Alkene Addition Reactions: Complete Study Guide

Overview — The Five Reactions at a Glance

All five reactions are electrophilic additions to the alkene π bond. The table below summarizes the key outcomes — refer back to it as you work through each section.

Reaction	Reagent	Product	Regiochem.	Stereochem.	Rearrange?
Hydrohalogenation	HX	Alkyl halide	Markovnikov	Racemization	Yes
Halogenation	Br₂ / Cl₂	Vicinal dihalide	Anti-Markov. (X)*	Anti	No
Acid Hydration	H₂O / H₂SO₄	Alcohol	Markovnikov	Racemization	Yes
Oxymercuration	Hg(OAc)₂/H₂O; NaBH₄	Alcohol	Markovnikov	Mixture†	No
Hydroboration-Ox.	BH₃; H₂O₂/NaOH	Alcohol	Anti-Markovnikov	Syn	No

*Markovnikov OH regiochemistry applies in halohydrin formation. †NaBH₄ step scrambles the original anti stereochemistry, giving a mixture.

Hydrohalogenation adds a hydrogen halide (HX) across the double bond to give an alkyl halide. It is the entry point to carbocation chemistry and Markovnikov's rule.

Feature	Detail
Reagent	HCl, HBr, or HI (reactivity: HI > HBr > HCl)
Product	Alkyl halide
Regiochemistry	Markovnikov — H to less-substituted, X to more-substituted carbon
Stereochemistry	Racemization at any new chiral center
Rearrangements	Yes — always check

Mechanism

Step 1 — Protonation of the Alkene (Rate-Determining)

The π electrons attack the electrophilic H of HX. A C–H bond forms at the less-substituted carbon; X⁻ departs; a carbocation forms at the more-substituted carbon. This is the slow, rate-limiting step.

Carbocation Type	Relative Stability
Tertiary (3°)	Most stable — hyperconjugation from 3 alkyl groups
Secondary (2°)	Moderate stability
Primary (1°)	Unstable — avoid when possible
Methyl	Least stable

Step 2 — Nucleophilic Attack by Halide

X⁻ attacks the planar (sp²) carbocation from either face — if a new chiral center is created, a racemic mixture results.

Carbocation Rearrangements

Always check whether the initial carbocation can rearrange to a more stable one before the halide attacks:

• Hydride shift (1,2-H shift): H migrates with its bonding electrons to an adjacent carbocation.
• Alkyl shift (1,2-alkyl shift): An entire alkyl group migrates, sometimes changing the carbon skeleton.

Treatment of an alkene with Br₂ or Cl₂ gives a vicinal dihalide via a cyclic halonium ion — the defining example of anti addition in alkene chemistry.

Feature	Detail
Reagents	Br₂ or Cl₂ in CH₂Cl₂ or CCl₄ (F₂ too reactive; I₂ unfavorable)
Product	Vicinal dihalide (or halohydrin if water present)
Intermediate	Cyclic bromonium/chloronium ion — NOT a flat carbocation
Stereochemistry	Anti addition exclusively — no exceptions
Rearrangements	No

Mechanism

Step 1 — Bromonium Ion Formation

The π electrons attack one Br of Br₂, polarizing the Br–Br bond. One Br bridges both alkene carbons simultaneously, forming a strained three-membered cyclic bromonium ion while Br⁻ departs.

Step 2 — Backside Attack by Bromide

Br⁻ attacks from the backside (opposite the bridging Br), analogous to SN2 inversion. Both halogens end up on opposite faces: anti addition.

Stereospecificity: E vs Z Alkenes

Starting Alkene	Product after Anti Addition
(E)-2-butene (trans)	Meso compound: (2R,3S)-2,3-dibromobutane only
(Z)-2-butene (cis)	Racemic mixture: (2R,3R) + (2S,3S) dibromobutane (50:50)

Halohydrin Formation (X₂ + H₂O)

When Br₂ is added in water, water attacks the bromonium ion as the nucleophile. OH installs on the more-substituted carbon (Markovnikov-like); Br on the less-substituted. Stereochemistry is still anti. No rearrangements.

Acid-catalyzed hydration adds water across the double bond using H₂SO₄ as catalyst to give the Markovnikov alcohol. It is the reverse of acid-catalyzed dehydration.

Feature	Detail
Reagents	H₂O (excess) + dilute H₂SO₄
Product	Alcohol — OH to more-substituted carbon
Regiochemistry	Markovnikov
Stereochemistry	Racemization (planar carbocation)
Rearrangements	Yes — always check
Reversible?	Yes — reverse is acid-catalyzed dehydration

Three-Step Mechanism

Step 1 (slow, rate-determining): H₃O⁺ protonates the π bond, placing H on the less-substituted carbon and generating the most stable carbocation at the more-substituted carbon.

Step 2 (fast): Water attacks the planar carbocation with a lone pair, forming an oxonium ion (R–OH₂⁺). Attack from either face → racemic mixture.

Step 3 (fast): A second water molecule deprotonates the oxonium ion, regenerating H₃O⁺ (true catalyst) and yielding the neutral alcohol.

Equilibrium: Hydration vs Dehydration

Desired Product	Conditions to Favor (Le Chatelier)
Alcohol (hydration)	Excess water, lower temperature, dilute acid
Alkene (dehydration)	Concentrated acid, high temperature, remove water

Oxymercuration-demercuration is a two-step method for Markovnikov hydration without rearrangements — the go-to method when a clean, predictable alcohol is needed from a substrate prone to carbocation shifts.

Feature	Detail
Step 1 reagents	Hg(OAc)₂ / H₂O
Step 2 reagent	NaBH₄ (replaces Hg with H)
Product	Markovnikov alcohol — OH to more-substituted carbon
Rearrangements	NO — most important advantage
Conditions	Mild — no strong acid required

Mechanism

Step 1a — Mercurinium Ion Formation

Mercury (δ+) in Hg(OAc)₂ is the electrophile. The π electrons attack Hg, forming a three-membered cyclic mercurinium ion (analogous to bromonium) while OAc⁻ departs.

Step 1b — Nucleophilic Attack by Water

Water attacks the more-substituted carbon of the mercurinium ion from the backside (anti), installing –OH on the more-substituted carbon.

Step 2 — Demercuration (NaBH₄)

NaBH₄ reduces the C–Hg bond, replacing mercury with hydrogen. This step is not stereospecific, giving a mixture at the C–H carbon. Net result: Markovnikov addition of H and OH, no rearrangements.

Acid Hydration vs. Oxymercuration — Key Differences

Feature	Acid Hydration	Oxymercuration
Regiochemistry	Markovnikov	Markovnikov
Intermediate	Discrete carbocation	Mercurinium ion
Rearrangements?	Yes	No
Conditions	Strong acid	Mild, Hg(OAc)₂
Reversible?	Yes	No

Useful Variant: Alkoxymercuration

Replace water with an alcohol (ROH) as solvent. ROH acts as the nucleophile, installing –OR on the more-substituted carbon. NaBH₄ reduction gives the Markovnikov ether — with no rearrangements.

Developed by Herbert C. Brown (Nobel Prize, 1979), hydroboration-oxidation is the premier method for anti-Markovnikov, syn-selective addition of water to an alkene — the direct complement to acid hydration and oxymercuration.

Feature	Detail
Step 1 reagent	BH₃·THF (or 9-BBN for enhanced selectivity)
Step 2 reagents	H₂O₂ / NaOH
Product	Anti-Markovnikov alcohol — OH to less-substituted carbon
Regiochemistry	Anti-Markovnikov
Stereochemistry	Syn addition — H and OH to the same face
Rearrangements	Never — no carbocation intermediate

Step 1 — Hydroboration (Concerted Syn Addition)

Boron's empty p-orbital (Lewis acid) interacts with the alkene π electrons in a four-membered cyclic transition state where B and H add simultaneously to the same face. There is no intermediate — this is a concerted process.

Why Anti-Markovnikov?

• Electronic: Boron (δ+) goes to the less-substituted carbon, which better accommodates a partial positive charge — opposite of Markovnikov carbocation chemistry.
• Steric: Boron's substituents are bulky and prefer the less hindered carbon.

Step 2 — Oxidation (H₂O₂ / NaOH)

H₂O₂ in basic solution replaces the C–B bond with C–OH with complete retention of configuration. Because hydroboration was syn and oxidation retains, the overall result is syn addition of H and OH across the double bond.

Three-Way Comparison: Routes to Alcohol from Alkene

Feature	Acid Hydration	Oxymercuration	Hydroboration-Ox.
Regiochemistry	Markovnikov	Markovnikov	Anti-Markovnikov
Stereochemistry	Racemization	Mixture	Syn (stereospecific)
Rearrangements?	Yes	No	Never
Intermediate	Carbocation	Mercurinium ion	Concerted cyclic TS

Master Comparison — All Five Reactions

	Hydrohalog.	Halogenation	Acid Hydration	Oxymercuration	Hydroboration-Ox.
Key reagent	HCl, HBr, HI	Br₂ / Cl₂	H₂SO₄ / H₂O	Hg(OAc)₂; NaBH₄	BH₃; H₂O₂/NaOH
Product	Alkyl halide	Dihalide / halohydrin	Alcohol	Alcohol	Alcohol
Intermediate	Carbocation	Halonium ion	Carbocation + oxonium	Mercurinium ion	Concerted 4-membered TS
Regiochem.	Markovnikov	Anti-Markov. (X)*	Markovnikov	Markovnikov	Anti-Markovnikov
Stereochem.	Racemization	Anti	Racemization	Mixture	Syn
Rearrangements?	Yes	No	Yes	No	No
Reversible?	No	No	Yes	No	No

Choosing Your Reaction

What do you need?	Use this reaction
Alkyl halide (Markovnikov)	Hydrohalogenation (HX)
Vicinal dihalide (anti addition)	Halogenation (Br₂ or Cl₂)
Markovnikov alcohol; rearrangement acceptable	Acid-catalyzed hydration (H₂SO₄/H₂O)
Markovnikov alcohol; NO rearrangement	Oxymercuration (Hg(OAc)₂/H₂O then NaBH₄)
Anti-Markovnikov alcohol; syn stereochemistry	Hydroboration-oxidation (BH₃ then H₂O₂/NaOH)