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CHAPTER TWENTY
Enols and Enolates
LEARNING BY MODELING
Spartan 20.1 Make models of acetaldehyde and its enol and replace both with their Database structures. What are their calculated energies? Which is more stable? Which has the more acidic hydrogen
on the basis of calculated charges? Which has the greater dipole moment?
Spartan 20.2 (a) Make models of both enols of 3-methyl-2-butanone (isopropyl methyl ketone), Min-
imize, and compare their energies. Which is the more stable enol according to molecular mechanics
(strain energy minimization)?
H
O
H
O
O
3-Methyl-2-butanone
3-Methyl-1-buten-2-ol
3-Methyl-2-buten-2-ol
(b) Replace the enols with their calculated models from the Database. How do the relative
enol stabilities compare with those of (a)?
(c) Does the generalization that more-substituted double bonds are more stable than lesssubstituted ones apply to these enols?
Spartan 20.3 Early in the 20th century Julius Bredt (University of Aachen) formulated what we now
call Bredt’s Rule. Bredt found that reactions that produce double bonds at bridgeheads in bicyclic ring
systems occur reluctantly, if at all, and concluded that bridgehead double bonds were strained and unstable. Test Bredt’s rule by making Minimized molecular models of the two enols capable of being formed
from bicyclo[2.2.1]heptan-2-one. Which is more stable? What is the energy difference between them?
O
OH
Bicyclo[2.2.1]heptan-2-one
OH
Bicyclo[2.2.1]hept-1-en-2-ol
(violates Bredt’s rule)
Bicyclo[2.2.1]hept-2-en-2-ol
Spartan 20.4 Structures A and B are conformations of the E enol of propanedial (OPCHCH2CHPO).
Structures C and D are conformations of the Z enol. Make models of each and Minimize their energies. Which enol is the most stable? Which is the least stable? Explain.
H
H
H
O
O
O
H
O
O
O
H
H
O
H
A
H
O
H
H
H
H
H
H
H
H
B
C
D
Spartan 20.5 Make a model of 1,3-cyclohexanedione and replace it with the calculated model from the
Database. Compare the calculated charges for the various hydrogens. Which hydrogens are most acidic?
Which are least acidic? Does the charge distribution correspond to what you expect?
Spartan 20.6 Build molecular models of 2- and 3-cyclohexenone and replace them with the calculated
structures from the Database. Before looking at their Properties, predict which is more stable and
which has the greater dipole moment. Check your answers against the calculated values.
O
O
2-Cyclohexenone
3-Cyclohexenone
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Learning By Modeling
Spartan 20.7 1,2- versus 1,4-addition to ,-unsaturated aldehydes and ketones was explained in Section 18.12 in terms of 1,2-addition being faster (kinetic control) but 1,4-addition yielding a more stable
product (thermodynamic control).
(a) Make Minimized models of the 1,2- and 1,4-addition products for the reaction of acrolein
with HCN. Which product is more stable on the basis of molecular mechanics?
O
OH
CHCH H2C
Acrolein
HCN
H2C
Hydrogen cyanide
O
N N
CHCHC
Acrolein cyanohydrin
CCH2CH2CH
3-Cyanopropanal
(b) Replace each product with its Database structure. Which product has the lower calculated
energy?
Spartan 20.8 Consider electron delocalization in the enolate ions derived from acetaldehyde
(CH3CHPO) and thioacetaldehyde (CH3CHPS). Recall that double bonds between carbon and secondrow elements (such as O) are stronger than those involving third-row elements (such as S).
O
O
C
H2C
H2C
H
S
C
and
S
C
H2C
H
C
H2 C
H
Enolate of acetaldehyde
H
Enolate of thioacetaldehyde
Which enolate would you expect to have a more nucleophilic carbon? Which enolate would you
expect to have a shorter carbon–carbon bond? Compare your predictions with the calculated values
in the Database.
Spartan 20.9 Retrieve the models from the Database to learn which is the more stable enolate of
ethyl acetate, E or Z.
O
O
H3C
NaOCH2CH3
CH3COCH2CH3
C
O
H
C
OCH2CH3
H
Ethyl acetate
C
C
OCH2CH3
H3C
(E)-enolate
(Z)-enolate
Spartan 20.10 (a) Make a model of acetoacetic acid in which both carbonyl groups lie in the same
plane and Minimize it. What is its energy? Replace it with the model from the Database and Minimize. Do the carbonyl groups lie in the same plane? What is the dihedral angle defined by the four
designated atoms? What is the energy of this conformation?
*
O
O
O
CH3CCH2CO2H
*
*
* OH
Acetoacetic acid (acetylacetic acid; 3-oxobutanoic acid)
(b) What do you predict will happen to the dihedral angle in (a) when the methyl group in
acetoacetic acid is replaced by H? When it is replaced by tert-butyl?
Spartan 20.11 Repeat part (a) of Spartan 20.2 for the enolate of ethyl acetoacetate. Suggest a reason
for the difference in geometry between acetoacetic acid and the enolate of its ethyl ester.
Spartan 20.12 Enolization of -keto esters can involve either the ketone or ester carbonyl group.
H
H
O
O
O
OCH2CH3
Ethyl acetoacetate
O
O
OCH2CH3
Ketone carbonyl enolized
O
OCH2CH3
Ester carbonyl enolized
Compare the calculated energies of the two enols in the Database. Which is more stable? Why?
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