Difficulty: Intermediate-Advanced | Prerequisites: Electrophilic addition to alkenes (first half of Ch. 5), carbocation stability, bond polarity
Alkynes are the triple-bond cousins of alkenes, and many of the same addition reactions apply, but with twists. Alkyne hydration introduces the enol-keto tautomerism concept, which recurs throughout organic chemistry. Free-radical reactions introduce an entirely different mechanism class where the peroxide effect flips the usual regiochemistry of HBr addition. Bond dissociation energy ties the thermodynamics together, letting you calculate whether a reaction is exothermic or endothermic from first principles.
Alkynes undergo acid-catalysed hydration to give ketones (via an enol intermediate). Free-radical addition of HBr reverses the usual Markovnikov selectivity (anti-Markovnikov) because the bromine radical, not a proton, adds first. Bond dissociation energy (ΔH°) tells you the energy cost of breaking a bond homolytically, and the difference between bonds broken and bonds formed determines whether a reaction releases or absorbs energy.
Alkyne
A hydrocarbon containing a carbon-carbon triple bond. The triple bond consists of one sigma bond and two pi bonds, making alkynes even more electron-rich than alkenes.
Enol
A structural isomer in which a hydroxyl group (-OH) is attached to a carbon that is part of a C=C double bond. Enols are usually unstable and tautomerise rapidly to the corresponding ketone or aldehyde.
Think of it as the "halfway house" between the alkyne and the final ketone product.
Keto-enol tautomerism
The equilibrium interconversion between an enol form (OH on a C=C) and a keto form (C=O with adjacent C-H). The keto form is almost always more stable and therefore dominant at equilibrium.
In simple terms, the molecule prefers to have C=O rather than C=C-OH.
Free radical
A species with an unpaired electron. Formed by homolytic cleavage of a bond (each atom gets one electron). Stability order: tertiary > secondary > primary, though the effect of alkyl substitution is less pronounced than for carbocations.
Peroxide effect
The reversal of regioselectivity in HBr addition to alkenes caused by small amounts of peroxides (ROOR) or other radical initiators. Peroxides generate bromine radicals, which add to the less substituted carbon, giving anti-Markovnikov products.
Think of it as the presence of peroxides flipping the normal HBr selectivity.
Bond dissociation energy (BDE)
The energy required to break a covalent bond homolytically (into two radicals). Measured in kJ/mol or kcal/mol. Used to calculate ΔH° of a reaction: ΔH° = Σ(BDE of bonds broken) - Σ(BDE of bonds formed).
Addition polymerisation
The process by which alkene monomers join together by successive addition reactions, forming long-chain polymers (e.g. polyethylene from ethylene). Many industrial polymerisations are free-radical processes.
Reagents: H₂O, H₂SO₄, often with HgSO₄ as catalyst.
The triple bond undergoes Markovnikov addition of water, forming an enol intermediate.
The enol rapidly tautomerises to the more stable ketone. This process is irreversible in practice because ketones are significantly more stable than enols.
For a terminal alkyne (R-C≡CH), the product is a methyl ketone (R-CO-CH₃).
The enol-to-ketone conversion involves migration of a proton from oxygen to carbon, with the double bond shifting from C=C to C=O.
Without peroxides: follows Markovnikov's rule, just like alkene addition. The first equivalent of HBr adds to give a vinyl halide. A second equivalent can add to give a geminal dihalide.
The first addition is fast; the second is slow.
In the presence of peroxides (or light, or AIBN), HBr adds to alkenes with reversed (anti-Markovnikov) regiochemistry.
The mechanism involves three stages:
Initiation: peroxide undergoes homolytic cleavage, generating radicals that abstract H from HBr, producing Br•.
Propagation: Br• adds to the less substituted carbon of the alkene (placing the radical on the more substituted, more stable carbon). Then H is abstracted from another HBr molecule.
Termination: two radicals combine, ending the chain.
The peroxide effect operates only with HBr, not with HCl or HI (for thermodynamic reasons related to bond energies).
The result is a faster reaction with anti-Markovnikov regiochemistry.
Reaction with a pi bond (radical addition)
Atom abstraction (radical pulls an atom, usually H, from another molecule)
Recombination with another radical (the reverse of bond homolysis)
BDE measures the energy to break one specific bond homolytically.
To calculate ΔH° for a reaction:
ΔH° = Σ(BDE of bonds broken) - Σ(BDE of bonds formed)
If ΔH° < 0, the reaction is exothermic (energetically favourable). If ΔH° > 0, the reaction is endothermic.
Alkenes serve as monomers. The double bond opens and each monomer links to the next, forming a long carbon chain.
Many polymerisations (e.g. polyethylene production) proceed by free-radical mechanisms.
Radical initiators start the chain; the growing polymer chain is itself a radical until termination.
Polyethylene, polypropylene, and PVC are all produced by addition polymerisation of alkene monomers, making this one of the most commercially important reaction types in all of chemistry. Free-radical chemistry also underpins processes in atmospheric chemistry (ozone depletion) and biology (oxidative stress and antioxidant action).
Students often think the peroxide effect works with HCl and HI as well as HBr. It does not. The bond energies only make the radical chain thermodynamically favourable for HBr.
A common error is forgetting that alkyne hydration goes through an enol intermediate. Students write the ketone product directly without showing or understanding the tautomerism step.
Students sometimes confuse homolytic cleavage (radicals, one electron each) with heterolytic cleavage (ions, both electrons to one atom). Bond dissociation energy specifically refers to homolytic cleavage.
Some students assume free-radical stability follows exactly the same pattern as carbocation stability. The ordering (tertiary > secondary > primary) is the same, but the effect of substitution on radical stability is considerably smaller than its effect on carbocation stability.
⚠️ Keto-enol tautomerism is tested repeatedly in organic chemistry, not just in this chapter. Understand the mechanism now and it will pay dividends later.
⚠️ The peroxide effect is a favourite exam topic. Be able to draw the full free-radical mechanism (initiation, propagation, termination) and explain why the regiochemistry reverses.
⚠️ Know how to calculate ΔH° from bond dissociation energies. This is a straightforward calculation question that appears often.
⚠️ Be able to classify a reaction mechanism into one of the five types from the chapter summary: carbocation intermediates, cyclic ion intermediates, concerted mechanisms, free-radical intermediates, or catalytic.
Fill in the blank: Acid-catalysed hydration of a terminal alkyne produces a ______ via an enol intermediate.
True or False: The peroxide effect reverses the regiochemistry of HCl addition to alkenes.
Fill in the blank: ΔH° = Σ(BDE bonds ______) - Σ(BDE bonds ______).
True or False: Free-radical stability is affected by alkyl substitution to the same degree as carbocation stability.
True or False: Alkyne hydration is irreversible because the ketone product is much more stable than the enol.
Answers: 1. Methyl ketone. 2. False (only HBr). 3. Broken; formed. 4. False (the effect is considerably smaller for radicals). 5. True.
Q: What is the product of acid-catalysed hydration of 1-hexyne?
A: 2-hexanone (a methyl ketone). Water adds in a Markovnikov fashion to give an enol at C1-C2, which tautomerises to the ketone with the carbonyl at C2.
Q: Draw the mechanism for free-radical addition of HBr to propene in the presence of peroxides. What is the major product?
A: Initiation: peroxide cleaves homolytically, the radical abstracts H from HBr to give Br•. Propagation: Br• adds to C1 (less substituted), placing the radical on C2 (more substituted, more stable). The carbon radical abstracts H from HBr, giving 1-bromopropane (anti-Markovnikov). The major product is 1-bromopropane.
Q: Using BDE values, determine whether the addition of HBr to ethene is exothermic or endothermic. (BDE: C=C pi bond ~264 kJ/mol, H-Br ~366 kJ/mol, C-H ~410 kJ/mol, C-Br ~285 kJ/mol.)
A: Bonds broken: pi bond (264) + H-Br (366) = 630 kJ/mol. Bonds formed: C-H (410) + C-Br (285) = 695 kJ/mol. ΔH° = 630 - 695 = -65 kJ/mol. The reaction is exothermic.
Q: Why does the peroxide effect not operate with HCl or HI?
A: For HCl, the Cl• addition step is endothermic (unfavourable). For HI, the H-abstraction step from HI is endothermic. Only with HBr are both propagation steps exothermic, sustaining the radical chain.
Q: Name the three fundamental reactions of free radicals.
A: (1) Reaction with a pi bond. (2) Atom abstraction. (3) Recombination with another radical.
Keto-enol tautomerism recurs extensively in carbonyl chemistry (Chapters 17-19 and beyond). Understanding it here makes those later chapters significantly easier. Free-radical chemistry connects to atmospheric chemistry, polymer science, and biochemistry (lipid peroxidation, antioxidants).
Bond dissociation energy calculations connect to thermochemistry from general chemistry. The ΔH° approach here is the same one used to evaluate any reaction's energetics from tabulated bond energies.
Alkyne hydration, enol, keto-enol tautomerism, tautomerisation, methyl ketone, free radical, radical chain mechanism, peroxide effect, anti-Markovnikov addition, bond dissociation energy, BDE, ΔH° calculation, homolytic cleavage, radical initiator, AIBN, addition polymerisation, polyethylene, vinyl halide, geminal dihalide, radical stability, organic chemistry chapter 5