Universals Of Free Radical Reactions

The splitting of chemical bonds produces free radicals. Reactions triggered by free radicals are called free radical reactions, or free radical type chain reactions. Radical reactions generally go through three stages: chain initiation, chain propagation, or chain formation, and chain termination. The chain initiation stage is the stage of producing free radicals. Since the homolysis of the bond requires energy, the chain initiation phase requires heat or light.
Some compounds are very active and easily produce active particle free radicals, which are called initiators. Free radicals can sometimes be produced by REDOX reactions of single electron transfer. The chain transfer stage is the stage of transformation from one free radical to another free radical, like a relay race, free radicals are constantly passed on, like a chain one by one, so it is called a chain reaction. The chain termination stage is the stage of disappearance of free radicals. Free radicals pair together to form bonds. All the free radicals are gone, and the free radical reaction stops.
Free radical reaction is characterized by no obvious solvent effect, acid, base and other catalysts have no obvious effect on the reaction, when there is oxygen in the reaction system (or there are some impurities that can trap free radicals), the reaction often has an induction period.
Thermal cracking reaction
In the absence of oxygen, the carbon-carbon bond breaks in alkanes at high temperatures (around 800 ° C), and large molecular compounds are transformed into small ones, a reaction called pyrolysis. After petroleum processing, in addition to gasoline, there are paraffin with relatively large molecular weight such as kerosene and diesel; Through the thermal cracking reaction, it can be transformed into small molecule compounds such as gasoline, methane, ethane, ethylene and propylene. The process is very complicated, and the products are also complex. Both carbon-carbon bonds and carbon-hydrogen bonds can be broken, and the break can occur in the middle of the molecule or on one side of the molecule. The larger the molecule, the easier it is to break, and the molecule after hot cracking can also be hot cracked again. The reaction mechanism of hot cracking reaction is the free radical reaction under the action of heat, and the raw material used is a mixture.
The free radicals produced after thermal cracking can bind to each other. The free radicals produced by thermal cracking can also be broken through C-H bonds to produce alkenes.
The overall result is the thermal cracking of large alkane molecules into smaller alkanes and alkenes. This reaction is difficult to perform in the laboratory, but very important in industry. In industrial hot cracking, paraffin is mixed with water vapor in a tube through a heating device of about 800 °C, and then cooled to 300 ~ 400°C, which is completed in less than one second, and then the hot cracking products are separated by freezing method. Raw materials such as plastics, rubber and fibers can be obtained through this reaction.
For example, the thermal cracking reaction with a catalyst can reduce the temperature, but the reaction mechanism is not a free radical reaction but an ionic reaction.
Oxidation reaction and combustion
In life, we often encounter this phenomenon, people have wrinkles when they are old, rubber products become hard and sticky after a long time, plastic products become hard and easy to crack after a long time, and edible oil deteriorates after a long time. These phenomena are called aging. The aging process is very slow, and the reason for aging is that the oxygen in the air enters various molecules with active hydrogen and occurs autoxidaticm, and then other reactions occur.
All alkanes can be burned, and when fully burned, the reactants are completely destroyed, forming carbon dioxide and water, and releasing a lot of heat.
When burning, the flame is light blue, not bright.