Alkylation

Source: Wikipedia, the free encyclopedia.
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.[2] 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

Suzuki couplings
.

Aryl
).

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.

Alkylation by carbon electrophiles

C-alkylation

C-alkylation is a process for the formation of carbon-carbon bonds. The largest example of this takes place in the

aluminium trichloride
. This approach is rarely used industrially as alkyl halides are more expensive than alkenes.

N-,P-, S- alkylation

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

alkyl halide
. Similar reactions occur when tertiary phosphines are treated with alkyl halides, the products being phosphonium salts.

Menshutkin-reaction
Menshutkin-reaction

sulfonium ions
.

O-alkylation

Alcohols alkylate to give ethers:

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]

(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.

Oxidative addition to metals

In the process called

cross coupling reactions
proceed via oxidative addition as well.

Electrophilic alkylating agents

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.

Methylation with diazomethane

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]

Hazards

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

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.

Catalysts

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

Electrophilic alkylation uses

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]

In biology

Main article: methylation

Alkylation in biology causes

nitrogenous bases. It is caused by alkylating agents such as EMS (Ethyl Methyl Sulphonate). Bifunctional alkyl groups which have two alkyl groups in them cause cross linking in DNA. Alkylation damaged ring nitrogen bases are repaired via the Base Excision Repair (BER) pathway.[10]

SAM cofactor
and cytosine base are shown for simplicity.

Commodity chemicals

Several commodity chemicals are produced by alkylation. Included are several fundamental benzene-based feedstocks such as

linear alkylbenzene sulfonates (for detergents).[11]

linear alkylbenzene sulfonate
detergents.

Gasoline production

Main article: alkylation unit
Typical acid-catalyzed route to 2,4-dimethylpentane.

In a conventional

isooctane). Alkylate is a premium gasoline blending stock because it has exceptional antiknock properties and is clean burning. Alkylate is also a key component of avgas. By combining fluid catalytic cracking, polymerization, and alkylation, refineries can obtain a gasoline yield of 70 percent. The widespread use of sulfuric acid and hydrofluoric acid in refineries poses significant environmental risks.[12] Ionic liquids are used in place of the older generation of strong Bronsted acids.[13][14]

Dealkylation

Complementing alkylation reactions are the reverse, dealkylations. Prevalent are

methyl amines
.

See also

References

  1. ISBN 0-471-85472-7
  2. ISBN 0-8247-8701-3
    .
  3. PMID 16466262
    .
  4. doi:10.15227/orgsyn.058.0143
    .
  5. ^ G. S. Hiers and F. D. Hager (1941). "Anisole". Organic Syntheses; Collected Volumes, vol. 1, p. 58.
  6. ISBN 978-0471936237
    .
  7. doi:10.1021/op020049k
    .
  8. ISBN 0471936235
    .
  9. doi:10.1016/j.cattod.2008.10.054
    .
  10. PMID 9701511
    .
  11. ISBN 0471238961
    .
  12. doi:10.1002/14356007.a01_185
  13. S2CID 225512999
    .
  14. ^ "Oil & Gas Engineering | Ionic liquid alkylation technology receives award". 2 January 2018.

External links

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