Despite the fact that alkanes are inactive, they are capable of releasing large amounts of energy when interacting with halogens or other free radicals. Alkanes and reactions with them are constantly used in many industries.
Alkanes facts
Alkanes occupy an important place in organic chemistry. The formula of alkanes in chemistry is C H2n+2. Unlike aromatics, which have a benzene ring, alkanes are considered aliphatic (acyclic).
In the molecule of any alkane, all elements are connected by a single bond. Therefore, this group of substances has the ending "-an". Accordingly, alkenes have one double bond, and alkynes have one triple bond. Alcodienes, for example, have two double bonds.
Alkanes are saturated hydrocarbons. That is, they contain the maximum number of H (hydrogen) atoms. All carbon atoms in an alkane are in position sp3 – hybridization. This means that the alkane molecule is built according to the tetrahedral rule. The methane molecule (CH4) resembles a tetrahedron,and the remaining alkanes have a zigzag structure.
All C atoms in alkanes are connected using ơ - bonds (sigma - bonds). C–C bonds are nonpolar, C–H bonds are weakly polar.
Properties of alkanes
As mentioned above, the alkane group has little activity. The bonds between two C atoms and between C and H atoms are strong, so they are difficult to destroy by external influences. All bonds in alkanes are ơ bonds, so if they break, it usually results in radicals.
Halogenation of alkanes
Due to the special properties of the bonds of atoms, alkanes are inherent in substitution and decomposition reactions. In substitution reactions in alkanes, hydrogen atoms replace other atoms or molecules. Alkanes react well with halogens - substances that are in group 17 of the periodic table of Mendeleev. The halogens are fluorine (F), bromine (Br), chlorine (Cl), iodine (I), astatine (At) and tennessine (Ts). Halogens are very strong oxidizing agents. They react with almost all substances from the table of D. I. Mendeleev.
Chlorination reactions of alkanes
In practice, bromine and chlorine usually take part in the halogenation of alkanes. Fluorine is too active an element - with it the reaction will be explosive. Iodine is weak, so the substitution reaction will not go with it. And astatine is very rare in nature, so it is difficult to collect enough of it for experiments.
Halogenation steps
All alkanes go through three stages of halogenation:
- The origin of the chain or initiation. Under influencesunlight, heat, or ultraviolet radiation, the chlorine molecule Cl2 breaks down into two free radicals. Each has one unpaired electron in the outer layer.
- Development or growth of the chain. Radicals interact with methane molecules.
- Chain termination is the final part of alkane halogenation. All radicals begin to combine with each other and eventually disappear completely.
Alkanes bromination
When halogenating higher alkanes after ethane, the difficulty is the formation of isomers. Different isomers can be formed from one substance under the action of sunlight. This happens as a result of a substitution reaction. This is proof that any H atom in the alkane can be substituted by a free radical during halogenation. A complex alkane decomposes into two substances, the percentage of which can vary greatly depending on the reaction conditions.
Propane bromination (2-bromopropane). In the reaction of halogenation of propane with a Br2 molecule under the influence of high temperatures and sunlight, 1-bromopropane - 3% and 2-bromopropane - 97% are released.
Bromation of butane. When butane is brominated under the action of light and high temperatures, 2% 1-bromobutane and 98% 2-bromobutane come out.
The difference between chlorination and bromination of alkanes
Chlorination is more commonly used in industry. For example, for the production of solvents containing a mixture of isomers. Upon receipt of the haloalkanedifficult to separate from each other, but on the market the mixture is cheaper than the pure product. In laboratories, bromination is more common. Bromine is weaker than chlorine. It has low reactivity, so bromine atoms have high selectivity. This means that during the reaction, the atoms "choose" which hydrogen atom to replace.
The nature of the chlorination reaction
When chlorinating alkanes, isomers are formed in approximately equal amounts in their mass fraction. For example, chlorination of propane with a catalyst in the form of an increase in temperature to 454 degrees gives us 2-chloropropane and 1-chloropropane in a ratio of 25% and 75%, respectively. If the halogenation reaction takes place only with the help of ultraviolet radiation, 43% of 1-chloropropane is obtained, and 57% of 2-chloropropane. Depending on the reaction conditions, the ratio of the obtained isomers may vary.
The nature of the bromination reaction
As a result of bromination reactions of alkanes, an almost pure substance is easily released. For example, 1-bromopropane - 3%, 2-bromopropane - 97% of the n-propane molecule. Therefore, bromination is often used in laboratories to synthesize a specific substance.
Sulfation of alkanes
Alkanes are also sulfonated by the mechanism of radical substitution. For the reaction to occur, oxygen and sulfur oxide SO2 (sulphurous anhydride) simultaneously act on the alkane. As a result of the reaction, the alkane is converted into an alkyl sulfonic acid. Example of butane sulfonation:
CH3CH2CH2CH3+ O2 +SO2 → CH3CH2CH2CH 2SO2OH
General formula for sulfoxidation of alkanes:
R―H + O2 + SO2 → R―SO2OH
Sulfochlorination of alkanes
In the case of sulphochlorination, instead of oxygen, chlorine is used as an oxidizing agent. Alkanesulfonic chlorides are obtained in this way. The sulfochlorination reaction is common to all hydrocarbons. It occurs at room temperature and sunlight. Organic peroxides are also used as a catalyst. Such a reaction affects only secondary and primary bonds related to carbon and hydrogen atoms. The matter does not reach tertiary atoms, as the reaction chain breaks.
Konovalov's reaction
The nitration reaction, like the halogenation reaction of alkanes, proceeds according to the free-radical mechanism. The reaction is carried out using highly dilute (10 - 20%) nitric acid (HNO3). Reaction mechanism: as a result of the reaction, alkanes form a mixture of compounds. To catalyze the reaction, an increase in temperature up to 140⁰ and normal or elevated ambient pressure is used. During nitration, C–C bonds are destroyed, and not only C–H, in contrast to the previous substitution reactions. This means that cracking is taking place. That is the splitting reaction.
Oxidation and combustion reactions
Alkanes are also oxidized according to the free radical type. For paraffins, there are three types of processing using an oxidative reaction.
- In the gas phase. Soget aldehydes and lower alcohols.
- In the liquid phase. Use thermal oxidation with the addition of boric acid. With this method, higher alcohols are obtained from С10 to С20.
- In the liquid phase. Alkanes are oxidized to synthesize carboxylic acids.
In the process of oxidation, the free radical O2 completely or partially replaces the hydrogen component. Complete oxidation is combustion.
Good-burning alkanes are used as fuel for thermal power plants and internal combustion engines. Burning alkanes produce a lot of heat energy. Complex alkanes are placed in internal combustion engines. Interaction with oxygen in simple alkanes can lead to an explosion. Asph alt, paraffin and various lubricants for industry are made from waste products resulting from reactions with alkanes.
