Alkenes......

1. Addition of Strong Brønsted Acids

As illustrated by the preceding general equation, strongBrønsted acids such as HCl, HBr, HI & H2SO4, rapidly addto the C=C functional group of alkenes to give products in which newcovalent bonds are formed to hydrogen and to the conjugate base of theacid. Using the above equation as a guide, write the addition productsexpected on reacting each of these reagents with cyclohexene.

Weak Brønsted acids such as water (pKa = 15.7) and acetic acid(pKa = 4.75) do not normally add to alkenes. However, the addition of astrong acid serves to catalyze the addition of water, and in this wayalcohols may be prepared from alkenes. For example, if sulfuric acid isdissolved in water it is completely ionized to the hydronium ion,H3O(+), and this strongly acidic (pKa = -1.74) species effectshydration of ethene and other alkenes.
CH2=CH2 + H3O(+) ——> HCH2–CH2OH + H(+)
The importance of choosing an appropriate solvent for these additionreactions should now be clear. If the addition of HCl, HBr or HI isdesired, water and alcohols should not be used. These strong acids willionize in such solvents to give ROH2(+) and the nucleophilic oxygen ofthe solvent will compete with the halide anions in the final step,giving alcohol and ether products. By using inert solvents such ashexane, benzene and methylene chloride, these competing solventadditions are avoided. Because these additions proceed by way of polaror ionic intermediates, the rate of reaction is greater in polarsolvents, such as nitromethane and acetonitrile, than in non-polarsolvents, such as cyclohexane and carbon tetrachloride.

Regioselectivity and the Markovnikov Rule

Only one product is possible from the addition of these strongacids to symmetrical alkenes such as ethene and cyclohexene. However,if the double bond carbon atoms are not structurally *****alent, as inmolecules of 1-butene, 2-methyl-2-butene and 1-methylcyclohexene, thereagent conceivably may add in two different ways. This is shown for2-methyl-2-butene in the following equation.
(CH3)2C=CHCH3 + H-Cl (CH3)2CH–CHClCH3or(CH3)2CCl–CHHCH3 2-methyl-2-butene
2-chloro-3-methylbutane
2-chloro-2-methylbutane
When addition reactions to such unsymmetrical alkenes are carried out,we find that one of the two possible constitutionally isomeric productsis formed preferentially. Selectivity of this sort is termed regioselectivity.In the above example, 2-chloro-2-methylbutane is nearly the exclusiveproduct. Similarly, 1-butene forms 2-bromobutane as the predominantproduct on treatment with HBr.
After studying many addition reactions of this kind, the Russian chemist Vladimir Markovnikov noticed a trend in the structure of the favored addition product. He formulated this trend as an empirical rule we now call The Markovnikov Rule:When a Brønsted acid, HX, adds to an unsymmetricallysubstituted double bond, the acidic hydrogen of the acid bonds to thatcarbon of the double bond that has the greater number of hydrogen atomsalready attached to it.
In more homelier vernacular this rule may be restated as, "Them that has gits."
It is a helpful exercise to predict the favored product in examples such as those shown below:


Empirical rules like the Markovnikov Rule are useful aids forremembering and predicting experimental results. Indeed, empiricalrules are often the first step toward practical mastery of a subject,but they seldom constitute true understanding. The Markovnikov Rule,for example, suggests there are common and important principles at workin these addition reactions, but it does not tell us what they are. Thenext step in achieving an understanding of this reaction must be toconstruct a rational mechanistic model that can be tested by experiment.
All the reagents discussed here are strong Brønsted acids so,as a first step, it seems sensible to find a base with which the acidcan react. Since we know that these acids do not react with alkanes, itmust be the pi-electrons of the alkene double bond that serve as thebase. As shown in the diagram on the right, the pi-orbital extends intothe space immediately above and below the plane of the double bond, andthe electrons occupying this orbital may be attracted to the proton ofa Brønsted acid. The resulting acid-base equilibrium generatesa carbocation intermediate (the conjugate acid of the alkene) whichthen combines rapidly with the anionic conjugate base of theBrønsted acid. This two-step mechanism is illustrated for thereaction of ethene with hydrogen chloride by the following equations.
An energy diagram for this two-step addition mechanism is shown to theleft. From this diagram we see that the slow or rate-determining step(the first step) is also the product determining step (the anion willnecessarily bond to the carbocation site). Electron donating doublebond substituents increase the reactivity of an alkene, as evidenced bythe increased rate of hydration of 2-methylpropene (two alkyl groups)compared with 1-butene (one alkyl group). Evidently, alkyl substituentsact to increase the rate of addition by lowering the activation energy,ΔE‡1 of the rate determining step, and it is here we should look for arationalization of Markovnikov's rule.
As expected, electron withdrawing substituents, such as fluorine orchlorine, reduce the reactivity of an alkene to addition by acids(vinyl chloride is less reactive than ethene).

Energy
higher

begin

lower

George Hammond formulated a useful principle that relates thenature of a transition state to its ******** on the reaction path. ThisHammond Postulate states that atransition state will be structurally and energetically similar to thespecies (reactant, intermediate or product) nearest to it on thereaction path. In strongly exothermic reactions the transitionstate will resemble the reactant species. In strongly endothermicconversions, such as that shown to the right, the transition state willresemble the high-energy intermediate or product, and will track theenergy of this intermediate if it changes. This change in transitionstate energy and activation energy as the stability of the intermediatechanges may be observed by clicking the higher or lower buttons to theright of the energy diagram. Three examples may be examined, and thereference curve is changed to gray in the diagrams for higher (magenta)and lower (green) energy intermediates.
The carbocationintermediate formed in the first step of the addition reaction nowassumes a key role, in that it directly influences the activationenergy for this step. Independent research shows that the stability ofcarbocations varies with the nature of substituents, in a mannersimilar to that seen for alkyl radicals. The exceptional stability ofallyl and benzyl cations is the result of charge delocalization, andthe stabilizing influence of alkyl substituents, although lesspronounced, has been interpreted in a similar fashion.
Carbocation
Stability CH3(+) <CH3CH2(+)< (CH3)2CH(+)≈ CH2=CH-CH2(+)<C6H5CH2(+)≈ (CH3)3C(+)
From this information, applying the Hammond Postulate, we arrive at a plausible rationalization of Markovnikov's rule. Whenan unsymmetrically substituted double bond is protonated, we expect themore stable carbocation intermediate to be formed faster than the lessstable alternative, because the activation energy of the path tothe former is the lower of the two possibilities. This is illustratedby the following equation for the addition of hydrogen chloride topropene. Note that the initial acid-base equilibrium leads to api-complex which immediately reorganizes to a sigma-bonded carbocationintermediate. The more stable 2º-carbocation is formedpreferentially, and the conjugate base of the Brønsted acid(chloride anion in the example shown below) then rapidly bonds to thiselectrophilic intermediate to form the final product.
The following energy diagram summarizes these features. Note that thepi-complex is not shown, since this rapidly and reversibly formedspecies is common to both possible reaction paths.


2. Rearrangement of Carbocations

The formation of carbocations is sometimes accompanied by astructural rearrangement. Such rearrangements take place by a shift ofa neighboring alkyl group or hydrogen, and are favored when therearranged carbocation is more stable than the initial cation. Theaddition of HCl to 3,3-dimethyl-1-butene, for example, leads to anunexpected product, 2-chloro-2,3-dimethylbutane, in somewhat greateryield than 3-chloro-2,2-dimethylbutane, the expected Markovnikovproduct. This surprising result may be explained by a carbocationrearrangement of the initially formed 2º-carbocation to a3º-carbocation by a 1,2-shift of a methyl group. To see thisrearrangement click the "Show Mechanism" button to the right of the equation.

Another factor that may induce rearrangement of carbocationintermediates is strain. The addition of HCl to α-pinene, the majorhydrocarbon component of turpentine, gives the rearranged product,bornyl chloride, in high yield. As shown in the following equation,this rearrangement converts a 3º-carbocation to a2º-carbocation, a transformation that is normally unfavorable.However, the rearrangement also expands a strained four-membered ringto a much less-strained five-membered ring, and this relief of strainprovides a driving force for the rearrangement. A three-dimensionalprojection view of the rearrangement may be seen by clicking the "Other View"button. The atom numbers (colored red) for the pinene structure areretained throughout the rearrangement to help orient the viewer. Thegreen numbers in the final product represent the proper numbering ofthis bicyclic ring system.

The propensity for structural rearrangement shown by certain molecularconstitutions, as illustrated above, serves as a useful probe for theintermediacy of carbocations in a reaction. We shall use this testlater.
An extensive and more detailed discussion of cation induced rearrangements may be accessed by Clicking Here.

3. Addition of Lewis Acids (Electrophilic Reagents)

The proton is not the only electrophilic species that initiatesaddition reactions to the double bond. Lewis acids like the halogens,boron hydrides and certain transition ****l ions are able to bond tothe alkene pi-electrons, and the resulting complexes rearrange or areattacked by nucleophiles to give addition products. The electrophiliccharacter of the halogens is well known. Although fluorine isuncontrollably reactive, chlorine, bromine and to a lesser degreeiodine react selectively with the double bond of alkenes. The additionof chlorine and bromine to alkenes, as shown in the following generalequation, proceeds by an initial electrophilic attack on thepi-electrons of the double bond. Iodine adds reversibly to doublebonds, but the equilibrium does not normally favor the additionproduct, so it is not a useful preparative method. Dihalo-compounds inwhich the halogens are juxtaposed in the manner shown are called vicinal, from the Latin vicinalis, meaning neighboring.
R2C=CR2 + X2 ——> R2CX-CR2X Other halogen containing reagents which add to double bonds include hypohalous acids, HOX,and sulfenyl chlorides, RSCl. These reagents are unsymmetrical, sotheir addition to unsymmetrical double bonds may in principle takeplace in two ways. In practice, these addition reactions areregioselective, with one of the two possible constitutionally isomericproducts being favored. The electrophilic moiety of these reagents isthe halogen.
(CH3)2C=CH2 + HOBr ——> (CH3)2COH-CH2Br
(CH3)2C=CH2 + C6H5SCl ——> (CH3)2CCl-CH2SC6H5
The regioselectivity of the above reactions may be explained by thesame mechanism we used to rationalize the Markovnikov rule. Thus,bonding of an electrophilic species to the double bond of an alkeneshould result in preferential formation of the more stable (more highlysubstituted) carbocation, and this intermediate should then combinerapidly with a nucleophilic species to produce the addition product.This is illustrated by the following equation.
To apply this mechanism we need to determine the electrophilic moiety in each of the reagents. By using electronegativity differences we can dissect common additionreagents into electrophilic and nucleophilic moieties, as shown on theright. In the case of hypochlorous and hypobromous acids (HOX), theseweak Brønsted acids (pKa's ca. 8) do not react as protondonors; and since oxygen is more electronegative than chlorine orbromine, the electrophile will be a halide cation. The nucleophilicspecies that bonds to the intermediate carbocation is then hydroxideion, or more likely water (the usual solvent for these reagents), andthe products are called halohydrins. Sulfenyl chlorides add in theopposite manner because the electrophile is a sulfur cation, RS(+),whereas the nucleophilic moiety is chloride anion (chlorine is moreelectronegative than sulfur).
If you understand this mechanism you should be able to write products for the following reactions:


The addition products formed in reactions of alkenes with mercuricacetate and boron hydrides (compounds shown at the bottom of of thereagent list) are normally not isolated, but instead are converted toalcohols by a substitution reaction. These important synthetictransformations are illustrated for 2-methylpropene by the followingequations, in which the electrophilic moiety is colored red and thenucleophile blue. The top reaction sequence illustrates the oxymercuration procedure and the bottom is an example of hydroboration.
The light blue vertical line separates the addition reaction on theleft from the substitution on the right. The atoms or groups that havebeen added to the original double bond are colored orange in the finalproduct. In both cases the overall reaction is the addition of water tothe double bond, but the regioselectivity is reversed. Theoxymercuration reaction gives the product predicted by Markovnikov'srule; hydroboration on the other hand gives the "anti-Markovnikov"product. Complementary reactions such as these are important becausethey allow us to direct a molecular transformation whichever way isdesired.
Mercury and boron are removed from the organic substrate in the secondstep of oxymercuration and hydroboration respectively. These reactionsare seldom discussed in detail; however, it is worth noting that themercury moiety is reduced to ****llic mercury by borohydride (probablyby way of radical intermediates), and boron is oxidized to borate bythe alkaline peroxide. Addition of hydroperoxide anion to theelectrophilic borane generates a tetra-coordinate boron peroxide,having the general formula R3B-O-OH(-). This undergoes successiveintramolecular shifts of alkyl groups from boron to oxygen, accompaniedin each event by additional peroxide addition to electron deficientboron. The retention of configuration of the migrating alkyl group isattributed to the intramolecular nature of the rearrangement.
Since the oxymercuration sequence gives the same hydration product as acid-catalyzed addition of water (see Brønsted acid addition),we might question why this two-step procedure is used at all. Thereason lies in the milder reaction conditions used for oxymercuration.The strong acid used for direct hydration may not be tolerated by otherfunctional groups, and in some cases may cause molecular rearrangement (see above).
The addition of borane, BH3, requires additional comment. In pure formthis reagent is a dimeric gas B2H6, called diborane, but in ether orTHF solution it is dissociated into a solvent coordinated monomer,R2O-BH3. Although diborane itself does not react easily with alkenedouble bonds, H.C. Brown(Purdue, Nobel Prize 1979) discovered that the solvated monomer addsrapidly under mild conditions. Boron and hydrogen have rather similarelectronegativities, with hydrogen being slightly greater, so it is notlikely there is significant dipolar character to the B-H bond. Sinceboron is electron deficient (it does not have a valence ****l electronoctet) the reagent itself is a Lewis acid and can bond to thepi-electrons of a double bond by displacement of the ether moiety fromthe solvated monomer. As shown in the following equation, this bondingmight generate a dipolar intermediate consisting of anegatively-charged boron and a carbocation. Such a species would not bestable and would rearrange to a neutral product by the shift of ahydride to the carbocation center. Indeed, this hydride shift isbelieved to occur concurrently with the initial bonding to boron, asshown by the transition state drawn below the equation, so the discreteintermediate shown in the equation is not actually formed.Nevertheless, the carbocation stability rule cited above remains auseful way to predict the products from hydroboration reactions. You may correct the top equation by clicking the button on its right.Note that this addition is unique among those we have discussed, inthat it is a single-step process. Also, all three hydrogens in boraneare potentially reactive, so that the alkyl borane product from thefirst addition may serve as the hydroboration reagent for twoadditional alkene molecules.



To examine models of B2H6. and its dissociation in THF
Stereoselectivity in Addition Reactions to Double Bonds

As illustrated in the drawing on the right, the pi-bond fixesthe carbon-carbon double bond in a planar configuration, and does notpermit free rotation about the double bond itself. We see then that addition reactions to this function might occur inthree different ways, depending on the relative orientation of theatoms or groups that add to the carbons of the double bond: (i) theymay bond from the same side, (ii) they may bond from opposite sides, or(iii) they may bond randomly from both sides. The first twopossibilities are examples of stereoselectivity, the first being termed syn-addition, and the second anti-addition.Since initial electrophilic attack on the double bond may occur equallywell from either side, it is in the second step (or stage) of thereaction (bonding of the nucleophile) that stereoselectivity may beimposed.
If the two-step mechanism described above is correct, and if thecarbocation intermediate is sufficiently long-lived to freely-rotateabout the sigma-bond component of the original double bond, we wouldexpect to find random or non-stereoselective addition in the products.On the other hand, if the intermediate is short-lived and factors suchas steric hindrance or neighboring group interactions favor one side inthe second step, then stereoselectivity in product formation is likely.The following table summarizes the results obtained from many studies,the formula HX refers to all the strong Brønsted acids. Theinteresting differences in stereoselectivity noted here provide furtherinsight into the mechanisms of these addition reactions.
ReagentH–XX2HO–XRS–ClHg(OAc)2BH3 Stereoselectivitymixedantiantiantiantisyn


1. Brønsted Acid Additions

The stereoselectivity of Brønsted acid addition issensitive to experimental conditions such as temperature and reagentconcentration. The selectivity is often anti, but reports of synselectivity and non-selectivity are not uncommon. Of all the reagentsdiscussed here, these strong acid additions (E = H in the followingequation) come closest to proceeding by the proposed two-step mechanismin which a discrete carbocation intermediate is generated in the firststep. Such reactions are most prone to rearrangement when this isfavored by the alkene structure.

2. Addition Reactions Initiated by Electrophilic Halogen

The halogens chlorine and bromine add rapidly to a wide variety ofalkenes without inducing the kinds of structural rearrangements notedfor strong acids (first example below). The stereoselectivity of theseadditions is strongly anti, as shown in many of the following examples.
An important principle should be restated at this time. The alkenesshown here are all achiral, but the addition products have chiralcenters, and in many cases may exist as enantiomeric stereoisomers. Inthe absence of chiral catalysts or reagents, reactions of this kindwill always give racemic mixtures if the products are enantiomeric. Onthe other hand, if two chiral centers are formed in the addition thereaction will be diastereomer selective. This is clearly shown by theaddition of bromine to the isomeric 2-butenes. Anti-addition tocis-2-butene gives the racemic product, whereas anti-addition to thetrans-isomer gives the meso-diastereomer.
We can account both for the high stereoselectivity and the lack ofrearrangement in these reactions by proposing a stabilizing interactionbetween the developing carbocation center and the electron rich halogenatom on the adjacent carbon. This interaction, which is depicted forbromine in the following equation, delocalizes the positive charge onthe intermediate and blocks halide ion attack from the syn-********.
The stabilization provided by this halogen-carbocation bonding makesrearrangement unlikely, and in a few cases three-membered cyclichalonium cations have been isolated and identified as trueintermediates. A resonance description of such a bromonium ionintermediate is shown below. The positive charge is delocalized overall the atoms of the ring, but should be concentrated at the moresubstituted carbon (carbocation stability), and this is the site towhich the nucleophile will bond.
Becausethey proceed by way of polar ion-pair intermediates, chlorine andbromine addition reactions are faster in polar solvents than innon-polar solvents, such as hexane or carbon tetrachloride. However, inorder to prevent solvent nucleophiles from competing with the halideanion, these non-polar solvents are often selected for these reactions.In water or alcohol solution the nucleophilic solvent may open thebromonium ion intermediate to give an α-halo-alcohol or ether, togetherwith the expected vic-dihalide. Such reactions are sensitive to pH andother factors, so when these products are desired it is necessary tomodify the addition reagent. Aqueous chlorine exists as the followingequilibrium, Keq ≈ 10-4. By adding AgOH, the concentration of HOCl canbe greatly increased, and the chlorohydrin addition product obtainedfrom alkenes.
The more widely used HOBr reagent, hypobromous acid, is commonly madeby hydrolysis of N-bromoacetamide, as shown below. Both HOCl and HOBradditions occur in an anti fashion, and with the regioselectivitypredicted by this mechanism (OH bonds to the more substituted carbon ofthe alkene).
Vicinal halohydrins provide an alternative route for the epoxidation of alkenes over that of reaction with peracids. As illustrated in the following diagram, a base induced intramolecular substitution reactionforms a three-membered cyclic ether called an epoxide. Both thehalohydrin formation and halide displacement reactions arestereospecific, so stereoisomerism in the alkene will be reflected inthe epoxide product (i.e. trans-2-butene forms atrans-disubstituted epoxide). A general procedure for forming theseuseful compounds will be discussed in the next section.
4. Hydrogenation

Addition of hydrogen to a carbon-carbon double bond is called hydrogenation.The overall effect of such an addition is the reductive removal of thedouble bond functional group. Regioselectivity is not an issue, sincethe same group (a hydrogen atom) is bonded to each of the double bondcarbons. The simplest source of two hydrogen atoms is molecularhydrogen (H2), but mixing alkenes with hydrogen does not result in anydiscernible reaction. Although the overall hydrogenation reaction isexothermic, a high activation energy prevents it from taking placeunder normal conditions. This restriction may be circumvented by theuse of a catalyst, as shown in the following diagram.

Catalysts are substances that changes the rate (velocity) of a chemicalreaction without being consumed or appearing as part of the product.Catalysts act by lowering the activation energy of reactions, but theydo not change the relative potential energy of the reactants andproducts. Finely divided ****ls, such as platinum, palladium andnickel, are among the most widely used hydrogenation catalysts.Catalytic hydrogenation takes place in at least two stages, as depictedin the diagram. First, the alkene must be adsorbed on the surface ofthe catalyst along with some of the hydrogen. Next, two hydrogens shiftfrom the ****l surface to the carbons of the double bond, and theresulting saturated hydrocarbon, which is more weakly adsorbed, leavesthe catalyst surface. The exact nature and timing of the last events isnot well understood.
As shown in the energy diagram, the hydrogenation of alkenes isexothermic, and heat is released corresponding to the ΔE (coloredgreen) in the diagram. This heat of reaction can be used to evaluatethe thermodynamic stability of alkenes having different numbers ofalkyl substituents on the double bond. For example, the following tablelists the heats of hydrogenation for three C5H10 alkenes which give thesame alkane product (2-methylbutane). Since a large heat of reactionindicates a high energy reactant, these heats are inverselyproportional to the stabilities of the alkene isomers. To a roughapproximation, we see that each alkyl substituent on a double bondstabilizes this functional group by a bit more than 1 kcal/mole.
Alkene Isomer (CH3)2CHCH=CH2
3-methyl-1-butene CH2=C(CH3)CH2CH3
2-methyl-1-butene(CH3)2C=CHCH3
2-methyl-2-butene Heat of Reaction
( ΔHº ) –30.3 kcal/mole–28.5 kcal/mole–26.9 kcal/mole
From the mechanism shown here we would expect the addition of hydrogento occur with syn-stereoselectivity. This is often true, but thehydrogenation catalysts may also cause isomerization of the double bondprior to hydrogen addition, in which case stereoselectivity may beuncertain.
The formation of transition ****l complexes with alkenes has beenconvincingly demonstrated by the isolation of stable platinum complexessuch as Zeise's salt, K[PtCl3(C2H4)].H2O, andethylenebis(triphenylphosphine)platinum, [(C6H5)3P]2Pt(H2C=CH2). In thelatter, platinum is three-coordinate and zero-valent, whereas Zeise'ssalt is a derivative of platinum(II). A model of Zeise's salt and adiscussion of the unusual bonding in such complexes may be viewed by clicking here.Similar complexes have been reported for nickel and palladium, ****lswhich also function as catalysts for alkene hydrogenation.
Anon-catalytic procedure for the syn-addition of hydrogen makes use ofthe unstable compound diimide, N2H2. This reagent must be freshlygenerated in the reaction system, usually by oxidation of hydrazine,and the strongly exothermic reaction is favored by the elimination ofnitrogen gas (a very stable compound). Diimide may exist as cis-transisomers; only the cis-isomer serves as a reducing agent. Examples ofalkene reductions by both procedures are shown on the right.

5. Oxidations

(i) Hydroxylation

Dihydroxylated products (glycols) are obtained by reaction withaqueous potassium permanganate (pH > 8) or osmium tetroxide inpyridine solution. Both reactions appear to proceed by the samemechanism (shown below); the ****llocyclic intermediate may be isolatedin the osmium reaction. In basic solution the purple permanganate anionis reduced to the green manganate ion, providing a nice color test forthe double bond functional group. From the mechanism shown here wewould expect syn-stereoselectivity in the bonding to oxygen, andregioselectivity is not an issue.
When viewed in context with the previously discussed additionreactions, the hydroxylation reaction might seem implausible.Permanganate and osmium tetroxide have similar configurations, in whichthe ****l atom occupies the center of a tetrahedral grouping ofnegatively charged oxygen atoms. How, then, would such a speciesinteract with the nucleophilic pi-electrons of a double bond? Apossible explanation is that an empty d-orbital of the electrophilic****l atom extends well beyond the surrounding oxygen atoms andinitiates electron transfer from the double bond to the ****l, in muchthe same fashion noted above for platinum. Back-bonding of thenucleophilic oxygens to the antibonding π*-orbital completes thisinteraction. The result is formation of a ****llocyclic intermediate,as shown below.

(ii) Epoxidation

Some oxidation reactions of alkenes give cyclic ethers in whichboth carbons of a double bond become bonded to the same oxygen atom.These products are called epoxides or oxiranes.An important method for preparing epoxides is by reaction withperacids, RCO3H. The oxygen-oxygen bond of such peroxide derivatives isnot only weak (ca. 35 kcal/mole), but in this case is polarized so thatthe acyloxy group is negative and the hydroxyl group is positive(recall that the acidity of water is about ten powers of ten weakerthan that of a carboxylic acid). If we assume electrophilic characterfor the OH moiety, the following equation may be written.

It is unlikely that a dipolar intermediate, as shown above, is actuallyformed. The epoxidation reaction is believed to occur in a single stepwith a transition state incorporating all of the bonding events shownin the equation. Consequently, epoxidations by peracids always havesyn-stereoselectivity, and seldom give structural rearrangement. Youmay see the transition state by clicking the Change Equation button.Presumably the electron shifts indicated by the blue arrows induce acharge separation that is immediately neutralized by the green arrowelectron shifts.
The previous few reactions have been classified as reductions or oxidations, depending on the change in oxidation state of the functional carbons.It is important to remember that whenever an atom or group is reduced,some other atom or group is oxidized, and a balanced equation mustbalance the electron gain in the reduced species with the electron lossin the oxidized moiety, as well as numbers and kinds of atoms. Startingfrom an alkene (drawn in the box), the following diagram shows ahydrogenation reaction on the left (the catalyst is not shown) and anepoxidation reaction on the right. Examine these reactions, and foreach identify which atoms are reduced and which are oxidized.

Epoxides may be cleaved by aqueous acid to give glycols that are often diastereomeric with those prepared by the syn-hydroxylation reactiondescribed above. Proton transfer from the acid catalyst generates theconjugate acid of the epoxide, which is attacked by nucleophiles suchas water in the same way that the cyclic bromonium ion described aboveundergoes reaction. The result is anti-hydroxylation of thedouble bond, in contrast to the syn-stereoselectivity of the earliermethod. In the following equation this procedure is illustrated for acis-disubstituted epoxide, which, of course, could be prepared from thecorresponding cis-alkene. This hydration of an epoxide does not changethe oxidation state of any atoms or groups.

(iii) Oxidative Cleavage of Double Bonds

Ozonolysis
In determining the structural formula of an alkene, it is oftennecessary to find the ******** of the double bond within a given carbonframework. One way of accomplishing this would be to selectively breakthe double bond and mark the carbon atoms that originally formed thatbond. For example, there are three isomeric alkenes that all give2-methylbutane on catalytic hydrogenation. These are 2-methyl-2-butene(compound A), 3-methyl-1-butene (compound B) and 2-methyl-1-butene(compound C), shown in the following diagram. If the double bond iscleaved and the fragments marked at the cleavage sites, the ******** ofthe double bond is clearly determined for each case. A reaction thataccomplishes this useful transformation is known. It is called ozonolysis, and its application to each of these examples may be seen by clicking the "Show Reaction" button.

Ozone, O3, is an allotrope of oxygen that adds rapidly to carbon-carbondouble bonds. Since the overall change in ozonolysis is more complexthan a simple addition reaction, its mechanism has been extensivelystudied. Reactive intermediates called ozonides have been isolated fromthe interaction of ozone with alkenes, and these unstable compounds maybe converted to stable products by either a reductive workup (Zn dustin water or alcohol) or an oxidative workup (hydrogen peroxide). Theresults of an oxidative workup may be seen by clicking the "Show Reaction"button a second time. Continued clicking of this button repeats thecycle. The chief difference in these conditions is that reductiveworkup gives an aldehyde product when hydrogen is present on a doublebond carbon atom, whereas oxidative workup gives a carboxylic acid orcarbon dioxide in such cases. The following equations illustrateozonide formation, a process that is believed to involve initialsyn-addition of ozone, followed by rearrangement of the extremelyunstable molozonide addition product. They also show the decompositionof the final ozonide to carbonyl products by either a reductive oroxidative workup.