methyl benzene)
c. 1-Hexene
d. 2-Hexene
e. Cyclohexene
The benzylic cation is highly resonance stabilized, which gives it the approximate stability
of a 2o carbocation. Notice that the cation formed has stability more like a 3o
carbocation since it 2o to begin with (C+ bonded to benzene ring as well as the terminal C)
ADDITION OF HYDROGEN HALIDES/CARBOCATION STABILITY/CARBOCATION REARRANGEMENTS
23. Which of the following carbocations would not undergo rearrangement?
a.
CH3CHCHCH3
CH3 b.
CH3CHCCH3
CH3
CH3
c.
CH3CCH2CH3
CH3
d.
CH3CHCH2
CH3
e.
CH3CCHCH2CH3
CH3
CH3
The one that is already a 3o carbocation. (a) and (d) would undergo hydride shifts, while
(b) and (e) undergo methide shifts
ADDITION OF HYDROGEN HALIDES/CARBOCATION STABILITY/CARBOCATION REARRANGEMENTS
24. Treating 1-methylcyclohexene with H3O+ would produce which of the following as the
principal product(s)?
II III
IV V
I
HO HO
HO
HOOH
a. I & V
b. II
c. III & IV
d. IV
e. I, III, & V
Direct formation of a tertiary carbocation followed by the addition of water
ADDITION OF HYDROGEN HALIDES/CARBOCATION STABILITY/CARBOCATION REARRANGEMENTS
25. What is the principal product of the following reaction?
a.
b.
c.
d.
Br2, CH3OH
+ enantiomerOH
Br
+ enantiomer
OCH3
Br
+ enantiomer
Br
H3CO
e. More than one of the above
Variation on halohydrin formation where alcohol is the solvent, leading to an ether (as
opposed to an alcohol product). Other principles apply – since you are adding Br2, not
HBr, do not expect a rearrangement. 2o carbocation is more stable, so we expect
alcohol addition adjacent the quaternary positon
ADDITION OF HYDROGEN HALIDES/CARBOCATION STABILITY/CARBOCATION REARRANGEMENTS
26. Reacting 1-methylcyclopentene with bromine monochloride (BrCl) yields which of the
following as principal products?
I II III
Cl
Br
CH3 CH3
Br
Cl
Cl
Br
CH3
IV
CH3
Br
Cl
V
CH2Cl
Br
+
enantiomer
+
enantiomer
+
enantiomer
+
enantiomer
+
enantiomer
a. I
b. II
c. III
d. IV
e. V
Too hard? Not if you apply fundamental principles! Chlorine is more electronegative than
bromine, making the bromine end of the molecule more electrophilic. Thus, a bromonium
ion bridges between the [formerly] alkene carbons in question, with a large degree of
carbocation character at the 3o position. Chloride attacks this site of greatest
electrophilicity from “below”, leaving the chloro and bromo groups on opposite faces of
the ring
+ enantiomer
OCH3
Br
REACTION RATE CONSIDERATIONS
27. Which of the following correctly expresses the relationship between rate and the
standard free energy of activation?
a. log(rate) = 2.3RTGo‡
b. log(rate) = -2.3RTGo‡
c. log(rate) = -Go‡/2.3RT
d. log(rate) = Go‡/2.3RT
e. ln(rate) = -Go‡/2.3RT
Notice the negative value associated with the free energy change – larger free energy
barriers lead to larger negative exponents and much slower rates of reaction. To wit, just
as the 1.4 kcal/mol difference changes the equilibrium concentration by a factor of 10, a
1.4 kcal/mol increase in free energy of activation slows the reaction by a factor of 10
REACTION RATE CONSIDERATIONS
28. Briefly, what is the difference between the standard free energy of activation and the
standard free energy of dissociation?
The standard free energy of activation pertains to the relationship between free
energy changes and rate of reaction, whereas the standard free energy of
dissociation relates overall free energy changes to equilibrium concentrations of
reactants and products. Importantly, there are only positive values for Go‡, since
it is the difference between the transition state and reactant system free energy.
For Gao, negative free energy changes (release of free energy) between product
and reactant free energy levels are common, if not expected, with large negative
changes driving the reaction to completion, as should be evident with an overall
positive value of Gao in Ka = 10^-Ga
o/2.3RT
CATALYSIS/CATALYTIC HYDROGENATION OF ALKENES
29. Under usual hydrogenation conditions, how many moles of diatomic hydrogen will react with
the compound immediately below
a. 2
b. 3
c. 4
d. 5
e. 6
5 bonds and 1 ring, and the ring won’t react
CATALYSIS/CATALYTIC HYDROGENATION OF ALKENES
30. Which of the following will convert cyclohexene into cyclohexane?
a. H2/Ni/high pressure
b. H2/Pd/C
c. H2O/Ni
d. More than 1 of the above
e. All of the above
Hydrogenation of an alkene requires hydrogen (and a metal catalyst). LiAlH4 and NaBH4
provide hydride, which effectively amounts to hydrogenation upon acidic workup, but only
for more electrophilic systems like carbonyls, and NOT for alkenes. Similarly, carbonyls
are typically resistant to conditions that hydrogenate alkenes – a very useful difference
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