Reactions of Alkenes

Addition of HOX; X: Cl, Br CCl4 Ozonolysis Ozonolysis

Reaction Mechanism A reaction mechanism describes how a reaction occurs which bonds are broken and which new ones are formed the order in which bond-breaking and bond-forming steps take place the role of the catalyst (if any is present) the energy of the entire system during the reaction

Energy Diagram Energy diagram: a graph showing the changes in energy that occur during a chemical reaction Reaction coordinate: a measure of the change in position of atoms during a reaction Heat of reaction, H: the difference in energy between reactants and products exothermic: products are lower in energy than reactants; heat is released endothermic: products are higher in energy than reactants; heat is absorbed

Energy Diagram Transition state: an energy maximum represents an unstable species formed during the course of a reaction Activation Energy, Ea: the difference in energy between reactants and the transition state determines the rate of reaction if Ea is large, only a few molecular collisions occur with sufficient energy to reach the transition state, and the reaction is slow if Ea is small, many collisions generate sufficient energy to reach the transition state, and the reaction is fast

Activation Energy Activation energy Is the difference in energy between the reactants and the transition state. Is the minimum energy required to a reaction to occur or happen This will determine the rate of the reaction How fast the reaction occurs

Energy Diagram an energy diagram for a two-step reaction involving formation of an intermediate

Nucleophile (nucleus loving): An electron rich species that seeks a region of low electron density. Electrophile (electron loving): A low electron-density species that seeks a region of high electron density.

Addition of HX (X=halogens) Hydrohalogenation Carried out with pure reagents or in a polar solvent such as acetic acid Addition is regioselective regioselective reaction: a reaction in which one direction of bond-forming or bond-breaking occurs in preference to all other directions Markovnikov’s rule: in additions of HX, H adds to the carbon with the greater number of hydrogens

HCl + 2-Butene A two-step mechanism Step 1: formation of the sec-butyl cation, a 2° carbocation intermediate Step 2: reaction of the sec-butyl cation (an electrophile) with chloride ion (a nucleophile) completes the reaction

Carbocations Carbocation: a species in which a carbon atom has only six electrons in its valence shell and bears positive charge Carbocations are: classified as 1°, 2°, or 3° depending on the number of carbons bonded to the carbon bearing the positive charge electrophiles; that is, they are electron-loving Lewis acids; that is, they are electron-pair acceptors

Carbocations a 3° carbocation is more stable than a 2° carbocation, and requires a lower activation energy for its formation a 2° carbocation is, in turn, more stable than a 1° carbocation, and requires a lower activation energy for its formation methyl and 1° carbocations are so unstable that they are never observed in solution

Addition of H2O Addition of H2O is called hydration acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the more substituted carbon of the double bond

Step 1: proton transfer to the alkene gives a carbocation Step 2: a Lewis acid/base reaction gives an oxonium ion Step 3: proton transfer to solvent gives the alcohol

Carbon Rearrangements Rearrangement: change in connectivity of atoms in a product compared to starting material 1,2-shift Hydride ion is migrating group More stable carbocation is formed

Addition of Cl2 and Br2 Carried out with either the pure reagents or in an inert solvent such as CH2Cl2

Addition of Cl2 and Br2 addition is stereoselective stereoselective reaction: a reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed addition to a cycloalkene, for example, gives only a trans product

Addition of Cl2 and Br2 Step 1: formation of a bromonium ion intermediate Step 2: halide ion opens the three-membered ring

Addition of Cl2 and Br2 Anti coplanar addition to a cyclohexene corresponds to trans-diaxial addition

Ozonolysis Cleavage of a Carbon-Carbon double bond Two Carbonyl groups in place

Reduction of Alkenes Alkenes react with H2 in the presence of a transition metal catalyst to give alkanes the most commonly used catalysts are Pd, Pt, and Ni the reaction is called catalytic reduction or catalytic hydrogenation

Reduction of Alkenes the most common pattern is syn addition of hydrogens; both hydrogens add to the same side of the double bond