Difficulty: Intermediate | Prerequisites: Lewis structures, bond polarity, acid-base concepts, basic alkene structure (Chapter 5-6 notes recommended).
Electrophilic addition is the signature reaction of alkenes. The carbon-carbon double bond is electron-rich (a nucleophile), so it attacks electrophiles such as H-Br, Br2 and H3O+. Every reaction in this set follows the same logic: the pi bond breaks, two new sigma bonds form, and the electrophile ends up on one carbon while the nucleophile ends up on the other. Understanding which atom lands where (regioselectivity) and which face of the molecule it approaches from (stereochemistry) is the core skill this unit tests.
Alkenes react with hydrogen halides, water, halogens and hypohalous acids by adding those reagents across the double bond. Markovnikov's rule predicts where the H and the electrophile land. Halogenation goes through a bromonium (or chloronium) ion intermediate, which forces anti addition and controls stereochemistry.
Electrophile
A species that accepts an electron pair. In simple terms, it is the "electron-poor" reagent that is attracted to the pi bond.
Nucleophile
A species that donates an electron pair. The alkene's pi bond acts as the nucleophile in electrophilic addition.
Markovnikov's rule
In the addition of H-X across a double bond, the hydrogen adds to the carbon that already bears more hydrogens, and X adds to the more substituted carbon. Think of it as: the rich get richer, the carbon with more H's gets another one.
Carbocation
A positively charged carbon intermediate. Stability order: tertiary > secondary > primary > methyl. The reaction proceeds through the more stable carbocation, which is why Markovnikov's rule works.
Bromonium ion (halonium ion)
A three-membered ring intermediate formed when Br2 adds to an alkene. The bromine bridges both carbons of the former double bond, preventing free rotation and forcing anti addition.
Vicinal dihalide
A compound with two halogen atoms on adjacent (neighbouring) carbons. This is the product of halogenation of an alkene with Br2 or Cl2.
Halohydrin
A compound with a halogen and an -OH group on adjacent carbons. Formed by treating an alkene with HOBr or HOCl (or Br2/Cl2 in water).
Regioselectivity
The preference for a reaction to form one constitutional isomer over another. Markovnikov vs anti-Markovnikov addition is a question of regioselectivity.
Anti addition
Addition where the two groups attach from opposite faces of the double bond. Halogenation gives anti addition because of the halonium ion intermediate.
Reagents: HBr or HCl
Product: alkyl halide
Follows Markovnikov's rule: H adds to the less substituted carbon, halide to the more substituted carbon
Mechanism: two steps
Step 1: Pi electrons of the alkene attack the H of H-X, forming a carbocation on the more substituted carbon
Step 2: Halide ion (nucleophile) attacks the carbocation
The reaction proceeds through the most stable carbocation intermediate
Reagents: H2SO4, H2O (acid-catalysed)
Product: alcohol
Follows Markovnikov's rule: OH ends up on the more substituted carbon
Mechanism:
Step 1: Protonation of the alkene by H-OSO3H forms a carbocation
Step 2: Water (nucleophile) attacks the carbocation
Step 3: Deprotonation by a base (water or bisulfate) gives the alcohol
Carbocation rearrangements (hydride and methyl shifts) are possible if they lead to a more stable cation
Reagents: Br2 or Cl2
Product: vicinal dihalide (dibromoalkane or dichloroalkane)
Stereochemistry: anti addition only
Mechanism:
Step 1: Pi electrons attack the polarised Br-Br bond, forming a bromonium ion (three-membered ring with Br bridging both carbons)
Step 2: Bromide ion (the nucleophile) attacks from the opposite face of the bromonium ion (backside attack)
The bromonium ion is the reason anti addition occurs, not a regular carbocation
Result is a trans product (the two Br atoms are on opposite faces)
Reagents: Br2 or Cl2 in water (generates HOBr or HOCl in situ)
Product: halohydrin (OH and Br on adjacent carbons)
Proceeds through a bromonium ion intermediate, just like halogenation
Water acts as the nucleophile instead of the halide ion
Regioselectivity: OH adds to the more substituted carbon, Br to the less substituted carbon
Stereochemistry: anti addition (same reasoning as halogenation)
Reaction | Reagents | Product | Regiochemistry | Stereochemistry | Key Intermediate |
|---|---|---|---|---|---|
Hydrohalogenation | HBr or HCl | Alkyl halide | Markovnikov | Not controlled | Carbocation |
Hydration | H2SO4, H2O | Alcohol | Markovnikov | Not controlled | Carbocation |
Halogenation | Br2 or Cl2 | Vicinal dihalide | N/A (symmetric reagent) | Anti addition | Bromonium/chloronium ion |
Halohydrin formation | Br2/Cl2 in H2O | Halohydrin | OH on more substituted C | Anti addition | Bromonium/chloronium ion |
Hydration of alkenes is the industrial route to simple alcohols such as ethanol (from ethylene) and isopropanol (from propene). Halogenation is used as a classical test for unsaturation: if a solution of Br2 in CH2Cl2 decolourises when mixed with a compound, a double bond is present.
Students often think Markovnikov's rule is about the halide going to the carbon with more hydrogens. It is the opposite: the H goes to the carbon with more H's, the halide to the more substituted carbon.
Students sometimes draw a standard carbocation intermediate for halogenation with Br2. Halogenation goes through a bromonium ion, not an open carbocation, which is why it gives anti addition rather than a mixture of syn and anti.
Confusing halohydrin formation with simple halogenation. The difference is the solvent: Br2 in an inert solvent gives a dihalide; Br2 in water gives a halohydrin, because water outcompetes Br- as the nucleophile.
Forgetting that carbocation rearrangements can occur in hydrohalogenation and hydration (any reaction with a carbocation intermediate) but cannot occur in halogenation (no open carbocation).
⚠️ Predicting products from reagents and predicting reagents from products are the two most common question formats.
⚠️ Stereochemistry of halogenation (anti addition via bromonium ion) is heavily tested. Be ready to draw the bromonium ion intermediate and show why the product is trans.
⚠️ Markovnikov vs anti-Markovnikov regiochemistry is the central comparison for the entire unit. Know which reagent set gives which outcome.
⚠️ Mechanism questions will ask you to draw curved arrows. Practise the full mechanism for each reaction type.
True or False: In hydrohalogenation of propene with HBr, Br adds to C-1 (the less substituted carbon). (False, Br adds to the more substituted carbon, C-2.)
Fill in the blank: The intermediate in bromination of an alkene with Br2 is called a ________. (Bromonium ion.)
True or False: Acid-catalysed hydration of an alkene gives an anti-Markovnikov alcohol. (False, it gives Markovnikov addition.)
Fill in the blank: Halogenation of alkenes gives ________ addition stereochemistry. (Anti.)
True or False: Halohydrin formation uses the same intermediate as halogenation. (True, both go through a halonium ion.)
Q: What product forms when 2-methylpropene reacts with HBr? Name the rule that predicts the regiochemistry.
A: 2-Bromo-2-methylpropane (tert-butyl bromide). Markovnikov's rule: H goes to the less substituted carbon (C-1), Br to the more substituted carbon (C-2).
Q: Draw the intermediate formed when cyclohexene reacts with Br2. What stereochemistry does this intermediate enforce?
A: A bromonium ion, a three-membered ring with Br bridging carbons 1 and 2. It enforces anti addition, so the product is trans-1,2-dibromocyclohexane.
Q: An alkene is treated with Br2 in water. What is the product, and why does OH end up on the more substituted carbon?
A: A bromohydrin. The bromonium ion forms first; water (the nucleophile) then attacks the more substituted carbon because that carbon bears more positive charge (it can better stabilise partial positive charge), giving OH on the more substituted carbon and Br on the less substituted one.
Q: Explain why carbocation rearrangements are possible in hydration but not in halogenation.
A: Hydration proceeds through a free (open) carbocation, which can rearrange via hydride or methyl shifts to form a more stable cation. Halogenation proceeds through a halonium ion (bridged, cyclic), so no open carbocation ever forms and rearrangement does not occur.
This material connects directly to hydroboration-oxidation (anti-Markovnikov addition), which provides the complementary regiochemistry. If Markovnikov gives an alcohol on the more substituted carbon, hydroboration-oxidation gives it on the less substituted one. Together, the two methods let you place an OH on whichever carbon you need.
Electrophilic addition to alkynes follows the same principles but adds a layer of complexity: alkynes can react with one or two equivalents of reagent, and hydration of alkynes introduces keto-enol tautomerism.
The stereochemistry concepts here (syn vs anti addition) reappear in oxidation reactions such as dihydroxylation with OsO4 (syn) and in reduction with H2/Pd (syn).
Electrophilic addition, alkene reactions, Markovnikov's rule, Markovnikov addition, anti-Markovnikov, hydrohalogenation, HBr addition, HCl addition, acid-catalysed hydration, halogenation, bromination, chlorination, bromonium ion, chloronium ion, halonium ion, halohydrin, bromohydrin, chlorohydrin, anti addition, syn addition, vicinal dihalide, carbocation intermediate, regioselectivity, stereochemistry of addition, CHM 255, organic chemistry reactions, alkene addition reactions