In chemistry, radical substitution refers to reactions in which free radicals attack a molecule of a substance, replacing its individual atoms. In the substitution reaction, new radicals are formed. The chain reaction continues until all free radicals run out.
Radical definition
A radical is an atom or molecule having one or more unpaired electrons on an external electron layer. That is, such electrons that do not have a pair. A radical can form when a molecule acquires one electron, or vice versa, loses it. For the most part, free radicals are unstable, since their outer electronic layer is not complete. Therefore, radicals easily react with certain substances, forming new substances and free radicals.
What are radicals?
The main groups by which the classification of radicals occurs:
- stability: stable and unstable;
- charge: uncharged, negatively charged and positively charged;
- degree of connectivity: free and complex.
Stable radicals
Usually, radicals “live” little and are in a hurry to react more quickly. Such radicals exist for seconds or fractions of seconds and are called unstable. But there are those that are stable, the period of their existence can reach several years. In inorganic chemistry, O3, NO, ClO2, NO2, and others are classified as stable. In the organic section, there are more stable radicals. They are divided into several groups:
- hydrocarbon;
- hydrazyl;
- nitroxyl;
- amine;
- aroxyl;
- verdazyl.
Radical substitution reaction mechanism
Three stages are distinguished in the reaction mechanism:
- Initiation. Through external factors (heating, radiation, chemical and electrical catalysts), the bond in the molecule of the substance is destroyed, forming free radicals.
- The development of a chain or its growth. Free elements enter into interaction with molecules, due to which new substances and radicals are formed.
- Open circuit. In the third stage, the radicals are joined together. They recombine (the union of unpaired electrons that belong to different particles), due to which new independent molecules appear. There are no free radicals left, and the reaction chain is considered complete.
Typical Substitution Reactions
Typically, a radical substitution reaction is exemplified by halogenation of alkanes. The simplest alkane is methane - CH4, and the most common halogen is chlorine.
Alkanes
Alkanes are saturated hydrocarbons containing only simple bonds. The general formula of alkanes is CnH2n + 2. Saturated are those hydrocarbons that contain the maximum number of hydrogen atoms. Earlier, alkanes were called paraffins due to the fact that these substances did not react with acids, alkalis, etc. In fact, the resistance to interaction with strong reagents is explained by the strength of C — C and C — H bonds. Saturation of alkanes also suggests that they do not participate in addition reactions. They are characterized by decomposition, substitution, and others.
Halogens
In order to carry out a radical substitution reaction, it is necessary to define halogens. Halogens are elements of the 17th group of the periodic table. Halogens are Cl (chlorine), I (iodine), F (fluorine), Br (bromine) and At (astatine). All halogens are non-metals and strong oxidizing agents. Fluorine has the highest oxidative activity, and astatine has the lowest. In the process of halogenation of alkanes, one or more hydrogen atoms in a substance are replaced by halogen.
The substitution mechanism for the example of methane halogenation
Methane is considered to be the simplest alkane, therefore, the reactions of its halogenation are easy to remember, and on this basis to carry out radical substitution of other alkanes. Chlorine is usually taken as halogen. He has a medium response force. The alkanes do not react with iodine, since it is a weak halogen. The interaction with fluorine takes place with an explosion, because fluorine atoms are very active. Although, during the substitution reaction of alkanes with chlorine, an explosion can also occur.
The origin of the chain. Under the influence of solar, ultraviolet radiation or from heating, the chlorine molecule Cl2 decomposes into two free radicals. Each has one unpaired electron on the outer layer.
Cl 2 → 2Cl
The development or growth of the chain. Interacting with methane molecules, free radicals form new ones and continue the chain of transformations.
CH 4 + Cl · → CH 3 + HCl
CH 3 + Cl 2 → CH 3 Cl + Cl
Further, the reaction proceeds until all free radicals disappear.
Chain termination is the final stage of radical substitution of alkanes. The radicals combine with each other and form new molecules.
CH 3 · + · Cl → CH 3 Cl
CH 3 · + · CH 3 → CH 3 - CH 3
Methane chlorination
Under the influence of sunlight, chlorine radicals replace all hydrogen atoms in methane. To completely replace hydrogen, the proportion of chlorine in the mixture should be sufficient. Thus, four derivatives of methane can be obtained:
CH 3 Cl is chloromethane.
CH 2 Cl 2 - dichloromethane.
CHCl 3 - trichloromethane (chloroform).
CCl 4 - carbon tetrachloride.
Halogenation of other alkanes
Starting with propane (C 3 H 8 ), tertiary and secondary carbon atoms appear in alkanes. Halogenation of branched alkanes can give different results. As a result of the radical substitution reaction, isomers of alkanes are formed. The mass of each resulting substance can vary greatly depending on the temperature.
During thermal halogenation, the composition of the resulting product is determined based on the ratio of the number of C ― H – bonds of carbon atoms, which in primary alkanes are primary, secondary and tertiary. As a result of photochemical halogenation, the composition of the resulting products will depend on the rate at which halogen atoms replace hydrogen atoms. It is easiest for halogens to take the place of the tertiary hydrogen atom. It is more difficult to replace the secondary and primary.
Propane Chlorination
Upon chlorination of propane with a catalyst in the form of a temperature increase to 450 ° C, 2-chloropropane in an amount of 25% and 1-chloropropane in an amount of 75% are formed.
2CH 3 CH 2 CH 3 + 2Cl 2 → CH 3 CH (Cl) CH 3 + CH 3 CH 2 CH 2C l + 2HCl
If a radical alkane is substituted using sunlight, 57% of 2-chloropropane and 43% of 1-chloropropane are released.
The difference in the mass of the obtained substances between the first and second reactions is explained by the fact that in the second case, the substitution rate for the H atom of the secondary atom is 4 times higher than that of the primary atom, although the propane molecule has more primary C ― H bonds.
Oxidation reactions
Free radicals are again involved in alkane oxidation reactions. In this case, the O 2 radical is attached to the alkane molecule, and a complete or incomplete oxidation reaction occurs. Complete oxidation is called combustion:
CH 4 + 2O 2 → CO 2 + 2H 2 O
The alkane combustion reaction by the radical substitution mechanism is widely used in industry as a fuel for thermal power plants, for internal combustion engines. Only branched alkanes can be placed in such engine engines. Simple linear alkanes in ICE explode. Lubricants, asphalt, paraffin, etc. are produced from non-volatile sludge resulting from radical substitution.
Partial oxidation
In industry, mixtures that are formed during the partial oxidation of methane are used to make synthetic alkanes. Methyl alcohol (CH 3 OH), formaldehyde (HCNO), formic acid (HCOOH) can be obtained from methane with incomplete oxidation by air. And when butane is oxidized, acetic acid is produced in industry:
2C 4 H 10 + 5O 2 → 4CH 3 COOH + 2H 2 O
In order for the alkanes to be partially oxidized, catalysts (Co 2+ , Mn 2+ , etc.) are used at relatively low air temperatures.