Preparation
Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). Because this double bond is a site of high electron density, alkenes are among the most synthetically versatile organic compounds. Before we can exploit their reactivity, however, we must be able to make them reliably and selectively. Three preparation methods dominate introductory organic chemistry: acid-catalysed dehydration of alcohols, base-induced dehydrohalogenation of haloalkanes, and the partial hydrogenation of alkynes using Lindlar's catalyst. Each method works by eliminating two atoms or groups from adjacent carbons, creating the π bond. The key challenge in every case is regioselectivity — when more than one alkene product is geometrically or structurally possible, we need rules to predict which one predominates. Saytzeff's rule (also spelled Zaitsev) states that the more substituted (more stable) alkene is the major product. Understanding why requires knowledge of carbocation stability and the relative energies of alkene isomers. The third method, Lindlar's catalyst, addresses a different problem: converting an alkyne into a cis-alkene without over-reducing it to an alkane. This selective, stereospecific reduction is indispensable in pharmaceutical synthesis, where the geometry of a double bond can determine whether a drug is active or inactive. Together these three routes illustrate fundamental principles of elimination, regioselectivity, and stereoselectivity that recur throughout all of organic chemistry.
- Dehydration of Alcohols (Acid-Catalysed Elimination)
- Saytzeff's Rule and Dehydrohalogenation
- Lindlar's Catalyst — Stereoselective Reduction of Alkynes
Dehydration of Alcohols (Acid-Catalysed Elimination)
An alcohol (R–OH) is treated with a concentrated strong acid — most commonly concentrated H₂SO₄ or H₃PO₄ — and heated. The reaction proceeds via an E1 mechanism for secondary and tertiary alcohols. Step 1: The acid protonates the hydroxyl group, converting the poor leaving group OH⁻ into the excellent leaving group H₂O. Step 2: Water departs, generating a carbocation. Step 3: A base (often the bisulfate ion or another alcohol molecule) abstracts a proton from a carbon adjacent to the carbocation, and the electrons form the π bond. The temperature required increases with decreasing substitution: tertiary alcohols dehydrate readily at ~85 °C, secondary alcohols need ~170 °C, and primary alcohols (which cannot form stable carbocations and proceed via E2) may need >200 °C. A key complication is carbocation rearrangement — if a more stable carbocation can form by a 1,2-hydride or methyl shift, it will, and a rearranged alkene results. Always check whether the carbon skeleton changes before predicting the product.
Saytzeff's Rule and Dehydrohalogenation
Dehydrohalogenation removes HX (where X = Cl, Br, or I) from a haloalkane using a strong base such as KOH or NaOEt dissolved in ethanol. The base abstracts a β-hydrogen (a hydrogen on the carbon next to the one bearing the halogen) while the halide departs, in a concerted E2 mechanism. When the substrate has two different sets of β-hydrogens, two structural isomers of the alkene are possible. Saytzeff's rule (1875) states that the predominant product is the more substituted alkene — the one with more alkyl groups attached to the double-bond carbons. For example, dehydrobromination of 2-bromobutane with KOH/EtOH gives mainly but-2-ene (disubstituted) rather than but-1-ene (monosubstituted). The stability order of alkenes is: tetrasubstituted > trisubstituted > disubstituted > monosubstituted > ethene, explained by hyperconjugation and the inductive electron-donating effect of alkyl groups stabilising the π system. Note: bulky bases such as KOtBu (potassium tert-butoxide) favour the less substituted (Hofmann) alkene because steric hindrance prevents approach to the more hindered β-hydrogen.
Lindlar's Catalyst — Stereoselective Reduction of Alkynes
Alkynes can be partially hydrogenated to alkenes using Lindlar's catalyst: palladium deposited on calcium carbonate, poisoned with lead acetate and quinoline. The poisoning deactivates the catalyst just enough to stop the reaction after one equivalent of H₂ has been added, preventing over-reduction to the alkane. Because both hydrogen atoms are delivered from the same face of the catalyst surface (syn addition), the product is exclusively the cis (Z)-alkene. For example, hex-3-yne + H₂ / Lindlar's → (Z)-hex-3-ene. If the trans (E)-alkene is required instead, dissolving-metal reduction (Na or Li in liquid NH₃) is used — this proceeds via a radical anion and gives anti addition, yielding the trans product. Lindlar's catalyst is widely used in the synthesis of natural products such as insect pheromones, where the precise geometry of a double bond is biologically critical.
The Key Equation
- \ce{CH3CH(OH)CH3} — Propan-2-ol (isopropanol) (—): A secondary alcohol; the hydroxyl group is on the central carbon.
- \ce{H2SO4} — Concentrated sulfuric acid (—): Catalyst and proton source; protonates –OH to form –OH₂⁺, converting it into a good leaving group.
- \Delta — Heat (°C): Provides activation energy; ~170 °C typically required for secondary alcohols.
- \ce{CH3CH=CH2} — Propene (—): The alkene product containing one C=C double bond.
- \ce{H2O} — Water (—): Small-molecule by-product eliminated during the reaction; its removal by distillation drives the equilibrium toward the alkene.
Worked Example
Problem. 2-Bromopentane is treated with KOH dissolved in ethanol and heated. (a) Identify all possible alkene products. (b) Using Saytzeff's rule, predict the major product. (c) Write the balanced equation for the major reaction.
Answer. Major product is pent-2-ene (predominantly the trans/E isomer). Minor product is pent-1-ene.
Origins · 1 minAlexander Mikhailovich Saytzeff (Zaitsev) and how this was discovered
Saytzeff (also transliterated as Zaitsev) was a Russian organic chemist working at the University of Kazan. In 1875 he formulated the empirical rule that now bears his name: in an elimination reaction, the predominant alkene product is the more substituted one (i.e., the thermodynamically more stable isomer). He established this rule through systematic dehydrohalogenation experiments on a wide range of haloalkanes. His student Vladimir Markovnikov had earlier formulated the analogous Markovnikov's rule for addition reactions, making the Kazan school uniquely influential in establishing the foundational regiochemical rules of organic chemistry. Saytzeff's rule was initially controversial — the German chemist Hofmann held that the less substituted (less hindered) product should dominate, and for a time both rules were debated. We now know both men were right: Saytzeff's rule applies to small bases (E2 or E1), while Hofmann's rule applies to bulky bases. The debate led directly to our modern understanding of steric effects in elimination.
Related Concepts
- Addition Reactions of Alkenes — Alkenes prepared by the methods above undergo electrophilic addition (HX, H₂O, Br₂, H₂); understanding preparation and reactions together completes the picture of alkene chemistry.
- Alkynes: Structure and Bonding — Lindlar's catalyst converts alkynes to cis-alkenes; understanding alkyne structure explains why partial hydrogenation is possible and why the triple bond is more reactive than a double bond.
- Alcohols: Elimination vs Substitution — Dehydration of alcohols to form alkenes competes with substitution (SN1/SN2); conditions that favour elimination (high temperature, strong acid) versus substitution must be distinguished.
- E2 Elimination Mechanism — Dehydrohalogenation proceeds via an E2 mechanism requiring anti-periplanar geometry; this stereoelectronic requirement explains why certain substrates give specific stereoisomers of alkene.
- Carbocation Stability and Rearrangements — E1 dehydration proceeds through a carbocation intermediate; 1,2-hydride and methyl shifts can produce rearranged alkene products that differ from those predicted by simple Saytzeff analysis.