Initiation requires an input of energy but after that the reaction is self-sustaining. However, in the presence of organic peroxides, hydrogen bromide adds by a different mechanism. This forms an anion, which can be protonated by a hydrogen in an ammonia solvent. CH3-CH2-CH3 + Br2 → 3% CH3-CH2-CH2Br + 97% CH3-CHBr-CH3. Allylic chlorination has important practical applications in industry. In most cases, this is a homolytic cleavage event, and takes place very rarely due to the high energy barriers involved. By the way, we will address the question of what “radical conditions” means, i.e., what conditions are necessary in order to bring about the radical chain addition in preference to the competitive electrophilic addition. K. Peter C. Vollhardt, Neil E. Schore; Organic Chemistry: Structure and Function Fifth Edition; W. H. Freeman and Campany, 2007, Micheal Vokin; Nuffield Advance Chemistry Student’s Book Forth Edition; Person Education Limited, 2004. 87 kcal/mol). Strong sources of light can also serve cleavage this bond. q Radical inhibitors can be added to intercept chain-carrying radicals and stop the radical chains. Because of their high reactivity, free radicals have the potential to be both extremely powerful chemical tools and extremely harmful contaminants. q The first step of the propagation cycle is feasible because the newly formed C-Br bond is stronger than the pi bond which is broken (Use your equation for calculating approximate enthalpies of reaction in terms of the D’s of the bonds broken and formed.. q The second step is feasible because the newly formed C-H bond is stronger than the H-Br bond (D for this bond is ca. Learning Skills Center. That creates a new radical with the single electron on the other carbon atom. q Radical chain reactions typically do not have a single rds, but in this case the addition of bromine atoms to the pi bond is of special importance because it determines the regiochemistry. In this case, we isolate 1-bromobutane, formed by anti-Markovnikov addition. So the radical chain addition of HBr is also regiospecific, but has the opposite sense of regiospecificity, which we call anti-Markovnikov regiospecificity. Once a chain reaction gets started, it is able to keep itself going. If the alkene reactant is asymmetric, two distinct product isomers are formed. Since the second propagation step is so exothermic, it occurs very quickly. That is way bromine ends up on the least substituted carbon od alkene. The halogenation of propane discloses an interesting feature of these reactions. q Note that bromine atoms prefer to add to the less substituted end of the double bond, just as protons did, etc. q The resulting alkyoxy radicals are highly reactive and abstract a hydrogen atom from a molecule of HBr, to yield bromine atoms (which are also radicals). Further experiments showed that 3º-hydrogens are even more reactive toward halogen atoms. We have previously discussed electrophilic additions to alkenes (Ad, . In other words, the Gibbs free energy barrier is very high for this reaction, mostly due to entropic rather than enthalpic considerations. In the case of hydrogen bromide, both steps of the propagation stage are exothermic. It converts aromatic compounds having a benzenoid ring into a product, 1,4-cyclohexadienes, in which two hydrogen atoms have been attached on opposite ends of the molecule.  Also the use of tert-butyl alcohol has become common. One important antioxidant that you are no doubt familiar with is ascorbic acid, or vitamin C. It reacts with harmful radicals to produce the ascorbyl radical, which is significantly more stable than most other radical species due to resonance delocalization. This is known as Markovnikov’s Rule.