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Dive into the characteristic reactions of alkenes like hydrogenation and addition of hydrogen halides. Learn about stereochemistry and selectivity in alkene reactions. Explore mechanisms and stability principles.
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Addition Reactions of Alkenes • The most characteristic reaction of alkenes is addition to the double bond.
Hydrogenation of Alkenes • Exothermic reaction with heat given off (heat of hydrogenation). The reaction is slow due to a high activation energy andis normally catalyzed by finely divided transition metals.
Possible Stereochemistry of Addition • The alkene is planar so addition of A and B could be either syn (same side) or anti (opposite sides).
Mechanism of Hydrogenation Step 1. Hydrogen atoms bond to the catalyst surface. Step 2. Alkenes bond to the catalyst surface.
Mechanism of Hydrogenation Step 4. Transfer of second hydrogen atom Step 3. Hydrogen atomtransfer from the catalyst to the alkene.
Mechanism of Hydrogenation The syn addition of H2 is observed in many specific examples:
Heats of Hydrogenation and Stability • Since more substituted alkenes are more stable the heat of hydrogenation of substituted alkenes should be lower.
Heats of Hydrogenation and Stability less stable more stable
Addition of Hydrogen Halides to Alkenes • Addition of H-X to an alkene yields an alkyl halide according to the equation:
Addition of Hydrogen Halides to Alkenes • The p-bond of the alkene which is electron rich (red in the electrostatic potential map) reacts with the polarized partially positive hydrogen (purple) of the HX.
Addition of Hydrogen Halides to Alkenes • More substituted double bonds are more electron rich sothey react faster. HI is the strongest acid so it reacts fastest in the series.
Selectivity of Addition of HX to Alkenes • Addition of HX is highly selective:
Markovnikov’s Rule for Addition of HX • Markovnikov’s rule: when an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the more hydrogens, and the halogen adds to the carbon that has fewer hydrogens.
Mechanism of Addition of HX to Alkenes Step 1. Protonation. This step is slow and rate determining. Protonation gives the most stable cation. Therefore theproton adds to the alkene carbon that initially has the most protons.
Mechanism of Addition of HX to Alkenes Step 2. Combination of the anion and cation. In the rapid second step the bromide attaches to the carbocation.
Carbocation Rearrangements • Carbocations rearrange to form more stable cations. The fact that rearrangements occur on addition of HX to alkenes provides further support for the reaction mechanism.
Carbocation Rearrangements • The mechanism must explain the formation of both products.Here it involves a hydride shift. The final step has chloride attaching to either of the secondary or tertiary carbocations.
Addition of Water (H-OH) to Alkenes Electrophilic addition of water to alkenes is an acid catalyzed reaction. Markovnikov’s rule is followed.
Rate of the Hydration Reaction The indication that more substitued alkenes react fastestsuggests carbocation intermediates
Mechanism of Hydration: Step 1 Step 1: Protonation. Markovnikov’s rule states that the proton is added onto the alkene carbon that initially has the most hydrogen atoms.
Mechanism of Hydration: Steps 2 & 3 Step 2: Nucleophilic addition. Step 3: Deprotonation.
Hydration and Dehydration The hydration of alkenes is the reverse of the dehydration of alcohols. These are equilibrium reactions and the real question is:How can you ensure the reaction goes in the desired direction?
Le Chatelier’s Principle A system at equilibrium adjusts to minimize any stress applied to it. For the hydration-dehydration equilibria, the key stress is water. Adding water pushes the equilibrium toward more product (alcohol). Removing water pushes the equilibrium toward more reactant (alkene).
Anti-Markovnikov Addition of Water An indirect method to add water It involves sequential hydroboration and oxidation. The water is added in an anti-Markovnikov way.
Hydroboration-Oxidation of Alkenes Hydroboration is the addition of boron hydride to the alkene: Oxidation replaces the boron substituent with hydroxyl.
Hydroboration-Oxidation of Alkenes Boron adds to the least substituted C of the alkene: The oxidation then forms the alcohol:
Hydroboration-Oxidation of Alkenes The reaction equation is normally written as:
Hydroboration-Oxidation of Alkenes No carbocation intermediates so no rearrangements. Several different forms of BH3 can be used.
Stereochemistry of Hydroboration-Oxidation The H and OH are added in a syn orientation.
Mechanism of Hydroboration Step 1: Addition of BH3.
Mechanism of Hydroboration Step 2: p-Complex rearrangement.
Mechanism of Oxidation Step 1: Deprotonation. Step 2: Nucleophilic attack.
Mechanism of Oxidation Step 3: Carbon migration. Step 4: Hydrolytic cleavage.
Addition of Halogens to Alkenes Electrophilic addition of halogens to alkenes yields vicinal dihalides (X = Br, Cl)
Stereochemistry of Addition of Halogens Trans-dihalides are exclusively formed by anti addition.
Rates of Addition of Halogens to Alkenes Rate of reaction increases with substitution suggesting that flow of electrons from the alkene to bromine is ratedetermining.
Mechanism of Bromination Step 1: Addition.
Mechanism of Bromination Step 2: Bromide attack. Bromide must attach to the carbon from the opposite sideto the bromonium cation. This is why the reaction is an anti-addition reaction.
Halohydrin Formation Reaction of bromine with alkenes in the presence of water yields vicinal halohydrins (halogen and a hydroxyl on adjacent carbons).
