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. AlCl 3 b. H 3 O + c. FeCl 3 d. SO 3 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 acids 1 . Notice that SO 3 reacts with H 2 O to generate H 2 SO 4 , a principal component of acid rain 1 For the interested student, the Lewis acid Wiki is pretty good
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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