Addition of HX or X2 to Alkenes
Addition of Halogen Acids to Alkenes
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...
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.
Radical Addition of HBr to Alkenes
The addition of HBr to alkenes in the presence of peroxides converts the alkene into an alkyl bromide. The overall addition of HBr to the double bond is anti-Markovnikov, with the bromine being bonded to the alkene carbon which would form the least stable carbocationic center. The reaction involves attack of bromine radical to generate a radical on one of the alkene carbons. The selectivity arises due to the tendency to direct addition in order to form the most stable radical intermediate; rearrangements do not occur. In the example shown below, the reaction is initiated by the addition of bromine radical to the alkene in the cyclohexene ring to form a radical intermediate. The bromine bonds to the secondary carbon since this leaves the more stable tertiary radical on carbon (radical stability largely parallels carbocation stability). In a second step, this tertiary radical picks up a hydrogen atom (from HBr) to give the final product. Since the radical is planar, as shown above for carbocations, there is no stereochemical preference in the addition reaction.
Addition of Halogen to Alkenes
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).
Sample Reactions:
Predict the major organic product for each of the following reactions. Be sure to show stereochemistry, when appropriate, and to draw the final product in its most stable conformation.
Addition of HX to alkenes converts the alkene into an alkyl halide. The overall addition of HCl to the double bond follows the Markovnikov convention, with the bromine being bonded to the most stable carbocationic center. The reaction generally involves a carbocation intermediate and rearrangements are possible. Since this reaction would represent addition of bromide anion to a secondary carbocation in a molecule containing no tertiary carbons, a rearrangement is unlikely.
Addition of HBr to alkenes in the presence of peroxides converts the alkene into an alkyl bromide. The overall addition of HBr to the double bond is anti-Markovnikov, with the bromine being bonded to the alkene carbon which would form the least stable carbocationic center. The reaction involves attack of bromine radical to generate a radical on one of the alkene carbons. The selectivity arises due to the tendency to direct addition in order to form the most stable radical intermediate; rearrangements do not occur.
Addition of halogen to alkenes converts the alkene into a trans-1,2-dihalide. The reaction involves initial addition of bromine to the alkene p-cloud, followed by backside attack of bromide to form the trans-1,2-dibromide. In the initially formed addition product, both of the bromines must be di-axial; the structure shown above shows the ring-inversion product in which both halogens are in the equatorial positions.
Addition of HX to alkenes converts the alkene into an alkyl halide. The overall addition of HCl to the double bond follows the Markovnikov convention, with the bromine being bonded to the most stable carbocationic center. The reaction generally involves a carbocation intermediate and rearrangements are possible. Since this reaction would represent addition of bromide anion to a tertiary carbocation, no rearrangement is observed in the present instance.
Addition of HBr to alkenes in the presence of peroxides converts the alkene into an alkyl bromide. The overall addition of HBr to the double bond is anti-Markovnikov, with the bromine being bonded to the alkene carbon which would form the least stable carbocationic center. The reaction involves attack of bromine radical to generate a radical on one of the alkene carbons. The selectivity arises due to the tendency to direct addition in order to form the most stable radical intermediate; rearrangements do not occur.
Addition of halogen to alkenes converts the alkene into a trans-1,2-dihalide. The reaction involves initial addition of chlorine to the alkene p-cloud, followed by backside attack of chloride to form the trans-1,2-dichloride. In the initially formed addition product, both of the chlorines must be di-axial; the structure shown above shows the geometry in which both halogens are re-positioned to generate the least amount of ring-strain.
Addition of HX to alkenes converts the alkene into an alkyl halide. The overall addition of HCl to the double bond follows the Markovnikov convention, with the bromine being bonded to the most stable carbocationic center. The reaction generally involves a carbocation intermediate and rearrangements are possible. Since this reaction would represent addition of bromide anion to a secondary carbocation in a molecule containing no tertiary carbons, a rearrangement is unlikely.
Addition of HBr to alkenes in the presence of peroxides converts the alkene into an alkyl bromide. The overall addition of HBr to the double bond is anti-Markovnikov, with the bromine being bonded to the alkene carbon which would form the least stable carbocationic center. The reaction involves attack of bromine radical to generate a radical on one of the alkene carbons. The selectivity arises due to the tendency to direct addition in order to form the most stable radical intermediate; rearrangements do not occur.
Addition of halogen to alkenes converts the alkene into a trans-1,2-dihalide. The reaction involves initial addition of bromine to the alkene p-cloud, followed by backside attack of bromide to form the trans-1,2-dibromide. In the initially formed addition product, both of the bromines must be trans-coplanar; in the structure shown above, there is no stereochemistry.
Addition of HX to alkenes converts the alkene into an alkyl halide. The overall addition of HCl to the double bond follows the Markovnikov convention, with the bromine being bonded to the most stable carbocationic center. The reaction generally involves a carbocation intermediate and rearrangements are possible. Since this reaction would represent addition of bromide anion to a secondary carbocation in a molecule containing no tertiary carbons, a rearrangement is unlikely.
Addition of HBr to alkenes in the presence of peroxides converts the alkene into an alkyl bromide. The overall addition of HBr to the double bond is anti-Markovnikov, with the bromine being bonded to the alkene carbon which would form the least stable carbocationic center. The reaction involves attack of bromine radical to generate a radical on one of the alkene carbons. The selectivity arises due to the tendency to direct addition in order to form the most stable radical intermediate; rearrangements do not occur.