Alkylation

C-alkylation is a process for the formation of carbon-carbon bonds. The largest example of this takes place in the alkylation units of petrochemical plants, which convert low-molecular-weight alkenes into high octane gasoline components. Electron-rich species such as phenols are also commonly alkylated to produce a variety of products; examples include linear alkylbenzenes used in the production of surfactants like LAS, or butylated phenols like BHT, which are used as antioxidants. This can be achieved using either acid catalysts like Amberlyst, or Lewis acids like aluminium.[3] On a laboratory scale the Friedel–Crafts reaction uses alkyl halides, as these are often easier to handle than their corresponding alkenes, which tend to be gasses. The reaction is catalysed by aluminium trichloride. This approach is rarely used industrially as alkyl halides are more expensive than alkenes.

N-, P-, and S-alkylation are important processes for the formation of carbon-nitrogen, carbon-phosphorus, and carbon-sulfur bonds,

Amines are readily alkylated. The rate of alkylation follows the order tertiary amine < secondary amine < primary amine. Typical alkylating agents are alkyl halides. Industry often relies on green chemistry methods involving alkylation of amines with alcohols, the byproduct being water. Hydroamination is another green method for N-alkylation.

In the Menshutkin reaction, a tertiary amine is converted into a quaternary ammonium salt by reaction with an alkyl halide. Similar reactions occur when tertiary phosphines are treated with alkyl halides, the products being phosphonium salts.

Menshutkin-reaction

Thiols are readily alkylated to give thioethers via the thiol-ene reaction.[4] The reaction is typically conducted in the presence of a base or using the conjugate base of the thiol. Thioethers undergo alkylation to give sulfonium ions.

Alcohols alkylate to give ethers:

${\displaystyle {\ce {R-OH + R'-X -> R-O-R'}}}$

When the alkylating agent is an alkyl halide, the conversion is called the Williamson ether synthesis. Alcohols are also good alkylating agents in the presence of suitable acid catalysts. For example, most methyl amines are prepared by alkylation of ammonia with methanol. The alkylation of phenols is particularly straightforward since it is subject to fewer competing reactions.[5]

${\displaystyle {\ce {Ph-O- + Me2-SO4 -> Ph-O-Me + Me-SO4-}}}$

(with Na⁺ as a spectator ion)

More complex alkylation of a alcohols and phenols involve ethoxylation. Ethylene oxide is the alkylating group in this reaction.

In the process called oxidative addition, low-valent metals often react with alkylating agents to give metal alkyls. This reaction is one step in the Cativa process for the synthesis of acetic acid from methyl iodide. Many cross coupling reactions proceed via oxidative addition as well.

Triethyloxonium tetrafluoroborate is one of the most electrophilic alkylating agents.[6]

Electrophilic alkylating agents deliver the equivalent of an alkyl cation. Alkyl halides are typical alkylating agents. Trimethyloxonium tetrafluoroborate and triethyloxonium tetrafluoroborate are particularly strong electrophiles due to their overt positive charge and an inert leaving group (dimethyl or diethyl ether). Dimethyl sulfate is intermediate in electrophilicity.

Diazomethane is a popular methylating agent in the laboratory, but it is too hazardous (explosive gas with a high acute toxicity) to be employed on an industrial scale without special precautions.[7] Use of diazomethane has been significantly reduced by the introduction of the safer and equivalent reagent trimethylsilyldiazomethane.[8]

Electrophilic, soluble alkylating agents are often toxic and carcinogenic, due to their tendency to alkylate DNA. This mechanism of toxicity is relevant to the function of anti-cancer drugs in the form of alkylating antineoplastic agents. Some chemical weapons such as mustard gas (sulfide of dichloroethyl) function as alkylating agents. Alkylated DNA either does not coil or uncoil properly, or cannot be processed by information-decoding enzymes.

Friedel-Crafts alkylation of benzene is often catalyzed by aluminium trichloride.

Electrophilic alkylations use Lewis acids and Brønsted acids, sometimes both. Classically, Lewis acids, e.g., aluminium trichloride, are employed when the alkyl halide are used. Brønsted acids are used when alkylating with olefins. Typical catalysts are zeolites, i.e. solid acid catalysts, and sulfuric acid. Silicotungstic acid is used to manufacture ethyl acetate by the alkylation of acetic acid by ethylene:[9]

${\displaystyle {\ce {C2H4 + CH3CO2H -> CH3CO2C2H5}}}$