Difficulty: Intermediate | Prerequisites: Chapters 6–10 (substitution/elimination reactions, stereochemistry, alkene reactions)
Chapter 11 builds on everything you know about alkene reactivity and substitution/elimination and extends it to systems where double bonds interact with each other or with adjacent reactive centres. The central idea is conjugation: when p orbitals overlap across three or more atoms, the resulting delocalisation changes stability, reactivity, and product distribution in ways that a single isolated double bond cannot. You will need a solid grasp of carbocation stability, resonance structures, and the SN1/SN2/E1/E2 framework from earlier chapters. This material feeds directly into aromatic chemistry (Chapter 12) and the carbonyl chemistry that dominates the second semester.
Conjugated systems (alternating single and double bonds) stabilise intermediates through resonance, making allylic carbocations, radicals, and anions more stable than their isolated counterparts. This conjugation controls whether electrophilic addition gives a 1,2 (kinetic) or 1,4 (thermodynamic) product. The Diels–Alder reaction, a [4+2] cycloaddition between a conjugated diene and an electron-poor dienophile, is the chapter's centrepiece and a major exam topic.
Allyl group
The H₂C=CHCH₂ fragment. The carbon adjacent to the double bond is the allylic carbon.
In simple terms, it is a three-carbon unit with a double bond on one end and a reactive site on the other.
Allylic carbon
The sp³ carbon directly attached to a C=C double bond. This position is special because a carbocation, radical, or anion formed here is stabilised by resonance with the adjacent pi bond.
Think of it as the carbon "next door" to the double bond, which can borrow electron density from it.
Conjugation
The overlap of p orbitals on three or more adjacent atoms, typically seen when double bonds alternate with single bonds. Conjugation delocalises electrons and lowers overall energy.
In simple terms, the electrons are not stuck on one bond but spread across several atoms, making the whole system more stable.
Resonance
The representation of electron delocalisation by drawing two or more Lewis structures (resonance contributors) for the same molecule. The true structure is a weighted average. The more stable contributor carries more weight.
1,2-addition (kinetic product)
Addition of H and X across the same double bond of a conjugated diene. Favoured at lower temperatures because it forms through the more stable carbocation intermediate and has a lower activation energy.
1,4-addition (thermodynamic product)
Addition of H and X across the ends of the entire conjugated system (positions 1 and 4). A single bond between C2 and C3 becomes a double bond. Favoured at higher temperatures because the product alkene is more substituted and therefore more stable.
Diene
A molecule containing two C=C double bonds. Classified as conjugated (alternating single/double, most stable), isolated (double bonds separated by two or more single bonds), or cumulated (double bonds share a central carbon, least stable).
Dienophile
The electron-poor alkene (or alkyne) partner in a Diels–Alder reaction. Electron-withdrawing groups (C=O, C≡N) on the dienophile increase reactivity.
Think of it as the "diene lover" that slots into the diene to form a ring.
Diels–Alder reaction
A [4+2] cycloaddition: the four pi electrons of a conjugated diene combine with the two pi electrons of a dienophile to form a new six-membered ring in a single concerted step.
s-cis / s-trans conformations
Rotational conformers around the single bond connecting the two double bonds in a conjugated diene. s-cis has the double bonds on the same side (required for Diels–Alder); s-trans has them on opposite sides (more stable but unreactive in D.A.).
Endo rule (Alder rule)
In a Diels–Alder reaction, the major product places the dienophile's substituents under (endo to) the newly formed six-membered ring, stabilised by secondary orbital overlap in the transition state.
NBS (N-bromosuccinimide)
A reagent for selective allylic (or benzylic) bromination via a radical chain mechanism. Used with light (hν) or a radical initiator (ROOR).
Solvolysis
A substitution reaction in which the solvent itself acts as the nucleophile. In allylic systems, solvolysis often proceeds by SN1 through a resonance-stabilised carbocation.
Allene
A cumulated diene of the form R₂C=C=CR₂. Allenes can display axial chirality even without a traditional stereocentre.
EWG (electron-withdrawing group)
A substituent that pulls electron density away from a pi system (e.g. C=O, C≡N, NO₂). On a dienophile, an EWG makes it more reactive toward an electron-rich diene.
EDG (electron-donating group)
A substituent that pushes electron density into a pi system (e.g. OR, NR₂, alkyl). On a diene, an EDG makes it more reactive toward an electron-poor dienophile.
The allyl group (H₂C=CHCH₂) can form three reactive intermediates, each stabilised by resonance across the three-carbon pi system:
Allyl carbocation (2 electrons in the pi system): positive charge delocalised over C1 and C3
Allyl radical (3 electrons): the unpaired electron is shared across C1 and C3
Allyl anion (4 electrons): the negative charge is delocalised over C1 and C3
The allylic carbon is sp³-hybridised and sits adjacent to the double bond. A lone pair, radical, or empty p orbital at this position can overlap with the adjacent pi bond, giving resonance stabilisation.
The major resonance contributor is whichever structure places the charge (or radical) on the more substituted carbon.
Conjugation occurs when p orbitals overlap on three or more adjacent atoms. The hallmark pattern is alternating double and single bonds.
Conjugated systems are more stable than isolated ones because electron density is spread over a larger region, lowering the overall energy.
Methyl < primary < secondary ≈ primary allylic < tertiary ≈ secondary allylic < tertiary allylic
Resonance stabilisation can elevate a primary allylic cation to roughly the stability of a secondary carbocation.
A more stable carbocation means a faster reaction rate in SN1 and E1 pathways.
Allylic rearrangements involve the movement of electrons (via resonance), not the physical migration of H or C groups. This is a common source of confusion.
In solvolysis (where the solvent acts as the nucleophile), the allylic carbocation can be attacked at either end of the delocalised system, giving a mixture of products.
SN2 works on primary and unhindered secondary allylic halides with good nucleophiles.
Relative SN2 rates: secondary alkyl halide < primary alkyl halide < secondary allylic halide < primary branched allylic halide < primary allylic halide.
Why allylic substrates are faster: during the backside attack, electron density spreads over the adjacent pi bond, stabilising the transition state. The shared electron density lowers the activation energy.
Practical note: when using an alkoxide nucleophile such as sodium ethoxide (NaOEt), remember to include the counterpart alcohol (ethanol) as the solvent.
Reagents: NBS with light (hν) or NBS with a radical initiator (ROOR).
The allylic C–H bond is weaker than a typical alkyl C–H bond, so radical abstraction occurs selectively at the allylic position.
Allylic radicals are more stable than simple alkyl radicals because of resonance delocalisation.
All allylic hydrogens in the starting alkene must be equivalent.
Both resonance forms of the resulting allylic radical must be equivalent.
If either condition fails, the reaction produces a mixture of constitutional isomers.
When the two resonance contributors of the radical are not equivalent, the major product comes from the more stable contributor (more substituted radical centre).
Conjugated (most stable): double bonds separated by exactly one single bond. Electron delocalisation lowers energy.
Isolated: double bonds separated by two or more single bonds. Each double bond behaves independently.
Cumulated (least stable): two double bonds share a central carbon (C=C=C). The central carbon is sp-hybridised.
s-trans: the two double bonds are on opposite sides of the connecting single bond. More stable (less steric strain) but cannot undergo Diels–Alder.
s-cis: the two double bonds are on the same side. Less stable but required for Diels–Alder (the diene ends must be close enough to bond with the dienophile).
Allenes (H₂C=C=CH₂) are cumulated dienes. The two terminal CH₂ groups lie in perpendicular planes.
An allene can have a chirality axis even without a traditional stereocentre. This is an exam favourite.
Elimination reactions from unsaturated alcohols or alkyl halides:
Dehydration (acid-catalysed loss of water)
Dehydrohalogenation (base-promoted loss of HX)
Adding HX (e.g. HBr) to a conjugated diene gives two possible products.
1,2-addition (kinetic product)
H and X add across the same double bond.
Favoured at lower temperatures.
Forms through the more stable carbocation intermediate (the allylic cation, attacked at the carbon bearing more of the positive charge).
Has a lower activation energy, so it forms faster.
1,4-addition (thermodynamic product)
H and X add to the 1 and 4 positions of the diene system. The internal single bond becomes a new double bond.
Favoured at higher temperatures (equilibrium control).
Gives the more substituted (more stable) alkene product.
The classic example is 1,3-butadiene + HBr: at low temperature the major product is 3-bromo-1-butene (1,2); at high temperature the major product is 1-bromo-2-butene (1,4).
A [4+2] cycloaddition: 4 pi electrons from the diene + 2 pi electrons from the dienophile form a new six-membered ring.
The diene is electron-rich; the dienophile is electron-poor.
An electron-withdrawing group (EWG) on the dienophile (C=O, C≡N) increases the reaction rate.
An electron-donating group (EDG) on the diene increases its reactivity.
The reaction is concerted (one step, no intermediates) and always syn addition.
The diene must be in the s-cis conformation (both double bonds on the same side of the single bond).
Electron-withdrawing groups on the dienophile increase the reaction rate.
The addition is syn: diene and dienophile approach from the same face.
The endo rule (Alder rule) governs the preferred stereochemistry of the major product.
Cis substituents on the dienophile remain cis in the product.
Trans substituents on the dienophile remain trans in the product.
The stereochemical relationships in the starting material are preserved in the cycloadduct.
Substituents that are cis in the s-cis diene (on the same side) end up cis in the product ring.
Substituents that are trans in the s-cis diene end up trans in the product.
To predict stereochemistry: draw the diene in its s-cis form and check whether the substituents of interest are on the same face or opposite faces.
"Endo" means the dienophile's substituent(s) point under the newly formed six-membered ring (toward the diene's pi system).
The endo product is kinetically favoured because secondary orbital overlap between the dienophile's EWG and the diene's pi electrons stabilises the transition state.
The exo product places the substituent on the outside (away from the ring). It is thermodynamically more stable but usually the minor product.
When the diene and dienophile are part of the same molecule, the reaction forms two rings at once.
Strategy: redraw the molecule so you can identify the diene portion and the dienophile portion, then apply the same stereochemical rules.
Keep careful track of carbon numbering.
Look for a six-membered ring containing one double bond.
Identify the two new sigma bonds formed in the cycloaddition.
Break those bonds to reveal the original diene and dienophile.
Check that the diene carries EDGs and the dienophile carries EWGs (if substituents are present).
Vinyl protons appear at 4.5–6.5 ppm in ¹H NMR.
C=C stretches appear near 1600 cm⁻¹ in IR.
Carbocation stability order: methyl < 1° < 2° ≈ 1° allylic < 3° ≈ 2° allylic < 3° allylic
SN2 rate order (allylic substrates): 2° alkyl < 1° alkyl < 2° allylic < 1° branched allylic < 1° allylic
Diels–Alder: diene (4π e⁻) + dienophile (2π e⁻) → six-membered ring ([4+2] cycloaddition)
Diene stability: conjugated > isolated > cumulated
NMR reference: vinyl protons at 4.5–6.5 ppm
IR reference: C=C stretch near 1600 cm⁻¹
The Diels–Alder reaction is one of the most widely used tools in synthetic chemistry. It builds complex six-membered rings with precise stereochemistry in a single step, which is why it appears in the synthesis of steroids, terpenes, and many pharmaceutical intermediates.
Conjugated diene polymers are the basis of synthetic rubber (polybutadiene, polyisoprene). The conjugation gives the polymer backbone its elasticity.
Students often confuse allylic rearrangements with hydride or methyl shifts. Allylic rearrangements are electron movements through resonance, not physical migration of atoms.
Many students assume the 1,2-product is always the major product. At high temperature, thermodynamic control takes over and the 1,4-product dominates.
Students forget that the diene must be in the s-cis conformation for a Diels–Alder reaction. A diene locked in s-trans (e.g. by a ring) cannot react.
A common error on exams is reversing endo and exo. Remember: endo = substituents point inward, under the ring. Endo is the kinetic (major) product.
⚠️ Diels–Alder stereochemistry (syn addition, endo rule, cis/trans preservation) is a near-certain exam topic. Practise drawing the s-cis diene, aligning it with the dienophile, and predicting the product's stereochemistry.
⚠️ 1,2 vs 1,4 addition: you need to be able to predict the major product at low temperature (kinetic, 1,2) and high temperature (thermodynamic, 1,4). Know how to draw the energy diagram.
⚠️ Retrosynthesis of Diels–Alder products: given a cyclohexene, break it back into diene + dienophile. Make sure EDGs are on the diene side and EWGs on the dienophile side.
⚠️ Allylic halogenation with NBS: know the conditions and be able to predict whether a clean single product or a mixture forms (check hydrogen equivalence and radical equivalence).
⚠️ Carbocation stability rankings regularly appear as multiple-choice or ranking questions.
True or false: a primary allylic carbocation is approximately as stable as a secondary carbocation. (True)
True or false: the s-trans conformation of a conjugated diene can undergo a Diels–Alder reaction. (False, s-cis is required)
Fill in the blank: at low temperature, HBr addition to 1,3-butadiene favours the ______ product. (1,2-addition / kinetic)
True or false: in a Diels–Alder reaction, trans substituents on the dienophile become cis in the product. (False, trans stays trans)
Fill in the blank: NBS with light or ROOR selectively brominates at the ______ position. (allylic)
Q: Rank the following in order of increasing carbocation stability: methyl, tertiary, primary allylic, secondary allylic.
A: Methyl < primary allylic < secondary allylic. Tertiary sits between primary allylic and secondary allylic (tertiary ≈ secondary allylic).
Q: 1,3-Butadiene reacts with HBr. Draw or name the major product at 0 °C and at 40 °C. Explain.
A: At 0 °C, the major product is 3-bromo-1-butene (1,2-addition, kinetic control). At 40 °C, the major product is 1-bromo-2-butene (1,4-addition, thermodynamic control). The 1,4-product has a more substituted, more stable internal double bond.
Q: What four requirements must be met for a Diels–Alder reaction to proceed?
A: (1) The diene must be in the s-cis conformation. (2) An EWG on the dienophile increases the rate. (3) Addition is syn. (4) The endo rule determines the major stereochemical outcome.
Q: A Diels–Alder product contains a six-membered ring with one double bond, a –COOH group endo, and two methyl groups cis on the ring. Work backwards: what were the starting diene and dienophile?
A: Break the ring at the two sigma bonds flanking the double bond. The dienophile carried the COOH (EWG). The diene had the two methyl groups on the same side when drawn in s-cis form (giving cis in the product). Endo COOH means it pointed under the forming ring in the transition state.
Q: Why does NBS with light selectively brominate the allylic position rather than a vinylic or alkyl position?
A: The allylic C–H bond is weaker than alkyl or vinylic C–H bonds because the resulting allylic radical is resonance-stabilised. The radical chain mechanism selectively abstracts the weakest C–H bond.
Q: Explain why primary allylic halides undergo SN2 faster than primary alkyl halides.
A: In the SN2 transition state, the incoming nucleophile's electron density can be delocalised into the adjacent pi bond, lowering the transition state energy and increasing the reaction rate.
This material connects directly to Chapter 12 (arenes and aromaticity), where the same principles of conjugation and resonance stabilisation explain why benzene is so stable and why benzylic intermediates are unusually reactive. The carbocation stability rankings from this chapter carry forward into every electrophilic addition and substitution mechanism you will meet for the rest of the course. The Diels–Alder reaction reappears in more advanced synthesis problems, often as a retrosynthetic disconnection for six-membered rings in natural product synthesis.
Conjugated diene, isolated diene, cumulated diene, allyl group, allylic carbon, allylic carbocation, allylic radical, allylic anion, resonance stabilisation, delocalisation, 1,2-addition, 1,4-addition, kinetic product, thermodynamic product, kinetic vs thermodynamic control, Diels–Alder reaction, [4+2] cycloaddition, diene, dienophile, s-cis, s-trans, endo rule, Alder rule, exo product, syn addition, electron-withdrawing group, EWG, electron-donating group, EDG, NBS, N-bromosuccinimide, allylic bromination, radical halogenation, allene, axial chirality, retrosynthesis, SN2 allylic, solvolysis, pericyclic reaction, concerted mechanism, secondary orbital overlap, intramolecular Diels–Alder