In the context of alkenes, a free radical addition reaction involves the addition of atoms or groups to the double bond of an alkene through a mechanism that involves free radicals.
Free radicals are atoms, molecules, or ions with unpaired electrons, making them highly reactive.
The general process for free radical addition to alkenes involves three main steps: initiation, propagation, and termination.
Initiation:
This step involves the formation of free radicals, typically through the homolytic cleavage of a bond under the influence of heat, light, or a catalyst.
For example, the homolytic cleavage of a halogen molecule (like Br2 or Cl2) can generate two halogen radicals.
Propagation:
In this step, the reactive radical reacts with the alkene, forming a new radical.
This newly formed radical can then react with another molecule, continuing the chain reaction.
This step is where the addition across the double bond occurs.
Termination:
The reaction stops when two free radicals combine to form a stable product, thus terminating the chain reaction.
Anti-Markovnikov's Orientation:
Anti-Markovnikov's rule is an important concept in the addition reactions of alkenes.
It predicts the regiochemistry of the addition of certain reagents to alkenes in a manner that is opposite to what is predicted by Markovnikov's rule.
Markovnikov's Rule states that in the addition of a protic acid (HX) to an unsymmetrical alkene, the hydrogen (H) attaches to the carbon with the most hydrogen atoms (and thus the least substituents), while the X (halide, for example) attaches to the carbon with more substituents. This rule is based on the stability of the carbocation intermediate formed during the reaction, where more substituted carbocations are more stable.
Anti-Markovnikov's Orientation occurs in certain conditions where the addition of a reagent to an alkene results in the hydrogen attaching to the carbon with more substituents, opposite of what Markovnikov's rule predicts. This orientation is typical in free radical addition reactions, where no carbocations are formed, and the regioselectivity is influenced by the stability of the free radical intermediates.
An example of Anti-Markovnikov's orientation is the hydroboration-oxidation reaction of alkenes, where BH3 and then H2O2/NaOH are added across the double bond in an anti-Markovnikov fashion.
However, for free radical additions specifically, the most classic example is the addition of HBr in the presence of peroxides (a condition known as peroxide effect or Kharasch effect), where the bromine radical adds to the less substituted carbon of the alkene, leading to the formation of an organic product where the bromine is attached to the less substituted carbon, and hydrogen is added to the more substituted carbon.