Mechanism of radical halogenation
Alkanes do not react with ordinary molecules, but actively interact with free radicals. A typical example is the halogenation reaction, which proceeds via a radical substitution mechanism ($S_R$). The process consists of three key stages:
- Chain initiation: Under the influence of ultraviolet light ($h\nu$), a halogen molecule splits into two active atoms—free radicals. For example: $Cl_2 \rightarrow 2Cl\bullet$.
- Chain propagation: The resulting chlorine radical abstracts a hydrogen atom from an alkane molecule (e.g., methane), forming hydrogen chloride ($HCl$) and an active alkyl radical (in the case of methane, $H_3C\bullet$). Then, this alkyl radical attacks a new, unreacted chlorine molecule. This yields the target product (chloromethane, $CH_3Cl$) and releases a new chlorine radical. This process is strictly cyclic: each new radical is capable of initiating a chain consisting of hundreds of thousands of consecutive steps.
- Chain termination: The reaction ceases when two free radicals collide with each other, forming a stable molecule (e.g., $H_3C\bullet + Cl\bullet \rightarrow CH_3Cl$). The probability of chain termination increases sharply with a decrease in vessel volume, as radicals collide with the walls more frequently and recombine on them.
Features of substitution reactions
Radical halogenation is characterized by several important regularities:
- Consecutive substitution: The reaction never stops at the mono-substituted product stage. If an excess of halogen is present in the system, methane is successively converted into chloromethane, dichloromethane, chloroform (trichloromethane), and finally carbon tetrachloride ($CCl_4$). Interestingly, in practice, carbon tetrachloride is produced not from methane, but from carbon disulfide ($CS_2$).
- Selectivity: The halogenation of methane homologs (e.g., propane) yields a mixture of isomers. The reactivity of carbon atoms decreases in the series: Tertiary > Secondary > Primary. Therefore, during the chlorination of propane, the proportion of 2-chloropropane will be significantly higher than that of 1-chloropropane. If a second hydrogen atom is substituted, it is directed predominantly to the same carbon atom where the halogen is already located.
In addition to halogens, alkanes react with nitric acid. This reaction is known as the Konovalov reaction. It proceeds upon heating to $150^\circ C$ with dilute nitric acid ($HNO_3$) or nitrogen(IV) oxide. Concentrated acid is not required for this process. The mechanism also remains radical, and the products are nitroalkanes.
Processing and alteration of the carbon skeleton
To alter the structure of alkane molecules, catalytic and thermal processes are applied, which are of paramount industrial importance:
- Isomerization and reforming: Normal alkanes (n-alkanes) can be converted into branched ones (iso-alkanes). In laboratory settings, this is achieved by heating with aluminum chloride ($AlCl_3$). In industry, a similar process is called reforming: it is carried out over a platinum catalyst supported on aluminum oxide ($Pt/Al_2O_3$) to increase the octane number of motor fuel.
- Cracking: This is the process of splitting a long alkane molecule into two shorter parts—a new alkane and an alkene. When aluminosilicate catalysts are used, the carbon chain cleavage occurs predominantly in the middle.
- Dehydrocyclization (aromatization): Alkanes containing 6 or more carbon atoms in the chain curl into a six-membered ring upon heating over oxide catalysts (e.g., $Cr_2O_3/Al_2O_3$). First, cycloalkanes are formed, which then lose hydrogen and convert into arenes. A classic example is the conversion of n-heptane into methylcyclohexane and subsequently into toluene with the release of hydrogen molecules.
- Functionalization: The conversion of inert alkanes into oxygen-containing compounds. A striking example is the industrial synthesis of acetic acid via the catalytic oxidation of butane with atmospheric oxygen.
Briefly: answers to topical questions
What are the differences between the mechanisms and products of thermal and catalytic cracking of alkanes?
Thermal and catalytic cracking of alkanes differ in their reaction conditions and the specific features of molecular cleavage.
| Characteristic | Thermal cracking | Catalytic cracking |
|---|---|---|
| Conditions | Heating (thermal decomposition) | Presence of catalysts (aluminosilicates) |
| Products | Alkenes | Alkane and alkene |
| Features | — | Chain cleavage occurs predominantly in the middle of the molecule |
Reaction mechanisms are not described in the presented materials.
What products are formed during the catalytic oxidation of methane under various conditions?
The catalytic oxidation of methane yields formaldehyde (methanal) and water.
The available materials describe only one pathway for this reaction, which is used for the industrial production of formaldehyde:
- Formaldehyde is the product of the catalytic oxidation of methane by oxygen.
Reaction equation: $$CH_4 + O_2 \rightarrow CH_2O + H_2O$$
Information regarding the formation of other products under different conditions is absent.
How does the state of aggregation of alkanes change depending on the length of the carbon chain?
The state of aggregation of normal alkanes can be estimated from the melting and boiling points given in the table.
At a temperature of about $20^\circ C$:
- Methane, ethane, propane, and butane are in a gaseous state, as their boiling points are below $20^\circ C$.
- Pentane to hexadecane are in a liquid state: their melting points are below $20^\circ C$, and their boiling points are above $20^\circ C$.
- Heptadecane and heavier normal alkanes from the table are in a solid state, as their melting points are above $20^\circ C$.
According to the table, as the number of carbon atoms increases, the boiling points of normal alkanes rise; melting points generally increase as well.
To easily remember the selectivity rule in radical substitution, imagine the hierarchy: "The tertiary carbon is the boss (it always takes the halogen first), the secondary is the deputy (gets what's left), and the primary is the rank-and-file employee."
Class and exam questions
Why does halogenation slow down when the reaction vessel volume is decreased?
A decrease in volume causes active free radicals to collide with vessel walls more frequently. Their recombination (joining each other) occurs on the walls, causing premature termination of the reaction chain.
Is it possible to obtain a 100% yield of chloromethane during methane chlorination?
No, the reaction is consecutive in nature. Dichloromethane, chloroform, and carbon tetrachloride will always be present in the mixture, as the halogen continues to attack already substituted molecules.
What conditions are required for the Konovalov nitration of alkanes?
For successful nitration, it is necessary to use dilute nitric acid (or nitrogen(IV) oxide) and heating to a temperature of about 150 °C. The use of concentrated acid is not required in this case.
What else to study on this topic
- Calculation of standard enthalpy of combustion and thermal effects of alkane reactions
- Specifics of industrial-scale production of carbon tetrachloride from carbon disulfide
- Detailed mechanisms of action of aluminosilicates in petroleum catalytic cracking
- The role of reforming in improving the octane number and quality of modern motor fuels
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