A Diels—Alder Reaction: Cyclopentadiene and Maleic
Anhydride
Introduction
Diels and Alder are the names of two German chemists who
discovered a reaction that makes new six-membered rings. The reaction
involves only  bonds in the reactants; no sigma bonds are broken. One
reactant is a 1,3-diene, or two  bonds separated by a sigma bond. The 1,3diene must be in a conformation that allows the formation of a new sixmembered ring. This conformation is called the s-cis conformation. The s
stands for sigma or single bond, and the two  bonds are on the same side of
the single bond. Hence, the conformation is s-cis, meaning the two  bonds
are cis with respect to the single bond. The other conformation that allows
the  system to be planar is called the s-trans conformation. A new sixmembered ring cannot form from the s-trans conformation of the diene.
single
bond
possible
s-cis conformation
1,3-butadiene
single
bond
not possible
s-trans conformation
1,3-butadiene
The s-cis conformation of a 1,3-diene permits the formation of a six-member ring.
Thus, one reactant is a 1,3-diene that can adopt an s-cis conformation.
The other reactant is called a dienophile because it is seeking the 1,3-diene.
The dieneophile must contain at least one  bond, usually an alkene or
carbon—carbon double bond. Although, a  bond of a triple bond may also
serve as the dienophile. Maleic anhydride is an example of a dienophile. The
equation for the reaction between 1,3-butadiene (the diene) and maleic
anhydride (the dienophile) is shown below.
Lab 2
1
O

Diels-Alder

O

diene
O
O
dienophile

O
reaction
O
new six-membered ring
Three  bonds in the reactants become one  bond in the addition product.
To arrive at the correct product, we must align the  bonds in the
reactants so they can form a new six-member ring. This involves placing the
diene on the left in an s-cis conformation facing the dienophile on the right.
The bonds are oriented so that you can see the outline of the new sixmembered ring.
O
left
diene
right
O dienophile
O
outline of new six-member ring
The above alignment shows us where the new ring originates.
Mechanism of Diels-Alder Reaction
The mechanism for the reaction is shown below. It is a concerted
reaction, meaning that all of the bond breaking and forming occurs in one
transition state. A row of dominos falling, one after another, as soon as you
topple the first one in the row is a concerted but not necessarily simultaneous
process. Six  electrons are involved in the reaction. All three of the  bonds
break, and three new bonds form. Only one of the new bonds is a  bond, the
other two are sigma bonds, which close the six-membered ring. Overall, the
dienophile adds to the diene at atoms number 1 and 4 of the diene. Thus, the
Diels-Alder reaction is a cycloaddition reaction (i.e., a ring is formed
during the addition reaction).
Lab 2
2
O
O
O
4
4
4
3
O
O
O
2
1
1
1
O
O
O
Mechanism of the Diels-Alder Addition Reaction
In the reaction above, we start with a dienophile (maleic anhydride)
that already contains a ring. Because we also make a new six-membered ring
during the reaction, the product contains two rings. That is, the product is
bicyclic. When we start with two acyclic reactants, the product is
monocyclic (contains one ring). If both reactants are monocyclic, the
product is tricyclic, etc.
Stereochemistry of the Diels—Alder Reaction
The reaction between 1,3-butadiene and maleic anhydride is shown
again below with the stereochemistry of the product shown.
N
O
X
X
X N H
O
H
O
O
H
S
O
acyclic
monocyclic
X S
H
O
bicyclic
The H atoms indicated by H, X, N and S have specific orientations in
the product. The groups N and S, oriented north and south in the diene on
the left, are both alpha (downward) in the product. The two X groups, which
point toward the dienophile, are oriented upward in the product. Finally, the
two H atoms on the dienophile are oriented upward or beta in the product.
This reaction is a template that can be used to determine the stereochemistry
of any Diels-Alder reaction you will encounter.
Directing Effects: Analysis of the Diene and Dienophile
Lab 2
3
We saw in previous lessons that some groups are electron donating
(EDG) and some are electron withdrawing (EWG). The manner of
substitution of electron-donating and electron-withdrawing groups in the
reactants governs how easily the reactants produce a Diels-Alder product.
When you give someone directions, you tell them where to go. In that
case, you are the “director.” A group that “tells another group where to go”
is called a directing group. A directing group can either donate or withdraw
electrons, depending on its structure. Thus, we have two kinds of directing
groups, EDG and EWG. An EDG “pushes” electron density (negative
charge) away from itself into the rest of the molecule, increasing the electron
density elsewhere in the molecule. This negative center facilitates a reaction
with a positive reagent (electrophile). A EWG “pulls” electron density
toward itself from the rest of the molecule, creating a positive center that
facilitates a reaction with a negative reagent (nucleophile).
Electron-Donating Groups
In order to donate electron density, these groups must have electrons
readily available. Consider the electron-donating groups shown below.
OCH3
NHCH3
OH
NH2
Electron-Donating Groups
These groups bond to the rest of the molecule by the squiggly.
Therefore, for discussion purposes, they push electron density through the
squiggly. The atom bonded to the squiggly in each case is a heteroatom with
at least one pair of nonbonding or n valence electrons. These nonbonding
electrons make these groups EDG. The electron donating effect of the –OMe
bonded to a benzene ring is shown below. A group with a pair of
nonbonding electrons on the heteroatom bonded to the squiggly can donate
electrons through the squiggly by a resonance effect.
Lab 2
4
CH3O
CH3O
center of negative
charge (attracts an
electrophile)
pushes electron
density into the ring
Electron-Donating Groups Push Electron Density
To determine whether a group is electron donating or withdrawing,
look at the first atom—the one bonded to the squiggly or the rest of the
molecule. If that atom has a pair of nonbonding or n electrons, it is an EDG.
Electron-Withdrawing Groups
These groups cannot have electrons available. Consider the electronwithdrawing groups shown below. For discussion purposes, these groups
pull electron density toward themselves through the squiggly. Note that the
O
C CH3
O
C OCH2CH3
O
NH3
N
O
Examples of Electron-Withdrawing Groups
atom bonded to the squiggly in each case either has a multiple bond (double
or triple bond) or a positive formal charge. Either of these two structural
features make the group an EWG. A positive charge simply “pulls” electron
density toward itself by an inductive effect. The multiple bonds pull electron
density toward themselves by a resonance effect. The electron-withdrawing
effect of a nitro group (NO2) bonded to a benzene ring is shown below.
Lab 2
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O
O
N
N
O
O
pulls electron
density out of ring
center of positive charge
(repels electrophiles)
Electron-Withdrawing Groups Pull Electron Density
Alkyl Groups are Electron-Donating Groups
Alkyl groups are a special case. They do not have a pair of
nonbonding electrons, nor do they have a multiple bond or positive charge.
How do they donate electron density? They donate electron density in the
same way they stabilize carbocations, by an effect known as
hyperconjugation. A methyl groups has three hydrogen atoms bonded to
carbon. The electron density in these bonds is “donated.”
Summary of Directing Groups
To determine whether a group is an EDG or a EWG, look at the first
atom of the group (the atom bonded to a squiggly if the group is alone or the
atom in the group that bonds the group to the rest of the molecule if the
group is part of a molecule). When this atom bears a positive formal charge
or a multiple bond, the groups is a EWG. When this atom does not bear
either a positive charge or multiple bonds, the group is an EDG.
Directing Groups in Diels—Alder Reactions
It has been determined by many experiments that EDGs bonded to the
diene and EWGs bonded to the dienophile facilitate Diels-Alder reactions.
EDG
EDG
EWG
EWG
Donating Groups in the Diene and Withdrawing Groups in the Dienophile
Lab 2
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The following box shows typical dienes for Diels-Alder reactions.
OCH3
Typical Dienes for Diels-Alder Reactions
The following box shows typical dienophiles for Diels-Alder
reactions.
O
CO2Et
O
O
O
O
O
CO2Et
O
Typical Dienophiles for Diels-Alder Reactions
How does one recognize a Diels-Alder reaction? These reactions are
easy to spot, because they involve two organic reactants and only heat as a
reactant. One of the reactants (the diene) must have a conjugated diene
system, and the other reactant (the dienophile) must contain a double bond
or triple bond. The diene might have EDGs and the dienophile might have
EWGs. You should be able to show the product of any diene with any
dienophile from the above two boxes.
The Experimental Reaction
Our laboratory reaction is one between cyclopentadiene and maleic
anhydride. Cyclopentadiene reacts with itself by a Diels-Alder reaction to
make a dimer of cyclopentadiene. A dimer is the product of a self-addition
reaction. Pure cyclopentadiene is obtained by “cracking” the dimer. In this
case, cracking means heating the dimer until it undergoes a retro-Diels-Alder
reaction. Retro means reverse, so a retro-Diels-Alder reaction converts the
Lab 2
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product of the forward reaction back into the reactants of the forward
reaction.
room temp.
+
heat
cyclopentadiene
cyclopentadiene
dimer
The dimer is formed by a Diels-Alder reaction and
cracked by a retro-Diels-Alder reaction.
Cyclopentadiene readily undergoes a Diels-Alder reaction with maleic
anhydride, making the tricyclic product cis-norbornene-5,6-endodicarboxylic anhydride.
O
+
cyclopentadiene
O
O
O
maleic
anhydride
O
O
cis-norbornene-5,6-endodicarboxylic anhydride
The name norbornene is derived from bornane, which is the common
name for the bicyclic compound 1,7,7-trimethyl[2.2.1]heptane. The prefix
nor- means that the methyl groups of bornane are absent in norbornane.
Lab 2
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bornane
Lab 2
norbornane
norbornene
9
Procedure
Only one Craig tube is needed.
1. Weigh 0.10 g of maleic anhydride on a creased weighing paper.
2. Transfer the weighed sample into a Craig tube.
Use the larger of the two Craig tubes in your microkit.
3. Add 10 drops of ethyl acetate to the Craig tube.
4. Gently shake the Craig tube until the solid dissolves. If the solid does not
dissolve, slowly add more ethyl acetate until only a small amount of solid
is visible.
5. Warm the Craig tube in a beaker of water on a hot plate. Use a 50-mL
beaker filled with 20-mL of water.
6. Add 4 drops of ligroin (bp 60-90 oC) to the Craig tube.
The kind of pet ether or ligroin is important, because the solubility of
the product is a key factor in its isolation. A pet ether in which the product
is too soluble will make it difficult to isolate crystals.
7. Add 7 drops (0.07 g) of recently distilled cyclopentadiene to the Craig
tube.
8. Hold the top of the Craig tube firmly in one hand and gently tap the tube
with the index finger of your other hand to thoroughly mix the contents
of the tube.
9. If a solid forms, heat the Craig tube in the hot-water bath until the solid
dissolves.
10. Add ligroin, one drop at a time, until the solution turns cloudy.
11. Place the Craig tube in an ice bath (mixture of ice and water) so that the
tube rests against the pouring spout of the beaker.
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12. When crystallization appears complete, collect your crystals by the
centrifugation technique as demonstrated by the instructor and described
below. Alternatively, collect your crystals on a Hirsh funnel.
Centrifugation Technique: Insert the large end of the plug from the
microkit into the Craig tube. Obtain a 12 cm piece of thin copper wire. Wrap
two or three turns of wire around the small end of the plug, which is
protruding from the Craig tube. Then extend the remainder of the wire
downward. Place a centrifuge tube over the narrow end of the plug. The
copper wire should protrude from the bottom of the centrifuge. Invert the
entire apparatus. Place the centrifuge tube, which now holds the Craig
tube, plug and wire, into a centrifuge. Make sure the wire does not interfere
with the operation of the centrifuge. Balance your centrifuge tube with that
of another student or another centrifuge tube partially filled with water.
Centrifuge the sample and collect the crystals on the large end of the plug.
13. Save the crystals for the lab on hydrolysis.
14. Clean and replace all glassware. Return all apparatus to the correct
storage location. Check the balance area. Return all chemicals to their
original locations and turn off all balances.
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Diels-Alder Questions
Student last name________________________, First name__________________________
1. Draw structures of a diene and a dienophile in the spaces below.
A diene ______________________________ A dieneophile ___________________________
2. Maleic acid loses water when it is heated strongly (pyrolyzed) to form maleic anhydride.
Write an equation for the pyrolysis of maleic acid.
3. Cyclopentadiene dimerizes when it is stored for long periods of time. A dimer is an adduct
(addition product) between two molecules. The cyclopentadiene dimer is a Diels-Alder adduct
between two cyclopentadiene molecules. Write an equation for the dimerization of cyclopentadiene.
4. Show the product of a Diels-Alder reaction between 1,3-cyclopentadiene and cis-2-butene.
5. Show the product of a Diels-Alder reaction between 1,3-butadiene and cyclopentene.
6. In problems 4 and 5, one of the reactants is a cyclic compound and the other is not. When one
of the reactants is cyclic, how many rings are found in the Diels-Alder cycloaddition product?
Ans.______
7. Show the product of a Diels-Alder reaction between 1,3-cyclohexadiene and cyclohexene.
8. Circle the best (one that reacts most readily) dieneophile among the following structures.
OCH3
CHO
CHO
CHO
9. Circle the most reactive diene for a Diels-Alder reaction among the following structures.
CHO
CHO
OCH3
CHO
10. Draw the structure of norbornane (or bicyclo[2.2.1] heptane).
Lab 2
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