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Alkylation is the introduction of a hydrocarbon group onto a chemical. Hydrocarbons are molecules with a carbon atom bound 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 different types of alkylation. These types are classified based on the character of the alkylating agent. Nucleophilic alkylating agents deliver a negatively charged alkyl group to the hydrocarbon, while electrophilic alkylating groups deliver a positively charged alkyl group to the hydrocarbon.
Electrophilic alkylation is often highly toxic, due to its ability to alkylate the bases of DNA. DNA which has been subject to alkylation either does not coil or uncoil properly, or cannot be decoded. This property is taken advantage of by alkylating antineoplastic agents, which are used in chemotherapy to attack the DNA of cancer cells. A less scrupulous use of these agents is as mustard gas poisons.
One specialized type of alkylation is methylation, in which the one carbon methyl group replaces a hydrogen atom. In cells, this reaction is mediated by enzymes and frequently targets DNA or proteins. Humans have hundreds of different methylation reactions that take place. They frequently cause a change in a reaction, such as the activation of gene expression or enzyme activity. Methylation can be a way of regulating the inheritance of genes outside of the usual method of DNA inheritance; this is known as epigenesis.
In oil refining, the term alkylation is used to indicate the production of high octane gasolines, although not all countries use alkylation plants. The use of alkyl units in refining gasoline was started in the 1930s. With the advent of World War II, the process was accelerated with the need for aviation fuel. In the United States, alkylation units are important for helping oil refineries meet standards set by the Clean Air Act, since high octane gasoline burns more cleanly in high performance engines.
Petroleum alkylation involves combining lighter gaseous hydrocarbons that are known as isoparaffins, such as isobutane, by alkylating them in a nucleophilic manner with olefins, alkenes such as propylene and butylene, in a reactor under the influence of an acid catalyst. After alkylation, heavier hydrocarbons are produced in a mixture, and the liquid fraction is called alkylate. If propylene and butylene are the alkylating agents, it is primarily composed of isopentane and isooctane.
The octane number of the gasoline depends on the compounds used and operating conditions. An octane rating of 100 would be gasoline comprised 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, since isooctane is not the most knock-resistant fuel available.