exp 6

Diels-Alder Reaction

The Diels-Alder reaction is an important synthetic reaction in organic chemistry where a diene reacts with a dienophile to form a ring of 6 carbons with one double bond (essentially, a cyclohexene).  The double bond forms on the center bond of the diene (blue in below figure).  Two new bonds are formed, which are illustrated in black in the following figure:

Figure 5.1 The Diels-Alder 4+2 Cycloaddition Reaction and Transition State

This reaction is useful since it is ring-forming and is stereoselective.  The stereoselectivity is related to its mechanism — as it happens in one concerted step, there is really only one outcome.  This is seen in the transition state from the above reaction (Figure 5.1), illustrating that the π electrons move in one step.  The stereochemistry of the product is primarily based upon that of the dienophile: (Z) results in a syn-cyclohexene while (E) results in an anti-cyclohexene, as illustrated in the next figure:

Figure 5.2 Stereochemistry in a Diels-Alder Reaction

In this experiment, you will conduct a Diels-Alder reaction between anthracene (the diene) and maleic anhydride (the dienophile) to produce the 9,10-dihydroanthracene-9,10-⍺,β-succinic acid anhydride:

Figure 5.3 Reaction between Anthracene and Maleic Anhydride 

Note that the central ring loses its aromaticity as a result of this reaction!

Reaction Techniques 

This reaction requires a high amount of activation energy, thus, this reaction will need to take place at high temperatures for a long time.  This necessitates the use of a solvent that is stable at those temperatures (xylene), and requires the reflux technique to minimize solvent loss (refer to Exp. 1 background).  
The product will then be crystallized in an ice bath, rinsed, and vacuum filtered.

Infrared Spectroscopy

The product of this reaction will be tested using infrared (IR) spectroscopy.  Spectroscopy is used to study something using light.  IR light waves are just below the visible light range’s energies, which happens to be the right amount of energy when interacting with a sample, to cause the molecular bonds to bend and/or stretch, as illustrated below: 

Figure 5.4 Molecular Bending and Stretching

When the light waves interact with the sample, they are not transmitted through, and so will appear as a “valley” (called a band) in the IR spectrum output since the light has been absorbed at these frequencies.  The wavenumber or frequency of this band in the IR spectrum matches the energy of specific functional groups.  See following table for the types of bands and their corresponding functional groups:

The provided wavenumber is a range, this means that the band (or valley) can occur anywhere within this range.  A “strong” band means that there is low transmittance (i.e., almost complete absorbance or a deep valley).  This means that a “weak” band conversely has high transmittance.  Figure 5.5 below may also help with visualizing the expected outcome of an IR spectrum. 

Figure 5.5 Illustration of Typical IR Absorption Bands for Common Bond Types

Thus, this analytical technique is beneficial at identifying the types of functional groups present.  Its limitation is that there is no significant information about the relative location or proximities of the functional groups to one another, nor the number of these functional groups (e.g., if there was more than one carbonyl group). 

Learning Goals

• Conduct an organic synthetic reaction.

• Use a reflux apparatus for a reaction.

• Learn Infrared (IR) spectroscopy – how to use the instrument and interpret results.