POLYMERIZATION OF ALKENES

Alkenes: Polymerization

Polymerization is a process by which an organic compound reacts with itself to form a high‐molecular‐weight compound composed of repeating units of the original compound. The polymerization of ethene by an ionic, or free‐radical, reagent A−B is an example.

Polymerization reactions proceed via either cationic or free‐radical mechanisms. In both processes, π bonds are converted to σ bonds, and energy is liberated. Cationic polymerization is less efficient than free‐radical polymerization due to the caustic nature of cation‐producing reagents. An example of a cation‐initiated polymerization is the reaction of ethene with sulfuric acid.

The reaction continues and gives

 which finally reacts with HSO ₄ ⁻ to create the polymer

The more effective free‐radical polymerization can be initiated by oxygen or other free‐radical compounds, such as peroxides. The free‐radical polymerization of ethene by an alkoxide radical is a typical reaction.

The reaction continues and gives

The reaction may end by one of two termination steps. One is the bonding of two free radicals 

and the other is the internal stabilization of the polymer by double‐bond formation.

OXIDATION AND CLEAVAGE OF ALKENES

Alkenes: Oxidation and Cleavage Reactions

Alkenes can easily be oxidized by potassium permanganate and other oxidizing agents. What products form depend on the reaction conditions. At cold temperatures with low concentrations of oxidizing reagents, alkenes tend to form glycols.

This reaction is sometimes referred to as the Baeyer test. Because potassium permanganate, which is purple, is reduced to manganese dioxide, which is a brown precipitate, any water‐soluble compound that produces this color change when added to cold potassium permanganate must possess double or triple bonds. This reaction involves syn addition, leading to a cis‐glycol (a vicinal dihydroxy compound). A cis‐glycol can also be produced by reacting the alkene with osmium tetroxide, OsO ₄.

When more concentrated solutions of potassium permanganate and higher temperatures are employed, the glycol is further oxidized, leading to the formation of a mixture of ketones and carboxylic acids.

Oxidation of alkenes by ozone leads to destruction of both the σ and π bonds of the double‐bond system. This cleavage of an alkene double bond, generally accomplished in good yield, is called ozonolysis. The products of ozonolysis are aldehydes and ketones.

This reaction is often used to find the double bond in an alkene molecule. For example, the isomers of C ₄H ₈ can be distinguished from one another via oxidative cleavage.

By identifying the products of the reaction, one isomer can be distinguished from another, and the position of the bonds in the original compound can be determined.

HYDROHALOGENATION OF ALKENES

Alkenes: Hydrohalogenation

Unlike halogens, hydrogen halides are polarized molecules, which easily form ions. Hydrogen halides also add to alkenes by electrophilic addition.

The addition of hydrogen halides to asymmetrically substituted alkenes leads to two products.

The major product is predicted by the Markovnikov rule, which states that when a hydrogen halide is added to an asymmetrically substituted alkene, the major product results from the addition of the hydrogen atom to the double‐bonded carbon that is attached to more hydrogen atoms, while the halide ion adds to the other double‐bonded carbon. This arrangement creates a more stable carbocation intermediate.

Hydrohalogenation mechanisms. The first step in the addition of a hydrogen halide to an alkene is the dissociation of the hydrogen halide.

The H ⁺ ion is attracted to the π‐bond electrons of the alkene, which forms a π complex.

The π complex then breaks, creating a σ single bond between one carbon of the double‐bonded pair and the hydrogen. The carbon atom that loses a share of the π bond then becomes a carbocation. In asymmetrically substituted alkenes, two different carbocations are possible. The major product is generated from the more stable carbocation, while the minor product forms from the less stable one.

Thus, the major product is 2‐bromopropane.

Hydrogen bromide can also be added to an alkene in an anti‐Markovnikov fashion. In anti‐Markovnikov additions, the hydrogen atom of the hydrogen halide adds to the carbon of the double bond that is bonded to fewer hydrogen atoms. For this to result, the reaction must proceed by a noncarbocation intermediate; thus in the presence of peroxide, the reaction proceeds via a free‐radical mechanism, with the major product being generated from the more stable free radical.

The mechanism for this reaction starts with the generation of a bromine free radical by the reaction of hydrogen bromide with peroxide.

The bromine free radical adds to the alkene, forming a more stable carbon free radical.

The secondary free radical is more stable than the primary free radical because the secondary molecule is better able to delocalize the stress placed on the carbon atom by the free‐radical electron. The major product then forms from the intermediates by reacting with hydrogen bromide.

In all additions of hydrogen halides across carbon‐carbon double bonds, the major product always comes from the more stable intermediate. In Markovnikov additions, the major product results from the more stable carbocation, while in anti‐Markovnikov additions, such as the hydrogen bromide addition in the presence of peroxide, the major product results from the more stable free radical.

HALOGENATION OF ALKENES

Alkenes: Halogenation

Halogenation is the addition of halogen atoms to a π‐bond system. For example, the addition of bromine to ethene produces the substituted alkane 1,2‐dibromoethane.

The reaction proceeds via a trans addition, but because of the free rotation possible around the single bond of the resulting alkane, a trans product cannot be isolated. If, however, the original alkene structure possesses restricted rotation due to a factor other than a double bond, a trans‐addition product can be isolated. For instance, ring structures possess restricted rotation. In a ring structure, the carbon backbone is arranged so there is no beginning or ending carbon atom. If cyclohexene, a six‐carbon ring that has one double bond, is halogenated, the resulting cycloalkane is trans substituted.

Mechanism and stereochemistry of halogenation. Alkenes and halogens are nonpolar molecules. However, both types of molecules, under proper conditions, can undergo induced‐dipole formation, which leads to the generation of forces of attraction between the molecules.

The bromoethyl carbocation that forms mid reaction in this example is often internally stabilized by cyclization into a three‐membered ring containing a positively charged bromine atom (bromonium ion).

This intermediate is more stable than the corresponding linear carbocation because all the atoms have a complete octet of electrons.

The bromonium ion shares the electrons in the carbon‐bromine covalent bond unevenly, with the overlap region being closer to the more electronegative bromine. This generates a partial positive charge (δ ⁺) on the carbon atoms of the ring. The charge delocalization stabilizes the ring structure, and the resulting partial positive charges on the carbon atoms attract the nucleophilic bromide ion.

The second bromide ion must approach a partially positive carbon atom from the side of the carbocation opposite where the bromonium ion attached. The reason for this is that the bromonium ion blocks access to the carbon atoms along an entire side, due to bond formation with the two carbon atoms. Such blocking is referred to as steric hindrance. Because of steric hindrance, only a trans addition is possible.

ELECTROPHILIC ADDITION REACTIONS

Alkenes: Electrophilic Addition Reactions

The most common reactions of the alkenes are additions across the double bond to form saturated molecules. Such reactions are represented by the following general equation, where X and Y represent elements in a compound that are capable of being added across the π‐bond system of an alkene to form a substituted alkane.

REACTION OF EPOXIDES

Alkenes: Epoxide Reactions

Alkenes are capable of reacting with oxygen in the presence of elemental silver to form a series of cyclic ethers called epoxides. Epoxides are three‐atom cyclic systems in which one of the atoms is oxygen. The simplest epoxide is epoxyethane (ethylene oxide).

Epoxyethane belongs to a class of chemicals called heterocyclic compounds. These compounds are cyclic structures in which one (or more) of the ring atoms is a hetero atom, that is, an atom of an element other than carbon. In the laboratory, epoxyethane is prepared by reacting 1‐chloro‐2‐hydroxyethane with a base.

The mechanism for this reaction starts with the base reacting with the acidic hydrogen of the OH group.

The oxygen anion is then attracted to the carbon that is bonded to the chlorine atom. This carbon bears a strong partial positive charge due to the great differences in electronegativity between the carbon and chlorine atoms.

As can be seen in the structural formula above, the oxygen atom must be located anti to the departing chlorine atom for the reaction to occur. The overall reaction is a syn addition. A third method of preparing epoxyethane is by the reaction of an alkene with peroxy acids.

CARBENE ADDITION

Alkenes: Addition of Carbenes

Carbenes are intermediates of the general formula R ₂C:. In this configuration, the carbon atom possesses only a sextet of electrons, and is therefore highly reactive and electrophilic. Carbenes are generally prepared by reacting a haloform, such as chloroform, with a strong base, such as sodium ethoxide.

Carbene (H ₂C:), however, is prepared by exposing diazomethane to ultraviolet light.

Due to the high reactivity of carbenes, they cannot be isolated. All carbene reactions are run by generating the carbene “in situ,” that is, generating the carbene in the presence of a reagent with which it will immediately react. Alkenes, which are ready sources of electrons, are such reagents. When alkenes react with carbenes, three‐membered rings are formed.

The insertion of a carbene into a π‐bond system is the most common way of preparing cyclopropanes. The addition of the methylene unit, CH ₂, to the carbon‐carbon double bond of the alkene is a syn addition.

Some chemicals, namely the carbenoids, behave like carbenes, even though they are not. The most common carbenoid is the Simmons‐Smith reagent, a mixture of iodomethane and a zinc‐copper couple. This reagent also reacts with alkenes to form a cyclopropane ring.

The mechanisms of carbene and carbenoid reactions show the difference between the two. The mechanism for a carbene reaction is a concerted process in which all bonds are broken and formed at one time.

The mechanism for the Simmons‐Smith reaction also shows a concerted addition; however, a carbene is never formed.

CATALYTIC HYDROGENATION

Alkenes: Catalytic Addition of Hydrogen

Hydrogenation is the addition of hydrogen to an alkene. Although this reaction is exothermic, it is very slow. The addition of a metal catalyst, such as platinum, palladium, nickel, or rhodium, greatly increases the reaction rate. Although this reaction seems simple, it is a highly complex addition. The reaction takes place in four steps.

In the first step, a hydrogen molecule reacts with the metal catalyst. This reaction breaks the σ bond between the hydrogen atoms and creates weak hydrogen‐metal bonds. Next, the π bond of an alkene molecule contacts the metal catalyst. The π bond is destroyed and two weak carbon‐metal single bonds are created.

 Finally, the weakly bound hydrogen atoms transfer one at a time from the catalyst surface to the carbon atoms of the former alkene molecule, forming an alkane. Upon formation of the two new carbon‐hydrogen bonds, the alkane molecule can move away from the catalyst.

Because both of the added hydrogen atoms were bound to the surface of the catalyst, they normally approach the alkene molecule from the same side, or face. This approach of hydrogen atoms to the same face of an alkene molecule is called a syn addition.

When hydrogen atoms approach alkene molecules from opposite sides, the reaction is called an anti addition. Anti addition most likely occurs when double‐bond isomerization occurs more rapidly than the catalytic addition of the second hydrogen in the hydrogenation.

HYDROBORATION-OXIDATION OF ALKENES

Alkenes: Hydroboration‐Oxidation

Water can be added to an alkene in such a way that the major product is not that predicted by the Markovnikov rule. An example of such a reaction is the indirect addition of water to an alkene via a hydroboration‐oxidation reaction. In this reaction, a disubstituted boron hydride is added across the carbon‐carbon double bond of an alkene. The resulting organoborane compound is oxidized to an alcohol by reaction with hydrogen peroxide in a basic media, such as aqueous sodium hydroxide solution.

No carbocation intermediate forms during this reaction. Although the elements of water are added to an alkene, water is not a reactant; the hydrogen comes from a boron hydride molecule, and the hydroxide group comes from a peroxide molecule.

The first step in the hydroboration mechanism is the formation of the organoborane molecule from the alkene. This reaction occurs rapidly. The boron atom generally bonds to the less substituted, and thus less sterically hindered, carbon. This first step proceeds via a reaction between the disubstituted organoborane and the π bond of the alkene, followed by formation of a C−H bond via a four‐center interaction. A four‐center interaction is a reaction in which bonds between four atoms are created and broken simultaneously.

The alkylborane then undergoes a three‐stage oxidation reaction to form the alcohol. In the first step, a hydroperoxide anion, formed by the reaction of a hydroxide ion with a peroxide molecule, adds to the electron‐deficient boron atom.

This intermediate is unstable and rearranges, losing a hydroxide ion to form a borate ester.

The borate ester then reacts with alkaline hydrogen peroxide to produce a trialkyl borate.

Finally, the trialkyl borate is hydrolyzed (which means split by the elements of water) to alcohols and a borate ion by the aqueous hydroxide ion.

HYDRATION OF ALKENES

Alkenes: Hydration (Direct Addition of Water)

The addition of water to an alkene in the presence of a catalytic amount of strong acid leads to the formation of alcohols (hydroxy‐alkanes).

                               

This reaction proceeds via a standard carbocation mechanism and follows the Markovnikov rule. The mechanism for the addition of water to ethene follows.

    1. The hydrogen ion is attracted to the π bond, which breaks to form a σ bond with one of the double‐ bonded carbons. The second carbon of the original double‐bonded carbons becomes a carbocation.

    2.An acid‐base reaction occurs between the water molecule and the carbocation, forming an oxonium  ion.

     

    3. The oxonium ion stabilizes by losing a hydrogen ion, with the resulting formation of an alcohol.