Reactions
Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). Unlike alkanes, alkenes are highly reactive because the π (pi) bond in the double bond is weaker than the σ (sigma) bond and is more exposed to attack by electron-seeking species. This makes alkenes ideal candidates for a class of reactions known as electrophilic addition, where the π electrons act as a nucleophile, attracting an electrophile to break the double bond and add atoms across it. The two most important concepts governing the selectivity of these reactions are Markovnikov's rule and the Peroxide (Anti-Markovnikov) effect. Markovnikov's rule predicts which carbon in an unsymmetrical alkene receives the hydrogen atom during addition of a protic reagent such as HBr, while the peroxide effect — discovered through the study of free-radical mechanisms — reverses this regioselectivity. Understanding these rules allows chemists to predict and control product formation with precision. Ozonolysis is a powerful oxidative cleavage reaction in which ozone (O₃) breaks the C=C double bond entirely, producing carbonyl compounds — aldehydes or ketones depending on the substitution of the alkene. Together, electrophilic addition, Markovnikov/Anti-Markovnikov selectivity, and ozonolysis form the foundation of alkene reaction chemistry and are essential tools in organic synthesis.
- Electrophilic Addition: General Mechanism
- Markovnikov's Rule
- Anti-Markovnikov Addition: The Peroxide Effect
- Ozonolysis
Electrophilic Addition: General Mechanism
In electrophilic addition, the electron-rich π bond of an alkene attacks an electrophile (E⁺), breaking the π bond and forming a new C–E bond on one carbon. This generates a carbocation intermediate on the adjacent carbon. In the second step, a nucleophile (Nu⁻) attacks the carbocation to complete the addition. Overall, E–Nu adds across the double bond: both carbons of the C=C gain a new bond. Example: addition of HBr to ethene gives bromoethane (CH₃CH₂Br). The rate-determining step is formation of the carbocation; its stability (3° > 2° > 1° > methyl) controls regioselectivity.
Markovnikov's Rule
When HX (where X = Cl, Br, I, or OH from H₂SO₄/H₂O) adds to an unsymmetrical alkene, the hydrogen attaches to the carbon bearing the greater number of hydrogen atoms (the less-substituted carbon), and X attaches to the more-substituted carbon. This is because protonation of the more-substituted carbon forms the more stable (higher-order) carbocation intermediate, which is energetically favored. Example: addition of HBr to propene (CH₃CH=CH₂) gives 2-bromopropane (CH₃CHBrCH₃, the Markovnikov product) as the major product, NOT 1-bromopropane.
Anti-Markovnikov Addition: The Peroxide Effect
When HBr is added to an alkene in the presence of peroxides (ROOR) or UV light, the mechanism changes from ionic to free-radical. The peroxide initiates homolytic cleavage of Br–Br or the O–O bond to generate bromine radicals (Br•). The Br• radical adds to the less-substituted carbon of the double bond — forming the more stable (more-substituted) carbon radical intermediate — and then a hydrogen atom is abstracted from HBr. The net result is that Br ends up on the less-substituted carbon: Anti-Markovnikov selectivity. Crucially, this peroxide effect applies ONLY to HBr, not to HCl or HI (due to bond energy considerations). Example: propene + HBr / peroxides → 1-bromopropane (Anti-Markovnikov product).
Ozonolysis
Ozonolysis cleaves the C=C double bond using ozone (O₃). The reaction proceeds in two stages. In Stage 1, ozone adds across the double bond to form an unstable molozonide (1,2,3-trioxolane), which rearranges to a more stable ozonide. In Stage 2, the ozonide is worked up with either a reductive work-up (using Zn/H₂O or Me₂S) or an oxidative work-up (using H₂O₂). Reductive conditions give aldehydes from terminal =CH₂ groups and ketones from internal =CR₂ groups. Oxidative conditions convert any aldehyde further to a carboxylic acid. Ozonolysis is invaluable for deducing alkene structure from fragment analysis and for synthesizing carbonyl compounds. Example: ozonolysis of 2-butene (CH₃CH=CHCH₃) with reductive work-up gives two molecules of acetaldehyde (ethanal, CH₃CHO).
The Key Equation
- \ce{CH3CH=CH2} — Propene (—): A three-carbon alkene with the double bond between C1 and C2; the starting material for this reaction.
- \ce{HBr} — Hydrogen bromide (—): The reagent providing both hydrogen and bromine; in radical conditions the H–Br bond undergoes homolytic cleavage to supply Br• radicals.
- \text{peroxide} — Peroxide initiator (ROOR) (—): A catalyst (e.g. benzoyl peroxide) that undergoes homolytic cleavage on heating or UV irradiation to generate radicals, switching the mechanism from ionic to free-radical.
- \ce{CH3CH2CH2Br} — 1-Bromopropane (—): The Anti-Markovnikov product; bromine is on C1 (the terminal carbon), opposite to what Markovnikov's rule would predict under ionic conditions.
Worked Example
Problem. Pent-1-ene (CH₂=CHCH₂CH₂CH₃) reacts with ozone (O₃) followed by reductive work-up with dimethyl sulfide (Me₂S). Identify the organic products of this ozonolysis reaction.
Answer. The products are methanal (formaldehyde) and butanal (butyraldehyde).
Origins · 1 minVladimir Vasilyevich Markovnikov and how this was discovered
Markovnikov formulated his famous rule in 1869 while working at the University of Kazan under Alexander Butlerov, the founder of structural organic chemistry. His rule — that in the addition of HX to an alkene, hydrogen adds to the carbon bearing the most hydrogen atoms — was derived empirically from experimental data long before carbocation theory existed to explain it mechanistically. His insight correctly predicted product selectivity for hundreds of reactions and remains a cornerstone of organic chemistry education worldwide. Markovnikov reportedly had a difficult personality and was involved in heated scientific disputes with contemporaries. Despite this, he was fiercely dedicated to developing Russian chemistry and founded a major chemical laboratory at Moscow University that became a leading European research centre.
Related Concepts
- Alkenes: Structure and Bonding — Understanding the σ and π bond framework of alkenes — especially the exposed, weaker π bond — is essential to explain why alkenes undergo electrophilic addition rather than substitution.
- Carbocations and Stability — The regioselectivity of Markovnikov addition is governed by the relative stability of carbocation intermediates (3° > 2° > 1°); this concept directly explains why the Markovnikov product is favoured in ionic addition.
- Alkynes: Reactions — Alkynes contain two π bonds and also undergo electrophilic addition reactions following Markovnikov's rule; comparison with alkene reactions deepens understanding of unsaturated hydrocarbon reactivity.
- Aldehydes and Ketones: Introduction — Ozonolysis of alkenes produces aldehydes and ketones as products, making knowledge of carbonyl compound structure and nomenclature essential for interpreting ozonolysis results.
- Free Radical Reactions — The Anti-Markovnikov (peroxide) effect proceeds via a free-radical chain mechanism involving initiation, propagation, and termination steps; understanding radical chemistry is prerequisite to explaining this reversed selectivity.