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 1,3-diene must be in a conformation that allows the formation of a new six-membered 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 six-membered ring cannot form from the s-trans conformation of the diene.

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.

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 six-membered ring.

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).

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.

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

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.

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.

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 electron-withdrawing groups shown below. For discussion purposes, these groups pull electron density toward themselves through the squiggly. Note that the

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.

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.

The following box shows typical dienes for Diels-Alder reactions.

The following box shows 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 product of the forward reaction back into the reactants of the forward reaction.

Cyclopentadiene readily undergoes a Diels-Alder reaction with maleic anhydride, making the tricyclic product cis-norbornene-5,6-endo-dicarboxylic 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.

Procedure

Only one Craig tube is needed.

  1. Weigh 0.10 g of maleic anhydride on a creased weighing paper.
  1. Transfer the weighed sample into a Craig tube.

Use the larger of the two Craig tubes in your microkit.

  1. Add 10 drops of ethyl acetate to the Craig tube.
  1. 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.
  1. Warm the Craig tube in a beaker of water on a hot plate. Use a 50-mL beaker filled with 20-mL of water.
  1. 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.

  1. Add 7 drops (0.07 g) of recently distilled cyclopentadiene to the Craig tube.
  1. 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.

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 Craigtube, plug and wire, into a centrifuge. Make sure the wire does notinterfere 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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Lab 2