1. Reactions of Alkenes

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

3. 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

4. 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

5. 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

6. 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

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

8. 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.

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

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

18. 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

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

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

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

22. 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

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