1、Chapter 8 Reactions of Alkenes 8-1 Addition Reactions of Alkenes The most common chemical transformation of a carbon-carbon double bond is the addition reaction. A large number of reagents, both inorganic and organic, have been found to add to this functional group. A majority of these reactions are

2、 exothermic, due to the fact that the C-C pi-bond is relatively weak (ca. 63 kcal/mole) relative to the sigma-bonds formed to the atoms or groups of the reagent. Remember, the bond energies of a molecule are the energies required to break (homolytically) all the covalent bonds in the molecule. Conse

3、quently, if the bond energies of the product molecules are greater than the bond energies of the reactants, the reaction will be exothermic. The following calculations for the addition of H-Br are typical. Note that by convention exothermic reactions have a negative heat of reaction.,Addition of Str

4、ong Brnsted Acids As illustrated by the preceding general equation, strong Brnsted acids such as HCl, HBr, HI and since oxygen is more electronegative than chlorine or bromine, the electrophile will be a halide cation. The nucleophilic species that bonds to the intermediate carbocation is then hydro

5、xide ion, or more likely water (the usual solvent for these reagents), and the products are called halohydrins. Sulfenyl chlorides add in the opposite manner because the electrophile is a sulfur cation, RS(+), whereas the nucleophilic moiety is chloride anion (chlorine is more electronegative than s

6、ulfur).,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 mercuric acetate and boron hydrides (compounds shown at the bottom of of the reagent list) are normally not isolated, but instead are c

7、onverted to alcohols by a substitution reaction. These important synthetic transformations are illustrated for 2-methylpropene by the following equations, in which the electrophilic moiety is colored red and the nucleophile blue. The top reaction sequence illustrates the oxymercuration procedure and

8、 the bottom is an example of hydroboration.,The light blue vertical line separates the addition reaction on the left from the substitution on the right. The atoms or groups that have been added to the original double bond are colored orange in the final product. In both cases the overall reaction is

9、 the addition of water to the double bond, but the regioselectivity is reversed. The oxymercuration reaction gives the product predicted by Markovnikovs rule; hydroboration on the other hand gives the anti-Markovnikov product. Complementary reactions such as these are important because they allow us

10、 to direct a molecular transformation whichever way is desired. Mercury and boron are removed from the organic substrate in the second step of oxymercuration and hydroboration respectively. These reactions are seldom discussed in detail; however, it is worth noting that the mercury moiety is reduced

11、 to metallic mercury by borohydride (probably by way of radical intermediates), and boron is oxidized to borate by the alkaline peroxide.,Addition of hydroperoxide anion to the electrophilic borane generates a tetra-coordinate boron peroxide, having the general formula R3B-O-OH(-). This undergoes su

12、ccessive intramolecular shifts of alkyl groups from boron to oxygen, accompanied in each event by additional peroxide addition to electron deficient boron. The retention of configuration of the migrating alkyl group is attributed to the intramolecular nature of the rearrangement. Since the oxymercur

13、ation sequence gives the same hydration product as acid-catalyzed addition of water, we might question why this two-step procedure is used at all. The reason lies in the milder reaction conditions used for oxymercuration. The strong acid used for direct hydration may not be tolerated by other functi

14、onal groups, and in some cases may cause molecular rearrangement. The addition of borane, BH3, requires additional comment. In pure form this reagent is a dimeric gas B2H6, called diborane, but in ether or THF solution it is dissociated into a solvent coordinated monomer, R2O-BH3. Although diborane

15、itself does not react easily with alkene double bonds, H.C. Brown (Purdue, Nobel Prize 1979) discovered that the solvated monomer adds rapidly under mild conditions.,Boron and hydrogen have rather similar electronegativities, with hydrogen being slightly greater, so it is not likely there is signifi

16、cant dipolar character to the B-H bond. Since boron is electron deficient (it does not have a valence shell electron octet) the reagent itself is a Lewis acid and can bond to the pi-electrons of a double bond by displacement of the ether moiety from the solvated monomer. As shown in the following eq

17、uation, this bonding might generate a dipolar intermediate consisting of a negatively-charged boron and a carbocation. Such a species would not be stable and would rearrange to a neutral product by the shift of a hydride to the carbocation center.,Indeed, this hydride shift is believed to occur conc

18、urrently with the initial bonding to boron, as shown by the transition state drawn below the equation, so the discrete intermediate shown in the equation is not actually formed. Nevertheless, the carbocation stability rule cited above remains a useful way to predict the products from hydroboration r

19、eactions. Note that this addition is unique among those we have discussed, in that it is a single-step process. Also, all three hydrogens in borane are potentially reactive, so that the alkyl borane product from the first addition may serve as the hydroboration reagent for two additional alkene mole

20、cules.,Stereoselectivity in Addition Reactions to Double Bonds,As illustrated in the drawing, the pi-bond fixes the carbon-carbon double bond in a planar configuration, and does not permit free rotation about the double bond itself.,We see then that addition reactions to this function might occur in

21、 three different ways, depending on the relative orientation of the atoms or groups that add to the carbons of the double bond: (i) they may bond from the same side, (ii) they may bond from opposite sides, or (iii) they may bond randomly from both sides.,The first two possibilities are examples of s

22、tereoselectivity, the first being termed syn-addition, and the second anti-addition. Since initial electrophilic attack on the double bond may occur equally well from either side, it is in the second step (or stage) of the reaction (bonding of the nucleophile) that stereoselectivity may be imposed.

23、If the two-step mechanism described above is correct, and if the carbocation intermediate is sufficiently long-lived to freely-rotate about the sigma-bond component of the original double bond, we would expect to find random or non-stereoselective addition in the products. On the other hand, if the

24、intermediate is short-lived and factors such as steric hindrance or neighboring group interactions favor one side in the 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

25、Brnsted acids. The interesting differences in stereoselectivity noted here provide further insight into the mechanisms of these addition reactions.,1. Brnsted Acid Additions The stereoselectivity of Brnsted acid addition is sensitive to experimental conditions such as temperature, solvent and reagen

26、t concentration. The selectivity is often anti, but reports of syn selectivity and non-selectivity are not uncommon. Of all the reagents discussed here, these strong acid additions (E = H in the following equation) come closest to proceeding by the proposed two-step mechanism in which a discrete car

27、bocation intermediate is generated in the first step. Such reactions are most prone to rearrangement when this is favored by the alkene structure.,2. Addition Reactions Initiated by Electrophilic Halogen The halogens chlorine and bromine add rapidly to a wide variety of alkenes without inducing the

28、kinds of structural rearrangements noted for strong acids (first example below). The stereoselectivity of these additions is strongly anti, as shown in many of the following examples.,An important principle should be restated at this time. The alkenes shown here are all achiral, but the addition pro

29、ducts have chiral centers, and in many cases may exist as enantiomeric stereoisomers. In the absence of chiral catalysts or reagents, reactions of this kind will always give racemic mixtures if the products are enantiomeric. On the other hand, if two chiral centers are formed in the addition the rea

30、ction will be diastereomer selective. This is clearly shown by the addition of bromine to the isomeric 2-butenes. Anti-addition to cis-2-butene gives the racemic product, whereas anti-addition to the trans-isomer gives the meso-diastereomer.,We can account both for the high stereoselectivity and the

31、 lack of rearrangement in these reactions by proposing a stabilizing interaction between the developing carbocation center and the electron rich halogen atom on the adjacent carbon. This interaction, which is depicted for bromine in the following equation, delocalizes the positive charge on the inte

32、rmediate and blocks halide ion attack from the syn-location.,The stabilization provided by this halogen-carbocation bonding makes rearrangement unlikely. In a few cases three-membered cyclic halonium cations have been isolated and identified as true intermediates. A resonance description of such a b

33、romonium ion intermediate is shown below. The positive charge is delocalized over all the atoms of the ring, but should be concentrated at the more substituted carbon (carbocation stability), and this is the site to which the nucleophile will bond.,Because they proceed by way of polar ion-pair inter

34、mediates, chlorine and bromine addition reactions are faster in polar solvents than in non-polar solvents, such as hexane or carbon tetrachloride. However, in order to prevent solvent nucleophiles from competing with the halide anion, these non-polar solvents are often selected for these reactions.

35、In water or alcohol solution the nucleophilic solvent may open the bromonium ion intermediate to give an -halo-alcohol or ether, together with the expected vic-dihalide. Such reactions are sensitive to pH and other factors, so when these products are desired it is necessary to modify the addition re

36、agent. Aqueous chlorine exists as the following equilibrium, Keq 10-4. By adding AgOH, the concentration of HOCl can be greatly increased, and the chlorohydrin addition product obtained from alkenes.,Cl2 + H2O HOCl + HCl,The more widely used HOBr reagent, hypobromous acid, is commonly made by hydrol

37、ysis of N-bromoacetamide, as shown below. Both HOCl and HOBr additions occur in an anti fashion, and with the regioselectivity predicted by this mechanism (OH bonds to the more substituted carbon of the alkene).,CH3CONHBr + H2O HOBr + CH3CONH2,Vicinal halohydrins provide an alternative route for the

38、 epoxidation of alkenes over that of reaction with peracids. As illustrated in the following diagram, a base induced intramolecular substitution reaction forms a three-membered cyclic ether called an epoxide. Both the halohydrin formation and halide displacement reactions are stereospecific, so ster

39、eoisomerism in the alkene will be reflected in the epoxide product (i.e. trans-2-butene forms a trans-disubstituted epoxide).,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 the double bo

40、nd functional group. Regioselectivity is not an issue, since the same group (a hydrogen atom) is bonded to each of the double bond carbons. The simplest source of two hydrogen atoms is molecular hydrogen (H2), but mixing alkenes with hydrogen does not result in any discernable reaction. Although the

41、 overall hydrogenation reaction is exothermic, a high activation energy prevents it from taking place under normal conditions. This restriction may be circumvented by the use of a catalyst, as shown in the following diagram.,Catalysts are substances that changes the rate (velocity) of a chemical rea

42、ction without being consumed or appearing as part of the product. Catalysts act by lowering the activation energy of reactions, but they do not change the relative potential energy of the reactants and products. Finely divided metals, such as platinum, palladium and nickel, are among the most widely

43、 used hydrogenation catalysts. Catalytic hydrogenation takes place in at least two stages, as depicted in the diagram. First, the alkene must be adsorbed on the surface of the catalyst along with some of the hydrogen. Next, two hydrogens shift from the metal surface to the carbons of the double bond

44、, and the resulting saturated hydrocarbon, which is more weakly adsorbed, leaves the catalyst surface. The exact nature and timing of the last events is not well understood.,As shown in the energy diagram, the hydrogenation of alkenes is exothermic, and heat is released corresponding to the E (color

45、ed green) in the diagram. This heat of reaction can be used to evaluate the thermodynamic stability of alkenes having different numbers of alkyl substituents on the double bond. For example, the following table lists the heats of hydrogenation for three C5H10 alkenes which give the same alkane produ

46、ct (2-methylbutane). Since a large heat of reaction indicates a high energy reactant, these heats are inversely proportional to the stabilities of the alkene isomers. To a rough approximation, we see that each alkyl substituent on a double bond stabilizes this functional group by a bit more than 1 k

47、cal/mole.,From the mechanism shown here we would expect the addition of hydrogen to occur with syn-stereoselectivity. This is often true, but the hydrogenation catalysts may also cause isomerization of the double bond prior to hydrogen addition, in which case stereoselectivity may be uncertain. A no

48、n-catalytic procedure for the syn-addition of hydrogen makes use of the unstable compound diimide, N2H2. This reagent must be freshly generated in the reaction system, usually by oxidation of hydrazine, and the strongly exothermic reaction is favored by the elimination of nitrogen gas (a very stable

49、 compound). Diimide may exist as cis-trans isomers; only the cis-isomer serves as a reducing agent. Examples of alkene reductions by both procedures are shown.,5. Oxidations (i) Hydroxylation Dihydroxylated products (glycols) are obtained by reaction with aqueous potassium permanganate (pH 8) or osm

50、ium tetroxide in pyridine solution. Both reactions appear to proceed by the same mechanism (shown below);,The metallocyclic intermediate may be isolated in the osmium reaction. In basic solution the purple permanganate anion is reduced to the green manganate ion, providing a nice color test for the

51、double bond functional group. From the mechanism shown here we would expect syn-stereoselectivity in the bonding to oxygen, and regioselectivity is not an issue.,When viewed in context with the previously discussed addition reactions, the hydroxylation reaction might seem implausible. Permanganate a

52、nd osmium tetroxide have similar configurations, in which the metal atom occupies the center of a tetrahedral grouping of negatively charged oxygen atoms. How, then, would such a species interact with the nucleophilic pi-electrons of a double bond? A possible explanation is that an empty d-orbital o

53、f the electrophilic metal atom extends well beyond the surrounding oxygen atoms and initiates electron transfer from the double bond to the metal. Back-bonding of the nucleophilic oxygens to the antibonding pi orbital completes this interaction. The result is formation of a metallocyclic intermediat

54、e, as shown.,(ii) Epoxidation Some oxidation reactions of alkenes give cyclic ethers in which both 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 with peracids, RCO3H. The oxyge

55、n-oxygen bond of such peroxide derivatives is not only weak (ca. 35 kcal/mole), but in this case is polarized so that the acyloxy group is negative and the hydroxyl group is positive (recall that the acidity of water is about ten powers of ten weaker than that of a carboxylic acid). If we assume ele

56、ctrophilic character for the OH moiety, the following equation may be written.,It is unlikely that a dipolar intermediate, as shown above, is actually formed. The epoxidation reaction is believed to occur in a single step with a transition state incorporating all of the bonding events shown in the e

57、quation. Consequently, epoxidations by peracids always have syn-stereoselectivity, and seldom give structural rearrangement. Presumably the electron shifts indicated by the blue arrows induce a charge separation that is immediately neutralized by the green arrow electron shifts.,The previous few rea

58、ctions 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 must balance the electron gain in the re

59、duced species with the electron loss in the oxidized moiety, as well as numbers and kinds of atoms. Starting from an alkene (drawn in the box), the following diagram shows a hydrogenation reaction on the left (the catalyst is not shown) and an epoxidation reaction on the right. Examine these reactions, and for each 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-hydrox