Difficulty: Intermediate to Advanced | Prerequisites: Ch. 1–8 (bonding, stereochemistry, substitution, elimination, alkene additions)
These chapters round out the semester. Alkynes (Ch. 9) extend the electrophilic addition logic of alkenes to triple bonds. Free radicals (Ch. 10) introduce a completely different reaction mechanism (chain reactions with initiation, propagation, termination). Conjugated dienes (Ch. 11) show what happens when double bonds interact with each other. Arenes and aromaticity (Ch. 12) introduce the most stable class of unsaturated compounds and explain why benzene does substitution rather than addition.
Alkyne
A hydrocarbon containing a carbon-carbon triple bond. General formula CnH2n-2. Terminal alkynes have a hydrogen on the triple-bond carbon; internal alkynes do not.
Acetylide anion
The conjugate base of a terminal alkyne (RC triple bond C-), formed by deprotonation with a strong base (NaNH2, NaH, or n-BuLi). A useful nucleophile in SN2 reactions for building longer carbon chains.
Free radical
A species with an unpaired electron. Radicals are typically highly reactive and short-lived. Radical stability follows the same order as carbocations: tertiary > secondary > primary > methyl.
Radical chain mechanism
A three-phase mechanism: initiation (radicals are generated, usually by heat or light breaking a weak bond), propagation (radicals react and regenerate, carrying the chain), and termination (two radicals combine, ending the chain).
Allylic position
The carbon adjacent to a carbon-carbon double bond. Allylic radicals and allylic carbocations are stabilised by resonance (the unpaired electron or empty orbital is delocalised across the pi system).
Conjugated diene
A molecule with two double bonds separated by exactly one single bond (e.g. 1,3-butadiene). The pi electrons are delocalised across all four carbons.
1,2-addition vs. 1,4-addition
Two modes of addition to a conjugated diene. 1,2-addition places both groups on adjacent carbons. 1,4-addition places groups on the terminal carbons, with the double bond shifting to the centre. 1,2-addition is the kinetic product; 1,4-addition is usually the thermodynamic product.
Diels-Alder reaction
A concerted [4+2] cycloaddition between a conjugated diene (4 pi electrons) and a dienophile (2 pi electrons, usually an alkene) to form a six-membered ring. One of the most important ring-forming reactions in organic chemistry.
Arene (aromatic compound)
A compound containing a benzene ring or a ring system that obeys Huckel's rule. Benzene is the parent arene.
Aromaticity
The special stability of cyclic, planar, fully conjugated molecules with 4n+2 pi electrons (Huckel's rule, where n = 0, 1, 2, 3...). Aromatic compounds are far more stable than expected from their structure.
Huckel's rule
A molecule is aromatic if it is cyclic, planar, fully conjugated, and has 4n+2 pi electrons. Anti-aromatic compounds are cyclic, planar, fully conjugated, but have 4n pi electrons and are unusually unstable.
Alkynes contain a carbon-carbon triple bond: one sigma bond and two pi bonds. The triple-bond carbons are sp-hybridised (linear, 180 degree bond angles).
IUPAC naming: longest chain containing the triple bond, change -ane to -yne, number from the end nearest the triple bond. When both double and triple bonds are present, use -en-yne and give the lower number to whichever comes first (double bond gets priority only if there is a tie).
Terminal alkynes (RC triple bond CH) have a pKa of about 25, making them enormously more acidic than alkenes (pKa about 44) or alkanes (pKa about 50).
The increased acidity comes from the sp hybridisation of the C-H bond: the 50% s character holds the electrons closer to the carbon, stabilising the conjugate base (the acetylide anion).
Deprotonation with NaNH2 (sodium amide), NaH, or n-BuLi gives the acetylide anion, RC triple bond C-.
Addition of HX: one equivalent gives a vinyl halide (Markovnikov); two equivalents give a geminal dihalide. Addition follows Markovnikov's rule at each step.
Addition of X2: one equivalent gives a trans-dihaloalkene (anti addition); two equivalents give a tetrahalide.
Hydration:
Acid-catalysed (H2SO4, HgSO4): Markovnikov addition of water to give an enol, which tautomerises to a ketone (keto-enol tautomerism). Terminal alkynes give methyl ketones.
Hydroboration-oxidation (disiamylborane, then H2O2/NaOH): anti-Markovnikov addition. Terminal alkynes give aldehydes.
Catalytic hydrogenation: H2/Pt reduces the triple bond all the way to an alkane. To stop at the cis-alkene, use Lindlar's catalyst (Pd on CaCO3, poisoned with lead acetate and quinoline). To get the trans-alkene, use dissolving metal reduction (Na or Li in liquid NH3).
Acetylide anions as nucleophiles: RC triple bond C- can do SN2 reactions on primary alkyl halides and methyl halides. This is a key carbon-carbon bond-forming reaction.
A free radical has an unpaired electron. Carbon radicals are sp2-hybridised (planar), with the unpaired electron in a p orbital.
Radical stability: tertiary > secondary > primary > methyl (same order as carbocations, for similar reasons: hyperconjugation stabilises the radical).
Allylic and benzylic radicals are especially stable due to resonance delocalisation of the unpaired electron.
Three phases:
Initiation: a radical is generated, typically by homolytic cleavage of a weak bond (e.g. Br2 with heat or light, or a peroxide ROOR).
Propagation: two or more steps that consume a radical and produce a new one, carrying the chain. The product is formed during propagation.
Termination: two radicals combine to form a covalent bond, ending the chain.
Chlorination: not very selective; gives a mixture of products. Every hydrogen in the molecule is roughly equally likely to be replaced, with a slight preference for tertiary over secondary over primary.
Bromination: highly selective for the weakest C-H bond (tertiary > secondary >> primary). Bromine radicals are less reactive and therefore more selective (reactivity-selectivity principle).
Selectivity ratios (relative reactivity per hydrogen): for bromination, tertiary : secondary : primary is approximately 1600 : 82 : 1.
In the presence of peroxides (ROOR), HBr adds to alkenes in anti-Markovnikov fashion.
Mechanism: the peroxide generates bromine radicals, which add to the less substituted carbon of the double bond (forming the more stable radical on the more substituted carbon). Then H-Br provides a hydrogen to complete the addition.
This works only with HBr, not HCl or HI. HCl radical addition is endothermic in one propagation step (unfavourable). HI has too weak a bond for the chain to propagate.
Conjugated dienes: double bonds separated by one single bond (1,3-butadiene). Extra stability from pi-electron delocalisation.
Isolated dienes: double bonds separated by two or more single bonds. No interaction between the pi systems; each behaves independently.
Cumulated dienes (allenes): double bonds share a carbon (C=C=C). Unusual geometry; the two end groups are perpendicular to each other.
When one equivalent of HBr adds to 1,3-butadiene, two products form:
1,2-addition product: both H and Br on adjacent carbons (3-bromo-1-butene). Kinetic product (forms faster).
1,4-addition product: H and Br on the terminal carbons, with the double bond in the centre (1-bromo-2-butene). Thermodynamic product (more stable, favoured at higher temperature or longer reaction time).
The intermediate allylic carbocation can be attacked at either end, which is why both products form.
A [4+2] cycloaddition: a conjugated diene (4 pi electrons) reacts with a dienophile (2 pi electrons, typically an alkene with electron-withdrawing groups) to form a cyclohexene ring.
Concerted, one-step mechanism: all bond-making and bond-breaking happen simultaneously. No intermediates.
The diene must be in the s-cis conformation (both double bonds on the same side of the single bond) to react. s-trans dienes cannot undergo Diels-Alder.
Stereochemistry: syn addition on both components. Substituents that are cis on the dienophile remain cis in the product. The endo rule: when there is a choice, the kinetic product usually has the endo orientation (substituents on the dienophile point toward the diene pi system in the transition state).
Electron demand: the reaction is accelerated by electron-donating groups on the diene and electron-withdrawing groups on the dienophile (normal electron demand). Inverse electron demand (the reverse) also works but is less common at this level.
Huckel's rule: a compound is aromatic if it is cyclic, planar, fully conjugated (every atom in the ring has a p orbital contributing to the pi system), and has 4n+2 pi electrons (n = 0, 1, 2, 3...).
Benzene (6 pi electrons, n = 1) is the simplest and most important aromatic compound.
Anti-aromatic: meets all criteria but has 4n pi electrons. Cyclobutadiene (4 pi electrons) is the classic example. Anti-aromatic compounds are less stable than the corresponding open-chain conjugated system.
Non-aromatic: fails one or more criteria (not cyclic, not planar, not fully conjugated). Simply a normal compound with no special stability or instability.
Monosubstituted benzenes: many have common (retained) names: toluene (methylbenzene), aniline (aminobenzene), phenol (hydroxybenzene), anisole (methoxybenzene), styrene (vinylbenzene), benzaldehyde, benzoic acid, acetophenone.
Disubstituted benzenes: use ortho (1,2-), meta (1,3-), para (1,4-) prefixes.
The phenyl group (C6H5-) is benzene as a substituent. The benzyl group is C6H5CH2-.
Benzene's heat of hydrogenation is about 150 kJ/mol less than expected for a hypothetical "cyclohexatriene" with three isolated double bonds. This difference is the resonance energy (or stabilisation energy) of benzene.
All C-C bonds in benzene are the same length (1.40 angstroms), intermediate between a single bond (1.54) and a double bond (1.34).
Benzene does not undergo the typical addition reactions of alkenes. Instead, it undergoes substitution reactions that preserve the aromatic ring.
Electrophilic aromatic substitution (EAS) is the characteristic reaction of benzene. An electrophile replaces one hydrogen on the ring. The mechanism has two steps: (1) the electrophile attacks the pi system to form an arenium ion (also called a sigma complex or Wheland intermediate); (2) a proton is lost to restore aromaticity.
Common EAS reactions: halogenation (Br2/FeBr3), nitration (HNO3/H2SO4), sulfonation (SO3/H2SO4), Friedel-Crafts alkylation (RCl/AlCl3), and Friedel-Crafts acylation (RCOCl/AlCl3).
These reactions are covered in greater depth in CHEM 2302, but the basics are introduced here.
Electron-donating groups (EDGs): activate the ring toward further EAS and direct incoming electrophiles to the ortho and para positions. Examples: -OH, -OR, -NH2, -NHR, -NR2, alkyl groups.
Electron-withdrawing groups (EWGs): deactivate the ring and direct to the meta position. Examples: -NO2, -CN, -COR, -COOR, -SO3H.
Halogens are the exception: they are deactivating (by induction) but ortho/para-directing (by resonance donation of lone pairs).
Huckel's rule
4n+2 pi electrons = aromatic (n = 0, 1, 2, 3...)
So: 2, 6, 10, 14... pi electrons are aromatic. 4, 8, 12... pi electrons in a cyclic, planar, fully conjugated system are anti-aromatic.
Alkyne partial reduction summary
Triple bond to cis-alkene: H2 / Lindlar's catalyst (Pd/CaCO3, poisoned)
Triple bond to trans-alkene: Na or Li in liquid NH3 (dissolving metal reduction)
Triple bond to alkane: H2 / Pt (excess)
Alkyne hydration summary
Markovnikov (gives ketone from internal alkyne, methyl ketone from terminal): H2O, H2SO4, HgSO4
Anti-Markovnikov (gives aldehyde from terminal alkyne): (1) disiamylborane or 9-BBN, (2) H2O2/NaOH
Radical halogenation selectivity
Chlorination: low selectivity (tertiary : secondary : primary per H is roughly 5 : 4 : 1)
Bromination: high selectivity (tertiary : secondary : primary per H is roughly 1600 : 82 : 1)
Diels-Alder requirements checklist
Conjugated diene in s-cis conformation
Dienophile (electron-poor alkene works best)
Heat (thermal reaction, no catalyst needed)
Syn addition on both components
Endo product is the kinetic product
Directing effects summary
Ortho/para directors (activating): -OH, -OR, -NH2, -NHR, -NR2, -NHCOR, alkyl groups
Ortho/para directors (deactivating): halogens (-F, -Cl, -Br, -I)
Meta directors (deactivating): -NO2, -CN, -SO3H, -COR, -COOR, -COOH, -CHO
The Diels-Alder reaction is one of the most widely used reactions in pharmaceutical synthesis and materials science. It builds six-membered rings in a single, predictable step with excellent control over stereochemistry. Nobel Prizes were awarded for its discovery (Diels and Alder, 1950) and for further development of asymmetric variants.
Aromatic compounds are everywhere: benzene rings form the backbone of most pharmaceuticals, dyes, polymers (polystyrene, polyester, Kevlar), and explosives (TNT). Understanding how substituents direct electrophilic attack is essential for designing synthetic routes to these compounds.
Free-radical chemistry underpins the degradation of polymers, the spoilage of food (lipid peroxidation), and the action of antioxidants like vitamin E, which work by trapping radicals and breaking the chain.
Students often assume that any cyclic compound with double bonds is aromatic. It is not. The compound must be cyclic, planar, fully conjugated, and have 4n+2 pi electrons. Cyclooctatetraene (8 pi electrons) is not aromatic; it is tub-shaped and non-planar specifically to avoid anti-aromaticity.
A persistent error is thinking that Lindlar's catalyst gives the trans-alkene. Lindlar's gives cis (syn addition from the catalyst surface). Dissolving metal reduction (Na/NH3) gives trans.
Students frequently forget that the Diels-Alder diene must be in the s-cis conformation. If the diene is locked in s-trans (as in some rigid bicyclic systems), the reaction cannot occur.
When counting pi electrons for Huckel's rule, students sometimes count sigma-bond electrons or include electrons that are not part of the conjugated pi system. Only electrons in p orbitals that are part of the continuous ring of conjugation count. Lone pairs on nitrogen or oxygen count only when they are in a p orbital aligned with the ring's pi system.
⚠️ Alkyne reactions often appear as multi-step synthesis problems: "Starting from acetylene and any reagents, synthesise 2-hexanone." You need to combine acetylide SN2 reactions with hydration. Practise retrosynthetic thinking.
⚠️ Radical halogenation selectivity is a classic calculation question: given the structure, predict the ratio of monobromination products using the per-hydrogen selectivity factors.
⚠️ The Diels-Alder reaction is heavily tested. Be able to identify the diene and dienophile, predict the product (including regiochemistry and stereochemistry), and work backwards from a product to determine the starting materials.
⚠️ Huckel's rule questions often present unusual-looking ring systems and ask you to determine whether they are aromatic, anti-aromatic, or non-aromatic. Count pi electrons carefully and check all four criteria.
⚠️ Directing effects and activating/deactivating groups are foundational for CHEM 2302 but are introduced here and may appear on the final exam.
True or false: A terminal alkyne is more acidic than an alkene because of the greater s character in the sp C-H bond. (True)
Fill in the blank: Lindlar's catalyst reduces a triple bond to a ______ alkene. (cis)
True or false: In radical bromination, a primary hydrogen is just as likely to be abstracted as a tertiary hydrogen. (False: bromine is highly selective for tertiary hydrogens.)
Fill in the blank: The Diels-Alder reaction requires the diene to be in the ______ conformation. (s-cis)
True or false: Cyclopentadienyl anion (5-membered ring, 6 pi electrons) is aromatic. (True: it is cyclic, planar, fully conjugated, and has 4n+2 = 6 pi electrons with n = 1.)
Q: Propose a two-step synthesis of 1-hexyne from acetylene and 1-bromobutane.
A: Step 1: Deprotonate acetylene with NaNH2 to form the acetylide anion (HC triple bond C-). Step 2: The acetylide anion attacks 1-bromobutane via SN2, forming 1-hexyne (CH3CH2CH2CH2C triple bond CH). This works because 1-bromobutane is a primary substrate, ideal for SN2.
Q: What products would you expect from the monobromination of 2-methylbutane using Br2 and light? Which is the major product?
A: 2-Methylbutane has one tertiary hydrogen (on C2), two types of secondary hydrogens, and two types of primary hydrogens. Because bromine is highly selective for tertiary C-H bonds, the major product is 2-bromo-2-methylbutane (substitution at the tertiary position), formed in large excess over all other monobrominated products.
Q: Draw the product of the Diels-Alder reaction between 1,3-butadiene and maleic anhydride (a cyclic dienophile with two C=O groups cis to each other).
A: The product is a bicyclic compound: a cyclohexene ring fused to the anhydride ring. The two C=O groups of the anhydride, which were cis in the dienophile, remain cis in the product (syn addition). The endo product (C=O groups pointing toward the larger bridge) is the kinetic product.
Q: Is pyridine (a six-membered ring with one nitrogen replacing a CH) aromatic? Explain.
A: Yes. Pyridine is cyclic, planar, and fully conjugated. The nitrogen contributes one electron to the pi system via its p orbital (the lone pair is in an sp2 orbital in the plane of the ring, not part of the pi system). Total pi electrons: 6 (4n+2, n = 1). Pyridine is aromatic.
Q: Explain why the 1,4-addition product of HBr to 1,3-butadiene is favoured at high temperature, while the 1,2-product is favoured at low temperature.
A: The 1,2-product forms faster (lower activation energy) and is the kinetic product, favoured at low temperature where the reaction is irreversible. The 1,4-product is thermodynamically more stable (more substituted double bond), so at higher temperature, where the reaction becomes reversible and equilibrium is reached, the 1,4-product accumulates.
Alkyne chemistry extends the electrophilic addition patterns you learned with alkenes (Ch. 8). Every alkene reaction has an alkyne analogue, and the question is always whether the reaction stops at one addition or proceeds to two. Thinking of alkynes as "double alkenes" is a useful simplification.
Free-radical reactions introduce a third mechanistic category (alongside ionic substitution/elimination and polar addition). Recognising whether a reaction is ionic or radical is the first step in predicting its behaviour. If peroxides, heat, or light are mentioned in the conditions, think radicals.
The Diels-Alder reaction connects forward to CHEM 2302, where pericyclic reactions and orbital symmetry (Woodward-Hoffmann rules) are discussed in more detail. The endo rule and stereochemical control of Diels-Alder products are directly useful in synthesis planning.
Aromaticity is the gateway to all of aromatic chemistry in CHEM 2302: electrophilic aromatic substitution, nucleophilic aromatic substitution, and the chemistry of heterocyclic aromatics (pyridine, pyrrole, furan, thiophene) all depend on understanding why aromatic rings behave so differently from ordinary alkenes.
alkyne, triple bond, sp hybridisation, terminal alkyne, internal alkyne, acetylide anion, NaNH2, sodium amide, acidity of alkynes, Lindlar's catalyst, dissolving metal reduction, Na NH3, cis alkene, trans alkene, keto-enol tautomerism, enol, HgSO4, hydroboration of alkynes, disiamylborane, free radical, radical, homolytic cleavage, initiation propagation termination, radical chain, radical halogenation, chlorination, bromination, selectivity, reactivity-selectivity principle, anti-Markovnikov HBr, peroxide effect, allylic radical, allylic carbocation, resonance stabilisation, conjugated diene, 1 3-butadiene, isolated diene, cumulated diene, allene, 1 2-addition, 1 4-addition, kinetic product, thermodynamic product, Diels-Alder reaction, cycloaddition, 4+2, diene, dienophile, s-cis, s-trans, endo rule, exo, arene, aromatic, aromaticity, benzene, Huckel's rule, 4n+2, anti-aromatic, 4n, cyclopentadienyl, pyridine, pyrrole, phenol, aniline, toluene, electrophilic aromatic substitution, EAS, arenium ion, Wheland intermediate, ortho para meta, directing effects, activating deactivating, Friedel-Crafts, Carey organic chemistry chapter 9 10 11 12, CHEM 2301 UMN