Source: Carey, Organic Chemistry, 11th ed., Ch. 11 (skip 11.16) and Ch. 12 (skip 12.12-12.15)
Tags: conjugation, 1 3-butadiene, allylic systems, Diels-Alder reaction, diene, dienophile, aromaticity, Huckel rule, benzene, resonance energy, aromatic stability, electrophilic aromatic substitution, CHEM 2301 UMN
Difficulty: Advanced | Prerequisites: Chapters 1-10 study notes (all prior reaction mechanisms, orbital theory, resonance)
Big picture: Chapter 11 explores what happens when double bonds are separated by exactly one single bond (conjugated systems). Conjugation introduces new reactivity patterns, including 1,2- vs 1,4-addition and the powerful Diels-Alder cycloaddition. Chapter 12 takes conjugation to its logical extreme with benzene and aromatic compounds, where a special kind of cyclic conjugation produces extraordinary stability. Aromaticity is one of the most important concepts in all of organic chemistry, and understanding it is essential for CHEM 2302 as well.
Conjugated dienes are more stable and more reactive than isolated dienes because their overlapping p orbitals allow delocalisation of pi electrons. This leads to 1,4-addition products alongside the expected 1,2-addition products, and enables the Diels-Alder reaction (a [4+2] cycloaddition). Aromatic compounds like benzene are unusually stable because their cyclic, planar, fully conjugated pi systems contain 4n+2 pi electrons (Huckel's rule), which creates a large resonance stabilisation energy. This stability means benzene undergoes substitution (keeping the aromatic ring intact) rather than addition (which would destroy it).
Conjugated diene
A molecule containing two double bonds separated by a single bond (C=C-C=C). The p orbitals on all four carbons overlap, creating a continuous pi system. Example: 1,3-butadiene.
Isolated diene
A molecule containing two double bonds separated by two or more single bonds. The double bonds behave independently because their p orbitals do not overlap.
Cumulated diene (allene)
A molecule in which two double bonds share a common carbon atom (C=C=C). The central carbon is sp-hybridised. Not a focus of this course.
S-cis and s-trans conformations
Conformations of a conjugated diene about the single bond connecting the two double bonds. S-cis: the double bonds are on the same side (needed for Diels-Alder). S-trans: the double bonds are on opposite sides (more stable for 1,3-butadiene).
1,2-Addition
Addition of a reagent across one double bond only. The other double bond remains unchanged. This is the kinetic product (formed faster).
1,4-Addition (conjugate addition)
Addition across the entire conjugated system, with atoms adding to positions 1 and 4 and a new double bond forming between positions 2 and 3. This is the thermodynamic product (more stable).
Kinetic vs thermodynamic control
At low temperatures, the 1,2-addition product (kinetic product) predominates because it forms faster. At higher temperatures, the 1,4-addition product (thermodynamic product) predominates because equilibrium favours the more stable product.
Allylic cation / allylic radical
A carbocation or radical located on a carbon adjacent to a double bond. Stabilised by resonance: the positive charge or unpaired electron is delocalised across three atoms (through the adjacent pi bond).
Diels-Alder reaction
A [4+2] cycloaddition between a conjugated diene (4 pi electrons) and a dienophile (2 pi electrons, typically an alkene or alkyne) to form a six-membered ring. One of the most important ring-forming reactions in organic chemistry.
Dienophile
The two-pi-electron component in a Diels-Alder reaction, usually an alkene or alkyne bearing electron-withdrawing groups.
Endo rule
In the Diels-Alder reaction, when the dienophile has substituents, the endo product (substituents oriented toward the diene in the transition state) is generally the major product (kinetic preference).
Conjugated dienes are more stable than isolated dienes. The heat of hydrogenation of 1,3-butadiene is about 15 kJ/mol less than twice that of 1-butene, indicating stabilisation from conjugation.
The extra stability arises from overlap of p orbitals across the single bond connecting the two double bonds, which allows partial pi-bonding character in that connecting bond.
Addition of HBr to 1,3-butadiene gives two products:
1,2-addition product: 3-bromo-1-butene (Br on C2, double bond between C3-C4 remains)
1,4-addition product: 1-bromo-2-butene (Br on C1, new double bond between C2-C3)
The allylic cation intermediate is resonance-stabilised, with the positive charge shared between C2 and C4. Bromide attacks at either position.
Low temperature favours the 1,2-product (kinetic control).
Higher temperature or longer reaction times favour the 1,4-product (thermodynamic control), because the internal double bond is more stable (more substituted) than the terminal double bond.
A concerted, one-step pericyclic reaction. No intermediates.
Requirements:
The diene must be in the s-cis conformation (both double bonds on the same side of the connecting single bond).
Electron-rich dienes and electron-poor dienophiles react fastest (normal electron demand).
Stereochemistry:
Syn addition with respect to both the diene and the dienophile. Substituents that are cis on the dienophile remain cis in the product (and trans remain trans).
The endo product is the kinetic product (usually major).
Regiochemistry: the "ortho/para" rule. When both the diene and dienophile are unsymmetrical, the major product has the 1,2- (ortho) or 1,4- (para) relationship of substituents.
The Diels-Alder reaction is reversible at high temperatures (retro-Diels-Alder).
Allylic cations, radicals, and anions are all stabilised by resonance with the adjacent double bond.
SN1 reactions at allylic positions are faster than at comparable non-allylic positions because the allylic carbocation is resonance-stabilised.
SN2 reactions at allylic positions are also somewhat faster due to the partial p-orbital character of the allylic C-LG bond.
Allylic bromination with NBS (Ch. 10) is selective for the allylic position because of the stability of the allylic radical.
Aromatic compound
A cyclic, planar, fully conjugated molecule (or ion) that has 4n+2 pi electrons (where n is a non-negative integer) and exhibits special thermodynamic stability. Benzene is the classic example.
Huckel's rule
A molecule (or ion) is aromatic if it is cyclic, planar, fully conjugated (every atom in the ring contributes a p orbital), and contains 4n+2 pi electrons (n = 0, 1, 2, 3...). That means 2, 6, 10, 14... pi electrons.
Antiaromatic
A cyclic, planar, fully conjugated molecule with 4n pi electrons (4, 8, 12...). Antiaromatic species are destabilised relative to the open-chain conjugated form and are extremely rare. Cyclobutadiene is the classic example.
Nonaromatic (non-aromatic)
A compound that does not satisfy all the criteria for aromaticity, usually because it is not fully conjugated or not planar. Nonaromatic compounds have "normal" stability; they are neither stabilised nor destabilised by the Huckel count.
Resonance energy (delocalisation energy)
The difference in energy between the actual aromatic compound and a hypothetical structure with localised double bonds. For benzene, this is approximately 150 kJ/mol, a substantial stabilisation.
Kekulé structures
The two equivalent resonance structures of benzene, each showing alternating single and double bonds. The real benzene is the hybrid, with all C-C bonds identical (bond order 1.5).
Electrophilic aromatic substitution (EAS)
The general reaction type for benzene: an electrophile replaces one hydrogen on the ring, preserving aromaticity. This is favoured over addition because substitution retains the aromatic pi system while addition would destroy it.
Arenium ion (sigma complex, Wheland intermediate)
The carbocation intermediate formed when the electrophile bonds to the ring in the first step of EAS. The ring temporarily loses aromaticity, then a proton is lost to restore it.
Benzene (C6H6) has three degrees of unsaturation, which could imply a triene. But benzene does not behave like a typical alkene: it does not decolourise bromine water, does not undergo Markovnikov addition, and is far more stable than expected.
All six C-C bonds are identical (1.40 angstroms, between a single bond at 1.54 and a double bond at 1.34).
The resonance energy of benzene (~150 kJ/mol) represents the extra stability from the delocalised pi system.
The pi electrons are delocalised in a continuous ring of overlapping p orbitals above and below the plane of the ring.
Step 1: Is the molecule cyclic? If not, it cannot be aromatic.
Step 2: Is it planar? If not (e.g. a large ring that twists), it cannot be aromatic.
Step 3: Is every atom in the ring contributing a p orbital to the pi system? (This includes atoms with lone pairs in p orbitals.)
Step 4: Count the pi electrons. If the total is 4n+2 (2, 6, 10, 14...), the compound is aromatic. If 4n (4, 8, 12...), it is antiaromatic.
The cyclopentadienyl anion (C5H5-) has 6 pi electrons and is aromatic. This is why cyclopentadiene is unusually acidic (pKa ~15): the conjugate base is aromatic and therefore very stable.
The cycloheptatrienyl cation (tropylium cation, C7H7+) has 6 pi electrons and is aromatic.
The cyclopentadienyl cation (C5H5+) would have 4 pi electrons and is antiaromatic, which is why it does not form readily.
Pyridine: a six-membered ring with one nitrogen. The nitrogen's lone pair is in an sp2 orbital in the plane of the ring (not part of the pi system). The ring has 6 pi electrons from three double bonds.
Pyrrole: a five-membered ring with one nitrogen. The nitrogen's lone pair is in a p orbital and is part of the pi system, contributing 2 pi electrons. Total: 6 pi electrons, aromatic.
Furan and thiophene follow the same logic as pyrrole: the heteroatom lone pair is in the pi system.
Monosubstituted benzene: name the substituent + "benzene" (e.g. chlorobenzene, nitrobenzene). Some have retained common names: toluene (methylbenzene), aniline (aminobenzene), phenol (hydroxybenzene), anisole (methoxybenzene).
Disubstituted benzene: use ortho (1,2-), meta (1,3-), or para (1,4-) prefixes, or number the positions.
The phenyl group (C6H5-) is benzene as a substituent. The benzyl group is C6H5CH2-.
General mechanism (two steps):
Step 1: The electrophile (E+) attacks the pi system, forming the arenium ion (a non-aromatic carbocation intermediate).
Step 2: A proton is lost from the carbon bearing the electrophile, restoring aromaticity.
Why substitution and not addition? Substitution restores the aromatic pi system. Addition would leave a non-aromatic product, sacrificing ~150 kJ/mol of resonance energy.
Common EAS reactions (details covered more fully in CHEM 2302):
Halogenation: Br2/FeBr3 or Cl2/AlCl3
Nitration: HNO3/H2SO4
Sulfonation: SO3/H2SO4
Friedel-Crafts alkylation: RCl/AlCl3
Friedel-Crafts acylation: RCOCl/AlCl3
Note: Sections 12.12-12.15 are excluded from this course.
Huckel's rule
Aromatic if 4n + 2 pi electrons (n = 0, 1, 2, 3...) Gives: 2, 6, 10, 14 pi electrons
Antiaromatic if 4n pi electrons Gives: 4, 8, 12 pi electrons
Resonance energy of benzene
~150 kJ/mol (36 kcal/mol)
Pi electron counts for common aromatic species
Benzene: 6 (three double bonds)
Cyclopentadienyl anion: 6 (two double bonds + lone pair)
Tropylium cation: 6 (three double bonds, empty p orbital does not add electrons)
Pyridine: 6 (three double bonds; N lone pair is not in the pi system)
Pyrrole: 6 (two double bonds + N lone pair in p orbital)
The Diels-Alder reaction is one of the most powerful tools in synthetic chemistry. It forms two new C-C bonds and up to four stereocentres in a single step and is used in the synthesis of steroids, terpenes, and many pharmaceutical compounds. The 1950 Nobel Prize in Chemistry was awarded to Otto Diels and Kurt Alder for this reaction.
Aromatic compounds are everywhere: benzene rings appear in amino acids (phenylalanine, tyrosine, tryptophan), in drugs (aspirin, ibuprofen, paracetamol), in polymers (polystyrene, PET), and in dyes and pigments. Understanding aromaticity is essential for biochemistry and medicinal chemistry.
Students often confuse 1,2- and 1,4-addition products. Remember: 1,4-addition leaves a new double bond between the internal carbons. If the product has a double bond that was not present in the starting material at that position, it is a 1,4-product.
A common error with Huckel's rule is counting all electrons on the heteroatom rather than only the ones in the pi system. In pyridine, the nitrogen lone pair is in the plane of the ring (in an sp2 orbital) and does not count. In pyrrole, the lone pair is in a p orbital perpendicular to the ring and does count.
Students sometimes think any cyclic conjugated compound is aromatic. Cyclooctatetraene (8 pi electrons) is not aromatic; it puckers out of planarity to avoid being antiaromatic.
The Diels-Alder reaction requires the diene to be in the s-cis conformation. Dienes locked in the s-trans conformation (e.g. trans,trans-2,4-hexadiene when the single bond cannot rotate to s-cis) will not react.
⚠️ 1,2- vs 1,4-addition and the distinction between kinetic and thermodynamic control is a guaranteed exam topic. Know which conditions (temperature) favour which product.
⚠️ Be able to predict the product of a Diels-Alder reaction given a diene and a dienophile, including stereochemistry (endo rule, retention of dienophile stereochemistry).
⚠️ Huckel's rule problems appear frequently. Practise counting pi electrons for rings with heteroatoms, cations, and anions.
⚠️ Know why benzene undergoes substitution rather than addition. Be able to draw the arenium ion intermediate for EAS.
⚠️ The final exam is comprehensive (Chapters 1-12 and 14). This means SN1/SN2/E1/E2, alkene additions, and all earlier material will be tested again alongside Ch. 11-12 content.
True or False: 1,4-Addition to a conjugated diene is the kinetic product.
Fill in the blank: The Diels-Alder reaction requires the diene to be in the ______ conformation.
True or False: Cyclopentadienyl anion is aromatic.
Fill in the blank: Benzene has ______ pi electrons, satisfying Huckel's rule with n = ______.
True or False: In electrophilic aromatic substitution, the arenium ion intermediate is aromatic.
Answers: 1. False (1,4-addition is the thermodynamic product; 1,2-addition is the kinetic product). 2. S-cis. 3. True (6 pi electrons, cyclic, planar, fully conjugated). 4. 6; n = 1. 5. False (the arenium ion has lost aromaticity; aromaticity is restored when a proton is lost in the second step).
Q: Predict the major product when 1,3-butadiene reacts with HBr at -80 degrees C.
A: At low temperature, kinetic control gives the 1,2-addition product: 3-bromo-1-butene. The proton adds to C1 (Markovnikov), forming the allylic cation. Bromide attacks the more reactive (less delocalised) C2 position faster.
Q: Draw the product of the Diels-Alder reaction between 1,3-butadiene and maleic anhydride.
A: The product is cis-4-cyclohexene-1,2-dicarboxylic anhydride (a bicyclic structure). The two carbonyl-bearing carbons of maleic anhydride are cis in the starting material and remain cis in the product (syn addition). The endo product is the major isomer.
Q: Is the cyclopropenyl cation (C3H3+) aromatic? Explain.
A: Yes. It is cyclic, planar, fully conjugated (three overlapping p orbitals), and has 2 pi electrons (one double bond; the empty p orbital on the cationic carbon contributes no electrons but does contribute an orbital). 4n+2 = 2 when n = 0, so it is aromatic.
Q: Why is pyrrole a much weaker base than pyridine, even though both contain nitrogen?
A: In pyrrole, the nitrogen lone pair is part of the aromatic pi system (6 pi electrons). Protonating this nitrogen would remove the lone pair from the pi system, destroying aromaticity and costing ~150 kJ/mol of stabilisation. In pyridine, the nitrogen lone pair is in an sp2 orbital in the plane of the ring and is not part of the pi system, so protonation does not disturb aromaticity.
Conjugation and aromaticity are the logical extension of the orbital overlap concepts from Chapter 1 and the resonance framework from Chapters 1-5. Allylic cation and radical stability (Ch. 11) builds directly on the carbocation stability ideas from Ch. 6-8. Aromaticity is the foundation for CHEM 2302, where electrophilic aromatic substitution, directing effects, and the chemistry of aromatic side chains are covered in depth.
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