Difficulty: Intermediate to Advanced | Prerequisites: Resonance, substitution/elimination, stereochemistry (R/S, E/Z, cis/trans)
This material brings together several threads from the semester. Aromaticity explains a special kind of stability that governs reactivity across many ring systems. Addition reactions (to alkenes, alkynes, and dienes) and free-radical mechanisms show up in both product prediction and mechanism-writing questions. The Diels-Alder reaction is a pericyclic reaction with strict stereochemical rules. Much of this is flagged as "most likely tested" and "new material" in the source notes.
Aromatic compounds follow the 4n+2 rule (cyclic, planar, fully conjugated, with 4n+2 pi electrons); anti-aromatic compounds follow 4n. Addition reactions to alkenes and alkynes proceed through carbocation or radical intermediates, and diene additions can be 1,2 (kinetic) or 1,4 (thermodynamic). Free-radical reactions have three phases: initiation, propagation, termination. The Diels-Alder reaction joins a diene and dienophile in one concerted step with predictable stereochemistry.
Aromatic
A cyclic, planar, fully conjugated molecule whose pi-electron count satisfies 4n+2 (where n is a non-negative integer: 0, 1, 2, ...). In simple terms, it is an unusually stable ring because its electrons are perfectly delocalised.
Anti-aromatic
A cyclic, planar, fully conjugated molecule whose pi-electron count satisfies 4n. These are unusually unstable and tend to distort out of planarity to avoid anti-aromaticity.
Non-aromatic
Any compound that fails one or more of the four criteria (cyclic, planar, fully conjugated, correct electron count). It has ordinary stability.
Diene
A molecule with two conjugated double bonds. In a Diels-Alder reaction, the diene must adopt the s-cis conformation to react.
Dienophile
The reaction partner of the diene in a Diels-Alder reaction, typically an alkene or alkyne with an electron-withdrawing group.
1,2-addition (kinetic product)
Addition across the two carbons of one double bond in a conjugated system. Favoured at low temperature (around -80 °C).
1,4-addition (thermodynamic product)
Addition across carbons 1 and 4 of a conjugated diene. Favoured at high temperature (around 40 °C) because the more substituted alkene product is more stable.
Free radical
A species with an unpaired electron. Radicals are formed during initiation (by homolytic cleavage of a bond, often with heat or light) and carry chain reactions through propagation steps.
Allylic position
The carbon adjacent to a double bond. Allylic radicals and carbocations are stabilised by resonance with the pi system.
Must be cyclic. Open-chain conjugated systems cannot be aromatic.
All atoms in the ring must participate in conjugation. Every atom needs a p orbital available for overlap. An sp³-hybridised carbon in the ring breaks conjugation because it lacks a vacant p orbital.
A lone pair on a heteroatom (N, O, S) that is not already part of a pi bond can be donated into the pi system and counted as pi electrons.
An empty p orbital (such as on C⁺) participates in conjugation but contributes zero pi electrons.
Must be planar. The p orbitals need to overlap continuously around the ring.
Count the pi electrons. If the total fits 4n+2 for some non-negative integer n, the compound is aromatic. If it fits 4n, it is anti-aromatic. Common counts: 2 (n=0), 6 (n=1), 10 (n=2).
Compounds that fail any one of rules 1 through 4 are classified as non-aromatic (ordinary stability).
Acid-catalysed hydration. H₃O⁺ adds water across the double bond following Markovnikov's rule (H goes to the less substituted carbon, OH to the more substituted).
Addition of hydrogen halides (HX). Markovnikov addition. Carbocation intermediate forms at the more substituted carbon.
Halogenation (X₂). Without water, gives a vicinal dihalide through a cyclic halonium ion. With water present, gives a halohydrin (anti addition).
Acid-catalysed hydration. Follows Markovnikov's rule. The initial enol product tautomerises to a ketone (keto-enol tautomerism).
With HX: At low temperature (around -80 °C), 1,2-addition is the kinetic product. At high temperature (around 40 °C), 1,4-addition is the thermodynamic product.
With X₂: 1,4-addition is the major product, forming exclusively the E (trans) alkene.
Identifying 1,2 vs 1,4 from the product: If the halogen ends up at the more substituted position, it is 1,2-addition. If it ends up at the primary/terminal position, it is 1,4-addition.
Every radical chain reaction has three phases: initiation, propagation, and termination.
Initiation: Homolytic cleavage of X₂ (using heat or light) produces two halogen radicals.
Propagation step 1: X radical abstracts an H from the alkane, producing HX and a carbon radical.
Propagation step 2: The carbon radical reacts with X₂, producing the haloalkane and regenerating the X radical.
Bromination is selective for the most substituted C (most stable radical). Chlorination is less selective.
Initiation (two steps): ROOR undergoes homolytic cleavage to give two RO radicals. Then RO radical abstracts H from HBr, producing ROH and a Br radical.
Propagation: Br radical adds to the alkene (anti-Markovnikov, giving the more stable radical intermediate). The carbon radical then abstracts H from HBr.
This is the only radical addition that works reliably in practice (HBr with peroxides). HCl and HI radical additions are not useful.
With NBS or Cl₂/heat: The radical forms at the allylic carbon (not on the double bond carbon).
Chlorination propagation: Cl radical abstracts the allylic H, producing an allylic radical. The allylic radical then reacts with Cl-Cl.
NBS (N-bromosuccinimide): Provides a low, steady concentration of Br₂ for allylic bromination. Products can be 1,2-addition or 1,4-addition (low T favours 1,2; high T favours 1,4).
The allylic radical mechanism is different from simple alkane halogenation because the starting material is an alkene, and Br-Br is directly involved in the propagation step.
Metal-ammonia reduction: Uses Na or Li in liquid NH₃ to reduce an alkyne to a trans alkene. This is not a chain radical process in the usual sense, but it proceeds through radical anion intermediates.
The Diels-Alder is a concerted [4+2] cycloaddition: a conjugated diene reacts with a dienophile to form a six-membered ring in a single step.
Diene geometry: If the diene is cyclic, a bridged bicyclic ("flap") product forms. If the diene has substituents on each carbon, their geometry is preserved:
Substituents on the same side of the diene (both outside or both inside) give cis products.
Substituents on opposite sides (inside/outside) give trans racemic products.
Convention: outside substituents appear as dashes; inside substituents appear as wedges in the product.
Dienophile geometry is preserved: A trans dienophile gives trans substituents in the product. A cis dienophile gives cis substituents.
Endo rule: Substituents on the dienophile prefer to be tucked under the newly forming ring (endo position). This applies only when the dienophile has a double bond (not a triple bond). Even cyclic dienophiles follow the endo rule.
The Diels-Alder reaction is one of the most powerful tools in synthetic chemistry for building six-membered rings with precise stereochemistry. It is used industrially and in pharmaceutical synthesis to construct complex ring systems in a single step.
Students often count all electrons in the ring, not just pi electrons, when applying the 4n+2 rule. Sigma-bond electrons are never counted. Only electrons in p orbitals that are part of the conjugated pi system count.
A lone pair on a heteroatom is only counted as pi electrons if it is in a p orbital contributing to conjugation. If the lone pair is already involved in a pi bond (e.g. the nitrogen in pyridine has its lone pair in an sp² orbital in the plane, not in the pi system), do not count it.
Many students assume anti-aromatic compounds are common. In practice, they are rare because molecules distort to avoid anti-aromaticity (e.g. cyclooctatetraene adopts a tub shape to become non-aromatic rather than remaining planar and anti-aromatic).
A frequent Diels-Alder error is forgetting the endo rule, or applying it to a dienophile with a triple bond (where endo/exo distinction does not exist).
⚠️ Mechanism questions on free radicals are flagged as "most likely tested." Be able to write full initiation and propagation steps for alkane bromination, HBr/ROOR addition to alkenes, and allylic chlorination/NBS.
⚠️ Aromaticity classification (aromatic, anti-aromatic, or non-aromatic) requires checking all four criteria. Expect at least one tricky heterocyclic example.
⚠️ Diels-Alder stereochemistry is a high-value exam topic. You need to predict whether the product is cis or trans based on the diene and dienophile geometry, and apply the endo rule.
⚠️ Diene addition temperature dependence: -80 °C means kinetic (1,2) product; 40 °C means thermodynamic (1,4) product. This is tested directly.
⚠️ Reagent matching: NBS with light (hv) adds halogen at the allylic position while retaining the double bond. Do not confuse this with simple alkene addition.
True or false: A compound with 8 pi electrons in a planar, fully conjugated ring is aromatic. (False. 8 = 4n where n=2, so it is anti-aromatic.)
Fill in the blank: At -80 °C, HBr addition to a conjugated diene gives the ______ product. (1,2 / kinetic)
True or false: The endo rule applies to dienophiles with triple bonds. (False. Endo/exo distinction only exists for dienophiles with double bonds.)
Fill in the blank: In alkane radical bromination, the radical forms preferentially at the ______ substituted carbon. (most)
True or false: An sp³-hybridised carbon in a ring breaks conjugation and prevents aromaticity. (True.)
Q: Pyrrole (a five-membered ring with one NH) has 6 pi electrons and is aromatic. Explain where the 6 pi electrons come from.
A: The four carbon atoms each contribute one pi electron from their p orbitals (4 electrons from two C=C double bonds). The nitrogen's lone pair occupies a p orbital perpendicular to the ring and is donated into the pi system, adding 2 more electrons. Total: 4 + 2 = 6, which satisfies 4n+2 with n=1.
Q: 1,3-butadiene reacts with HBr at 40 °C. Predict the major product and explain.
A: At 40 °C, the thermodynamic product dominates: 1,4-addition gives 1-bromo-2-butene (the more substituted, more stable internal alkene). The carbocation intermediate forms at C2, and bromide attacks at C4.
Q: Write the propagation steps for the radical chlorination of propane at the secondary carbon.
A: Step 1: Cl radical abstracts an H from the secondary carbon, producing HCl and a secondary carbon radical. Step 2: The secondary carbon radical reacts with Cl₂, producing 2-chloropropane and regenerating a Cl radical.
Q: A Diels-Alder reaction uses a cis-substituted dienophile. What is the stereochemistry of the substituents in the product?
A: Cis. The dienophile's geometry is preserved in the product. Cis substituents on the dienophile remain cis on the newly formed ring.
Q: Cyclopentadienyl anion (C₅H₅⁻) is aromatic. How many pi electrons does it have?
A: 6 pi electrons. Three double bonds would give 6, but this is a five-membered ring with two double bonds (4 electrons) plus the carbanion's lone pair in a p orbital (2 electrons). Total = 6, satisfying 4n+2 with n=1.
Aromaticity draws on resonance (Exam 1): the 4n+2 rule is fundamentally about electron delocalisation across a cyclic system. If your resonance drawing skills are shaky, revisit those notes first.
Allylic substitution ties back to SN1/SN2 (Exam 3): the mechanism is identical, but the rate is faster because the allylic carbocation/transition state is stabilised by the adjacent double bond.
Free-radical additions (HBr/ROOR) give anti-Markovnikov products, which contrasts with the Markovnikov selectivity of ionic HBr addition. Knowing which conditions apply is a common exam distinction.
Related Terms / Search Tags
aromaticity, anti-aromatic, non-aromatic, Huckel's rule, 4n+2 rule, pi electrons, conjugation, cyclopentadienyl anion, pyrrole, Diels-Alder, diene, dienophile, s-cis conformation, endo rule, exo product, 1,2-addition, 1,4-addition, kinetic product, thermodynamic product, free radical, radical chain reaction, initiation, propagation, termination, NBS, allylic halogenation, allylic radical, anti-Markovnikov, HBr with peroxide, ROOR, metal-ammonia reduction, tautomerism, keto-enol