Despite the fact that alkanes are inactive, they are capable of releasing a large amount of energy when interacting with halogens or other free radicals. Alkanes and reactions with them are constantly used in many industries.
Alcan facts
In organic chemistry, alkanes occupy an important place. The formula of alkanes in chemistry is C n H 2n + 2. Unlike aromatic substances that 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. Alkodienes, 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 the alkane are in the position of sp 3 - hybridization. This means that the alkane molecule is built according to the tetrahedron rule. The methane molecule (CH 4 ) resembles a tetrahedron, and the remaining alkanes have a zigzag structure.
All C atoms in the alkanes are connected using the Ζ‘ - bond (sigma - bond). The bonds C β C are nonpolar, the bonds C β H are weakly polar.
Alkane Properties
As mentioned above, a group of alkanes has little activity. The bonds between the two C atoms and between the C and H atoms are strong, therefore it is difficult to destroy them by external influences. All bonds in alkanes are Ζ‘ - bonds, therefore, if they break, this usually leads to the appearance of radicals.
Halogenation of alkanes
Due to the special properties of atomic bonds, substitution and decomposition reactions are inherent in alkanes. In alkane substitution reactions, hydrogen atoms are replaced by other atoms or molecules. Alkanes react well with halogens - substances in the 17th group of the periodic table. The fluorine (F), bromine (Br), chlorine (Cl), iodine (I), astatine (At) and tennessin (Ts) are halogens. Halogens are very strong oxidizing agents. They react with almost all substances from the table of D. I. Mendeleev.
Alkane chlorination reactions
In practice, usually bromine and chlorine 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 small in nature, so it is difficult to collect a sufficient amount for conducting experiments.
Halogenation steps
All alkanes undergo three stages of halogenation:
- Chain initiation or initiation. Under the influence of sunlight, heat or ultraviolet radiation, the chlorine molecule Cl 2 breaks up into two free radicals. Each has one unpaired electron on the outer layer.
- The development or growth of the chain. Radicals interact with methane molecules.
- Open chain is the final part of the halogenation of alkanes. All radicals begin to unite with each other and eventually completely disappear.
Alkane bromination
When halogenating higher alkanes following ethane, the formation of isomers is a difficulty. From one substance under the influence of sunlight, different isomers can form. This occurs as a result of the substitution reaction. This is proof that when halogenated with a free radical, any H atom in the alkane can be substituted. A complex alkane breaks down into two substances, the percentage of which can vary greatly depending on the reaction conditions.
Bromination of propane (2-bromopropane). In the reaction of halogenation of propane with the Br2 molecule under the influence of high temperatures and sunlight, 1-bromopropane - 3% and 2-bromopropane - 97% are released.
Bromination of butane. When butane is brominated under the influence of light and high temperatures, 2% 1-bromobutane and 98% 2-bromobutane are obtained.
The difference between chlorination and bromination of alkanes
Chlorination is more commonly used in industry. For example, for the production of solvents that contain a mixture of isomers. It is difficult to separate halogen alkanes from each other, but on the market the mixture is cheaper than a pure product. Bromination is more common in laboratories. Bromine is weaker than chlorine. It has low reactivity, so bromine atoms have high selectivity. This means that during the reaction, the atoms βchooseβ which hydrogen atom to replace with them.
The nature of the chlorination reaction
When alkanes are chlorinated, isomers are formed in approximately equal amounts in their mass fraction. For example, chlorination of propane with a catalyst in the form of a temperature increase 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, 1-chloropropane is obtained 43%, and 2-chloropropane - 57%. Depending on the reaction conditions, the ratio of the resulting isomers may vary.
The nature of the reaction bromination
As a result of the reactions of bromination of alkanes, an almost pure substance easily leaves. For example, 1-bromopropane - 3%, 2-bromopropane - 97% of the n-propane molecule. Because bromination is often used in laboratories for the synthesis of a certain substance.
Sulfonation of alkanes
Alkanes are also sulfonated by a radical substitution mechanism . In order for the reaction to occur, oxygen and sulfur oxide SO 2 (sulfur dioxide) are simultaneously affected by the alkane. As a result of the reaction, the alkane is converted to alkyl sulfonic acid. An example of butane sulfonation:
CH 3 CH 2 CH 2 CH 3 + O 2 + SO 2 β CH 3 CH 2 CH 2 CH 2 SO 2 OH
General formula for sulfonation of alkanes:
R β H + O 2 + SO 2 β R β SO 2 OH
Sulfochlorination of alkanes
In the case of sulfochlorination, chlorine is used as the oxidizing agent instead of oxygen. In this way, alkanesulfonyl chlorides are obtained. The sulfochlorination reaction is common to all hydrocarbons. It occurs at room temperature and in sunlight. Organic peroxides are also used as a catalyst. This reaction affects only secondary and primary bonds related to carbon and hydrogen atoms. It does not reach the tertiary atoms, since the reaction chain is broken.
The reaction of Konovalov
The nitration reaction, as well as the alkane halogenation reaction, proceeds according to the free-radical mechanism. The reaction is carried out using highly diluted (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 to 140β° and normal or elevated ambient pressure are used. Upon nitration, the bonds C β C are destroyed, and not only C β H, unlike previous substitution reactions. This means that cracking is taking place. That is a cleavage reaction.
Oxidation and combustion reactions
Alkane oxidation reactions also occur in a free radical type. For paraffins, there are three types of processing using an oxidative reaction.
- In the gas phase. So get 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 C 10 to C 20 .
- In the liquid phase. Alkanes are oxidized to synthesize carboxylic acids.
In the oxidation process, the free radical O 2 completely or partially replaces the hydrogen component. Complete oxidation is combustion.
Well-burning alkanes are used as fuel for thermal power plants and internal combustion engines. Burning alkanes produce a lot of thermal energy. Complex alkanes are placed in internal combustion engines. Interactions with oxygen in simple alkanes can lead to an explosion. Asphalt, paraffin and various lubricants for industry are made from waste products resulting from reactions with alkanes.