Vollhardt 6e Lecture PowerPoints - Chapter 12

CHAPTER 12

Reactions of Alkenes

Why Addition Reactions Proceed: Thermodynamic Feasibility

12-1

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 + DHo

A-B) – (DHoC-A + DHo

C-B)

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

Catalytic Hydrogenation12-2

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)

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.

Nucleophilic Character of the Pi Bond: Electrophilic Addition of Hydrogen Halides

12-3

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.

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:

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 the proton.

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:

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

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).

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.

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.

All steps are reversible in the hydration of alkenes. The proton serves as a catalyst only: it is regenerated in the reaction.

The reversibility of alkene protonation leads to alkene equilibration.

Protonation-deprotonation reactions may interconvert related alkenes and produce 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:

Electrophilic Addition of Halogens to Alkenes12-5

Halogen molecules also act 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 Alkenes12-5

Bromination takes place through anti addition.

Consider the bromination of cyclohexene. No cis-1,2-dibromocyclohexane is formed.

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.

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 a 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.

The Generality of Electrophilic Addition12-6

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.

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.

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:

Oxymercuration-Demercuration: A Special Electrophilic Addition

12-7

The electrophilic addition of a mercuric salt to an alkene is called mercuration. The product formed 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.

The 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.

Oxymercuration-demercuration in an alcohol solvent yields an ether:

Hydroboration-Oxidation: A Stereospecific Anti-Markovnikov Hydration

12-8

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.

Because the borane is electron poor, and the alkene is electron rich, an initial Lewis acid-base complex similar to the bromonium ion can 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. The boron binds to the less hindered (substituted) carbon.

The oxidation of alkylboranes gives alcohols.

The oxidation of a trialkylborane by hydrogen peroxide produces an alcohol in which the 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, an alkyl group migrates with its electron pair (with retention of configuration) to the neighboring oxygen atom.

After all three alkyl groups have migrated to oxygen atoms, the trialkyl borate is hydrolyzed by base to the alcohol and sodium borate.

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.

Diazomethane, Carbenes and Cyclopropane Synthesis

12-9

Cyclopropanes can be readily prepared by the addition of a carbene to the double bond of an alkene.

A carbene has the general structure, R2C:, in which the central carbon is surrounded by six electrons (sextet), and is thus electron deficient.

The electron-deficient carbene readily adds to an electron rich alkene.

Diazomethane forms methylene, which converts alkenes into cyclopropanes.

The highly reactive species methylene, H2C: (the simplest carbene) can be produced from the decomposition of diazomethane:

When methylene is generated in the presence of an alkene, an addition reaction occurs producing a cyclopropane. This reaction is usually stereospecific, with retention of the original double bond configuration.

Halogenated carbenes and carbenoids also give cyclepropanes.

Halogenated carbenes, prepared from halomethanes, can also be used to synthesize cyclopropanes.

Treatment of trichloromethane (chloroform) with strong base causes an elimination reaction in which both a proton and a chlorine atom are removed from the same carbon.

The resulting product is a dichlorocarbene which reacts with alkenes to produce cyclopropanes.

To avoid the hazards associated with diazomethane preparation, an alternate route using diiodomethane and zinc (Simmons-Smith reagent) to produce ICH2ZnI is used.

This substance is an example of a carbenoid, a carbenelike substance that converts alkenes into cyclopropanes stereospecifically.

Oxacyclopropane (Epoxide) Synthesis: Epoxidation by Peroxycarboxylic Acids

12-10

Oxacyclopropanes contain a single oxygen atom connected to two carbons to form a three-membered ring.

Oxacyclopropanes may be converted into vicinal anti diols.

Peroxycarboxylic acids deliver oxygen atoms to double bonds.

Peroxycarboxylic acids have the general formula:

These compounds react with double bonds because one of the oxygen atoms is electrophilic.

The resulting products are an oxacyclopropane and a carboxylic acid.

This reaction is referred to as an “epoxidation.” The older common name of an oxacyclopropane was an “epoxide.”

Commonly used peroxycaraboxylic acids for this reaction are meta-chloroperoxybenzoic acid (MCPBA) which is somewhat shock sensitive, and magnesium monoperoxyphthalate (MMPP).

The mechanism of this epoxidation reaction involves a cyclic transition state:

The peroxycarboxylic acid reactivity with double bonds increases with alkyl substitution, allowing for selective oxidations:

Hydrolysis of oxacyclopropanes furnishes the products of anti dihydroxylation of an alkene.

Ring opening of oxacyclopropanes with water produces anti vicinal diols.

Vicinal Syn Dihydroxylation with Osmium Tetroxide

12-11

The reaction of osmium tetroxide with alkenes yields syn vicinal diols in a two step process:

The reaction mechanism involves the concerted addition of the osmium tetroxide to the bond of the alkene:

Catalytic amounts of osmium tetroxide in the presence of an oxidizing agent (H2O2) to regenerate the spent osmium tetroxide are often used, due to the expense and toxicity of OsO4.

An older reagent for vicinal syn dihydroxylation of alkenes is KMnO4.

This reagent is less useful than OsO4 because of its tendency towards overoxidation.

The deep purple KMnO4 is converted into a brown precipitate, (MnO2) during the reaction, which can serve as a useful test for the presence of alkenes.

Oxidative Cleavage: Ozonolysis12-12

The mildest reagent capable of breaking both the and bonds in a double bond is ozone, O3. This process is known as “ozonolysis.”

Ozone is produced by an electrical discharge in dry oxygen in a instrument called an ozonator.

The initial product of the reaction of ozone with an alkene is an ozonide which is then directly reduced to two carbonyl products.

The mechanism of ozonolysis proceeds through a molozonide, which breaks apart into two fragments, which then recombine to form the ozonide:

Radical Additions: Anti-Markovnikov Product Formation

12-13

Hydrogen bromide can add to alkenes in anti-Markovnikov fashion: a change in mechanism.

The reaction products from the treatment of 1-butene with HBr depend upon the presence or absence of molecular oxygen in the reaction mixture:

In the presence of oxygen, a radical chain sequence mechanism leads to the anti-Markovnikov product.

Small amounts of peroxides (RO-OR) are formed in alkene samples stored in the presence of air (O2).

The peroxides initiate the radical chain sequence mechanism, which is much faster than the ionic mechanism operating in the absence of peroxides.

The halogen’s attack is regioselective, generating the more stable secondary radical rather than the primary one.

The alkyl radical subsequently abstracts a hydrogen from HBr which regenerates the chain-carrying bromine atom.

Both propagation steps are exothermic.

Termination is by radical recombination or by some other removal of the chain carriers.

Commonly used peroxides for initiating radical additions include:

Are radical additions general?

HCl and HI do not give anti-Markovnikov addition products with alkenes. The chain propagation steps involving these hydrogen halides are endothermic, which leads to very slow reactions and chain termination.

HCl and HI give Markovnikov products by ionic mechanisms regardless of the presence of radicals.

Other reagents, such as thiols, do undergo successful radical additions to alkenes:

Dimerization, Oligomerization, and Polymerization of Alkenes

12-14

Alkenes can react with one another in the presence of an appropriate catalyst: an acid, a radical, a base, or a transition metal.

Polymer synthesis is of great industrial importance:

Carbocations attack pi bonds.

Protonation of 2-methylpropene by hot aqueous sulfuric acid leads to the formation of two dimers:

The initial protonation produces a 1,1-dimethylethyl (tert-butyl) cation which then attacks the double bond of a second 2-methylpropene molecule.

The cation addition proceeds according to the Markovnikov rule to generate the more stable carbocation.

Deprotonation of the addition product from either adjacent carbon leads to a mixture of two products.

Repeated attack can lead to oligomerization and polymerization.

When 2-methylpropene is treated with mineral acid under more stringent conditions, higher oligiomers can be obtained through repeated addition reactions:

At higher temperatures, polymers containing many subunits are formed.

Synthesis of Polymers12-15

Polymerization reactions can be categorized as cationic, radical, anionic, and metal catalyzed.

Acid-catalyzed cationic polymerizations have already been covered. Initiators include H2SO4, HF, and BF3.

Radical polymerizations lead to commercially useful materials.

The polymerization of ethene in the presence of an organic peroxide at high pressures and temperatures proceeds by a radical polymerization process.

Polyethene (polyethylene) polymerized in this way is actually a branched polymer. Branching occurs as a result of hydrogen abstraction along the growing chain by another radical center.

The average molecular weight of polyethene is almost 1 million.

Polychloroethene (PVC or polyvinylchloride) is a polymer of chloroethene (vinyl chloride).

The peroxide initiator and the intermediate radical chains add only to the unsubstituted end of the monomer (producing the most stable radical) which results in a very regular head-to-tail structure of molecular weight over 1.5 million.

Pure PVC is fairly hard and brittle. It can be softened by the addition of carboxylic acid esters (plasticizers) for use in elastic materials such as vinyl leather, plastic covers, and garden hoses.

Polypropenenitrile (polyacrylonitrile) can be prepared from propenenitrile (acrylonitrile) using hydrogen peroxide with FeSO4 as a catalyst.

Polypropenenitrile, -(CH2CHCN)n-, also known as Orlon, is used to make fibers.

Anionic polymerizations require initiation by bases.

Anionic polymerizations are initiated by strong bases such as alkyllithiums, amides, alkoxides, and hydroxide.

The adhesive properties of “Super Glue” result from the hydroxide initiated polymerization of 2-cyanopropenoate.

The electron withdrawing natures of the carbonyl and nitrile groups create a partially positive carbon center at which the hydroxide can initially attack.

The negative charge on the resulting anion is then resonance stabilized by both the carbonyl and nitrile groups.

Metal-catalyzed polymerizations produce highly regular chains.

Ziegler-Natta catalysts are important initiators for metal-catalyzed polymerizations. They are typically made from titanium tetrachloride and a trialkylaluminum such as Al(CH2CH3)3.

Polymers produced using a Ziegler-Natta catalyst are characterized by regularity of construction and high linearity. This results in much higher density and strength than similar polymers obtained from radical polymerization.

Ethene: An Important Industrial Feedstock12-16

Ethene is the basis for the production of polyethene (polyethylene).

The major source of ethene is the pyrolysis of petroleum, or hydrocarbons derived from natural gas.

Ethene is the starting material for the production of many other industrial chemicals:

Alkenes in Nature: Insect Pheromones12-17

Pheromones are chemical substances used for communication within a living species. Pheromones are used for sex, trail, alarm, and defense signaling, to name a few.

The sex attractant for the male silkworm moth is 10-trans-12-cis-hexadecadien-1-ol (bombykol).

The natural pheromone is 10 billion times more active in eliciting a response than is the 10-cis-12-trans isomer, and 10 trillion times more active than the trans, trans isomer.

Important Concepts12

1. Double Bond Reactivity – exothermic addition reactions leading to saturated products

2. Hydrogenation of Alkenes – immeasurably slow unless a catalyst is used

• Palladium on carbon, PtO2, Raney nickel• H2 preferentially added to the least hindered face of

the double bond

3. Bond – attacked by acid and electrophiles• If the initial intermediate is a carbocation, the more

highly substituted carbocation is formed (Markovnikov’s Rule).

• If the initial intermediate is cyclic onium ion, nucleophilic ring opening is at the more substituted carbon (control of both regio- and stereochemistry).

Important Concepts12

4. Hydroboration – mechanistically between hydrogenation and electrophilic addition

• Step 1: complexation to boron• Step 2: concerted transfer of hydrogen to carbon• Hydroboration-oxidation: anti-Markovnikov hydration

of alkenes

5. Carbenes and Carbenoids – useful for synthesis of cyclopropanes from alkenes

6. Peroxycarboxylic Acids – contains oxygen atom transferable to alkenes to give oxacyclopropanes (epoxidation)

7. Osmium Tetroxide – addition to alkenes in a concerted syn manner to give vicinal diols

Important Concepts12

8. Ozonolysis – when followed by reduction, yields carbonyl compounds by cleavage of the double bond

9. Radical Chain Additions To Alkenes –• Chain carrier adds to the bond to form the more

highly substituted radical• Allows for anti-Markovnikov hydrobromination of

alkenes• Allows for the addition of thiols and some

halomethanes

10. Polymers – Alkenes react with themselves to form polymers.

• Initiation by charged species, radicals or some transition metals