Difficulty: Intermediate | Prerequisites: Chapter 5-6 alkene structure, CHEM 211 acid-base concepts, carbocation stability.
Alkene addition reactions are the mirror image of elimination. Where elimination creates a double bond by removing atoms, addition breaks a double bond by adding atoms across it. These reactions are central to organic synthesis because they let you build new C-H, C-X, and C-O bonds in a controlled way.
The key challenge is selectivity: when two different carbons of the double bond could receive the new atom or group, which one wins? The answer depends on the mechanism, specifically whether a carbocation intermediate forms (Markovnikov selectivity) or whether the addition is concerted (anti-Markovnikov selectivity, as in hydroboration-oxidation). You should already understand carbocation stability and be comfortable drawing curved-arrow mechanisms.
Alkenes react with HX, water (with acid catalyst), and borane to add new atoms across the double bond. HX and acid-catalysed hydration go through carbocation intermediates and follow Markovnikov's rule (the H goes to the carbon with more hydrogens). Hydroboration-oxidation is the opposite: it gives anti-Markovnikov, syn addition. Knowing which mechanism operates tells you both where the atoms end up (regiochemistry) and which face they add to (stereochemistry).
Electrophilic addition
The general reaction type where an electrophile (electron-poor species) reacts with the electron-rich pi bond of an alkene. The pi bond acts as the nucleophile. This is the fundamental pattern for all alkene addition reactions covered here.
Markovnikov's rule
In the addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon that already has more hydrogens, and the X (halide, OH) adds to the more substituted carbon. This arises because the reaction proceeds through the more stable (more substituted) carbocation. In simple terms, "the rich get richer" with hydrogens.
Anti-Markovnikov addition
The opposite regiochemistry: X (or OH) ends up on the less substituted carbon. This occurs in hydroboration-oxidation because no carbocation intermediate is involved.
Regioselectivity
Preference for one constitutional isomer over another. In alkene additions, this means which carbon of the double bond gets the H and which gets the X or OH.
Stereoselectivity
Preference for one stereoisomer over another. Syn addition means both new groups add to the same face of the double bond. Anti addition means they add to opposite faces.
Carbocation rearrangement
When a carbocation can become more stable by a 1,2-hydride shift (moving H with its bonding electrons to the adjacent carbon) or a 1,2-methyl shift, it will rearrange. This can change the position of the positive charge and therefore the position of the final product.
Hydroboration-oxidation
A two-step sequence: (1) BH3 (or BH3·THF) adds across the double bond in a single concerted step, placing B on the less substituted carbon and H on the more substituted carbon (anti-Markovnikov, syn addition); (2) treatment with H2O2/NaOH replaces B with OH, retaining the stereochemistry. The net result is anti-Markovnikov addition of water with syn stereochemistry.
Syn addition
Both new atoms or groups add to the same face of the double bond. Hydroboration is a syn addition because both B and H are delivered from the same side in one concerted step.
Anti addition
The two new atoms or groups add to opposite faces of the double bond. This occurs when the mechanism involves a cyclic intermediate that is opened from the back side, as in halogenation (Br2 addition).
Every electrophilic addition to an alkene follows the same basic logic:
The pi bond of the alkene acts as a nucleophile and attacks an electrophile (H⁺ from HX, H⁺ from H3O⁺, or BH3)
This breaks the pi bond and forms a new sigma bond, generating an intermediate (usually a carbocation, except in hydroboration)
A nucleophile attacks the intermediate to complete the addition
The worksheet's five-question framework applies here too. Ask yourself: what is changing (the pi bond is breaking, two new sigma bonds are forming), what are the conditions (acidic means carbocation pathway; BH3 means concerted), what intermediates can form (carbocations follow Markovnikov), what are the reactive sites (the more and less substituted carbons of the alkene), and what mechanism is reasonable.
When HX (HCl, HBr, HI) adds to an alkene, the mechanism is electrophilic addition through a carbocation intermediate. The conditions are acidic, which should immediately signal carbocation chemistry.
Step 1 (proton transfer): The pi bond attacks the H of HX. The proton adds to the less substituted carbon of the double bond (Markovnikov's rule), generating the more stable carbocation on the more substituted carbon.
Step 2 (nucleophilic attack): The halide ion (X⁻) attacks the carbocation to give the product.
Puzzle 2: HCl addition to a vinyl arene
This puzzle shows HCl adding to a styrene-type alkene. The conditions are acidic (HCl is a proton source). In Step 1, the proton adds to the terminal carbon of the double bond. This generates a benzylic carbocation, which is resonance-stabilised (the positive charge is delocalised into the aromatic ring). The worksheet notes this carbocation is "better because its resonance stabilised." Chloride then attacks the benzylic carbon. Because the carbocation is planar, Cl⁻ can attack from either face, giving a racemic mixture.
The regioselectivity here is Markovnikov: the Cl ends up at the benzylic position (the more substituted carbon) because that carbocation is stabilised by resonance with the ring.
Puzzle 7: HBr addition with a hydride shift
This puzzle illustrates what happens when the initial carbocation can rearrange. After HBr protonates the alkene, the initial carbocation is secondary. A 1,2-hydride shift converts it to a more stable tertiary (or otherwise more substituted) carbocation. Bromide then attacks the rearranged carbocation.
The major product comes from the rearranged carbocation. The minor product comes from direct trapping of the initial, unrearranged carbocation. Whenever you see an initial secondary carbocation adjacent to a tertiary centre, expect a hydride shift.
Both reactions add water (H and OH) across a double bond, but they give opposite regiochemistry. This contrast is a favourite exam topic.
Acid-catalysed hydration (Markovnikov)
Conditions: dilute H2SO4 or H3O⁺ in water. The mechanism is electrophilic addition through a carbocation, identical in logic to HX addition. The proton adds to the less substituted carbon (Markovnikov), forming the more stable carbocation. Water then attacks the carbocation as a nucleophile. A final deprotonation gives the alcohol. The OH ends up on the more substituted carbon.
Because a carbocation intermediate is involved, carbocation rearrangements are possible, and the stereochemistry is not controlled (the carbocation is planar, so attack can occur from either face).
Hydroboration-oxidation (anti-Markovnikov, syn)
Conditions: (1) BH3 or BH3·THF, (2) H2O2, NaOH. The first step is concerted (no carbocation): boron and hydrogen add simultaneously across the double bond in a single transition state. Boron, being the electrophilic atom, adds to the less substituted (less sterically hindered) carbon, and hydrogen adds to the more substituted carbon. Because both add in one step from the same face of the alkene, the addition is syn.
In the second step (oxidation), the C-B bond is replaced by a C-OH bond with retention of configuration. The net result: OH on the less substituted carbon (anti-Markovnikov) with syn stereochemistry.
Summary of the contrast
If you need OH on the more substituted carbon: use acid-catalysed hydration. If you need OH on the less substituted carbon: use hydroboration-oxidation. This complementarity is the reason both reactions exist in the synthetic toolkit.
Whenever the mechanism passes through a carbocation, the regiochemistry is determined by which carbocation is more stable. The stability ranking: 3° > 2° > 1° > methyl, with additional stabilisation from resonance (benzylic, allylic).
How to apply this in practice:
Draw both possible carbocations that could form from protonation of the alkene
Identify which is more stable (more substituted, or resonance-stabilised)
The nucleophile attacks that carbocation
Check for possible rearrangements: if the initial carbocation is secondary and a neighbouring carbon is tertiary, a hydride or methyl shift will produce the more stable cation
Puzzle 2 demonstrates this with benzylic stabilisation. The proton adds so that the positive charge lands on the benzylic carbon, which benefits from resonance delocalisation into the aromatic ring. This is more stable than a primary carbocation on the terminal carbon, even though the benzylic carbon is technically secondary.
Puzzle 7 shows that even when the initial protonation gives the Markovnikov carbocation, a rearrangement can shift the position of the charge. The product then reflects the rearranged cation, not the initial one. The minor product comes from the unrearranged pathway.
Reaction | Reagents | Regiochemistry | Stereochemistry | Intermediate |
|---|---|---|---|---|
HX addition | HCl, HBr, or HI | Markovnikov | Not controlled (racemic if stereocentre) | Carbocation |
Acid-catalysed hydration | H2SO4/H2O or H3O⁺ | Markovnikov (OH on more substituted C) | Not controlled | Carbocation |
Hydroboration-oxidation | (1) BH3·THF (2) H2O2/NaOH | Anti-Markovnikov (OH on less substituted C) | Syn addition | None (concerted) |
Halogenation (Br2/Cl2) | Br2 or Cl2 | N/A (symmetrical addition) | Anti addition | Bromonium/chloronium ion |
Students often confuse Markovnikov's rule with "the halide goes to the carbon with more hydrogens." It is the opposite: the hydrogen adds to the carbon with more hydrogens, and the halide (or OH) adds to the more substituted carbon.
Students sometimes assume hydroboration-oxidation gives Markovnikov products because "it adds water." It does not. The concerted mechanism bypasses the carbocation, giving anti-Markovnikov regiochemistry.
Students forget to check for carbocation rearrangements. If your initial carbocation is secondary and there is a tertiary centre next door, a hydride shift is not optional: it will happen, and the product reflects the rearranged cation.
Students sometimes draw the hydroboration step as two separate steps (protonation then nucleophilic attack). It is concerted: B and H add simultaneously, which is why it gives syn addition. Drawing it in two steps leads to the wrong stereochemical prediction.
⚠️ Predicting the product of HX addition to an asymmetric alkene is one of the most common exam questions. You must identify the more stable carbocation to get the regiochemistry right.
⚠️ The contrast between acid-catalysed hydration and hydroboration-oxidation is a guaranteed exam topic. Know both sets of reagents and both outcomes cold.
⚠️ Carbocation rearrangements frequently appear as "trick" questions. If you see a secondary carbocation forming next to a tertiary centre, draw the rearrangement.
⚠️ Stereochemistry of addition (syn vs anti) is often tested alongside regiochemistry. Hydroboration = syn, anti-Markovnikov. Halogenation = anti. HX addition = not stereospecific (racemic).
True or False: Markovnikov's rule states that the hydrogen adds to the carbon with fewer hydrogens.
Fill in the blank: Hydroboration-oxidation gives __________ (Markovnikov/anti-Markovnikov) addition with __________ (syn/anti) stereochemistry.
True or False: A carbocation rearrangement can only occur during SN1 reactions, not during alkene additions.
Fill in the blank: In acid-catalysed hydration, the -OH group ends up on the __________ (more/less) substituted carbon.
True or False: The first step of hydroboration is a concerted process with no carbocation intermediate.
Q: Draw the major product of the reaction of 1-methylcyclohexene with HBr.
A: The proton adds to C-2 (the less substituted carbon of the double bond, which already bears more H atoms). This places the positive charge on C-1, the more substituted (tertiary) carbon. Bromide attacks C-1. The major product is 1-bromo-1-methylcyclohexane (Markovnikov addition).
Q: You need to convert 1-hexene into 1-hexanol (OH on C-1). Which reagent set do you use, and why?
A: Hydroboration-oxidation: (1) BH3·THF, (2) H2O2/NaOH. This gives anti-Markovnikov addition, placing -OH on the less substituted carbon (C-1). Acid-catalysed hydration would place -OH on C-2 (Markovnikov), which is the wrong product.
Q: An alkene reacts with HCl, but the product has the chlorine on a carbon that was not part of the original double bond. What happened?
A: A carbocation rearrangement occurred. The initial protonation formed a less stable carbocation, which rearranged via a 1,2-hydride shift (or methyl shift) to a more stable carbocation. Chloride then attacked the rearranged carbocation, placing Cl on a carbon that was adjacent to, not part of, the original double bond.
Q: Explain why hydroboration gives syn addition while HBr addition does not control stereochemistry.
A: In hydroboration, B and H add in a single concerted step from the same face of the alkene (syn). There is no intermediate, so no opportunity for rotation or attack from the opposite face. In HBr addition, a planar carbocation intermediate forms, and Br⁻ can attack from either face, so stereochemistry is not controlled.
Alkene additions connect directly to elimination reactions: they are the reverse process. Dehydration of an alcohol (E1 under acidic conditions) creates an alkene; hydration of that alkene re-forms the alcohol. Understanding both directions lets you think about synthetic routes forward and backward.
Carbocation rearrangements appear here and in substitution/elimination. The same principles of hydride and methyl shifts apply regardless of whether the carbocation was formed by loss of a leaving group (SN1/E1) or by protonation of a double bond.
The regio- and stereoselectivity concepts from alkene additions reappear in epoxide ring-opening reactions, dihydroxylation, and other transformations later in the course. Building intuition for syn vs anti addition now will pay off repeatedly.
alkene addition, electrophilic addition, Markovnikov rule, Markovnikov's rule, anti-Markovnikov, hydrohalogenation, HBr addition, HCl addition, hydration, acid-catalysed hydration, hydroboration-oxidation, BH3, syn addition, anti addition, regioselectivity, stereoselectivity, carbocation, carbocation rearrangement, 1,2-hydride shift, 1,2-methyl shift, benzylic carbocation, allylic carbocation, resonance stabilisation, pi bond, alkene reactions, CHEM 212, organic chemistry II, predict the products, reaction mechanisms, Chapter 6