Markovnikov's Rule
Markovnikov's Rule states that when a protic acid (HX) adds to an unsymmetrical alkene, the hydrogen atom bonds to the carbon that already bears the greater number of hydrogen atoms, while the halide or nucleophile bonds to the more substituted carbon. This regioselectivity arises because the reaction proceeds through the more stable (more substituted) carbocation intermediate. The rule is foundational for predicting the major product of electrophilic addition reactions in organic synthesis.
Key Formula
CH2=CH2 + HX → CH3CHX (X adds to more substituted carbon)
LaTeX: \text{CH}_2{=}\text{CH}_2 + \text{HX} \rightarrow \text{CH}_3\text{CHX}
| Symbol | Meaning | Unit |
|---|---|---|
| HX | Protic acid (e.g., HBr, HCl, HI) | — |
| X | Halide or nucleophilic group | — |
| H | Proton added to less substituted carbon | — |
Worked Example
Problem
Predict the major product when HBr is added to propene (CH3-CH=CH2) under standard electrophilic addition conditions.
Solution
Step 1: Identify the two carbons of the double bond. C1 (=CH2) has 2 H atoms; C2 (=CH-) has 1 H atom. Step 2: Apply Markovnikov's Rule — H adds to C1 (more H atoms), Br adds to C2 (more substituted). Step 3: The intermediate carbocation at C2 is a secondary carbocation, which is more stable than a primary carbocation at C1. Step 4: Br⁻ attacks the secondary carbocation at C2. Step 5: Major product is 2-bromopropane (CH3-CHBr-CH3).
Answer
2-Bromopropane (CH3CHBrCH3) is the major product.
Markovnikov vs Anti-Markovnikov Addition Outcomes
| Reagent | Alkene | Markovnikov Product | Anti-Markovnikov Product | Condition |
|---|---|---|---|---|
| HBr | Propene | 2-Bromopropane | 1-Bromopropane | Standard (Markovnikov) |
| HCl | But-1-ene | 2-Chlorobutane | 1-Chlorobutane | Standard (Markovnikov) |
| HBr | Propene | 1-Bromopropane | 2-Bromopropane | Peroxide (anti-Markovnikov) |
| H2O (H+) | 2-Methylpropene | 2-Methyl-2-propanol | 2-Methyl-1-propanol | Acid-catalysed (Markovnikov) |
| HI | Styrene | 1-Iodo-1-phenylethane | 1-Iodo-2-phenylethane | Standard (Markovnikov) |
Interactive Tools
Khan Academy — Markovnikov's Rule
Video lesson on Markovnikov's Rule with carbocation stability.
Open ToolChemSpider — 2-Bromopropane
Chemical structure and data for the Markovnikov product of HBr + propene.
Open ToolBrilliant.org — Electrophilic Addition
Deep-dive into electrophilic addition mechanisms and regioselectivity.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Free Radical
A free radical is a highly reactive chemical species containing one or more unpaired electrons in its outermost orbital. Free radicals are typically short-lived and can be derived from atoms, molecules, or ions by homolytic cleavage of a covalent bond. They play key roles in combustion, atmospheric chemistry, polymerisation, and biological oxidative stress, where they can damage DNA, proteins, and lipids.
Condensation Reaction
A condensation reaction is a type of chemical reaction in which two molecules combine to form a larger molecule with the simultaneous loss of a small molecule, most commonly water (H2O) but sometimes methanol, HCl, or ammonia. Condensation reactions are fundamental in the synthesis of polymers, esters, amides, and biological macromolecules including proteins, nucleic acids, and polysaccharides. The reverse process, in which the small molecule is reincorporated to break the bond, is called hydrolysis.
Constitutional Isomer
Constitutional isomers (also called structural isomers) are molecules that share the same molecular formula but differ in the connectivity of their atoms — that is, in which atoms are bonded to which. They are distinct compounds with different physical and chemical properties, such as boiling point, melting point, and reactivity. Constitutional isomerism is the most basic form of isomerism and is distinct from stereoisomerism, where connectivity is the same but spatial arrangement differs.
Named after Russian chemist Vladimir Vasilyevich Markovnikov (1838–1904), who formulated the rule in 1870 based on his observations of HX addition to alkenes. The rule was later rationalised mechanistically via carbocation stability theory.