Chapter 5 – Alkene Addition Reactions 5.1 – An Overview of Electrophilic Addition Reactions 5.2 – Reactions of Alkenes with Halogens 5.3 – Writing Organic Reactions 5.4 – Conversion of Alkenes into Alcohols 5.5 – Ozonolysis of Alkenes 5.6 – Free-Radical Addition of Hydrogen Bromide to Alkenes 5.7 – Polymers: Free-Radical Polymerization of Alkenes 5.8 – Alkenes in the Chemical Industry

Overview of Addition Reactions The carbon of the alkene with fewer alkyl groups becomes bonded to the less electronegative atom (the positive end of the reagent dipole) 5.1 An Overview of Electrophilic Addition Reactions

Addition of Bromine and Chlorine p bond: electron rich; serves as a nucleophile Halogens: electrophilic via an induced dipole 5.2 Reactions of Alkenes with Halogens

Addition of Chlorine and Bromine The product and mechanism are solvent dependent. One of the most common mechanisms involved a halonium ion. Bromonium, chloronium, or iodonium ion. 5.2 Reactions of Alkenes with Halogens

Addition of Chlorine and Bromine Why is a bromonium ion formed rather than a bromo-substitued carbocation? 5.2 Reactions of Alkenes with Halogens

Addition of Chlorine and Bromine Mechanistic Details The alkene acts a nucleophile and Br2 acts as an electrophile. 5.2 Reactions of Alkenes with Halogens

Addition of Chlorine and Bromine Mechanism cont’d. Bromine addition is completed when a bromide ion donates an electron pair to either of the ring carbons of the bromonium ion. 5.2 Reactions of Alkenes with Halogens

Halohydrins and Haloetherification Formed when a solvent, present in great excess, with a nucleophilic oxygen is used (an hydroxyl group). 5.2 Reactions of Alkenes with Halogens

Halohydrins Other halogens can also be used. Result: net addition of a hypohalous acid (HO-X). While I2 addition to alkenes doesn’t occur, Iodohydrins can be produced and are useful in organic synthesis. /not what it says in the text / 5.2 Reactions of Alkenes with Halogens

Halohydrin Regioselectivity Regioselectivity is observed in unsymmetrical alkenes. High regioselectivity observed when one carbon of the double bond is disubstituted. 5.2 Reactions of Alkenes with Halogens

Regioselectivity: the rationale About 90% of the positive charge resides on the tertiary carbon. The C-Br bond is so long and weak that it is essentially a carbocation. 5.2 Reactions of Alkenes with Halogens

How do we know these things about how reactions happen How do we know these things about how reactions happen ? – intermediates and mechanisms. product distributions relative rates

Conventions for Writing Reactions Solvents are generally written under the arrow Reactants and catalysts are written over the arrow 5.3 Writing Organic Reactions

Conventions for Writing Reactions major organic product reactants & conditions organic starting material percent yield 5.3 Writing Organic Reactions

Conversion of Alkenes into Alcohols OH on most substituted C OH on least substituted C 5.4 Conversion of Alkenes into Alcohols

Oxymercuration-Reduction of Alkenes A two step reaction carried out in sequence. Oxymercuration Reduction Net reaction = hydration of an alkene. Highly regioselective. 5.4 Conversion of Alkenes into Alcohols

Oxymercuration - Intermediates Oxymercuration – the alkene reacts with mercuric acetate in aqueous solution, adding –HgOAc and –OH across the double bond. 5.4 Conversion of Alkenes into Alcohols

Oxymercuration – Mechanism, part 2 Oxymercuration proceeds via a mercurinium ion Very comparable to a bromonium ion. 5.4 Conversion of Alkenes into Alcohols

Oxymercuration – Mechanism cont’d. The mercurinium ion reacts with the solvent water: 5.4 Conversion of Alkenes into Alcohols

Oxymercuration – A late, fast step The addition is completed by the transfer of a proton to the acetate ion. The strongest base present is acetate ion, not water. The equilibrium lies far to the right. 5.4 Conversion of Alkenes into Alcohols

Oxymercuration – The reduction (NaBH4) Reduction Step – conversion of the oxymercuration adducts into alcohols. C-Hg bond is replaced by C-H bond, with the H coming from the borohydride. For now, think of BH4- as  to BH3 + H- 5.4 Conversion of Alkenes into Alcohols

Oxymercuration – Summary Highly regioselective. No rearrangements occur (since there are no carbenium ion intermediates) More convenient to run on a laboratory scale than hydration (acid-catalyzed reaction with water) as it is free of rearrangements and other side reactions. 5.4 Conversion of Alkenes into Alcohols

Hydroboration-Oxidation Borane (BH3) adds –H and –BH2 regioselectively to alkene. Subsequent oxidation replaces the boron with an OH group. Net result: the hydroxyl ends up bonded to the carbon with fewer alkyl substituents. 5.4 Conversion of Alkenes into Alcohols

Hydroboration-Oxidation Hydroboration Step: Boron, as the more electronegative end of B-H, becomes bonded to the carbon with fewer alkyl substituents. 5.4 Conversion of Alkenes into Alcohols

Hydroboration-Oxidation Hydroboration Step: Borane has three B-H bonds and each one of these can react in turn with an alkene. If excess borane is employed the second addition doesn’t occur; it’s not an essential element of the reaction. Second Addition 5.4 Conversion of Alkenes into Alcohols

Hydroboration-Oxidation Hydroboration Steps: Third Addition (not essential) 5.4 Conversion of Alkenes into Alcohols

Hydroboration-Oxidation The accepted mechanism is concerted and avoids a carbocation. Some degree of electron deficiency is present. This explains regioselectivity. 5.4 Conversion of Alkenes into Alcohols

Hydroboration-Oxidation Oxidation Step: Conversion of organoboranes into alcohols. or more generally this includes R-BH2 → R-OH The C-B bond is replaced with C-OH. The oxygen comes from H2O2. I’ll show the mechanism but you don’t need to be able to reproduce it. 5.4 Conversion of Alkenes into Alcohols

Show mechanism for C–B goes to C–OH

Comparing Hydration Methods Oxymercuration-reduction and hydroboration- oxidation are complementary reactions. Both are more selective than acid-catalyzed hydration. Oxymercuration-reduction results in the –OH on the more highly substituted C. Hydroboration-oxidation results in the –OH being predominantly on the less substituted C. 5.4 Conversion of Alkenes into Alcohols

Formation of Ozonides The addition of ozone to alkenes breaks the C-C p-bond to give an unstable cycloaddition product. This unstable product is spontaneously converted into an ozonide, breaking the second C-C bond. 5.5 Ozonolysis of Alkenes

Ozonolysis Addition of ozone is a concerted cycloaddition. Usual second step, reaction with DMS, (CH3)2S. NOTE rearranged ozonide. 5.5 Ozonolysis of Alkenes

Ozonolysis: one mechanistic rationale Spontaneous rearrangement of the initial product leads to the ozonide. (you don’t need this mechanism; besides, it’s not really correct) 5.5 Ozonolysis of Alkenes

Another Reaction of Ozonides Treatment of an Ozonide with Water If an ozonide is treated with water, hydrogen peroxide (H2O2) is formed, causing the following: Aldehydes are converted to carboxylic acids. This does not affect the ketone products. 5.5 Ozonolysis of Alkenes

Ozonolysis Summary 5.5 Ozonolysis of Alkenes

End of Quiz 2 coverage Radical addition will not tested until later

Free Radical Addition of HBr The peroxide effect - The addition of peroxides reverses the regioselectivity of HBr addition to alkenes. This is considered, but isn’t really, anti- Markovnikov regioselectivity 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

An Explanation of the Peroxide Effect Radical stabilities explain the observed reversed regiochemistry with peroxides present. Formation of a tertiary radical is faster than formation of a primary radical. 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Free-Radicals Free radical – any species with at least one unpaired electron. 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

tertiary > secondary > primary Carbon Free-Radicals Radical Stability: free-radicals are stabilized by alkyl-group substitution at sp2 hybridized carbons. tertiary > secondary > primary 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

“Fishhook” Notation of radical formation Heterolytic Bond Breaking: Homolytic Bond Breaking: A different curved-arrow notation (fishhook notation) is used for homolytic processes. 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Free-Radical Chain Reactions Most free radical reactions are classified as free- radical chain reactions. Three fundamental steps involved in free-radical chain reactions. Initiation: net number of radicals increases Propagation: net number of radicals remains constant Termination: net number of radicals decreases 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Free-Radical Reactions: Initiators Initiation: The free radicals that take part in subsequent steps of the reaction are formed from a free-radical initiator. 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Free-Radical H-Br addition reactions: Initiation: In the free-radical addition of HBr to alkenes, two initiation steps take place. 1. 2. 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Free-Radical H-Br addition reactions: steps Propagation: radicals react with non radical starting materials to give other radicals. a. b. Regenerated for another use in step a. 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Free-Radical H-Br addition reactions: steps Termination: two radicals react to give non radical products. 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Bond-Dissociation Energies Defined as the standard enthalpy of the reaction: Requires homolytic (not heterolytic) bond breakage. Measures the intrinsic strength of a chemical bond. 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Additional material Tables of bond dissociation energies (such as p 216 in the text) and radical heats of formation (p. 212) will likely be added here.

Bond-Dissociation Energies BDE analysis can be used to explain why a peroxide effect is not observed in the addition of HCl and HI to alkenes. Compare propagation steps: 5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

Polymers In the presence of free-radical initiators, many alkenes react to form polymers. This process is a free-radical polymerization. 5.7 Polymers: Free-Radical Polymerization of Alkenes

Polymers Initiation Step: First Propagation Step: 5.7 Polymers: Free-Radical Polymerization of Alkenes