Difficulty: Intermediate | Prerequisites: CHEM 225 Exam 1-2 material (alkene structure, stereochemistry, carbocation stability). Nucleophilic substitution notes (SN1/SN2) are helpful for understanding competing pathways.
Alkenes are among the most versatile functional groups in organic chemistry. The carbon-carbon double bond is electron-rich, making it a target for electrophilic addition reactions: acids, halogens, water, boranes, and oxidising agents all react with alkenes in predictable ways. This set of reactions forms a major portion of CHEM 225 and is heavily tested. You should already be comfortable drawing alkene structures, assigning E/Z stereochemistry, and understanding carbocation stability before diving in.
Alkene addition reactions follow a pattern: something adds across the double bond, and the regiochemistry (which atom goes where) and stereochemistry (syn vs. anti addition) depend on the reagent. Markovnikov addition puts the hydrogen on the less substituted carbon; anti-Markovnikov does the reverse. Knowing which reagent gives which outcome is the core skill tested here.
Heat of hydrogenation
The energy released when an alkene is reduced to an alkane by adding H₂ across the double bond. A higher heat of hydrogenation means the alkene is less stable. Think of it as: the more energy released, the more the molecule "wanted" to lose its double bond, meaning it was less happy as an alkene.
Markovnikov addition
In the addition of HX or H₂O to an alkene, the hydrogen attaches to the less substituted carbon (the one with more H atoms already), and the X or OH attaches to the more substituted carbon. In simple terms: "the rich get richer" with hydrogen.
Anti-Markovnikov addition
The opposite regiochemistry: the hydrogen ends up on the more substituted carbon, and the functional group (typically OH from hydroboration-oxidation) lands on the less substituted carbon.
Syn addition
Both new groups add to the same face of the double bond. Examples: hydroboration, catalytic hydrogenation, hydroxylation (with OsO₄ or KMnO₄), cyclopropanation (with Simmons-Smith).
Anti addition
The two new groups add to opposite faces of the double bond. Examples: bromination (Br₂), halohydrin formation.
Hydroboration-oxidation
A two-step sequence: (1) BH₃/THF adds borane across the double bond in a syn, anti-Markovnikov fashion; (2) H₂O₂/HO⁻ oxidises the C-B bond to a C-OH bond. Net result: anti-Markovnikov, syn addition of water.
Oxymercuration-demercuration
A two-step sequence: (1) Hg(OAc)₂/H₂O/THF forms a mercurinium ion that opens with Markovnikov regiochemistry; (2) NaBH₄/HO⁻ removes the mercury. Net result: Markovnikov addition of water with no rearrangement.
Hydroxylation (dihydroxylation)
Addition of two OH groups across the double bond. OsO₄ or cold, dilute KMnO₄ gives syn-dihydroxylation (both OH groups on the same face).
Bromination
Br₂ adds across the double bond through a bromonium ion intermediate, giving anti addition of two bromine atoms.
Cyclopropanation
Formation of a cyclopropane ring from an alkene, typically using the Simmons-Smith reaction (CH₂I₂/Zn-Cu). This is a syn addition: the CH₂ is delivered to one face of the double bond.
Epoxidation
Conversion of an alkene to an epoxide (a three-membered ring containing oxygen). Typically done with a peroxyacid (mCPBA). The oxygen is delivered in a syn fashion to one face of the double bond.
The heat of hydrogenation is a direct measure of alkene stability. Less stable alkenes release more energy upon hydrogenation.
Stability order for alkenes:
More substituted alkenes are more stable (tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted)
Trans (E) alkenes are more stable than cis (Z) alkenes of the same substitution level, due to reduced steric strain
Conjugated alkenes are more stable than isolated alkenes
Consequently, the least substituted alkene has the highest heat of hydrogenation. On the exam, a terminal, monosubstituted alkene releases the most energy upon hydrogenation because it is the least stabilised by hyperconjugation and inductive effects.
More substituted alkenes react faster with electrophiles (such as H₃O⁺ in acid-catalysed hydration) because the more substituted carbocation intermediate is more stable. A trisubstituted alkene reacts faster than a disubstituted one, which reacts faster than a monosubstituted one.
The exam tests this: among several alkenes reacting with H₂O under acid conditions, the most substituted alkene reacts fastest because it forms the most stable carbocation.
Protonation of the alkene forms a carbocation (Markovnikov regiochemistry: proton adds to the less substituted carbon)
Water attacks the carbocation
Result: Markovnikov addition of OH
Downside: carbocation rearrangements can occur
Step 1: Hg(OAc)₂ / H₂O / THF forms a mercurinium ion (cyclic, like a bromonium ion)
Step 2: NaBH₄ / HO⁻ replaces the mercury with hydrogen
Result: Markovnikov addition of OH, no rearrangement
The mercurinium ion prevents rearrangement because no free carbocation forms
Step 1: BH₃/THF adds across the double bond in a concerted, syn fashion. Boron goes to the less substituted carbon (anti-Markovnikov), hydrogen to the more substituted carbon.
Step 2: H₂O₂ / HO⁻ replaces the B with OH, retaining the stereochemistry
Result: anti-Markovnikov, syn addition of OH
The exam question asks which reagent set places OH on the less substituted carbon of a terminal alkene (anti-Markovnikov product). The answer is hydroboration-oxidation: 1. BH₃/THF; 2. H₂O₂, HO⁻.
H₂O/H₃O⁺ and oxymercuration both give Markovnikov products (OH on the more substituted carbon), so neither would produce the anti-Markovnikov alcohol shown.
In a syn addition, both new groups are delivered to the same face of the double bond.
Hydroboration: BH₃ adds B and H to the same face (syn). After oxidation, gives syn addition of OH and H.
Catalytic hydrogenation: H₂ with Pd, Pt, or Ni delivers both hydrogens to the same face (syn).
Hydroxylation (dihydroxylation): OsO₄ (or cold, dilute KMnO₄) adds two OH groups to the same face (syn). This produces a syn-1,2-diol. The stereochemistry of the starting alkene determines the diol's stereochemistry.
Cyclopropanation (Simmons-Smith): CH₂I₂/Zn(Cu) delivers a CH₂ unit to one face of the alkene (syn), forming a cyclopropane ring.
Bromination: Br₂ adds through a bromonium ion intermediate. The second Br⁻ attacks from the opposite face, giving anti addition of two bromines.
Halohydrin formation: Br₂/H₂O gives anti addition of Br and OH (the bromonium ion opens by backside attack of water).
The exam asks directly: which of the listed reactions is not a syn addition? The answer is bromination, which proceeds through a bromonium ion and gives anti addition.
Because hydroxylation is a syn addition, the geometry of the starting alkene controls the stereochemistry of the diol product.
E-2-hexene gives a syn diol where the two OH groups are on the same face, producing one pair of enantiomers
Z-2-hexene gives a different diol stereochemistry
The exam presents a specific diol and asks which alkene produced it. To work backwards: draw the diol, mentally remove both OH groups and restore the double bond, and determine which E/Z isomer would place both OH groups on the same face to give the observed stereochemistry.
The exam asks which reagents convert cyclohexene to cyclohexene oxide (an epoxide). The route: (1) Br₂/H₂O gives a bromohydrin (anti addition of Br and OH); (2) NaOH performs an intramolecular SN2, where the alkoxide displaces bromide to close the three-membered epoxide ring. Answer: 1. Br₂/H₂O; 2. NaOH.
The exam presents starting materials and target molecules and asks you to propose reagents. The approach:
Compare the starting material and product. What has changed? (A leaving group replaced by a functional group? A double bond added? An OH or OMe group appeared?)
Identify the type of reaction needed (substitution, elimination, addition).
Consider stereochemistry. Has configuration been retained or inverted? Is the addition syn or anti?
Choose reagents that give the correct regiochemistry and stereochemistry.
Secondary alkyl chloride to methyl ether (retention of configuration at the adjacent centre):
NaOCH₃ in a polar aprotic solvent (acetone or DMF) performs an SN2. If inversion is shown at the electrophilic carbon, SN2 is confirmed. If the carbon bearing Cl is secondary but not too hindered, SN2 can still proceed.
Alkyl chloride to methyl ether (with a different connectivity in the product, implying an E2 then addition):
Sometimes the target has a different carbon framework or an ether oxygen in a position that requires elimination to form an alkene, followed by addition. Look carefully at whether the product's skeleton matches the starting material.
Cyclopentyl bromide (with defined stereochemistry) to cyclopentanol (with retention of configuration):
Retention of configuration = two inversions. One approach: SN2 with a carboxylate nucleophile (inversion), then hydrolysis (retention). Alternatively, if the stereochemistry works out: an SN1 or a double-inversion pathway.
Cyclohexane to a 1,3-diene (elimination product):
Multiple eliminations may be needed. Strong base, high temperature, and a substrate with appropriate leaving groups.
Reagent | Regiochemistry | Stereochemistry | Product |
|---|---|---|---|
H₂O / H₃O⁺ | Markovnikov | Not controlled (via carbocation) | Alcohol |
Hg(OAc)₂/H₂O/THF, then NaBH₄/HO⁻ | Markovnikov | Not controlled | Alcohol (no rearrangement) |
BH₃/THF, then H₂O₂/HO⁻ | Anti-Markovnikov | Syn | Alcohol |
Br₂ | N/A | Anti | Vicinal dibromide |
Br₂ / H₂O | Markovnikov (OH on more substituted C) | Anti | Bromohydrin |
OsO₄ or cold KMnO₄ | N/A | Syn | 1,2-diol |
mCPBA | N/A | Syn | Epoxide |
CH₂I₂ / Zn(Cu) | N/A | Syn | Cyclopropane |
H₂ / Pd (or Pt, Ni) | N/A | Syn | Alkane |
HBr | Markovnikov | Not controlled (via carbocation) | Alkyl bromide |
HBr / ROOR (peroxides) | Anti-Markovnikov | Not controlled | Alkyl bromide |
Students often assume that all addition reactions to alkenes are syn additions. They are not. Bromination and halohydrin formation proceed through cyclic intermediates (bromonium ions) and give anti addition.
Students confuse hydroboration-oxidation with oxymercuration-demercuration. Both add water across a double bond, but they give opposite regiochemistry: hydroboration gives anti-Markovnikov, oxymercuration gives Markovnikov.
Students think the most stable alkene has the highest heat of hydrogenation. The relationship is inverse: the most stable alkene has the lowest heat of hydrogenation (it releases less energy because it was already in a lower energy state).
Students forget that acid-catalysed hydration can cause carbocation rearrangements. If you need a Markovnikov alcohol without rearrangement, use oxymercuration-demercuration instead.
⚠️ Heat of hydrogenation is tested as a ranking question. Remember: least stable = highest heat of hydrogenation.
⚠️ Identifying whether a reaction is syn or anti addition is a standalone question type. Memorise which reactions fall in each category.
⚠️ Reagent selection for a given transformation ("which reagent set gives this product?") appears repeatedly. The regiochemistry (Markovnikov vs. anti-Markovnikov) and the stereochemistry (syn vs. anti) narrow the choice.
⚠️ Working backwards from a diol to the starting alkene (hydroxylation stereochemistry) is tested. Syn addition + a specific diol stereochemistry constrains the starting alkene's E/Z geometry.
⚠️ Synthesis problems (Problem III) require you to propose reagents. You must combine what you know about substitution, elimination, and addition in sequence.
True or False: A trisubstituted alkene has a higher heat of hydrogenation than a monosubstituted alkene. (False: the trisubstituted alkene is more stable, so it has a lower heat of hydrogenation.)
Fill in the blank: Hydroboration-oxidation gives ______ regiochemistry and ______ stereochemistry. (Anti-Markovnikov; syn.)
True or False: Bromination of an alkene is a syn addition. (False: bromination proceeds through a bromonium ion and gives anti addition.)
Fill in the blank: To get a Markovnikov alcohol without carbocation rearrangement, use ______. (Oxymercuration-demercuration: Hg(OAc)₂/H₂O/THF, then NaBH₄/HO⁻.)
True or False: OsO₄ hydroxylation of E-2-butene and Z-2-butene gives the same diol. (False: syn addition to different alkene geometries gives different diol stereochemistries.)
Q: Among a terminal monosubstituted alkene, a disubstituted alkene, a trisubstituted alkene, and a tetrasubstituted alkene, which has the highest heat of hydrogenation?
A: The terminal monosubstituted alkene. It is the least stable, so it releases the most energy upon hydrogenation.
Q: Which reagent set converts an alkene to an anti-Markovnikov alcohol with syn stereochemistry?
A: 1. BH₃/THF; 2. H₂O₂, HO⁻ (hydroboration-oxidation).
Q: Which of the following is not a syn addition: hydroboration, bromination, cyclopropanation, hydroxylation?
A: Bromination. It proceeds through a bromonium ion intermediate and gives anti addition.
Q: You need to convert cyclohexene to cyclohexene oxide (an epoxide). Which two-step reagent sequence works?
A: 1. Br₂/H₂O (forms the bromohydrin by anti addition); 2. NaOH (intramolecular SN2 closes the epoxide ring).
Q: Syn hydroxylation of E-2-hexene produces a specific diol. Would Z-2-hexene give the same diol?
A: No. Because hydroxylation is a syn addition, different alkene geometries produce diols with different stereochemistries.
Q: An alkene reacts with H₂O under acid conditions. Among a monosubstituted, disubstituted, trisubstituted, and tetrasubstituted alkene, which reacts fastest?
A: The tetrasubstituted alkene (or whichever is most substituted). The more substituted the alkene, the more stable the carbocation intermediate, and the faster the reaction proceeds.
Alkene addition chemistry connects backwards to elimination reactions (E1/E2 produce the alkenes you then functionalise) and forwards to alcohol chemistry, epoxide ring-opening reactions, and eventually carbonyl chemistry in the next unit. The stereochemical principles here (syn vs. anti, Markovnikov vs. anti-Markovnikov) recur throughout the rest of organic chemistry. Understanding carbocation stability, first introduced in acid-base and SN1/E1 material, is essential for predicting regiochemistry in electrophilic additions.
Alkene reactions, addition reactions, heat of hydrogenation, Markovnikov rule, anti-Markovnikov, syn addition, anti addition, hydroboration-oxidation, BH₃/THF, oxymercuration-demercuration, Hg(OAc)₂, acid-catalysed hydration, bromination, bromonium ion, halohydrin, hydroxylation, dihydroxylation, OsO₄, KMnO₄, epoxidation, mCPBA, cyclopropanation, Simmons-Smith, CH₂I₂, Zaitsev rule, electrophilic addition, carbocation stability, alkene stability, catalytic hydrogenation, retrosynthesis, CHEM 225, organic chemistry II, University of Michigan