Difficulty: Advanced | Prerequisites: Conjugated dienes and MO theory (Ch. 14 Part 1), alkene addition reactions, stereochemistry (cis/trans, R/S), energy diagrams and kinetic vs thermodynamic control.
Conjugated dienes do not just sit there being stable. They undergo a distinctive set of reactions that isolated alkenes cannot: electrophilic addition can deliver products at two different positions (1,2 vs 1,4), and the concerted Diels-Alder [4+2] cycloaddition builds six-membered rings in a single step with predictable stereochemistry. This chapter also introduces radical addition of HBr to alkenes, which flips the usual Markovnikov regiochemistry. These reactions are some of the most heavily tested material in second-semester organic chemistry because they combine mechanism, regiochemistry, stereochemistry, and energy diagram analysis in a single problem.
When HBr adds to a conjugated diene, two products form: the 1,2-addition product (kinetic, favoured at low temperature) and the 1,4-addition product (thermodynamic, favoured at high temperature). The Diels-Alder reaction is a concerted [4+2] cycloaddition between a diene (in s-cis) and a dienophile that forms a cyclohexene ring in one step, with stereospecific retention of alkene geometry and endo selectivity. Radical addition of HBr to alkenes, initiated by peroxides or light, gives the anti-Markovnikov product.
1,2-addition
Addition of an electrophile and nucleophile across adjacent carbons (C1 and C2) of a conjugated diene, producing a monosubstituted alkene with a remaining double bond between C3 and C4. Think of it as: the reagent lands on the two carbons closest to where the first bond-forming event happened.
1,4-addition (conjugate addition)
Addition across the terminal carbons of the conjugated system (C1 and C4), producing a disubstituted alkene with a new double bond between C2 and C3. In simple terms, the reagent skips over to the far end of the diene, and the double bond migrates to the middle.
Kinetic product
The product that forms faster because it has the lower activation energy barrier. In diene addition, this is typically the 1,2-product. Think of it as: the product you get when the reaction runs cold and fast, before equilibrium sets in.
Thermodynamic product
The product that is more stable (lower in energy) overall, regardless of how fast it forms. In diene addition, this is typically the 1,4-product (more substituted internal alkene). In simple terms, the product you get when you give the reaction enough time or heat to find the lowest-energy outcome.
Diels-Alder reaction
A concerted [4+2] cycloaddition in which a conjugated diene (4 pi electrons) reacts with a dienophile (2 pi electrons, typically an alkene) to form a six-membered ring (cyclohexene) in a single mechanistic step. Think of it as: the diene wraps around the dienophile and all three pi bonds rearrange simultaneously to form two new sigma bonds and one new pi bond.
Dienophile
The two-carbon pi component in a Diels-Alder reaction, usually an alkene or alkyne. Electron-withdrawing groups on the dienophile accelerate the reaction by lowering its LUMO energy. In simple terms, the "diene-loving" partner, which is the smaller piece that the diene grabs.
Concerted reaction
A reaction in which all bond-making and bond-breaking events happen simultaneously in a single transition state, with no intermediates. Think of it as: everything happens at once, in one smooth step, with one energy barrier.
Endo rule
In a Diels-Alder reaction with a bridged bicyclic product, the major product is the endo diastereomer, in which the dienophile's substituents point towards the larger bridge (underneath the diene). This preference arises from additional stabilising orbital overlap (secondary orbital interactions) in the endo transition state. In simple terms, the bulky groups on the dienophile tuck underneath the diene rather than pointing away from it.
Stereospecific
A reaction in which the stereochemistry of the starting material determines the stereochemistry of the product. In Diels-Alder, a cis dienophile gives cis substituents in the product; a trans dienophile gives trans substituents. Think of it as: what you put in is what you get out, stereochemically speaking.
Radical addition
An addition reaction that proceeds through radical (single-electron) intermediates rather than ionic ones. Initiated by homolytic cleavage of a weak bond (e.g. O–O in peroxides or by UV light). In simple terms, instead of pushing pairs of electrons around, you are shuffling single electrons, which changes which carbon the halogen ends up on.
When a conjugated diene (e.g. 1,3-butadiene) reacts with one equivalent of HBr:
Step 1: The proton adds to a terminal carbon (C1), forming an allylic carbocation. This cation has two resonance forms of equal energy: positive charge on C2 (a secondary carbocation with resonance stabilisation) and positive charge on C4.
Step 2: Bromide can attack either electrophilic carbon.
Attack at C2 → 1,2-addition product (3-bromo-1-butene, a monosubstituted alkene)
Attack at C4 → 1,4-addition product (1-bromo-2-butene, a disubstituted internal alkene)
Kinetic vs thermodynamic control:
At low temperature and short reaction times, the 1,2-product (kinetic product) predominates. It forms faster because the bromide is physically closer to C2 in the allylic cation.
At high temperature and long reaction times, the 1,4-product (thermodynamic product) predominates. It is more stable because the internal, more substituted double bond has lower energy.
The energy diagram shows a common allylic cation intermediate with two different pathways: the 1,2 pathway has a lower activation energy (forms faster) but a higher-energy product, while the 1,4 pathway has a higher activation energy but a lower-energy product.
Examples with cyclic substrates:
The same logic applies to cyclic conjugated dienes. For example, reaction of a methylcyclohexadiene with HBr gives a 1,2-addition product (Br on C2, favoured at low temp) and a 1,4-addition product (Br on C4, favoured at high temp). The allylic cation intermediate has resonance, and the regiochemistry follows from which position the nucleophile attacks.
Overview:
A [4+2] cycloaddition: 4 pi electrons from the diene + 2 pi electrons from the dienophile.
Products: a cyclohexene ring (from an alkene dienophile) or a cyclohexadiene (from an alkyne dienophile).
Discovered in 1928 by Otto Diels and Kurt Alder; Nobel Prize awarded in 1950.
One of the most powerful C–C bond-forming reactions in organic chemistry.
Mechanism:
Concerted, one-step mechanism: all three pairs of electrons shift simultaneously.
Three pi bonds break (C1–C2, C3–C4 of the diene, and C5–C6 of the dienophile).
Two new sigma bonds form (C1–C6 and C4–C5).
One new pi bond forms (C2–C3).
Single transition state with a cyclic, six-membered geometry. No intermediates.
Requirements:
The diene must be in the s-cis conformation. Dienes locked in s-trans (e.g. cyclohexadiene) cannot react.
Electron-rich dienes and electron-poor dienophiles react fastest (normal electron demand). Electron-withdrawing groups (C=O, CN, NO₂, CO₂R) on the dienophile lower its LUMO, narrowing the HOMO-LUMO gap and accelerating the reaction.
Heat is typically required (e.g. refluxing xylenes at 138 °C), though highly reactive pairs can react at room temperature.
Orbital symmetry:
The reaction is controlled by the interaction of the diene HOMO (π₂) with the dienophile LUMO (π₂*). The symmetry of these orbitals matches constructively at both ends of the forming bonds, which is why the reaction is thermally allowed. This is a key concept from the Woodward-Hoffmann rules.
Stereospecificity (syn addition, dienophile geometry retained):
A cis-dienophile gives cis substituents in the product.
A trans-dienophile gives trans substituents in the product.
The reaction is a suprafacial, syn addition on both components: everything adds from the same face.
Endo rule (diastereoselectivity):
When the Diels-Alder product is a bridged bicyclic system (e.g. from cyclopentadiene + a dienophile), two diastereomers are possible: endo and exo.
The endo product is the kinetically favoured (major) product.
In the endo transition state, the dienophile's electron-withdrawing groups point towards the diene (underneath the forming ring), allowing secondary orbital interactions that stabilise the transition state.
In the exo transition state, those groups point away from the diene, so there is no additional stabilisation.
Predicting products:
Identify the diene (must be or be able to adopt s-cis) and dienophile (typically has electron-withdrawing groups).
Number the carbons: C1–C4 on the diene, C5–C6 on the dienophile.
Form bonds C1–C6 and C4–C5 to draw the six-membered ring.
Retain the stereochemistry of the dienophile (cis stays cis, trans stays trans).
Apply the endo rule for bicyclic products.
When the dienophile is an alkyne instead of an alkene, the product is a cyclohexadiene (1,3-cyclohexadiene) rather than a cyclohexene. Two pi bonds remain in the product ring. The endo rule and stereochemistry principles apply in the same way.
The contrast with ionic addition:
Normal (ionic) addition of HBr to an alkene follows Markovnikov's rule: H adds to the less substituted carbon, Br to the more substituted carbon (via the more stable carbocation intermediate).
In the presence of peroxides (H₂O₂ or ROOR) or UV light (hν), HBr adds in anti-Markovnikov fashion: Br ends up on the less substituted carbon.
Mechanism (radical chain):
Initiation: Peroxide undergoes homolytic cleavage (O–O bond breaks, one electron to each oxygen) to form two alkoxy radicals (HO· or RO·). The alkoxy radical then abstracts H from HBr, forming a bromine radical (Br·).
Propagation: The Br· radical adds to the alkene at the less substituted carbon (the position that generates the more stable carbon radical, which is more substituted). The resulting carbon radical then abstracts H from another HBr molecule, giving the anti-Markovnikov product and regenerating Br· to continue the chain.
Why anti-Markovnikov?
The bromine radical adds first (not H), and it adds to generate the more stable radical intermediate (tertiary > secondary > primary). Since Br goes to the less substituted end, the final product has Br on the terminal carbon.
Important note: This radical pathway works only with HBr. HCl and HI do not undergo useful radical addition because the energetics of the propagation steps are unfavourable (endothermic steps in the chain).
Alternative route to the same product:
Anti-Markovnikov placement of Br can also be achieved in two steps: (1) hydroboration-oxidation to give the anti-Markovnikov alcohol, then (2) conversion of the alcohol to the bromide with PBr₃.
Diels-Alder: diene (4π e⁻) + dienophile (2π e⁻) → cyclohexene
ΔH for s-trans → s-cis (1,3-butadiene): +2.8 kcal/mol
Typical Diels-Alder conditions: heat (Δ), often reflux in xylenes (~138 °C)
Radical initiation: HOOH → 2 HO· (homolysis, triggered by heat or hν)
The Diels-Alder reaction is used extensively in the synthesis of natural products and pharmaceuticals wherever a six-membered ring needs to be built with controlled stereochemistry. For example, the total synthesis of steroids and terpenes often features a Diels-Alder step as the key ring-forming event. Radical addition of HBr is used industrially when terminal (anti-Markovnikov) bromides are needed as synthetic intermediates.
Students often think 1,4-addition is always the major product. It depends on conditions. At low temperature, 1,2 (kinetic) dominates. At high temperature, 1,4 (thermodynamic) dominates.
A very common error is drawing the Diels-Alder mechanism with separate, stepwise arrow-pushing (as if it were an ionic addition). It is concerted: all arrows must be drawn in one step, with no intermediates.
Students sometimes forget that the diene must be s-cis to react. Drawing a Diels-Alder product from a diene locked in s-trans is a guaranteed wrong answer on an exam.
The endo rule is a kinetic preference, not a thermodynamic one. The exo product is often more thermodynamically stable (less steric strain), but the endo product forms faster and is the major product under standard conditions.
⚠️ Be able to draw the energy diagram for 1,2 vs 1,4 addition, label the kinetic and thermodynamic products, and predict which dominates under given conditions (temperature, reaction time).
⚠️ For any Diels-Alder problem: identify the diene and dienophile, check that the diene can adopt s-cis, number the carbons, draw the product with correct regiochemistry and stereochemistry (cis/trans retention), and apply the endo rule if a bicyclic product results.
⚠️ Know that the Diels-Alder is stereospecific: cis in → cis out, trans in → trans out. This is tested frequently.
⚠️ Radical addition of HBr: remember it requires peroxides or hν, works only with HBr (not HCl or HI), and gives anti-Markovnikov regiochemistry. Be able to draw the full radical chain mechanism (initiation, propagation).
⚠️ When asked "how else could you make this product?", the hydroboration-oxidation / PBr₃ route is a common alternative to radical HBr addition for placing Br on the less substituted carbon.
True or False: The 1,4-addition product of HBr to 1,3-butadiene is a disubstituted internal alkene.
Fill in the blank: The Diels-Alder reaction is a __________ [4+2] cycloaddition with no intermediates.
True or False: A trans-dienophile gives a cis product in a Diels-Alder reaction.
Fill in the blank: Radical addition of HBr gives the __________ (Markovnikov / anti-Markovnikov) product.
True or False: The endo product in a Diels-Alder reaction is the thermodynamic product.
Q: At low temperature, what is the major product of adding one equivalent of HBr to 1,3-butadiene? What about at high temperature?
A: At low temperature, the 1,2-addition product (3-bromo-1-butene, kinetic product) is major. At high temperature, the 1,4-addition product (1-bromo-2-butene, thermodynamic product) is major.
Q: Draw the product of the Diels-Alder reaction between 1,3-butadiene and ethene.
A: Cyclohexene. Two new C–C sigma bonds form between C1–C6 and C4–C5, and a pi bond remains between C2–C3.
Q: A cis-difluoroalkene reacts with 2,3-dimethyl-1,3-butadiene in a Diels-Alder reaction. What is the stereochemical relationship of the fluorines in the product?
A: The fluorines are cis in the product. The Diels-Alder reaction is stereospecific: the geometry of the dienophile is retained.
Q: Why does radical addition of HBr give anti-Markovnikov products, while ionic addition of HBr gives Markovnikov products?
A: In radical addition, the bromine radical (not H) adds first, and it adds to the terminal carbon to generate the more stable (more substituted) carbon radical. In ionic addition, the proton adds first to give the more stable carbocation on the more substituted carbon, and then bromide attacks that carbon.
Q: Cyclopentadiene reacts with maleic anhydride in a Diels-Alder reaction. Why is the endo product the major product?
A: In the endo transition state, the carbonyl groups of maleic anhydride point towards the diene, allowing secondary orbital interactions (overlap between the diene pi system and the dienophile's C=O pi system) that stabilise the transition state and lower the activation energy. The exo transition state lacks these interactions.
Q: Propose two different routes to place a bromine on the terminal carbon of 1-pentene.
A: Route 1: HBr with peroxides (radical addition, anti-Markovnikov, gives 1-bromopentane directly). Route 2: Hydroboration-oxidation (BH₃·THF, then NaOH/H₂O₂) to get 1-pentanol, then treat with PBr₃ to convert the alcohol to 1-bromopentane.
The kinetic vs thermodynamic control framework introduced for 1,2 vs 1,4 addition recurs in enolate chemistry (Ch. 18–19), where the kinetic enolate and the thermodynamic enolate give different alkylation products. The Diels-Alder reaction connects forward to pericyclic reactions and the Woodward-Hoffmann rules, which use the same orbital symmetry principles to predict whether a reaction is thermally or photochemically allowed. Radical addition connects to radical halogenation of alkanes (Ch. 4) and free-radical polymerisation, and understanding radical stability parallels carbocation stability from earlier chapters.
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