Ch. 16 - Conjugated SystemsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Sections
Conjugation Chemistry
Stability of Conjugated Intermediates
Allylic Halogenation
Conjugated Hydrohalogenation (1,2 vs 1,4 addition)
Diels-Alder Reaction
Diels-Alder Forming Bridged Products
Diels-Alder Retrosynthesis
Molecular Orbital Theory
Drawing Atomic Orbitals
Drawing Molecular Orbitals
HOMO LUMO
Orbital Diagram: 3-atoms- Allylic Ions
Orbital Diagram: 4-atoms- 1,3-butadiene
Orbital Diagram: 5-atoms- Allylic Ions
Orbital Diagram: 6-atoms- 1,3,5-hexatriene
Orbital Diagram: Excited States
Pericyclic Reaction
Thermal Cycloaddition Reactions
Photochemical Cycloaddition Reactions
Thermal Electrocyclic Reactions
Photochemical Electrocyclic Reactions
Cumulative Electrocyclic Problems
Sigmatropic Rearrangement
Cope Rearrangement
Claisen Rearrangement
Additional Practice
Conjugated Halogenation
Diels-Alder Inductive Effects
Diels-Alder Regiospecficity
Diels-Alder Asymmetric Induction
Diels-Alder Synthesis
Allylic SN1 and SN2
Cumulative Orbital Diagram Problems
Cumulative Cycloaddition Reactions
Cumulative Sigmatropic Problems
UV-Vis Spect Basics
UV-Vis Spect Beer's Law
Molecular Electronic Transition Therory
Woodward-Fieser Rules
Additional Guides
Diene
Johnny Betancourt

The Diels-Alder reaction is a [4+2] cycloaddition reaction that always produces a six-membered ring and sometimes produces a bicyclic compound. It is a pericyclic reaction between an electron-rich conjugated diene and an electron-poor dienophile. 

Diels-Alder reaction and mechanism

The Diels-Alder is a thermocyclic reaction between a conjugated diene and a dienophile. There are three arrows, and the important thing to remember is that they trace a circle in the same direction; that is, they either go clockwise or counterclockwise. I suggest picking one and sticking to it! Let’s check out the general mechanism

1,3-butadiene and ethene1,3-butadiene and ethene

Let’s look at this mechanism starting from the pi bond between carbons three and four. The pi bond attacks carbon 5 on the dienophile, which causes the pi bond between carbons five and 6 to attack carbon 1 in the dienophile, which causes the electrons in the pi bond between carbons 1 and 2 to move between carbons 2 and 3.

That’s a long-winded way to say that we end up with a six membered ring and our sigma bond between carbons 2 and 3 gets a pi bond. Of course, there is a transition state, so let’s see what it looks like:

Aromatic transition stateAromatic transition state

The transition state of the Diels-Alder reaction is 1) planar, 2) cyclic, 3) fully conjugated, and 4) has 6 pi electrons, which satisfies Huckel’s rule. That makes it aromatic!   

Adding complexity

Bridged molecules

In our simplest Diels-Alder, we form a six-membered ring. But what happens when our diene is already a six-membered ring like 1,3-hexadiene? We actually form a bicyclic compound! The diene atoms that don’t actually participate in the form a bridge like this: Building bridgesBuilding bridges

Notice that we end up with a new six-membered ring and our original one basically folds out of the way. The red lines in the product are the new bonds formed from the red arrows. 

Substituents

Ignoring stereochemistry for now, let’s check out how substituents affect the product. Let’s say we have 2-ethyl-3-methylcyclohexa-1,3-diene reacting with 1-butene. Depending on the molecules’ orientations, we can end up with two different products!

RegiospecificityRegiospecificity

Notice that in the first reaction, the blue ethyl group is near the green methyl group but it’s different in the second reaction; the blue ethyl group is near the green ethyl group in the second reaction. 

EDGs and EWGs

Electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) are often found in Diels-Alder reactions. EDGs (e.g. amines and alkoxy groups) are generally found on the diene, and EWGs (e.g. carbonyls and anhydrides) are generally found on the dienophile. 


EDGs and EWGsEDGs and EWGs

Of course, just like the ethyl group in the previous example, we could have two different molecules here. This is just to show there the groups are generally placed.

Heteroatoms and alkynes

Every once in a while you might come across an alkyne as a dienophile or a heteroatom in your diene. Let’s take a look at the reaction mechanism between a furan and ethyne as the dienophile:

Furan and acetyleneFuran and acetylene

Notice that it’s not very different even though there’s a triple bond and oxygen? All we did was kick the oxygen up to the bridge and react with only one of the pi bonds in the ethyne; the other pi bond stayed there between carbons 5 and 6!

Endo and exo

Diels-Alder reactions can produce diastereomers called endo and exo products. If all groups are on the same side of the newly formed ring, it’s an endo product; if substituents are on opposite sides of the ring, it’s an exo product! Check out this image to see what I mean: 

Endo and exoEndo and exo





Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.


Additional Problems
This is an example of a ____________ reaction. A) Electrophilic Addition B) Nucleophilic Substitution C) Woodward-Hoffman D) Diels-Alder E) None Of These
What is the product of the following Diels-Alder cycloaddition reaction: A) I B) II C) III D) IV E) None of these
Predict the major product for each of the following reactions by paying attention to region- and stereochemistry where appropriate.  
Predict the major product for the Diels-Alder reaction given below. 
Each of the following dienes does not react in a Diels Alder reaction.  Explain why for each case.
Two constitutional isomers of molecular formula C8H12O are formed in the following reaction. Ignoring stereochemistry, suggest reasonable structures for these Diels-Alder adducts.
The compound shown undergoes an intramolecular Diels—Alder reaction at room temperature. What is the structure of the product? (No need to show stereochemistry)  
Under which set of conditions is the reaction shown (below) best carried out? A) 6 M H2SO4 B) 6 M NaOH C) heating in hexane D) exposing to UV light in hexane
Which of the following compounds will react as a diene in a Diels-Alder reaction?
Which of the following compounds is an isolated diene?
Circle the compound below that is  not  capable of acting as the diene in a typical Diels-Alder reaction.
Predict the major product for each of the following reactions by paying attention to regio- and stereochemistry where appropriate.
The dienophile in the reaction is _____:
The diene in the reaction is _____:
Predict the product of the following reactions showing stereochemistry where appropriate:
Predict the product of the reaction below:
Predict the product of the reaction below:
Predict the product of the reaction below:
Show how the following compound could be prepared in a two-step synthesis in which the first step involves a Diels-Alder reaction between an appropriate diene and dienophile.
Predict the product(s) of the following Diels-Alder reaction:
Predict the product(s) of the following Diels-Alder reaction:
The following two dienes (H) and (I) do not effectively participate in Diels-Alder reactions. Why is this so?
Draw the correct product for the following Diels- Alder reaction:
Assume that the alkene reacts with the diene in a Diels-Alder reaction. 
Draw a structural formula for the product this Diels-Alder reaction, including all stereoisomers of the product.
Give the major organic product for the reaction.
Draw a structural formula for the product of this Diels-Alder reaction, including all stereoisomers of the product. Use the wedge/hash bond tools to indicate stereochemistry.
Draw a structural formula for the product of this Diels-Alder reaction, including all stereoisomers of the product. Use the wedge/hash bond tools to indicate stereochemistry. If a group is a chiral, do not use wedged or hashed bonds on it. Draw one structure per sketcher. Add additional sketchers using the drop-down menu in the bottom right corner. Separate multiple products using the +sign from the drop-down menu.
Draw the correct product for the following Diels-Alder reaction:
Predict the product for the following Diels-Alder reaction. (a) I(b) II(c) III(d) IV(e) none of these
Which of the following dienes can undergo the Diels-Alder reaction? (a) I(b) II(c) III(d) IV(e) None of the these