Isobutylene is one of the typical representatives of alkenes. By itself, it is not of particular value, but it is used in the chemical industry as one of the intermediate substances for the synthesis of rubbers or fuel additives.
General information
Isobutylene is an unsaturated hydrocarbon, that is, multiple bonds are present in the molecule of the substance. By its physical properties, it is a colorless gas with a pungent odor (it is inherent in most unsaturated hydrocarbons). The boiling point of isobutene is -7 o C, the melting point is -140 o C.
Isobutylene is poorly soluble in polar liquids (which most often means water), but it is very good in non-polar or weakly expressed polarity. These are mostly organic compounds; in this case, alcohol and ether.
The isobutylene formula is written as follows.
The prefix "iso" indicates the presence of a tertiary carbon atom - directly bonded to three other carbons. After studying the formula of isobutylene, you can notice that it has only two isomers of the carbon skeleton: 1-butene and 2-butene. They have a different structure, but are in the same class of substances (unsaturated hydrocarbons) and have the same chemical properties.
Being in nature
In general, unsaturated hydrocarbons are extremely rare in nature due to the fact that they are quite active due to their multiple bonds (in contrast to paraffins - saturated, in which all bonds are single). However, in small quantities they are part of the so-called associated petroleum gas. This is the name of a mixture of gaseous hydrocarbons, which is dissolved in oil deposits and released during its production or further processing. These gases can be used as fuel, or separate substances and fractions from them to be used in further organic synthesis. Isobutylene is found in the fraction of petroleum gas formed during cracking (thermal decomposition of "long" alkanes). The same fraction contains its isomers, as well as saturated butane, so that after its separation, further manipulations are necessary to obtain pure isobutylene.
Getting
There are several basic methods for producing isobutylene. First, get a mixture of butene isomers:
- Cracking or pyrolysis of a gasoline fraction. The yields of butene and isomers are extremely low: up to 10%.
- Dehydrogenation of butane. The process takes place on an aluminum-chromium catalyst at a temperature close to 600 o C and a pressure of about 0.1-0.2 MPa (megapascals). If isobutane is used in the reaction, then the yield of isobutylene will be 42-48%.
Isobutylene can be isolated from a mixture of isomers using sulfuric acid at a concentration of 65%. At room temperature, tert-butyl sulfuric acid is formed (with a sulfo group at the tertiary carbon atom). Next, the solution is diluted with water so that the sulfuric acid concentration reaches 45%, then, when heated and low pressure, isobutylene is separated from the mixture in pure form, which is then purified from impurities and liquefied (for storage or transportation).
There is also a method for the isolation of isobutylene by selective adsorption. Zeolites, minerals from the group of aluminosilicates (sodium, calcium) are used as adsorbent.
Chemical properties
The isobutylene formula shows that it has a branched carbon skeleton. This means that one of the carbon atoms (tertiary) has an increased electron density, which contributes to the passage of electrophilic reactions typical of compounds of its class. According to the formula of isobutylene, we can say that it is capable of all reactions characteristic of the class of alkenes (or olefins): multiple bond addition (hydrogenation, halogenation, hydrohalogenation), mild oxidation with oxygen on a silver catalyst (production of epoxides), mild oxidation of potassium permanganate to aqueous solution (obtaining glycols). It is also worth mentioning the Prince reaction, in which, when formaldehyde acts on an unsaturated hydrocarbon (alkene), dihydric alcohols, dioxanes or unsaturated alcohols are obtained . In the case of isobutylene, the resulting product, 4,4-dimethyl-1,3-dioxane, is used to produce isoprene (by decomposition).
Isoprene is used for the synthesis of isoprene rubber - one of the most important products of the chemical industry. However, a polymer can also be obtained from isobutylene itself: by copolymerizing isoprene and isobutene, butyl rubber with a very small percentage of isoprene is obtained.
Rubbers
As already mentioned, isobutylene is used to produce isoprene - a unsaturated hydrocarbon, which, in turn, is converted into isoprene rubber by polymerization. There are other applications for isoprene, but more than 95% of all production is used in rubber. Isoprene rubbers replace natural ones and are used in the manufacture of car tires, conveyor belts and various household goods.
Isobutylene is also used in the production of antioxidants, which, in turn, are also added to rubbers to prevent their aging and increase their useful life.
Fuel additives
No less interesting is the scope of isobutylene, as an additive to motor fuel. As you know, one of the most important characteristics of fuel is the octane number, which determines its detonation resistance. A number of compounds increase this resistance: firstly, of course, isooctane itself or 2,2,4-trimethylpentane, and isobutylene is used in its production. But the octane number is also affected by other additives: toxic tetraethyl lead, tert-butyl ethers and other compounds. In the synthesis of esters, namely, tert-butyl methyl and tert-butyl ethyl, isobutylene is again used.