Molecular Modeling Problems
Chapter 18: Enols and Enolates
1. Enolates in H/D Exchange Reactions. Treatment of the bicyclic enone
pictured below with deuterated sodium hydroxide in D2O, leads to exchange
of several hydrogen atoms for deuterium atoms.
NaOD
D2O
O
"enone"
O
D5
The reaction is presumed to involve initial deprotonation leading to an
enolate anion followed by deuteration by D2O. How many “reasonable”
enolate anions may result from deprotonation? Identify the “best” (lowest
energy) enolate. Identify any other enolates that would be expected to make
up at least 1% of a room temperature equilibrium mixture. How many
hydrogen atoms would you expect to be exchanged for deuterium atoms?
2. Using Enolate Stabilities to Anticipate Product Distributions. In the
presence of strong base, unsymmetrical ketones may give rise to two (and
possibly more) different enolates. Where the reaction is under
thermodynamic control, knowledge of which enolate is the most stable
allows assignment of the preferred product.
Assuming thermodynamic control, assign the product of each of the
following reactions.
To accompany Organic Chemistry, Sixth Edition by Vollhardt & Schore
Molecular Modeling Problems
Try to rationalize your results based on difference in the structures of the
different enolates.
3. Intramolecular Aldol Condensation. Aldol condensation involves
addition of an enolate formed upon deprotonation of an aldehyde or ketone
by strong base, to another aldehyde or ketone with subsequent loss of water,
for example, the aldol condensation of acetaldehyde.
CH3CHO
OH–
CH2CHO
–
CH3
OH
CH3CHO
H 2O
CH3CHCH2CHO
O
For some dicarbonyl compounds, intramolecular aldol condensation may
also occur, leading to the possibility of two or more different products. Thus,
aldol condensation of 2,7-octadione can give rise to either (or both) seven
and five-member ring products.
O
O
O
–
O
O
O
O
–
O
Identify the lower-energy enolate? Assuming that the product distribution
follows from the equilibrium distribution of enolates, what is the ratio of 5 to
7-member ring products at room temperature?
To accompany Organic Chemistry, Sixth Edition by Vollhardt & Schore