alkylation

thumb|upright=1.5|Typical route for alkylation of benzene with ethylene and ZSM-5 as a heterogeneous catalystAlkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting alkylation. Alkyl groups can also be removed in a process known as dealkylation. Alkylating agents are often classified according to their nucleophilic or electrophilic character. In oil refining contexts, alkylation refers to a particular alkylation of isobutane with olefins. For upgrading of petroleum, alkylation produces a premium blending stock for gasoline. In medicine, alkylation of DNA is used in chemotherapy to damage the DNA of cancer cells. Alkylation is accomplished with the class of drugs called alkylating antineoplastic agents.

==Nucleophilic alkylating agents== Nucleophilic alkylating agents deliver the equivalent of an alkyl anion (carbanion). The formal "alkyl anion" attacks an electrophile, forming a new covalent bond between the alkyl group and the electrophile. The counterion, which is a cation such as lithium, can be removed and washed away in the work-up. Examples include the use of organometallic compounds such as Grignard (organomagnesium), organolithium, organocopper, and organosodium reagents. These compounds typically can add to an electron-deficient carbon atom such as at a carbonyl group. Nucleophilic alkylating agents can displace halide substituents on a carbon atom through the SN2 mechanism. With a catalyst, they also alkylate alkyl and aryl halides, as exemplified by Suzuki couplings. thumb|upright=2.0|The Kumada coupling employs both a nucleophilic alkylation step subsequent to the oxidative addition of the aryl halide (L = [[Ligand, Ar = Aryl). The starting material, an aryl bromide (Ar-Br) reacts with nickel with ligands (NiL2). Then, a Grignard reagent (R-MgBr) alkylates the nickel center, replacing the bromide ligand (Br) with an alkyl ligand (R). This nickel-aryl-alkyl complex undergoes rearrangement and reductive elimination to expel an alkylated aryl (Ar-R). The outcome of this reaction is that the aryl group (Ar) is alkylated with an alkyl group (R), replacing bromide (Br), creating an alkylated aryl (Ar-R), the intended product. The bromide is expelled as magnesium bromide (MgBr2). Nickel with ligands (NiL2) acts as the catalyst, being reused multiple times.]] The SN2 mechanism is not available for aryl substituents, where the trajectory to attack the carbon atom would be inside the ring. Thus, only reactions catalyzed by organometallic catalysts are possible.