| Demonstration of Microscale Projection Experiments - Chemistry en miniature
Objective: Diene and Dienophile Components in Diels-Alder Products Peter Keusch |
German version
Note: The freshly distilled cyclopentadiene must be kept on ice or it will react with itself within an hour to form dicyclopentadiene (dimerization). Hazards and safety precautions:
Safety glasses, gloves. The experiment should be carried out in a portable fume hood giving all-round visibility! Experimental procedure: 1 mL of a solution of N-PTD is pipetted into each of two test tubes (T1, T2). A drop of anthracene solution is added to the solution in T1 and a drop of the solution of cyclopentadiene is given to the solution in T2. 1 mL of benzoquinone solution is added to two further test tubes (T3 and T4). The solution in T4 is mixed with 5 drops of freshly distilled cyclopentadiene.
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Discussion: · In a Diels-Alder cycloaddition a system with 2 p-electrons (dienophile) is added in 1,4-position to a conjugated p-system with 4 p-electrons (diene). The reaction is classified as a [4+2] process. This means that 4 p-electrons of the diene and 2 p-electrons of the dienophile are involved simultaneously in the reaction. A rearranging of the 6 electrons occurs. Two new s-bonds (colored blue in the equations below) and one p-bond are formed as three p-bonds (colored red in the equations below) are broken. The driving force of the reaction is the formation of the new s-bonds, which are energetically more stable than the p-bonds. The reaction product is a six-membered ring. A typical example is the reaction of 1.3 butadiene with ethene resulting in the formation of cyclohexene (1).  · The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile. Addition is syn on both components. Thus, the reactivity of open-chain dienes depends on the fraction of s-cis conformers available in the conformation equilibrium. Cyclic dienes which are locked by the ring in s-cis conformation e.g. cyclopentadiene and 1,3-cyclohexadiene are therefore especially reactive, while cyclic dienes that are permanently in the s-trans conformation and cannot adopt the s-cis conformation will not undergo the Diels-Alder reaction at all.  · The dimerization of cyclopentadiene already proceeding at room temperature is a Diels-Alder reaction. One molecule of cyclopentadiene acts as a 4 p-electron diene and the other as a 2 p-electron dienophile. This dimeric material can be 'cracked' back to cyclopentadiene by heating at 150°C for an hour and then distilling off the diene monomer.  · In general, electron-donating substituents on the diene (methyl, methoxy) promote cycloadditions as do electron-withdrawing groups (formyl, nitro) on the dienophile. · Cycloadditions belong to the pericyclic reactions. Breaking and forming of bonds takes place in a concerted manner (simultaneously) as the result of a cyclic arrangement of electrons in the transition state. · N-PTD and p-benzoquinone are dienophiles, cyclopentadiene and anthracene are dienes. The decolorization of the red solution of N-PTD or of the yellow solution of p-benzoquinone is due to the disruption of the conjugated double bond system in N-PTD and p-benzoquinone, respectively ((2) and (3)). 
 · The different reaction rates allow an evaluation of the dienophile and diene character: N-NPT is a better Diels-Alder dienophile than p-benzoquinone. Cyclopentadiene is a better Diels-Alder diene than anthracene. · Anthracene undergoes cycloaddition on the central ring (resonance stabilized cycloadduct). References: Pericyclic Reaction Chemistry General experimental instructions and index of experiments 
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