Alkenes: Addition & Oxidation Reactions

Forward: General Mechanistic Traits of Alkene Addition Reactions

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General Mechanisms of Alkene Addition Reactions:

The majority of the reactions of alkenes which will be described in this section fall into three basic categories:

  • ionic additions, which are initiated by an electrophilic agent interacting with the alkene p-cloud, activating the alkene carbons to nucleophilic additions,
  • syn addition reactions occurring on one side of the alkene p-cloud, by either radical or concerted mechanisms, and,
  • oxidative cleavage reactions in which the carbon-carbon double bond is cleaved to form di-carbonyl derivatives.
In the first of these, the alkene p-cloud, functioning as a Lewis base (an electron donor), donates electron density to a Lewis acid (in these examples, a proton, a halogen cation (halonium ion), or mercuric ion). The complex, bearing a positive charge, is now highly reactive towards nucleophiles in the system (anions or water) and undergoes attack anti to the activating Lewis acid, to form the final addition product. When the nucleophile is of low reactivity, the initial complex may rearrange to form a sigma bond to the Lewis acid, leaving a full carbocation on the adjacent carbon. As with all carbocations, this can react with nucleophiles from either face (being planar) and can undergo rearrangement reactions. Reactions involving these carbocation species are most common in the acid-catalyzed addition of water to alkenes, and with halogen acids (HCl and HBr).

Syn additions to the alkene p-cloud which are covered in this section include hydrogenation, gem-diol formation from MnO4-, organoborane formation from BH3 and carbene-dependent cyclopropanation reactions. In each of these, an electrophilic agent reacts with the p-system to (more or less) simultaneously form bonds to both carbons. In hydrogenation, the hydrogen gas is rendered electrophilic by adsorption to a metal surface (i.e., Pt). The metal surface also binds the alkene, activating the addition, and a variety of carbon-metal intermediate species are probably involved. Trivalent boron is a powerful Lewis acid and reaction with BH3 is probably initiated by the formation of a p-complex, as described above; this complex, however, seems to decompose in a concerted fashion to form the syn borane.

Oxidative cleavage of alkene generally involves the intermediate formation of a bridged oxygen species of some sort, followed by spontaneous or reductive cleavage. The examples covered in this section include ozonolysis, where the intermediate ozonide is decomposed by a dissolving-metal reduction, and acid-catalyzed cleavage of the intermediate metal di-ester formed during MnO4- oxidation.

As you work through each of these sets of reactions, you should pay particular attention to the common features of the mechanisms in each group. An understanding of a few basic chemical generalities will help you view reactions as broad classes, and help you avoid overt memorization.

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The addition of halogen acids to alkenes is a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the halogen acid donates a proton to the alkene p-system, which is functioning as a Lewis base. The protonated p-system has a short lifetime and can rapidly revert to starting materials, or can rearrange from a (cationic) protonated p-bond, to an sp3 sigma bond adjacent to an sp2 carbocation center. If the alkene is asymmetrical, the protonated p-cloud intermediate can break down by two pathways, as shown below, to potentially form carbocations having differing ground-state energies. The reaction pathways leading from this intermediate to the two carbocations will differ in energy, and, in general, the pathway leading to the more stable intermediate will be of lower energy, and will be the preferred pathway.

The resulting carbocation is formed on the carbon of the alkene which is best able to stabilize the cationic center. In simple unstrained non-conjugated systems, without adjacent heteroatoms, the order of stability of carbocations will be tertiary > secondary > primary. Since tertiary centers have no attached hydrogens, secondary centers have one and primary centers have two, there is an apparent inverse relationship between the "number of attached hydrogens" and the likelihood that the carbocation will form at that center. This is the origin of Markovnikov's Rule, which states that...

...in the addition of HX to an alkene, the proton will attach to the center having the greatest number of hydrogens...

often restated as "them that has, gets". While the rule is a useful guide, you should remember that the selectivity is actually to place the carbocation on the carbon which can best stabilize the charge.

Once the carbocation is formed, the most favorable reaction will involve the addition of a nucleophile to form an sp3 center. In the reaction with halogen acid (HCl and HBr), the most nucleophilic molecules in the system will be the chloride and bromide anions. Attack of these on the planar (sp2) carbocation can occur from either above, or below the plane defined by the sp2 center, and the net addition of HX can therefore occur either syn (cis; on the same side) or anti (trans; on the opposite side), relative to the hydrogen atom.

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The addition of HBr to an alkene in the presence of peroxides is a stepwise process involving radical intermediates. The radical chain reaction is initiated by the peroxide, and the most energetically favorable initiation reaction is abstraction of a hydrogen atom from HBr by the peroxy radical, to generate the bromine radical.

The bromine radical is electrophilic and attacks the alkene p-system, which donates an electron, forming a s>-bond to the bromine and leaving an unpaired electron (a radical) on one of the carbons of the alkene. If the alkene is asymmetrical, bond formation can occur by two pathways, as described above for halogen acid addition. Once again, the reaction pathways leading to the two radicals will differ in energy, and, in general, the pathway leading to the more stable intermediate will be of lower energy, and will be the preferred pathway.

The resulting radical is formed on the carbon of the alkene which is best able to stabilize the electrophilic site (the unpaired electron). In simple unstrained non-conjugated systems, without adjacent heteroatoms, the order of stability of carbon radicals parallels that of carbocations, with tertiary > secondary > primary. Since tertiary centers have no attached hydrogens, secondary centers have one and primary centers have two, there is an apparent inverse relationship between the "number of attached hydrogens" and the likelihood that the radical will form at that center. The carbon radical which is formed abstracts a hydrogen atom (most likely from HBr), propagating the chain and giving one mole of product. In this product, the "hydrogen" attached to the center which formed the radical, that is, the center with "the fewest number of hydrogens" (a secondary or tertiary center) and the bromine is attached to the carbon which is adjacent to the most stable radical ("the center with the most hydrogens"). This is opposite to Markovnikov's Rule, as described in the previous example, and the orientation in this reaction is often termed "anti-Markovnikov". While the rule is a useful guide, you should remember that the selectivity is actually to place the radical character on the carbon which can best stabilize the unpaired electron (the electrophilic center).

As with carbocation intermediates, carbon radicals are planar (sp2), and hydrogen abstraction from the second molecule of HBr can occur from either above, or below the plane defined by the sp2 center. The net addition of HBr can therefore occur either syn (cis; on the same side) or anti (trans; on the opposite side), as described in the previous example.

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The addition of halogen to alkenes is a stepwise process involving a "halonium" ion intermediate. The formation of this intermediate is initiated through attack of halogen on the alkene p-system, to form the cyclic halonium ion (i.e., bromonium or chloronium ion) and expel the halogen anion (i.e., bromide or chloride). This intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system; that is, the halide anion expelled in the previous step. Since the halonium ion effectively blocks attack by halide on the same side, attack must come from the backside (relative to the large halogen atom) to form the trans-1,2-dihalide. This is demonstrated below for the addition of bromine to 1-propene. The large bromine on the intermediate bromonium ion (shown as a space-filling overlay) effectively blocks attack from the top, forcing the addition to be anti (trans; from the opposite side). The attack of Br+ on the bromonium ion is an example of an SN2 reaction in which a nucleophile attacks at the carbon and displaces the leaving group is a single, smooth, concerted process..

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As with the addition of halogen, the addition of hypohalous acid (HOX) to alkenes is a stepwise process which also involves the "halonium" ion intermediate. As described previously for the addition of halogen (X2), the halonium ion intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system. In the case of HOX addition, the most powerful nucleophile in the system is hydroxide anion. Again, because of the steric bulk of the halonium ion, attack by hydroxide must come from the backside (relative to the large halogen atom) to form the trans product. If the alkene is asymmetrical, hydroxide can potentially attack at either carbon. The distribution of positive charge, however, will not necessarily be the same on both carbons, and attack will typically occur on the carbon which is best able to stabilize the partial positive charge. In general, this will also be the carbon which would form the most stable carbocation, and, as a general rule, hydroxide anion will attach to the carbon of the alkene which would form the most stable carbocation. Another way to view the selectivity is using Markovnikov's Rule; the carbon which would get the halogen in the addition of HX, will get the hydroxyl group in the addition of HOX.

As before, the large halogen on the intermediate halonium ion effectively blocks attack from the top, forcing the addition of halogen and hydroxide to be anti (trans; on the opposite side), relative to each other.

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The acid-catalyzed addition of water to alkenes is another example of a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the acid donates a proton to the alkene p-system, which is functioning as a Lewis base. The protonated p-system rearranges to form an sp3 sigma bond adjacent to an sp2 carbocation center. If the alkene is asymmetrical, two carbocations are possible and the addition will proceed to form the most stable carbocation. As before, tertiary centers will be favored over secondary, which are preferred over primary, and overall addition of water will follow the order predicted by Markovnikov's Rule.

Once the carbocation is formed, reaction with water (a relatively poor nucleophile) will be sufficiently slow such that the carbocation has a long enough lifetime to undergo rearrangement. In general, the molecule will rearrange by a combination of alkyl-, methyl- and hydride-transfers to form the most stable carbocation. Because of this, acid-catalyzed hydration can only be utilized to prepare alcohols where rearrangement is not considered to be a problem (i.e., tertiary and some secondary alcohols).

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The reaction of alkenes with mercuric acetate follows the general mechanism for Lewis acid activation of alkene addition reactions. Loss of acetate anion and chelation of the mercury with the alkene p-cloud generates a bridged, cationic intermediate. As in the previous examples, if the alkene is asymmetrical, the more positive charge will be localized on the carbon of the alkene which is best able to stabilize it (tertiary > secondary > primary). The steric bulk of the bridged mercury ion will direct attack to the opposite face of the alkene and the best nucleophile in the system (hydroxide anion) will attack at the carbon bearing the greatest positive charge. Again, this follows the Markovnikov convention.
The organomercurial formed is stable (one of the contributing factors to mercury toxicity) and must be removed by reduction with sodium borohydride. The borohydride replaces the mercury atom with a hydrogen with retention of configuration (front-side attack) to generate an alcohol with overall anti (trans) stereochemistry.

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The reaction of BH3 with an alkene begins with the Lewis acid chelation of the alkene p-system by the boron. This complex then rearranges in a more or less concerted manner to produce the alkyl borane, as shown below. The reaction seems to be dominated by steric effects and the boron attaches to the least hindered carbon. All three equivalents of the boron hydride can be utilized in separate reactions to give a trialkyl borane.

The organoborane which is formed can be oxidized by alkaline peroxide to form the alcohol by a mechanism which involves attack of peroxide anion on the boron, followed by alkyl migration to the oxygen, with loss of hydroxide anion. The resulting borate ester is rapidly hydrolyzed by the alkaline conditions. The overall stereochemistry of the addition reaction is syn (cis) and the regiochemistry of the product is generally anti-Markovnikov, due to the preference of the boron for the least hindered site (...the carbon with the most hydrogens ultimately gets the hydroxyl group...).

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Catalytic hydrogenation of alkenes produces the corresponding alkane, with syn (cis) addition of hydrogen. The reaction requires a metal catalyst, usually Pt, Pd or Ni, and the mechanism involves adsorption of hydrogen to the metal surface, followed by adsorption of the alkene (probably through chelation of the p-system). Hydrogen transfer occurs in a strictly cis manner, probably due to the geometric constraints of the metal surface. The detailed mechanism is not trivial, and probably involves several metal-carbon bonded species.

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Osmium tetroxide reacts rapidly with alkenes in ether to form a black precipitate, which is thought to be the cyclic osmium diester, as shown above. The intermediate can be readily converted to the 1,2-diol by reduction with bisulfite anion or H2S. The stereochemistry of the addition is strictly cis, since both oxygens come from the OsO4. The main disadvantage of the reaction, synthetically, is that OsO4 is both expensive and quite toxic.

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Cold, alkaline potassium permanganate will also convert alkenes into cis-1,2-diols. The reaction is often difficult to control and over-oxidation is a common side reaction. As with OsO4, the intermediate is thought to be a cyclic diester, as shown above. Unlike the osmium complex, the manganate ester is rapidly hydrolyzed under the reaction conditions to yield the diol without the need for the reduction step. As with OsO4, the addition is strictly cis, since both oxygens originate on the MnO4-. Permanganate is inexpensive and is much less toxic than OsO4, making it the reagent of choice for routine preparations where low yields and product mixtures are not significant problems.

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Chloroform (CHCl3) in alkaline solution produces a small equilibrium concentration of the highly electrophilic dichlorocarbene. If dichlorocarbene is generated in the presence of an alkene, it rapidly adds to the double bond to give a 1,1-dichlorocyclopropane derivative. You should note that carbenes are neutral, that they have a geometry that approximates sp2 (with the lone pair in one of these orbitals) and that they have empty p-orbitals. This combination makes them resemble both a carbanion (the lone pair) and a carbocation (a vacant p-orbital), all the same, highly reactive molecule.

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Unsubstituted carbene can be prepared by photolysis or pyrolysis of diazomethane (CH2N2; a somewhat dangerous undertaking) or more conveniently by the reaction of diiodomethane with a zinc-dust-copper alloy. The reaction probably involves the formation of carbon-zinc intermediates, which generate carbene by the elimination of zinc iodide. The reaction gives only moderate yields, but is useful for the preparation of cyclopropane rings, which are often difficult to prepare by other methods.

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Oxidation of Alkenes

Ozonolysis of alkenes, followed by dissolving metal reduction with Zn dust in acid, smoothly converts alkenes into aldehydes and ketones, depending on the nature of the groups attached to the sp2 carbons. The reaction involves the intermediate formation of an ozonide, in which the two halves of the alkene are joined by bridging oxygen atoms. Reduction the ozonide forms aldehydes from terminal alkenes and from sp2 centers attached to one additional carbon. Likewise, ketones are formed from sp2 centers attached to two additional carbons.

Acidic potassium permanganate is a powerful oxidant towards organic molecules and will readily cleave alkenes. Disubstituted sp2 carbons are converted into ketones, as with ozone, while sp2 carbons having lesser substitution will be converted into fragments having one higher oxidation state than that formed by reaction with ozone. Thus, terminal sp2 carbons are converted into CO2 instead of formaldehyde (H2C=O) and sp2 carbons attached to one additional carbon are converted into carboxylic acids, instead of aldehydes. Permanganate oxidation generally gives average-to-poor yields and over-oxidation is a major side reaction.

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