Physical Chemistry

Alkanes. Chemical properties. Radical substitution on the example of the halogenation reaction

Alkanes. Chemical properties. Radical substitution on the example of the halogenation reaction lesson will consider the topic: “Alkanes. Chemical properties. Radical substitution on the example of the halogenation reaction. You will learn a lot about the chemical properties of alkanes. Consider what constitutes a radical substitution, using the halogenation reaction as an example.

What explains the passivity of alkanes

Chemical properties of alkanes

Alkanes are hydrocarbons that do not have multiple bonds. One of the names of hydrocarbons of this series – paraffin, denotes their chemical passivity. Therefore, the chemical properties of alkanes are defined as follows:

  1. Due to the limiting nature of alkanes, addition reactions are not typical for them.
  2. Due to strong C-C and C-H bonds (short and practically non-polar), low reactivity follows (do not react with acids, alkalis, KMnO 4, etc.)
  3. The non-polarity of the bonds indicates that the reactions can proceed according to the free radical mechanism.

The main type of reaction is radical substitution  S R.

You can break the bond with heat or UV radiation. Typically, alkanes react at elevated temperatures or in sunlight.

halogenation reaction

The interaction of alkanes with halogens (elements of group VII-A) is collectively called the halogenation reaction. The reaction rate depends on the activity of the halogens, which decreases with increasing atomic radius.

Fluorine is too chemically active, and the reactions of alkanes with its participation are accompanied by an explosion and oxidation to CF 4.

The interaction of alkanes with iodine practically does not occur.

The reactions of chlorination and bromination are carried out at 300–400  o C or when irradiated with a UV lamp.

Mechanism of substitution reaction on the example of methane chlorination

During chlorination, the H atom in the methane molecule is replaced by a Cl atom. In excess of chlorine, further, substitution occurs:

СН4 + Сl2 CH3Cl + HCl;

CH3Cl + Сl2 CH2Cl2 + HCl;

CH2Cl2 + Сl2 CHCl3 + HCl;

CHCl3 + Сl2 CCl4 + HCl.

The mechanism of radical substitution

A reaction mechanism is a sequence of stages through which a chemical reaction passes.


For chemists

Stage 1 – the origin of the chain (initiation)

When a chlorine molecule is heated or irradiated, the weak bond between the chlorine atoms breaks, with each of the atoms getting one electron when the bond is broken.

As a result, uncharged particles with an unpaired electron are formed – radicalsAtoms in radicals contain an odd number of electrons at the outer level. Therefore, radicals are very unstable and reactive particles.

Cl:Сl  2 Сl·

Stage 2 – development (growth) of the chain

When a chlorine atom collides with an alkane molecule, it detaches an H atom from it, and an HCl molecule is formed. There is a new free radical · CH 3. It further reacts with another chlorine molecule. Such interactions can be repeated many times, forming a long chain.

Sl · + CH  → HCl + · SN 3

· СН 3 + Сl  → Сl · + CH 3 Сl

Stage 3 – chain termination (recombination)

Two radicals can meet and the chain will break:

· CH 3 + Сl ·  → CH 3 Сl

2Сl ·  → Сl 2

2 CH  → CH –CH 3

Please note: in the latter case, a substance arises in the molecule in which there are more carbon atoms than in the original alkane. Therefore, in a mixture of products of radical reactions of alkanes, there are always small amounts of substances with a longer carbon chain.


Features of radical substitution

Hydrogen atoms at the tertiary C atom enter into the radical substitution reaction most easily, then at the secondary one, and it is most difficult to break the C-H bond at the primary carbon atom. Therefore, during the bromination of 2-methyl butane, 2-Bromo-2-methyl butane is predominantly formed:

Nitration reaction

According to the mechanism of radical substitution, the reaction of nitration of alkanes in the gas phase proceeds (the Konovalov reaction). The conditions are elevated temperature and pressure.

In the vapor state, nitric acid decomposes:

2HNO 3  2NO 2 + ½O 2 + H 2 O

Nitric oxide (IV) is a radical · NO 2. It attacks the alkane molecule.

Final reaction:

CH4 HNO3 = CH3NO2 + H2O _ _

Burning alkanes

Rice. 1. Combustion of methane

The combustion of alkanes (Fig. 1) is also a radical chain reaction, but, in addition, it is a branched one. At the stage of chain development, there are processes that lead to the multiplication of radicals (branching). A fragment of the methane combustion mechanism:

• Initiation:

RH + O 2  = R HO 2 ( etc.)

• Chain development:

R· + O2 = ·RO2

RO 2 + RH = ROOH + R

• Branching:

ROOH = RO· + ·OH

ROOH + RH = RO · + R · + H 2 O

2ROOH = ·RO2 + RO· + H2O

A mixture of methane with oxygen (in a volume ratio of 1:2) or with air (1:10) burns with an explosion when ignited. Therefore, mixtures of methane, propane, ethane, and butane with air are very dangerous. They can sometimes form in mines, in workshops, and in residential areas. These gases must be handled with care. 

Summing up the lesson

In the lesson, the topic was considered: “Alkanes. Chemical properties. Radical substitution on the example of the halogenation reaction. You learned a lot about the chemical properties of alkanes, in particular, what constitutes radical substitution, using the halogenation reaction as an example.

Bibliography

  1. Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F. G. Feldman. – 14th edition. – M.: Enlightenment, 2012.
  2. Chemistry. Grade 10. Profile level: textbook. for general education institutions / V. V. Eremin, N. E. Kuzmenko, V. V. Lunin, et al. – M.: Bustard, 2008. – 463 p.
  3. Chemistry. Grade 11. Profile level: textbook. for general education institutions / V. V. Eremin, N. E. Kuzmenko, V. V. Lunin, et al. – M.: Bustard, 2010. – 462 p.
  4. Khomchenko G. P., Khomchenko I. G. Collection of problems in chemistry for applicants to universities. – 4th ed. – M.: RIA “New Wave”: Publisher Umerenkov, 2012. – 278 p.

Additional recommended links to Internet resources

  1. Internet portal “Chemist” ( Source )

Homework

  1. Nos. 13, 18 (p. 22) Rudzitis G. E., Feldman F. G. Chemistry: Organic chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F. G. Feldman. – 14th edition. – M.: Enlightenment, 2012.
  2. Write the reaction equations for the successive chlorination of propane.
  3. Which of the hydrocarbons is easier to enter into a radical substitution reaction: butane or 2-methylpropane? Why? Write reaction equations.

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