Alkylation is the addition of a hydrocarbon group to a substance, with alkyl groups being common organic molecules. There are different types of alkylation, including nucleophilic and electrophilic, with the latter being toxic and used in chemotherapy. Methylation is a specialized type of alkylation that targets DNA or proteins and can regulate gene inheritance. In petroleum refining, alkylation is used to produce high-octane gasolines, and involves combining isoparaffins with olefins under acidic catalysts. The resulting liquid fraction is called an alkylate and its octane number depends on the compounds used.
Alkylation is the introduction of a hydrocarbon group onto a chemical substance. Hydrocarbons are molecules with a carbon atom bonded to hydrogen atoms. Alkyl groups range from single carbon compounds such as methyl groups to much longer chains of hydrocarbons and are probably the most common type of organic molecule. Alkylation is of great importance both in cell biology and in industrial processes.
There are several types of alkylation. These types are classified according to the character of the alkylating agent. Nucleophilic alkylating agents provide a negatively charged alkyl group to the hydrocarbon, while electrophilic alkylating groups provide a positively charged alkyl group to the hydrocarbon.
Electrophilic alkylation is often highly toxic, due to its ability to alkylate DNA bases. DNA that has been subjected to alkylation does not coil or unwind properly, or it cannot be decoded. This property is exploited by alkylating antineoplastic agents, which are used in chemotherapy to attack the DNA of tumor cells. A less careful use of these agents is as mustard gas poisons.
A specialized type of alkylation is methylation, in which the single methyl carbon group replaces a hydrogen atom. In cells, this reaction is mediated by enzymes and often targets DNA or proteins. Humans have hundreds of different methylation reactions that take place. They often cause a change in a reaction, such as activation of gene expression or enzyme activity. Methylation may be a way to regulate gene inheritance outside the usual method of DNA inheritance; this is known as epigenesis.
In petroleum refining, the term alkylation is used to refer to the production of high-octane gasolines, although not all countries use alkylation plants. The use of alkyl units in gasoline refining started in the 1930s. With the advent of World War II, the process was accelerated by the need for jet fuel. In the United States, alkylation units are important in helping oil refineries meet standards set by the Clean Air Act, as high-octane gasoline burns cleaner in high-performance engines.
Petroleum alkylation involves combining lighter gaseous hydrocarbons known as isoparaffins, such as isobutane, nucleophilically alkylating them with olefins, alkenes such as propylene and butylene, in a reactor under the influence of an acidic catalyst. After alkylation, heavier hydrocarbons are produced in a mixture and the liquid fraction is called an alkylate. If propylene and butylene are the alkylating agents, it is mainly composed of isopentane and isooctane.
The octane number of gasoline depends on the compounds used and the operating conditions. An octane number of 100 would be a gasoline composed entirely of isooctane, a compound that is added to unleaded gasolines to prevent knocking. It is possible for a fuel to have a rating higher than 100, as isooctane is not the most impact resistant fuel available.
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