Study the passage, then select the single best answer to each question in the group.. The malonic ester synthesis is one of the most profitable methods of preparing substituted carboxyli
Trang 1ORGANIC CHEMISTRY TOPICAL:
Oxygen-Containing Compounds
Test 1
Time: 22 Minutes*
Number of Questions: 17
* The timing restrictions for the science topical tests are optional If you are using this test for the sole purpose of content reinforcement, you may want to disregard the time limit
Trang 2DIRECTIONS: Most of the questions in the following test
are organized into groups, with a descriptive passage preceding each group of questions Study the passage, then select the single best answer to each question in the group Some of the questions are not based on a descriptive passage; you must also select the best answer
to these questions If you are unsure of the best answer, eliminate the choices that you know are incorrect, then select an answer from the choices that remain Indicate your selection by blackening the corresponding circle on your answer sheet A periodic table is provided below for your use with the questions
PERIODIC TABLE OF THE ELEMENTS
1
H
1.0
2
He
4.0 3
Li
6.9
4
Be
9.0
5
B
10.8
6
C
12.0
7
N
14.0
8
O
16.0
9
F
19.0
10
Ne
20.2 11
Na
23.0
12
Mg
24.3
13
Al
27.0
14
Si
28.1
15
P
31.0
16
S
32.1
17
Cl
35.5
18
Ar
39.9 19
K
39.1
20
Ca
40.1
21
Sc
45.0
22
Ti
47.9
23
V
50.9
24
Cr
52.0
25
Mn
54.9
26
Fe
55.8
27
Co
58.9
28
Ni
58.7
29
Cu
63.5
30
Zn
65.4
31
Ga
69.7
32
Ge
72.6
33
As
74.9
34
Se
79.0
35
Br
79.9
36
Kr
83.8 37
Rb
85.5
38
Sr
87.6
39
Y
88.9
40
Zr
91.2
41
Nb
92.9
42
Mo
95.9
43
Tc
(98)
44
Ru
101.1
45
Rh
102.9
46
Pd
106.4
47
Ag
107.9
48
Cd
112.4
49
In
114.8
50
Sn
118.7
51
Sb
121.8
52
Te
127.6
53
I
126.9
54
Xe
131.3 55
Cs
132.9
56
Ba
137.3
57
La *
138.9
72
Hf
178.5
73
Ta
180.9
74
W
183.9
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.1
79
Au
197.0
80
Hg
200.6
81
Tl
204.4
82
Pb
207.2
83
Bi
209.0
84
Po
(209)
85
At
(210)
86
Rn
(222) 87
Fr
(223)
88
Ra
226.0
89
Ac †
227.0
104
Unq
(261)
105
Unp
(262)
106
Unh
(263)
107
Uns
(262)
108
Uno
(265)
109
Une
(267)
*
58
Ce
140.1
59
Pr
140.9
60
Nd
144.2
61
Pm
(145)
62
Sm
150.4
63
Eu
152.0
64
Gd
157.3
65
Tb
158.9
66
Dy
162.5
67
Ho
164.9
68
Er
167.3
69
Tm
168.9
70
Yb
173.0
71
Lu
175.0
†
90
Th
232.0
91
Pa
(231)
92
U
238.0
93
Np
(237)
94
Pu
(244)
95
Am
(243)
96
Cm
(247)
97
Bk
(247)
98
Cf
(251)
99
Es
(252)
100
Fm
(257)
101
Md
(258)
102
No
(259)
103
Lr
(260)
GO ON TO THE NEXT PAGE.
Trang 3Passage I (Questions 1–6)
β-keto esters and malonic esters (commonly known
as β-dicarbonyl compounds) are compounds that contain
two carbonyl groups separated by an intervening carbon
The central carbon of these compounds contains highly
acidic protons, known as α-hydrogens An α-hydrogen can
be abstracted with an appropriate base, forming an enolate
ion that can be either acylated or alkylated
Two common laboratory syntheses that rely on this
principle are the malonic ester synthesis (Reaction 1) and
the acetoacetic ester synthesis (Reaction 2) The malonic
ester synthesis is one of the most profitable methods of
preparing substituted carboxylic acids, while the acetoacetic
ester route is often used in the synthesis of methyl ketones
1) NaOEt
2) RX
1) H 3 O+ 2) R COOH
+CO2(g)
Reaction 1
O
R
1) H3O+ 2) RX
1) NaOEt
+CO2(g)
Reaction 2
OEt
O
OEt
O
OEt
O
O
OEt
O
R
OEt
O
OEt
O
O
R
2)
A chemist attempted to synthesize
cyclohexanecarboxylic acid (Figure 1) using a slight
variation of the malonic ester synthesis Diethyl malonate
(1 equiv) was dissolved in a solution of sodium ethoxide in
ethanol (2 equiv) The resulting solution was then added
dropwise, with constant stirring, to a solution of
1,5-dibromopentane (1 equiv) When the reaction was
complete, the reaction mixture was hydrolyzed with 6M
HCl(aq) and heated to remove any CO2 that formed After
the desired product was isolated, the final yield was
determined to be 75%
COOH
Figure 1
1 Which of the following best explains the chemistÕs
choice of ethanol as the solvent used in the synthesis described in the passage?
A The solvent chosen should have a boiling point
similar to that of the starting material
B The solvent chosen should have a boiling point
similar to that of the desired product
C Ethanol solvates the ethoxide anions, rendering
them less nucleophilic
D Ethanol is a better solvent than polar aprotic
solvents such as DMSO
2 Which of the following compounds can most readily
be synthesized via the acetoacetic ester pathway shown
in Reaction 2?
A Hexanoic acid
B 2-Hexanone
C 3-Hexanone
D Ethyl hexanoate
3 Which of the following best describes the mechanism
of both synthetic pathways shown in the passage?
A Alkylation followed by hydrogenation and then
carboxylation
B Deprotonation followed by alkylation and then
carboxylation
C Alkylation followed by protonation and then
decarboxylation
D Deprotonation followed by carboxylation and then
hydrolysis
GO ON TO THE NEXT PAGE.
Trang 44 Which of the following halides would be the most
suitable starting material for a malonic ester synthesis
of dibenzylacetic acid?
A Bromobenzene
B 2-Chlorophenol
C 4-Chlorophenol
D Benzyl bromide
5 Attempts to produce laboratory quantities of
asymmetrically substituted carboxylic acids such as
2-ethylpentanoic acid will result in lower yields than
those usually obtained in the case of symmetric
products This difference is primarily attributed to:
A reduced solubility of the participating reactants.
B competitive alkylation resulting in a mixture of
products
C decreased nucleophilicity of the anionic
intermediate
D increased difficulty in hydrolyzing the asymmetric
ester
6 The two synthetic pathways described in the passage
can best be described as:
A nucleophilic substitution reactions.
B nucleophilic addition reactions.
C alpha substitution reactions.
D simultaneous alpha substitution and nucleophilic
substitution reactions
GO ON TO THE NEXT PAGE.
Trang 5Passage II (Questions 7–12)
Triacylglycerols are fats or oils commonly found in
plant and animal systems These molecules are formed by
the attachment of three long chain carboxylic acids (fatty
acids) to glycerol via an ester linkage The carbon chain of
the fatty acids tends to be unbranched and even in number
(usually between 8 and 20 carbons) Fatty acids can be
either unsaturated or saturated: Unsaturated fats contain
double bonds, whereas saturated fats do not Since recent
research has supported the suggested link between dietary
saturated fats and serum cholesterol, the degree of saturation
in various animal fats and vegetable oils is of great interest
to nutritionists
Table 1 contains a number of vegetable oils consisting of
various fatty acids that differ in both chain length and
degree of saturation
Table 1 FATTY ACID CONTENT(%)
Palmitic Palmitoleic Steric Oleic Linoleic Linolenic
(C16)* (C16:1) (C18) (C18:1) (C18:2) (C18:3)
SOURCE:
*Cx :y
x = number of carbons
y = number of double bonds
The fatty acid content of these oils can be easily determined by employing the transesterification reaction (Reaction 1)
O
O
O
C
C
R' O
O
C
R'' + 3MeOH
R''' O
OH MeO
OH + MeO
OH MeO
O R''' C
R'' C
O
R' C O
H+/OHÐ
Triacylglycerol Methanol Glycerol Fatty acids
Reaction 1
A student attempted to determine the fatty acid composition of a commercially available cooking oil by using Reaction 1 The procedure involved dissolving the oil to be analyzed in toluene; when dissolution appeared to
be complete, a solution containing BF3 in methanol was added After heating for 30 minutes, the reaction mixture was cooled to room temperature and extracted with distilled water The organic layer was then retained for analysis
7 What is the purpose of using BF3 in the experiment described?
A To absorb excess water formed
B To catalyze the transesterification by polarizing
the carbon-oxygen double bond
C To catalyze the transesterification by acting as the
initial nucleophile
D To solvate the methanol molecules
8 If the student analyzes the product mixture by proton
NMR, which of the following observations would signify that the reaction had reached completion?
A The presence of several singlets in the region 3.0
to 45 ppm
B The absence of multiplets in the region 3.5 to 4.5
ppm
C The presence of peaks in the region 2.0 to 3.0
ppm
D The absence of a sharp singlet at 9.5 ppm
GO ON TO THE NEXT PAGE.
Trang 69 Triacylglycerol is a useful starting material in the
production of soaps (salts of long chain carboxylic
acids) via a process known as saponification If this
process was carried out by boiling the fat in aqueous
sodium hydroxide and then extraction with toluene,
what products would you expect to see in the aqueous
layer?
A A mixture of methyl esters and glycerol
B Glycerol and sodium chloride
C Both unreacted sodium chloride and triglyceride
D A mixture of glycerol and sodium salts of fatty
acids
1 0 Partial transesterification of triacylglycerol can result
in the formation of ÔdiolsÕ These compounds could
also be described as:
A methanol dimers.
B nonalkylated fatty acids.
C monoesters of glycerol
D diesters of glycerol
1 1 According to Table 1, which of the following oils has
the highest polyunsaturated fat content?
A Corn oil
B Linseed oil
C Soybean oil
D Castor bean oil
1 2 In order to determine the degree of unsaturation in the
oil being analyzed, the student treats a portion of the
reaction mixture with a solution of bromine in carbon
tetrachloride If it is found that 1.50 equivalents of
bromine react, what is the most likely source of the
oil?
A Soybean
B Safflower
C Corn
D Castor bean
GO ON TO THE NEXT PAGE.
Trang 7Questions 13 through 17 are NOT
based on a descriptive passage
1 3 Which of the following compounds is the strongest
acid?
A HOOC-CH2Cl
B HOOC-CH2OCH3
C HOOC-CH2F
D HOOC-CH3
1 4 Which of the following compounds is the most
susceptible to nucleophilic attack by OHÐ?
A Propanoic acid
B Ethanal
C Benzyl chloride
D Propionyl chloride
1 5 The Williamson synthesis of methoxybenzene is
outlined below
OH
1) NaOH
2) CH3Cl
OCH3
+ NaCl + H2O
Which of the following best describes the mechanism
of this reaction?
A Bimolecular nucleophilic substitution
B Unimolecular nucleophilic substitution
C Bimolecular elimination
D Unimolecular elimination
1 6 Reaction of benzoic acid with thionyl chloride
followed by treatment with ammonia will yield which
of the following compounds?
A Benzamide
B p-Aminobenzaldehyde
C p-Chlorobenzamide
D 3-Chloro-4-aminobenzaldehyde
1 7 The lH NMR spectrum below is characteristic of which of the following compounds?
ppm ( δ )
A Butanol
B Butanone
C Butanoic acid
D Butanal
END OF TEST
Trang 8ANSWER KEY:
Trang 9OXYGEN-CONTAINING COMPOUNDS TEST EXPLANATIONS
Passage I (Questions 1Ð6)
1 For question number 1, the correct answer is C This question requires an understanding of the malonic
ester synthesis described in the passage and the mechanism by which it occurs, as well as the concept of solvation
Ethanol is a protic solvent: a solvent whose molecules have a hydrogen attached to an oxygen or a nitrogen Protic solvents solvate anions very effectively by hydrogen bonding to them In this experiment, sodium ethoxide dissociates to produce an ethoxide anion which, depending on the solvent, can act as either a base, abstracting an acidic hydrogen, or as a nucleophile, attacking an electrophile However, as you can see from the reaction, sodium ethoxide abstracts an acidic alpha hydrogen from diethyl malonate, and does not attack the electrophilic carbonyl carbon In using the protic solvent ethanol, the ethoxide anion becomes hydrogen bonded and consequently solvated This "solvation sphere" renders the anion much less nucleophilic and so ensures that alpha deprotonation can proceed unhindered Therefore, choice C is the correct response
Now for the other answer choices Choices A and B discuss the boiling point of the solvent Of course, ethanol has
to be distilled off at the end of the reaction, but its boiling point was not the reason it was chosen Ethanol is used for its protic properties as we discussed, therefore A and B can be discarded Choice D is wrong, since a polar aprotic solvent would
be just as good a solvent as ethanol However, aprotic solvents are not suitable for this reaction: they do not possess a
hydrogen attached to an oxygen or a nitrogen Again, choice C is the correct answer
2 The correct answer to question 2 is choice B This question is testing your understanding of the
syntheses described in the passage, along with a bit of nomenclature It's mentioned in the passage that the acetoacetic ester synthesis is commonly used in the production of methyl ketones: ketones of the type CH3COR So, the question is really asking: "which of the following is a methyl ketone?" 2-Hexanone is the only answer choice that fits this definition and so is correct Hexanoic acid choice A and ethyl hexanoate choice D would be more easily synthesized by the malonic ester
synthesis, not the acetoacetic ester synthesis Finally choice C, 3-hexanone, has the carbonyl in the middle of the chain and would most likely be synthesized by methods other than those mentioned in the passage such as direct oxidation of
3-hexanol Again, choice B is the correct answer
3 The correct answer to question 3 is choice B Just like the previous question, this one tests your
understanding of the mechanism of both syntheses According to the reactions shown, both starting materials are reacted with
a base (in this case, an alkoxide) which abstracts a proton, forming a carbanion: deprotonation Choices A and C can be eliminated The carbanion, being a nucleophile, attacks the alkyl halide, displacing the halide: alkylation Choice D can be eliminated, leaving choice B as the correct response (By the way, since carbon dioxide is a product in both of the reactions, they also undergo decarboxylation.)
4 The correct answer to question 4 is choice D This question requires you to have a thorough
understanding of the malonic ester synthesis probably more so than the other questions It would probably be useful to draw-out the target compound, dibenzylacetic acid, and compare it to the cyclohexane carboxylic acid made by the chemist in the passage If you do this, you will get a figure showing an acetic acid molecule in which the alpha carbon is attached to two benzyl groups so the overall formula is C6H5CH2 twice and then CHCOOH The CH carbon is the original middle alpha carbon of the malonic ester or more specifically, the one that is deprotonated first by sodium ethoxide So, you can see that two benzyl groups have been substituted onto this carbon, so the alkyl halide that reacts must contain a benzyl group The only answer choice that contains this benzyl group is choice D, so this is the correct response When the alpha carbon of diethylmalonate is deprotonated, a benzyl group adds to this carbon, resulting in loss of bromide from the alkyl halide The same process occurs with a second benzyl bromide so overall, two equivalents of benzyl bromide and sodium ethoxide are needed Finally, the substituted acetic acid is formed by addition of acid and then heating in order to decarboxylate the
molecule All the other answer choices are aryl halides and since this reaction does not proceed by nucleophilic aromatic substitution, we could have quickly eliminated them Again, the correct answer is choice D
5 The correct answer to question 5 is choice B This question again tests your understanding of the malonic
ester synthesis The named product 2-ethylpentanoic acid could be made from diethyl malonate by first deprotonating the ester with a strong base, then adding one equivalent of propyl halide, deprotonating for a second time and then adding one equivalent of ethyl halide If this were done simultaneously, without removing the first alkylated product, we would expect to wind up with a mixture of 5, 6, and 7 carbon chains attached to the acid groups in the final products, since the propyl and ethyl halides compete for the same nucleophilic site To obtain the desired product, there would have to be a separation step
so that the first alkylated product could then be treated independently In either case the yield of desired product will be
reduced: by formation of undesired side products or by losses during separation Anyway, back to the question The primary
Trang 10Solubility problems choice A can be avoided by suitable choice of solvents, and are not related to the symmetry of the final product this answer choice is incorrect Choice C tries to distract you with the nucleophilicity of the anion which
is the deprotonated diethyl malonate molecule If sodium ethoxide is present, the nucleophilicity will be exactly the same, whether symmetric or asymmetric products are formed, so choice C is also wrong Meanwhile, choice D is, like some of the choices in earlier questions, looking at the wrong part of the reaction Hydrolysis is basically part of the work-up and so occurs after the alkylation steps are complete
6 The correct answer to question 6 is choice D, simultaneous alpha substitution and nucleophilic
substitution reactions The alpha position of both reactants is the position where the alkyl group has been substituted for one
of the hydrogens Now, in order for substitution to occur, the alkoxide must first abstract a proton from the alpha position, forming a nucleophilic carbanion This nucleophilic carbanion then attacks the alkyl halide, displacing the halide: a
nucleophilic substitution reaction Choice D is the correct response (Even though choices A and C could be considered true, the question asked you to choose the best description of the pathways, and that is choice D.) Choice B nucleophilic addition is a reaction characteristic of carbonyl compounds that have no leaving groups In these reactions the carbonyl is not
attacked, so choice B is incorrect Again, choice D is the correct response
Passage II (Questions 7–12)
7 The correct answer to question 7 is choice B This question tests your understanding of Reaction 1 The
most likely mechanism for a reaction like the one illustrated is nucleophilic attack by the oxygen of methanol on the
carbonyl carbon of the ester followed by the loss of a good leaving group which in this case is an OR group, where R is the glycerol "backbone" This will result in the formation of glycerol and three fatty acids the nature of which will depend on the alkyl chain Neutral alcohols are only slightly nucleophilic and react slowly with the carbonyl carbon Boron trifluoride
is a strong Lewis acid and can coordinate to the carbonyl oxygen, drawing even more electron density away from the carbon-oxygen double bond thereby making the carbon more susceptible to nucleophilic attack by methanol This is described best
by answer choice B As for the wrong choices, A misses the minor detail that water is not a product of the transesterification reaction Choice C wants us to believe that a strong Lewis acid like BF3 can act as a nucleophile, which is impossible by the definition of a Lewis acid as it is an electron pair acceptor, not a donor Choice D solvating the methanol molecule is incorrect as this would decrease its nucleophilicity, not enhance it Boron trifluoride has nothing to do with the methanol in the reaction as I said before, it simply polarizes the carbon-oxygen double bond in the triacylglycerol molecule Again,
choice B is the correct response
8 The correct answer to question 8 is choice B This question requires a basic understanding of NMR and
its relationship to the structural analysis of esters Correctly answering this question also requires a thorough understanding
of the experiment described in the passage Well, the product mixture retained for analysis was the organic layer
Presumably, this should be a mixture of fatty acid esters and an NMR spectrum should reflect this if the reaction did indeed go
to completion If the reaction was not complete then we would expect some water insoluble reactants to be present also Water insoluble reactants would have to be the triacylglycerol, and perhaps some mono- and di- esters of glycerol resulting from the incomplete, though partial, transesterification reaction With that level of understanding, we would be safe to hit the choices Choice A states that 3H singlets are observed This is evidence that methyl groups are present, but does not signify that nothing but methyl esters are present in the product mixture Choice B, on the other hand, indicates that no multiplets meaning no signals corresponding to the hydrocarbon "backbone" of triacylglycerol are observed If this signal were absent, there would also be no mono- or di-esters produced either This should make for pretty good evidence that starting material and partially reacted starting material are absent, and as such, it should make for a pretty good answer choice As for the rest
of the choices, choice C indicates signals in the region where protons alpha to a carbonyl would be observed Since the
initial triglyceride and the final fatty acid esters, as well as the partially reacted species, all have CH2 groups next to carbonyl carbons, this observation would not tell us anything about the progress of the reaction, or lack thereof Finally, choice D gives us a singlet at 9.5, or rather its absence to consider This resonance is characteristic of an aldehyde proton, so its
absence indicates the absence of aldehydes in the product mixture Since neither reactant nor product is an aldehyde, this
observation says nothing at all about the reaction or its progress Again, choice B is our answer
9 For question 9, the correct answer is choice D Saponification involves the basic hydrolysis of
triacylglycerol to yield glycerol and sodium carboxylates which are in fact soaps When triacylglycerol is boiled in aqueous sodium hydroxide, the hydroxide group attacks and adds to the carbonyl carbon of the ester, forming a tetrahedral intermediate, which in turn decomposes to give a carboxylic acid and an alkoxide ion The alkoxide then deprotonates the acid yielding the sodium salt of the acid and glycerol Choice D is the correct response
Because the question states that triacylglycerol is hydrolyzed in aqueous sodium hydroxide and then extracted with toluene, choices B and C can be eliminated since there is no mention of chloride ions As far as choice C is concerned, if