Core Concepts
In this organic chemistry tutorial, you will learn the basics of the Diels-Alder reaction, including the chemistry of its products and reactants, its mechanism, its regioselectivity, and some important reaction variants.
Topics Covered in Other Articles
- Resonance Structures
- Functional Groups
- Electrophilic Aromatic Substitution
- Cis and Trans Isomers
- Geometric Isomerism
Diels-Alder Reactants and Product
The Diels-Alder reaction forms a six-carbon cycloalkene from two smaller carbon structures. Specifically, the two reactants are called the diene and dienophile. The simplest Diels-Alder reaction uses 1,3-butadiene and ethene.
The diene, so-called by chemists due to its two double bonds, supplies four of the six carbons to the cyclic product. Interestingly, the diene’s two double bonds must be cis to one another to react with the dienophile and are conjugated, meaning that the pi electrons in the double bonds must move in concert. This concerted movement plays a crucial role in the mechanism of the Diels-Alder as we will see shortly. The dienophile, so-called because of its reactivity with the diene, supplies two of the carbons to the product and has a double bond between such carbons.
Besides the six carbons that compose the cyclic product structure, the diene and dienophile can have numerous different structures and still undergo a Diels-Alder reaction. In fact, a molecule can act as both a diene and dienophile, as shown below with cyclopentadiene.
Diels-Alder Reaction Mechanism
First, the diene and dienophile need to interact with a specific orientation that aligns each of the carbons of the dienophile with carbons 1 and 4 of the diene. Next, a benzene-like transition state forms, involving the circular movement of electrons to form two sigma bonds between the reactants. Also, a pi bond forms between carbons 2 and 3 of the diene. This yields the cyclic six-carbon product.
As we can see, the reaction nets two pi bonds broken and two sigma bonds formed. Because sigma bonds are lower in energy than pi bonds, this means that the Diels-Alder reaction is exothermic. However, the reaction also has a negative change in entropy, generally, because only one product forms from two reactants. Thermodynamically, this means that enthalpy drives the Diels-Alder reaction.
Kinetically, the Diels-Alder tends to proceed slowly, due to the exact position of the reactants required for transition state formation. This results in a highly negative change in entropy of transition state formation.
As shown above, the Diels-Alder requires no acid nor base catalyst, meaning that a variety of non-extreme pH conditions are acceptable for the reaction.
Diels-Alder Reaction Regioselectivity
The generic mechanism above implies that the dienophile can have either position relative to the diene, so long as one carbon binds with carbon 1 in the diene and the other carbon interacts with 4. This is true when either the diene or the dienophile is a meso compound (i.e., when either of the reactants is symmetrical).
However, when both the diene and dienophile lack symmetry, the reaction favors certain orientations of the diene relative to the dienophile, due to certain regions of the reactants having higher reactivity than others. Chemists call this phenomenon “regioselectivity”.
In both the diene and dienophile, the placement of electron-rich groups like amines or electron-poor groups like carboxylic acids affects the molecule’s resonance forms. These resonance forms place partial positive or negative charges on carbons at the ends of the molecule.
Specifically, this change in the molecule’s electron distribution is what affects the regional reactivity, favoring different products as a result. To better allow the pi electrons to move cyclically to undergo the Diels-Alder reaction, partially positive carbons from the diene favor forming sigma bonds with more negative carbons of the dienophile and vice versa.
Diels-Alder with Similar Groups
Interestingly, when both reactants have one electron-rich or one electron-poor group, the reaction favors the meta product. Meta, in this context, means the groups locate two carbons away from one another.
Diels-Alder with Different Groups
Further, when one molecule has an electron-rich group while the other has an electron-poor group, the reaction favors either the para product, where the groups are oppositely placed, or the ortho product, where the groups are adjacently placed. The formation of the para versus ortho product depends on whether the diene group locates at an exterior carbon (1 or 4) or an interior carbon (2 or 3).
For dienes and dienophiles with multiple attached groups, the regioselectivity of the Diels-Alder depends on the molecule’s resonance forms.
Alternative Diels-Alder Reactions
Since the Diels-Alder reaction was discovered by German chemists Otto Diels and Kurt Alder in 1928, the mechanism has proven versatile to later chemists. Consequently, later chemists have described and implemented many variants of the Diels-Alder. Some examples include the formation of heterocyclic molecules, that have a non-carbon atom as a ring member, which chemists make using Imine-Diels-Alder (IDA) or Oxo-Diels-Alder (ODA) reactions.
The Hexadehydro-Diels-Alder reaction (HDDA) represents another common variant, where instead of double bonds, the reactant molecules have triple bonds. The mechanism follows the same cyclic movement of electrons, forming what chemists call a benzyne intermediate. Then, a compound with both nucleophilic and electrophilic components performs an addition reaction across the final triple bond. Consequently, the HDDA forms an aromatic molecule. However, due to the peculiar molecular geometry of the mechanism, the reaction often requires a transition metal catalyst.
