Source: Carey, Organic Chemistry, 11th ed., Chapters 8-10
Tags: addition reactions, alkenes, Markovnikov, anti-Markovnikov, hydration, hydrohalogenation, halogenation, hydroboration, oxidation, epoxidation, ozonolysis, alkynes, terminal alkynes, free radicals, radical chain mechanism, CHEM 2301 UMN
Difficulty: Intermediate to Advanced | Prerequisites: Chapters 1-7 study notes (bonding, stereochemistry, SN1/SN2/E1/E2 mechanisms)
Big picture: Chapter 8 is the core "reaction catalogue" chapter of the course. It covers how alkenes (compounds with C=C double bonds) react with various reagents through addition reactions, each with its own regiochemistry and stereochemistry. Chapter 9 extends this to alkynes (C-C triple bonds), which undergo many of the same addition reactions but can stop at one or two stages. Chapter 10 introduces a fundamentally different type of mechanism, free radical reactions, where single electrons (rather than electron pairs) drive the chemistry. Together, these three chapters complete the major reaction types tested on Exam IV.
Alkenes are electron-rich at the double bond, making them nucleophiles that react with electrophiles. Addition reactions add atoms across the double bond, breaking the pi bond and forming two new sigma bonds. The key questions for each reaction: what adds, which way (Markovnikov or anti-Markovnikov), and what stereochemistry (syn or anti addition). Alkynes react similarly but can undergo one or two additions. Free radical reactions follow an initiation-propagation-termination chain mechanism and show anti-Markovnikov selectivity with HBr.
Alkene
A hydrocarbon containing at least one carbon-carbon double bond. General formula: CnH(2n) for acyclic monoenes. The double bond consists of one sigma bond and one pi bond.
Markovnikov's rule
In the addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon of the double bond that already bears more hydrogens (or equivalently, the X adds to the more substituted carbon). The underlying reason: the reaction proceeds through the more stable carbocation intermediate.
Anti-Markovnikov addition
Addition in which the hydrogen adds to the more substituted carbon and the functional group adds to the less substituted carbon. Occurs under specific conditions: hydroboration-oxidation and radical addition of HBr.
Syn addition
Both new groups add to the same face of the double bond. Hydroboration and catalytic hydrogenation are syn additions.
Anti addition
The two new groups add to opposite faces of the double bond. Halogenation (Br2/Cl2) and halohydrin formation are anti additions.
Electrophilic addition
The general mechanism for alkene reactions with electrophiles. The pi electrons of the alkene attack the electrophile, forming a carbocation (or cyclic intermediate), which is then captured by a nucleophile.
Carbocation intermediate
Formed in many electrophilic additions (e.g. HX addition, acid-catalysed hydration). Stability of the carbocation determines regioselectivity (Markovnikov's rule).
Bromonium ion (halonium ion)
A cyclic, bridged intermediate formed when Br2 (or Cl2) reacts with an alkene. The three-membered ring prevents rotation, forcing the nucleophile to attack from the opposite face (anti addition).
Halohydrin
The product of adding X2 in the presence of water. The halogen and OH end up on adjacent carbons with anti stereochemistry. Follows Markovnikov regiochemistry for the OH (OH goes to the more substituted carbon).
Hydroboration-oxidation
A two-step sequence: (1) addition of BH3 (or a borane reagent) across the double bond in a syn, anti-Markovnikov fashion; (2) oxidation of the C-B bond with H2O2/NaOH to give the alcohol. Net result: anti-Markovnikov, syn addition of water.
Catalytic hydrogenation
Addition of H2 across the double bond using a metal catalyst (Pt, Pd, or Ni). Syn addition. Converts the alkene to an alkane.
Epoxidation
Reaction of an alkene with a peroxyacid (mCPBA is common) to form an epoxide (a three-membered ring containing oxygen). Syn addition of the oxygen to the double bond.
Ozonolysis
Cleavage of the double bond using ozone (O3), followed by a reductive workup (typically Zn/H2O or dimethyl sulfide). The double bond is broken completely, producing two carbonyl compounds (aldehydes and/or ketones). Useful for determining the position of the double bond.
Dihydroxylation
Addition of two OH groups across a double bond. Osmium tetroxide (OsO4) gives syn dihydroxylation. Peroxyacid followed by acid-catalysed ring opening of the epoxide gives anti dihydroxylation.
Reagents: HCl, HBr, HI
Regiochemistry: Markovnikov (H to less substituted carbon, X to more substituted carbon)
Mechanism: protonation of the alkene to form the more stable carbocation, then halide attack
Stereochemistry: not controlled (both syn and anti products form, because the carbocation is planar)
Carbocation rearrangements are possible if a more stable cation is accessible by a 1,2-shift
Reagents: H3O+ (dilute H2SO4 or H3PO4 in water)
Regiochemistry: Markovnikov (OH on the more substituted carbon)
Mechanism: protonation of the alkene to form a carbocation, then water attacks the carbocation, then deprotonation gives the alcohol
This is the reverse of acid-catalysed dehydration (E1 elimination of alcohols)
Rearrangements possible
Reagents: Br2 or Cl2 (in an inert solvent like CH2Cl2)
Regiochemistry: both halogens add to the double bond
Mechanism: the alkene attacks Br2 to form a cyclic bromonium ion; bromide ion then opens the ring from the back side
Stereochemistry: anti addition. If the alkene is cyclic, the two bromine atoms end up trans to each other
Reagents: Br2 (or Cl2) in water
Regiochemistry: Markovnikov for the OH (OH on the more substituted carbon); halogen on the less substituted carbon
Mechanism: bromonium ion forms first, then water (the more abundant nucleophile) opens the ring at the more substituted position
Stereochemistry: anti (water attacks from the opposite side of the bromonium ion)
Reagents: (1) BH3 or BH3-THF; (2) H2O2, NaOH
Regiochemistry: anti-Markovnikov (OH ends up on the less substituted carbon)
Mechanism: BH3 adds across the double bond in a concerted, four-centre transition state. The boron goes to the less substituted carbon (steric control). Oxidation then replaces B with OH, retaining the stereochemistry.
Stereochemistry: syn addition
No carbocation intermediate, so no rearrangements
Reagents: H2, Pt (or Pd/C, or Ni)
Both H atoms add to the same face of the double bond (syn addition)
Converts alkene to alkane; removes all degrees of unsaturation from the double bond
Heat of hydrogenation can be used to compare alkene stabilities: more substituted alkenes are more stable (release less heat upon hydrogenation)
Reagent: mCPBA (meta-chloroperoxybenzoic acid) or another peroxyacid
Produces an epoxide (oxirane) with retention of the alkene's geometry (syn addition of the oxygen)
The epoxide can be opened under acidic or basic conditions to give a 1,2-diol (anti opening) or other products
Reagents: (1) O3; (2) Zn/H2O or (CH3)2S (reductive workup)
Breaks the double bond completely. Each carbon of the former double bond becomes a carbonyl:
A carbon with one H gives an aldehyde
A carbon with no H (disubstituted) gives a ketone
Useful for determining the location of a double bond in an unknown: identify the two carbonyl fragments, then mentally join them back together to find the original alkene
HX: Markovnikov, no stereochemical control
H2O / H+: Markovnikov, no stereochemical control
X2: anti addition
X2 / H2O: anti addition, Markovnikov for OH
BH3 then H2O2/NaOH: anti-Markovnikov, syn addition
H2 / Pt: syn addition
mCPBA: syn epoxidation
OsO4: syn dihydroxylation
O3 then Zn/H2O: cleavage to carbonyls
Alkyne
A hydrocarbon containing a carbon-carbon triple bond. General formula for acyclic monoalkynes: CnH(2n-2). The triple bond consists of one sigma bond and two pi bonds. The carbons are sp-hybridised (linear, 180 degree bond angles).
Terminal alkyne
An alkyne with the triple bond at the end of the chain (the triple-bond carbon bears a hydrogen). The terminal C-H is mildly acidic (pKa ~25), much more acidic than a typical C-H.
Internal alkyne
An alkyne with the triple bond between two carbon atoms, neither of which bears a hydrogen.
Acetylide anion
The conjugate base of a terminal alkyne, formed by treatment with a strong base such as NaNH2 (sodium amide) or n-BuLi. The acetylide is a good nucleophile and can be used in SN2 reactions to build larger carbon chains.
Terminal alkyne C-H: pKa ~25. Compare: alkane C-H ~50, alkene C-H ~44.
The acidity comes from the sp hybridisation: the more s-character in the orbital, the closer the electrons are held to the carbon nucleus, stabilising the resulting anion.
Bases strong enough to deprotonate a terminal alkyne: NaNH2 (pKa of NH3 ~38), NaH, n-BuLi.
NaOH and NaOEt are not strong enough (their conjugate acids have pKa ~16-18).
Acetylide anions (RC-C:-) can perform SN2 reactions on primary alkyl halides and methyl halides. This forms a new C-C bond and extends the carbon chain.
Example: sodium acetylide + 1-bromobutane gives 1-hexyne.
This is an important method for building larger carbon skeletons.
Does not work with secondary or tertiary substrates (elimination dominates).
Alkynes undergo many of the same electrophilic addition reactions as alkenes, but can react once (stopping at the alkene stage) or twice (going to the saturated product).
Addition of one equivalent of HX gives a vinyl halide (Markovnikov). Two equivalents give a geminal dihalide.
Addition of one equivalent of X2 gives a trans-dihaloalkene (anti addition). Two equivalents give a tetrahalide.
Catalytic hydrogenation with H2/Pt goes all the way to the alkane (two equivalents of H2 add).
Lindlar's catalyst (poisoned Pd) or dissolving metal reduction (Na/NH3) allow controlled partial reduction:
Lindlar's catalyst: syn addition of H2, giving the cis-alkene
Na/NH3: anti addition, giving the trans-alkene
Hydroboration of alkynes (using a bulky borane like disiamylborane) followed by oxidation gives an aldehyde (from terminal alkynes) or a ketone (from internal alkynes) via an enol intermediate.
Acid-catalysed hydration of alkynes (in the presence of HgSO4 catalyst) follows Markovnikov's rule, giving a ketone (from terminal alkynes) via an enol intermediate.
The enol form (C=C-OH) and the keto form (C-C=O) are tautomers: structural isomers that interconvert rapidly.
The keto form is almost always more stable and predominates at equilibrium.
Understanding tautomerism explains why hydration and hydroboration of alkynes give carbonyl products, not enols.
Free radical
A species with an unpaired electron. Radicals are extremely reactive and short-lived. Carbon radicals are classified as primary, secondary, or tertiary, with stability increasing in that order (same trend as carbocations, but less pronounced).
Homolytic cleavage (homolysis)
Bond breaking in which each atom retains one electron from the shared pair, producing two radicals. Shown with fishhook (single-barbed) arrows.
Bond dissociation energy (BDE)
The energy required to break a bond by homolysis. Higher BDE = stronger bond. BDE values are used to predict whether a radical reaction is exothermic or endothermic at each step.
Radical chain mechanism
A mechanism consisting of three phases: initiation (radicals are generated, typically by heat or light breaking a weak bond), propagation (radicals react with stable molecules to produce new radicals, sustaining the chain), and termination (two radicals combine, ending the chain).
Radical halogenation
The substitution of a hydrogen atom by a halogen atom via a radical chain mechanism. Chlorination is less selective; bromination is highly selective for the weakest (most substituted) C-H bond.
3 degree > 2 degree > 1 degree > methyl (same order as carbocations, but the energy differences are smaller)
Allylic and benzylic radicals are stabilised by resonance delocalisation
Initiation: X2 breaks apart under heat or UV light to give two X radicals. (X-X -> 2 X.)
Propagation (two steps):
Step 1: X. abstracts a hydrogen from the alkane, giving HX and an alkyl radical. (R-H + X. -> R. + HX)
Step 2: The alkyl radical reacts with X2, giving R-X and regenerating X. (R. + X2 -> R-X + X.)
The two propagation steps repeat, sustaining the chain.
Termination: any two radicals combine (X. + X. -> X2, R. + R. -> R-R, R. + X. -> R-X), ending the chain.
Chlorination is relatively unselective: it will abstract primary, secondary, and tertiary hydrogens with modest preference for weaker C-H bonds. Product mixtures are common.
Bromination is highly selective: it strongly prefers to abstract the weakest C-H bond (tertiary > secondary >> primary). A tertiary hydrogen is replaced roughly 1600 times faster than a primary hydrogen by bromine.
The reason: the transition state for hydrogen abstraction by Br. is late (product-like), so the stability of the radical being formed matters more. For Cl., the transition state is early (reactant-like), so radical stability matters less.
Reagents: HBr + peroxides (ROOR) or UV light
Only HBr undergoes this reaction (not HCl or HI).
Mechanism:
Initiation: peroxide decomposes to give RO. radicals; RO. abstracts H from HBr to give Br.
Propagation: Br. adds to the less substituted end of the alkene (giving the more stable carbon radical at the more substituted position). The carbon radical then abstracts H from HBr, giving the anti-Markovnikov product and regenerating Br.
Regiochemistry: anti-Markovnikov (Br on the less substituted carbon)
Stereochemistry: not controlled (mixture of syn and anti)
N-Bromosuccinimide (NBS) with light or peroxides selectively brominates the allylic position (the carbon adjacent to a double bond).
The allylic radical is resonance-stabilised, which lowers the activation energy for its formation.
The double bond is not consumed in this reaction; only the allylic C-H is replaced.
Heat of hydrogenation trends
More substituted alkenes have lower heats of hydrogenation (more stable): tetrasubstituted < trisubstituted < disubstituted < monosubstituted
(Lower value = more stable alkene)
Keto-enol tautomerism
R-C(=O)-CH2-R' <-> R-C(-OH)=CH-R'
The keto form predominates at equilibrium.
Relative reactivity of C-H bonds toward Br.
3 degree H : 2 degree H : 1 degree H is approximately 1600 : 82 : 1
The anti-Markovnikov hydroboration-oxidation is used in the synthesis of many pharmaceutical intermediates when the alcohol must be placed on the less substituted carbon. Catalytic hydrogenation is used industrially to convert vegetable oils (which contain C=C double bonds) into solid fats (margarine). Ozonolysis is a standard analytical technique for locating double bonds in natural products. Radical halogenation is used industrially for producing chlorinated solvents, though its poor selectivity means it works best on substrates where only one type of hydrogen is present.
Students often forget that Markovnikov's rule is about carbocation stability, not about "hydrogen goes to the carbon with more hydrogens." The hydrogen-counting shortcut works, but understanding the carbocation logic is essential for cases involving rearrangements.
A frequent mistake is assuming all addition reactions of HBr are anti-Markovnikov. Anti-Markovnikov addition requires radical conditions (peroxides or UV light). Without peroxides, HBr addition follows the normal Markovnikov pathway through a carbocation.
Students sometimes confuse syn and anti addition. A useful mnemonic: reactions that go through cyclic intermediates (bromonium ions, concerted hydroboration, OsO4) tend to be stereospecific. Reactions that go through open carbocations lose stereochemical control.
In alkyne chemistry, students forget that hydration of a terminal alkyne with HgSO4/H2SO4 gives a methyl ketone (Markovnikov), while hydroboration-oxidation of a terminal alkyne gives an aldehyde (anti-Markovnikov).
⚠️ You need to know the reagents, regiochemistry, and stereochemistry for every major alkene addition reaction listed above. This is memorisation, but understanding the mechanisms makes the patterns stick.
⚠️ The contrast between Markovnikov and anti-Markovnikov is tested heavily. Know which reagent combinations give which regiochemistry.
⚠️ Syn vs anti addition matters for cyclic alkenes, where the stereochemical outcome produces cis or trans products. Draw the 3D picture.
⚠️ Ozonolysis "working backwards" problems are classic: given the carbonyl fragments, deduce the original alkene. Practise these.
⚠️ For radical reactions, be able to draw a full initiation-propagation-termination mechanism with fishhook arrows. Know that only HBr (not HCl or HI) gives anti-Markovnikov radical addition.
⚠️ Bromination selectivity is a favourite calculation question. Given a substrate, count the different types of hydrogen, apply the relative reactivity factors, and predict the product distribution.
True or False: Hydroboration-oxidation of an alkene gives a Markovnikov alcohol.
Fill in the blank: The addition of Br2 to an alkene proceeds through a cyclic ______ intermediate, giving ______ addition stereochemistry.
True or False: Treatment of a terminal alkyne with NaOH in water will deprotonate the terminal hydrogen.
Fill in the blank: Radical bromination is more selective than radical chlorination because the transition state for hydrogen abstraction by Br. is ______ (early/late).
True or False: Lindlar's catalyst reduces an alkyne to a trans-alkene.
Answers: 1. False (hydroboration-oxidation gives anti-Markovnikov). 2. Bromonium ion; anti. 3. False (NaOH is not a strong enough base; you need NaNH2 or similar). 4. Late. 5. False (Lindlar's gives the cis-alkene; Na/NH3 gives the trans-alkene).
Q: Predict the product when 1-methylcyclohexene reacts with Br2 in CH2Cl2.
A: Anti addition of Br2 across the double bond. The product is trans-1,2-dibromo-1-methylcyclohexane. Both bromines add to opposite faces of the ring.
Q: An unknown alkene undergoes ozonolysis followed by reductive workup to give acetone (propan-2-one) and formaldehyde (methanal). What is the alkene?
A: Rejoin the carbonyl carbons with a double bond. Acetone contributes (CH3)2C= and formaldehyde contributes =CH2. The alkene is 2-methylpropene (isobutylene).
Q: Show how to convert 1-butyne into butanal (butyraldehyde) in one reaction sequence.
A: Hydroboration of 1-butyne with a bulky borane (e.g. disiamylborane, Sia2BH) followed by oxidation with H2O2/NaOH. The anti-Markovnikov addition places the oxygen on C1, giving, after tautomerism, the aldehyde butanal.
Q: Explain why radical addition of HCl to propene does not give anti-Markovnikov product, even in the presence of peroxides.
A: The propagation step in which Cl. adds to the alkene is endothermic (the C-Cl bond formed is not strong enough to offset the pi bond broken). Because this step is thermodynamically unfavourable, the radical chain does not sustain itself. Only HBr has the right energetics for a viable radical chain addition.
Alkene addition reactions are the reverse of elimination reactions (Ch. 7), and many problems require you to work in both directions. Alkynes connect to alkene chemistry because partial reduction of an alkyne is a common way to make a specific stereoisomer of an alkene (cis via Lindlar's, trans via Na/NH3). Radical mechanisms in Ch. 10 connect to conjugated systems (Ch. 11), where allylic radicals and allylic cations share the same resonance stabilisation logic.
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