CHAPTER 12 Reactions of Alkenes

Why Addition Reactions Proceed: Thermodynamic Feasibility 12-1 Why Addition Reactions Proceed: Thermodynamic Feasibility Because the C-C  bond is relatively weak, alkene chemistry is dominated by its reactions. The addition of a reagent, A-B, to give a saturated compound is the most common transformation of an alkene. Ho for the above reaction can be estimated from the relevant bond energies: Ho = (DHo bond + DHoA-B) – (DHoC-A + DHoC-B) 2

Most additions to alkenes should proceed to products with the release of energy.

Catalytic Hydrogenation 12-2 Catalytic Hydrogenation Hydrogenation takes place on the surface of a heterogeneous catalyst. In the absence of a catalyst, hydrogenations of alkenes, although exothermic, do not spontaneously occur, even at high temperatures. In the presence of a catalyst, the same hydrogenations proceed at a steady rate, even at room temperature. The most frequently used catalysts for hydrogenation reactions are: Palladium dispersed on carbon (Pd-C) Collodial platinum (Adam’s catalyst, PtO2) Nickel (Raney nickel, Ra-Ni) 4

The primary function of a catalyst in hydrogenation reactions is to provide metal-bound hydrogen atoms on the catalyst surface. Common solvents used for hydrogenations include methanol, ethanol, acetic acid, and ethyl acetate.

Hydrogenation is stereospecific. During a hydrogenation reaction, both atoms of hydrogen are added to the same face of the double bond (syn addition). In the absence of steric hindrance, addition to either face of the double bond can occur with equal probability which results in a racemic mixture of products. 6

12-3 Nucleophilic Character of the p Bond: Electrophilic Addition of Hydrogen Halides The  electrons of a double bond are more loosely held than those of the  bond. As a result, the  electrons, which extend above and below the molecular plane of the alkene, can act as a nucleophile in a manner similar to that of more typical Lewis bases. 2,3-dimethylbutene The electrophilic addition reactions of alkenes can be both regioselective and stereospecific.

Electrophilic attack by protons gives carbocations. A strong acid may add a proton to a double bond to give a carbocation. This reaction is simply the reverse of the last step in an E1 elimination reaction and has the same transition state. At low temperatures and with a good nucleophile, an electrophilic addition product is formed. Typically, the gaseous HX (HCl, HBr, or HI) is bubbled through the pure or dissolved alkene. The reaction can also be carried out in a solvent such as acetic acid. 8

The Markovnikov rule predicts regioselectivity in electrophilic additions. The only product formed during the reaction of propene with HCl is 2-chloropropane: Other addition reactions show similar results:

As a result, the halogen attaches to the more substituted carbon. If the carbon atoms participating in the double bond are not equally substituted, the proton from the hydrogen halide attaches itself to the less substituted carbon. As a result, the halogen attaches to the more substituted carbon. This result is known as “Markovnikov’s rule” and is based on the stability of the carbocation formed by the addition of proton. 10

Markovnikov’s rule can also be stated: HX adds to unsymmetric alkenes in a way that the initial protonation gives the more stable carbocation. Product mixtures will be formed from alkenes that are similarly substituted at both sp2 carbon atoms. If addition to an achiral alkene generates a chiral product, a racemic mixture will be obtained.

Carbocation rearrangements may follow electrophilic addition. In the absence of a good nucleophile, a rearrangement of the carbocation may occur prior to the addition of the nucleophile. An example of such a rearrangement is the addition of trifluoroacetic acid to 3-methyl-1-butene, where a hydride shift converts a secondary carbocation into a more stable tertiary carbocation: 12

The extent of carbocation rearrangement depends upon: alkene structure solvent strength and concentration of nucleophile temperature Rearrangements are generally favored under strongly acidic, nucleophile-deficient conditions.

Alcohol Synthesis by Electrophilic Hydration: Thermodynamic Control 12-4 Alcohol Synthesis by Electrophilic Hydration: Thermodynamic Control When other nucleophiles are present, they may also attack the intermediate carbocation. Electrophilic hydration results when an alkene is exposed to an aqueous solution of sulfuric acid (HSO4- is a poor nucleophile). 14

The addition of water by electrophilic hydration follows Markovnikov’s rule, however carbocation rearrangements can occur because water is a poor nucleophile. The electrophilic hydration process is the reverse of the acid-induced elimination of water (dehydration) of alcohols previously discussed.

Alkene hydration and alcohol dehydration are equilibrium processes. All steps are reversible in the hydration of alkenes. The proton serves as a catalyst only: it is regenerated in the reaction. In the absence of protons, alkenes are stable in water. The position of the equilibrium in the hydration reaction can be changed by adjusting the reaction conditions. 16

The reversibility of alkene protonation leads to alkene equilibration. Protonation-deprotonation reactions interconvert alkenes to give an equilibrium mixture of isomers. Under these conditions, a reaction is said to be under thermodynamic control.

This mechanism can convert less stable alkenes into their more stable isomers: 18

Electrophilic Addition of Halogens to Alkenes 12-5 Electrophilic Addition of Halogens to Alkenes Halogen acts as electrophiles with alkenes giving vicinal dihalides. The reaction with bromine results in a color change from red to colorless, which is sometimes used as a test for unsaturation. Halogenations are best carried out at or below room temperature and in inert halogenated solvents (i.e. halomethanes)

Electrophilic Addition of Halogens to Alkenes 12-5 Electrophilic Addition of Halogens to Alkenes Bromination takes place through anti addition. Bromination of cyclohexene: no cis-1,2-dibromocyclohexane Only anti addition is observed. The product is racemic since the initial attack of bromine can occur with equal probability at either face of the cyclohexene. 20

With acyclic alkenes the reaction is cleanly stereospecific:

Cyclic bromonium ions explain the stereochemistry. The polarizability of the Br-Br bond allows heterolytic cleavage when attacked by a nucleophile, forming cyclic bromonium ion: The bridging bromine atoms serves as the leaving group as the bromonium ion is attacked from the bottom by a Br- ion. In symmetric bromonium ions, attack is equally probable at either carbon atom leading to racemic or meso products. 22

The Generality of Electrophilic Addition 12-6 The Generality of Electrophilic Addition The bromonium ion can be trapped by other nucleophiles. Bromonation of cyclopentene using water as the solvent gives the vicinal bromoalcohol (bromohydrin). The water molecule is added anti to the bromine atom and the other product is HBr.

Vicinal haloalcohols are useful synthetic intermediates. 24

Vicinal haloethers can be produced if an alcohol is used as the solvent, rather than water.

Halonium ion opening can be regioselective. Mixed additions to double bonds can be regioselective: The nucleophile attacks the more highly substituted carbon of the bromonium ion, because it is more positively polarized. 26

Electrophilic additions of unsymmetric reagents add in a Markovnikov-like fashion: The electrophilic unit becomes attached to the less substituted carbon of the double bond. Mixtures of products are formed only when the two carbons are not sufficiently differentiated.

Reagents of the type A-B, in which A acts as the electrophile, A+, and B the nucleophile, B-, can undergo stereo- and regiospecific addition reactions to alkenes: 28

Oxymercuration-Demercuration: A Special Electrophilic Addition 12-7 Oxymercuration-Demercuration: A Special Electrophilic Addition Electrophilic addition of a mercuric salt to an alkene is called mercuration. The product is known as an alkylmercury derivative. A reaction sequence known as “oxymercuration-demercuration” is a useful alternative to acid-catalyzed hydration:

Oxymercuration is anti stereospecific and regioselective. Alcohol obtained from oxymercuration-demercuration is the same as that obtained from Markovnikov hydration, however, since no carbocation is involved in the reaction mechanism, rearrangements of the transition state do not occur. 30

Oxymercuration-demercuration in an alcohol solvent yields an ether:

Hydroboration-Oxidation: A Stereospecific anti-Markovnikov Hydration 12-8 Hydroboration-Oxidation: A Stereospecific anti-Markovnikov Hydration The boron-hydrogen bond adds across double bonds. Borane, BH3, adds to double bonds without catalytic activation: The borane is commercially available in an ether-tetrahydrofuran solvent. 32

Because the borane is electron poor, and the alkene is e-rich, an initial Lewis acid-base complex similar to the bromonium ion form: Because of the four center transition state, the addition reaction is syn. All three B-H bonds can react.

Hydroboration is regioselective as well as stereospecific (syn addition). Here, steric factors are more important than electronic factors. Boron binds to the less hindered (substituted) carbon. 34

The oxidation of alkylboranes gives alcohols. The oxidation of a trialkylborane by hydrogen peroxide produces an alcohol where a hydroxyl group has replaced the boron atom. In this reaction, the hydroxyl group ends up at the less substituted carbon: an anti-Markovnikov addition.

During the oxidation, alkyl group migrates with its electron pair (with retention of configuration) to the neighboring O atom. After all three alkyl groups have migrated to oxygen atoms, the trialkyl borate is hydrolyzed by base to the alcohol and sodium borate. 36

Hydroboration-oxidation of alkenes allows stereospecific and regioselective synthesis of alcohols. The reaction sequence exhibits anti-Markovnikov regioselectivity which complements acid-catalyzed hydration and oxymercuration-demercuration. The reaction mechanism does not involve a carbocation and thus rearrangements are not observed.