Difficulty: Intermediate | Prerequisites: Chapters 1–10 (carbocation stability, SN1/SN2 basics, radical reactions, pi bonding).
This chapter extends everything you know about substitution and radical reactions into systems where a double bond sits next door to the reactive site. That neighbouring pi bond changes stability, changes reactivity order, and opens up entirely new reaction pathways (conjugate addition, pericyclic reactions). If you are comfortable with carbocation stability rankings and basic SN1/SN2 mechanisms, this material builds directly on top of them. If those are shaky, revisit Chapters 7–8 first.
Allylic and conjugated systems are more stable than their plain alkyl counterparts because of resonance delocalisation. This extra stability flips the usual reactivity rankings for substitution, and it also creates new reaction types: 1,2 vs 1,4 addition to conjugated dienes (controlled by temperature), and the Diels-Alder cycloaddition (a powerful ring-forming reaction you will use constantly in synthesis problems).
Allylic position
The carbon directly adjacent to a C=C double bond. In simple terms, it is the carbon "next door" to the pi bond, and any group or radical sitting there benefits from resonance stabilisation.
Conjugated diene
A system of two C=C double bonds separated by exactly one sigma bond (e.g. 1,3-butadiene). Think of it as two double bonds close enough for their pi electrons to talk to each other.
Isolated diene
Two C=C double bonds separated by more than one sigma bond. The double bonds behave independently because they are too far apart for orbital overlap.
Cumulated diene (allene)
Two C=C double bonds directly adjacent, sharing a central carbon (C=C=C). The pi systems are perpendicular to each other, which gives these molecules unusual geometry.
1,2-Addition
Addition of HX across just one of the two double bonds in a conjugated diene. This is the kinetic product, favoured at low temperature.
1,4-Addition (conjugate addition)
Addition of HX across both ends of a conjugated diene system, with the double bond shifting to the middle. This is the thermodynamic product, favoured at higher temperature or longer reaction times.
Kinetic control
Conditions (low temperature, short time) under which the faster-forming product dominates, even if it is less stable.
Thermodynamic control
Conditions (higher temperature, longer time) under which the most stable product accumulates, because the reaction is reversible enough to reach equilibrium.
Diels-Alder reaction
A [4+2] cycloaddition between a conjugated diene (4 pi electrons) and a dienophile (2 pi electrons) to form a six-membered ring. Think of it as a one-step, heat-driven ring-building reaction.
Dienophile
The two-carbon pi component in a Diels-Alder reaction, typically an alkene bearing electron-withdrawing groups.
Endo rule
In a Diels-Alder reaction, large substituents on the dienophile prefer to "tuck under" the forming ring (the endo orientation) rather than point away from it.
NBS (N-bromosuccinimide)
A reagent that provides a low, steady concentration of Br2 for selective radical bromination at allylic (or benzylic) positions. Used with ROOR (radical initiator) or light.
Stability order for carbocations and radicals:
Methyl < primary < secondary ≈ primary allylic < tertiary ≈ secondary allylic < tertiary allylic
Allylic species get a stability boost from resonance: the charge or radical is delocalised across the pi system.
SN2 reactivity order for alkyl vs allylic halides (with NaOEt in EtOH, 45 °C):
Secondary alkyl < primary alkyl < secondary allylic < primary branched allylic < primary allylic
Allylic halides react faster in SN2 because the pi system stabilises the transition state.
For SN1, the reactivity order is reversed (the most stable carbocation forms fastest, so tertiary allylic is the most reactive).
The leaving group (halide) departs, forming a resonance-stabilised allylic carbocation.
The nucleophile attacks the major resonance contributor. It may attack the carbocation directly, or the pi bond may shift so the nucleophile attacks the other end of the allylic system.
Proton transfer occurs on the resulting alkyloxonium ion to give the final product.
Follows the same three-step radical chain mechanism: initiation, propagation, termination.
The allylic radical intermediate is resonance-stabilised, which makes allylic positions selectively targeted.
A radical on a more substituted carbon is more stable (same trend as carbocations).
Reagents: NBS/ROOR, NBS/light, Br₂/light, Cl₂/heat.
NBS is preferred because it keeps Br₂ concentration low, avoiding competing addition to the double bond.
Conjugated: pi bonds separated by one sigma bond. The p orbitals on all four carbons overlap, creating a continuous pi system. Most stable of the three types.
Isolated: pi bonds separated by more than one sigma bond. No orbital overlap between the double bonds; each reacts independently.
Cumulated: adjacent pi bonds (C=C=C). The central carbon is sp-hybridised and the two pi systems are perpendicular.
When HX adds to a conjugated diene, a proton first transfers to one end, forming an allylic carbocation.
The halide ion then attacks a positively charged carbon:
Attack at the nearer carbon gives the 1,2-addition product (kinetic product).
Attack at the far carbon gives the 1,4-addition product (thermodynamic product).
At low temperature, 1,2-addition dominates (kinetic control): the halide attacks the closest electrophilic carbon.
At higher temperature, 1,4-addition dominates (thermodynamic control): the more substituted (more stable) alkene product accumulates.
A concerted, one-step [4+2] cycloaddition. A 4-carbon conjugated diene reacts with a 2-carbon dienophile to form a six-membered ring. Requires heat.
Stereochemistry: syn addition. Both new sigma bonds form on the same face.
Endo rule: bulky substituents on the dienophile tuck underneath the forming ring (endo orientation is kinetically favoured).
Bond changes in the mechanism:
Carbons 1 and 4 of the diene connect to carbons 1 and 2 of the dienophile.
The pi bond on the dienophile disappears.
The two pi bonds on the diene are replaced by a single pi bond between carbons 2 and 3.
Spectroscopic evidence of Diels-Alder products:
Vinyl protons (sp² C-H) at 4.5–6.5 ppm in ¹H NMR.
C=C stretch near 1600 cm⁻¹ in IR.
Retrosynthesis tip: if you see a six-membered ring containing one pi bond, consider whether it could have come from a Diels-Alder reaction and work backwards to identify the diene and dienophile.
Carbocation / radical stability order
Methyl < 1° < 2° ≈ 1° allylic < 3° ≈ 2° allylic < 3° allylic
SN2 reactivity order (allylic halides, NaOEt/EtOH, 45 °C)
2° alkyl (0.19) < 1° alkyl (0.24) < 2° allylic (2.7) < 1° branched allylic (4.9) < 1° allylic unbranched (89–100)
Numbers are relative second-order rate constants.
Diels-Alder bond accounting
Two pi bonds in the diene + one pi bond in the dienophile → one new pi bond + two new sigma bonds + a six-membered ring.
The Diels-Alder reaction is one of the most widely used tools in synthetic organic chemistry. Pharmaceutical chemists rely on it to build complex ring systems in a single step, with predictable stereochemistry. The synthesis of steroids, terpenes, and many drug candidates features Diels-Alder steps.
Kinetic vs thermodynamic control matters well beyond dienes. Any time a reaction can give more than one product, the temperature and reaction time determine which one you get. This concept reappears in enolate chemistry, aldol reactions, and materials science (e.g. polymer crosslinking).
Students often assume that 1,4-addition is always the major product. It is not. At low temperature (kinetic control), 1,2-addition dominates.
Students sometimes confuse "allylic" with "vinylic." The allylic carbon is next to the double bond; the vinylic carbon is part of the double bond. Reactivity is very different.
The endo rule does not mean the endo product is more stable. It is the kinetic product of Diels-Alder, favoured by secondary orbital overlap in the transition state. The exo product is often more thermodynamically stable.
Students forget that SN2 reactivity order for allylic halides is the opposite of SN1. Allylic systems are faster for SN2 (transition state stabilisation), but for SN1 the most substituted allylic system wins (most stable carbocation).
⚠️ Expect to be asked: given a conjugated diene + HX, predict the 1,2 and 1,4 products AND state which is favoured under kinetic vs thermodynamic conditions.
⚠️ Diels-Alder retrosynthesis is a near-certainty on the exam. If you see a cyclohexene, ask yourself whether it could come from a [4+2] cycloaddition.
⚠️ Know the endo rule and be able to draw the endo vs exo products for a given diene + dienophile pair.
⚠️ Be comfortable predicting products of radical halogenation at an allylic position, including drawing all resonance structures of the intermediate radical.
⚠️ Spectroscopy tie-in: you may be given NMR or IR data and asked to confirm a Diels-Alder product (vinyl protons at 4.5–6.5 ppm, C=C at ~1600 cm⁻¹).
True or false: A primary allylic carbocation is more stable than a secondary alkyl carbocation. (True, roughly equal)
Fill in the blank: At low temperature, HBr addition to 1,3-butadiene gives mainly the ______ product. (1,2-addition)
True or false: The Diels-Alder reaction proceeds via anti addition. (False, it is syn addition)
Fill in the blank: In a Diels-Alder retrosynthesis, you look for a ______-membered ring containing one pi bond. (six)
True or false: NBS is used for radical halogenation because it provides a high concentration of Br₂. (False, it keeps Br₂ concentration low)
Q: What are the two possible products when HBr is added to 1,3-butadiene, and which is favoured at low temperature?
A: The 1,2-addition product (3-bromo-1-butene) and the 1,4-addition product (1-bromo-2-butene). At low temperature, 1,2-addition is favoured (kinetic control).
Q: Rank the following in order of increasing SN2 reactivity: primary alkyl halide, secondary allylic halide, primary allylic halide.
A: Primary alkyl halide < secondary allylic halide < primary allylic halide.
Q: In a Diels-Alder reaction between 1,3-butadiene and ethylene, describe the bond changes.
A: The two pi bonds of the diene and the one pi bond of ethylene are consumed. Two new C–C sigma bonds form (connecting C1 and C4 of the diene to C1 and C2 of ethylene), and a new pi bond appears between C2 and C3 of the original diene. The product is cyclohexene.
Q: Why is NBS preferred over Br₂ for radical allylic bromination?
A: NBS slowly releases Br₂ in low concentration, which favours radical substitution at the allylic position rather than electrophilic addition across the double bond.
Q: A student says the endo product of a Diels-Alder reaction is the thermodynamic product. Correct or incorrect?
A: Incorrect. The endo product is the kinetic product, favoured by secondary orbital interactions in the transition state. The exo product is often more thermodynamically stable.
Allylic stability connects directly to the carbocation and radical stability hierarchies from Chapters 7–8. If you understand why tertiary carbocations are stable (hyperconjugation), the resonance argument for allylic stability is the same idea applied through pi overlap.
Kinetic vs thermodynamic control reappears in Chapter 12 (benzylic reactivity) and later in enolate chemistry. The principle is universal: temperature and reversibility determine product distribution.
The Diels-Alder reaction is the foundation for retrosynthetic analysis of six-membered rings, a skill that carries through the rest of the organic chemistry sequence and into advanced synthesis courses.
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