WEEK 4:
DIELS-ALDER REACTION
PURPOSE:
This experiment will introduce the student to an important
cycloaddition reaction. The Diels Alder reaction is reversible
and the student will need to consider methods to increase the
yield, if possible.
IMPORTANT REACTIONS:
Diene
Dienophile
Adduct
H
O
H
H O
O
H
O
-phellandrene
(M. Mass 136
bp 175-176oC)
Maleic anhydride
(M. Mass 98
mp 54-56o)
O
O
endo-7-isopropyl-4-methylbicyclo[2.2.2]
oct-4-ene-1,2-dioic anhydride
(M. Mass 234, mp 126-127oC)
BACKGROUND INFORMATION:
The Diels Alder reaction is an example of a cycloaddition
reaction discovered by two German chemists (Otto Diels and Kurt
Alder) in the 1950’s. This reaction is an example of a
concerted reaction in which bonds are being made as other bonds
are being broken. Effective overlap of all bonding orbitals is
essential for the reaction to occur which leads to a specific
stereochemistry in the products. In this reaction, the endo
isomer is formed.
The Diels Alder reaction is an example of a 2-4
cycloaddition reaction in which an alkene adds in a 1,4 addition
to a diene. Neither Diels nor Alder fully understood the
generality or significance of cycloaddition reactions. This was
left for Roald Hofmann (now at Cornell University) and R. B.
Woodward from Harvard University to discover in the late 1960’s.
The Woodward Hofmann rules for cycloaddition reactions (which
are beyond the scope of this discussion) were perhaps one of the
major discoveries in organic synthesis in the 20th century.
In early work with this reaction, results were mixed and
yields varied quite a bit. Later it was found that this
reaction is reversible. If one considers LeChatelier’s rule, it
might be possible to shift the equilibrium in the desired
direction.
In the experiment that will be done here, alphaphellandrene will react with maleic anhydride. The alphaphellandrene is a natural product derived from cedar wood and
has a characteristic cedar wood odor. It is not 100% pure so
the directions call for an added amount to be used to compensate
for the impurity. Maleic anhydride is a white solid. It can
hydrolyze with water to give maleic acid so water should be
avoided if possible. With prolonged exposure to moisture, the
anhydride group in the product may convert to the diacid which
will have different properties. The final bicyclic product is
usually obtained without difficulty.
EXPERIMENTAL PROCEDURE:
This reaction must be done in a fume cupboard. Place maleic
anhydride (0.527 g, 5.4 mmoles) and -phellandrene (1 mL, 85 %
pure, 5.4 mmoles) into a 25 mL round bottomed flask and attach a
water-cooled condenser. Add anhydrous acetone (3 mL) through the
top of the condenser and swirl the reaction mixture. The
reaction is exothermic and a yellow colour should appear at this
point. Heat the reaction under reflux for 1 hour using a steam
bath. After this time, carefully remove the condenser and allow
the acetone solvent to evaporate off to approximately half of
its original volume. Allow the reaction to cool to room
temperature, then cool the reaction flask in an ice bath to
induce crystallization of your Diels-Alder adduct (scratching
with a glass rod may be required). Filter off your product using
a Hirsch funnel, washing the flask with a small amount of icecold acetone. Dry and weigh the final product, record the
melting point and determine the percent yield.
If required, (i.e. if the melting point of the product is
much lower than expected) recrystallize the product using the
minimum amount of warm methanol (be very careful to not overheat
as methanolysis of the anhydride group could occur).
IMPORTANT INFORMATION ABOUT THE REPORT:
The report for this experiment will follow the usual
format. Be sure the percent yield calculation is carefully
done. Also, record the melting point range of the final product
and compare that melting point to the reported melting point of
the product given above. Using these data, discuss the relative
success on the experiment.
END OF EXPERIMENT.
© 2007 STEPHEN ANDERSON AND ROBERT SHINE