The resulting radical is formed on the carbon of the alkene which is best abl 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 Markovni kov'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 (sp ),and hydrogen abstraction from the second molecule of HBr can occur from either above, or below the plane defined by the sp 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 The addition of halogen to alkenes is a stepwise process involving a"halonium"ion intermediate. The ormation of this intermediate is initiated through attack of halogen on the alkene T-system, to form the cyclic halonium ion (i.e, bromonium or chloronium ion) and expel the halogen anion (ie, 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 be 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 processThe 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. 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 -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 be 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