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Chemistry 251/253 |
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Homework 4 |
Questions 1-3 involve the following reaction sequence:
1. Draw the structure of compound A.CCC(=C)CC=C
2. Draw the structure of compound B.CCC(CO)CCCO
3. Draw the structure of compound C.CCC1CCCOCOC1
4. An unknown compound, D, contained 76.8% C and 10.5% H. It gave negative tests when treated with KMnO4 and with 2,4-D reagent. When subjected to aqueous acid, compound D produced 2-propanol and a new compound, E, whose 1H-NMR spectrum is attached. Compound E gave a negative iodoform test. Draw the structure of compound D.CCC(OC(C)C)(OC(C)C)c1ccc(C)cc1
5. How many of the following compounds would react with acetone to
produce an imine: 
3
The last compound would not.
6. Draw the enol of CH3CH2CHO.
CC=CO
,
which, in solution, exists in equilibrium with its enol tautomer.
CC(=CC#N)O
8. The 1H-NMR spectrum of this mixture is attached. To the nearest whole number, what is the percentage of the enol tautomer in the mixture? 25 The integration ratio of the signals at 2.02 and 4.78 ppm is 3/1. These signals are assigned to the methyl and methine protons of the enol form shown on the right above. The ratio of the signals at 2.26 and 3.60 ppm is 3/2, which is consistent with the methyl/methylene ratio in the keto tautomer. The ratio of the methyl signals in the enol and keto forms is approximately 25/75, which means that the mixture contains 25% of the enol tautomer.
9. How many of the following compounds would give a positive
iodoform test?
1
Only the 4th compound would give a positive test.
10.* Draw the structure of the enolate ion that would predominate
in the following equilibrium:
CC/C(=C/c1ccccc1)/[O-] The double bond has to be conjugated
to the aromatic ring, and the stereochemistry has to be Z, i.e. the
phenyl ring and the ethyl group have to be on opposite sides of the
double bond.
11. Select the base/solvent combination that would be best suited
to forming the kinetically controlled enolate ion from 
.
D Since a kinetically controlled reaction is irreversible, you want DpKa , and by extension Keq, to be large. This eliminates choices A and B. While deprotonation with methyl lithium, in principle, should have the largest value of Keq, we have already seen that this reagent adds tot he carbonyl group in preference to removing an a-hydrogen. You also want a non-protic solvent to minimize the chances of reprotonating the enolate ion, i.e. to prevent equilibration. Finally, you want a sterically hindered base because it will be more likely to deprotonate the less substituted a-carbon. Thus, D is the best choice.
12. Draw the structure of the product of the following
reaction:
CC12CCCCC2CCC=C1O[Si](C)(C)C
13.How many of the compounds shown below are enol ethers? 1 Only the first compound is an enol ether.
14. How many hydrogen atoms would be exchanged for deuterium atoms
if a sample of 
were
treated with a solution of NaOD in D2O?
1 There is only 1 a-hydrogen
in this molecule.
15. Draw the structure of the product of the
following reaction:
CC(C)(C)C(=O)C(C)(I)I
Only the two a-hydrogens
are removed by the base.
Questions 16 and 17 involve an analysis of
the deprotonation of 2-methyl-3-pentanone:
16.
When the enolate ion was quenched with
ClSi(CH3)3, the product gave the attached
1H-NMR
spectrum. Which carbon was deprotonated, C-2 or C-4? Enter the number
of the atom. 4 The splitting patterns in the
NMR spectrum are consistent with an isopropyl group and a
CH3-CH fragment.
17. Which of the following statements about the deprotonation of 2-methyl-3-pentanone is true?
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The enolate ion formed by removal of a proton from C-4 is a tri-substituted enolate, while that formed by deprotonation of C-2 is tetra-substituted. The tri-substituted isomer should be less stable. Therefore, the fact that it is formed preferentially must mean that its formation is kinetically controlled.
18. The aldol reaction may be described in general terms by the
equation 
.
How many of the structures shown below are consistent with this
formalism? 2 The second and fourth compounds
could be prepared using aldol chemistry.
19.
was prepared by an aldol reaction. Draw the structure of the starting
material.
CC(C)C(=O)C
20. 
was
prepared by an aldol reaction. Draw the structure of the starting
material. 
CCC(=O)CC
21. Consider the reaction
What
is the maximum number of crossed aldol reaction products that
could be formed, i.e. how many different products containing the
fragment
are possible? 3
22. Al D. Hyde wanted to perform a mixed aldol reaction, so he
devised the following scheme, 
,
by which Al meant to imply that he would add one reactant to a
mixture of the other reactant and NaOH. Select the combination of
reactants for which Al's scheme would be
successful.C The compound that Al mixes with
the NaOH can't have any a-hydrogens.
If it did, self-condensation would compete with crossed condensation.
Choices C and D fit the bill in this regard, but in choice D, the
second reactant does not have any a-hydrogens
either. Hence no condensation would take place between these two
reactants.
23. When Polly Ester attempted the
following reaction
,
she obtained a product that gave the attached 1H-NMR
spectrum. Draw the structure of the compound that Polly actually
obtained.CC(=O)CCCC(=O)C
In this case, the enolate ion adds to the b-carbon of the conjugated aldehyde.
24. Aldol chemistry involves joining together
two aldehydes, two ketones, or an aldehyde and a ketone. One of the
two reactants acts as a nucleophile, while the other serves as an
electrophile. Draw the structure of the compound that served as an
electrophile in the crossed aldol reaction that produced 
CC(=O)c1ccccc1
25. Draw the structure of the compound that you
would select to prepare 
using
aldol chemistry.CC(CCC(=O)C)C=O
26. Treatment of (CH3)2C=CHCH2Br with Li metal, followed by addition of D2O produces a mixture of isomers with the molecular formula C5H9D. Draw the structure of another alkyl bromide that would form the same mixture when subjected to the same reaction conditions. CC(C)(Br)C=C
27.Does the reduction of 4-methyl-3-penten-2-one to 4-methyl-2-pentanol shown below occur by path A or B? B NaBH4 does not reduce C-C double bond unless they are conjugated to an electron withdrawing group such as a carbonyl group, a cyano group, or a nitro group.
28. Select the picture that best represents the
pi orbital arrangement of the enolate ion formed by deprotonation of
4-penten-2-one,
.A
The protons on C-3 should be more acidic than those on C-1.When it is
deprotonated, C-3 rehybridizes to from sp3 to
sp2 in order to provide maximum pi orbital
overlap.
29. If you were to equilibrate a sample of
4-penten-2-one with excess D2O in basic solution,
,
what would the molecular formula of the product be?
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The NaOD will deprotonate C-1 and C-3. Deprotonation of C-3 will lead to the development of negative charge on C-3 and C-5 as shown by the resonance structures below. H/D exchange will occur, therefore, on C-1, C-3, and C-5. By using an excess of D2O, all of the H atoms on those carbons may be replaced with D atoms. Note that you never deveop a negative charge on C-4. Therefore, the proton on this carbon does not undergo exchange.
30. When Ethyl Ester tried to run the
following reaction, 
,
she isolated a product that gave the attached 1H-NMR
spectrum. Draw the structure of the compound that Ethyl actually
prepared.OCc1ccccc1O
