Everything about Alkylation totally explained
Alkylation is the transfer of an
alkyl group from one
molecule to another. The alkyl group may be transferred as an alkyl
carbocation, a
free radical, a
carbanion or a
carbene (or their equivalents).
Alkylating agents are widely used in chemistry because the alkyl group is probably the most common group encountered in organic molecules. Many biological target molecules or their synthetic precursors comprise of an alkyl chain, with specific
functional groups in a specific order. Selective alkylation, or adding parts to the chain with the desired functional groups, is used, especially if there's no commonly available biological precursor.
In oil refining contexts,
alkylation refers to a particular alkylation of isobutane with olefins. It is a major aspect of the
upgrading of
petroleum.
Alkylating agents
Alkylating agents are classified according to their
nucleophilic or
electrophilic character.
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 also displace
halide substituents on a carbon atom. In the presence of
catalysts, they also alkylate alkyl and aryl halides, as exemplified by
Suzuki couplings.
Electrophilic alkylating agents deliver the equivalent of an alkyl
cation. Examples include the use of
alkyl halides with a
Lewis acid catalyst to alkylate
aromatic substrates in
Friedel-Crafts reactions. Alkyl halides can also react directly with
amines to form C-N bonds; the same holds true for other nucleophiles such as alcohols, carboxylic acids, thiols, etc.
Electrophilic, soluble alkylating agents are often very toxic, due to their ability to alkylate DNA. They should be handled with proper
PPE. This mechanism of toxicity is also responsible for the ability of some alkylating agents to perform as anti-cancer drugs in the form of
alkylating antineoplastic agents, and also as
chemical weapons such as
mustard gas.
The electrophilic alkylating agents are commonly of concern as
alkylating antineoplastic agent that attaches an alkyl group to
DNA. The modify the information-encoding nucleic acids.
Radical alkylating agents
Carbene alkylating agents
Carbenes are extremely reactive and are known to attack even unactivated C-H bonds. Carbenes can be generated by elimination of a
diazo group. A metal can form a carbene equivalent called a
transition metal carbene complex.
In biology
Methylation is the most common type of alkylation, being associated with the transfer of a
methyl group. Methylation in nature is typically effected by
vitamin B12-derived enzymes, where the methyl group is carried by
cobalt. In
methanogenesis,
coenzyme M is methylated by
tetrahydromethanopterin.
Electrophilic compounds may alkylate different nucleophiles in the body. The toxicity, carcinogenity, and paradoxically, cancer cell-killing abilities of different DNA alkylating agents are an example.
Oil refining
In a standard
oil refinery process,
isobutane is alkylated with low-molecular-weight
alkenes (primarily a mixture of
propylene and
butylene) in the presence of a strong acid
catalyst, either
sulfuric acid or
hydrofluoric acid. In an oil refinery it's referred to as a
sulfuric acid alkylation unit (SAAU) or a
hydrofluoric alkylation unit, (HFAU). However, oil refinery employees may simply refer to the unit as the Alkyl or Alky unit. The catalyst is able to protonate the alkenes (propylene, butylene) to produce reactive
carbocations, which alkylate isobutane. The reaction is carried out at mild temperatures (0 and 30 °C) in a two-phase reaction. It is important to keep a high ratio of isobutane to olefin at the point of reaction to prevent side reactions that lead to a lower octane product, so the plants have a high recycle of isobutane back to feed. The phases separate spontaneously, so the acid phase is vigoriously mixed with the hydrocarbon phase to create sufficient contact surface.
The product is called alkylate and is composed of a mixture of high-
octane, branched-chain
paraffinic
hydrocarbons (mostly
isopentane and
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. The octane number of the alkylate depends mainly upon the kind of olefins used and upon operating conditions. For example,
isooctane results from combining butylene with isobutane and has an octane rating of 100 by definition. There are other products in the alkylate, so the
octane rating will vary accordingly.
Most
crude oils contain only 10 to 40 percent of their hydrocarbon constituents in the gasoline range, so refineries use
cracking processes, which convert high molecular weight hydrocarbons into smaller and more volatile compounds.
Polymerization converts small gaseous olefins into liquid gasoline-size hydrocarbons. Alkylation processes transform small olefin and iso-paraffin molecules into larger iso-paraffins with a high octane number.
Combining cracking, polymerization, and alkylation can result in a gasoline yield representing 70 percent of the starting crude oil. More advanced processes, such as
cyclicization of paraffins and
dehydrogenation of
naphthenes to form
aromatic hydrocarbons in a
catalytic reformer, have also been developed to increase the octane rating of gasoline. Modern refinery operation can be shifted to produce almost any fuel type with specified performance criteria from a single crude feedstock.
In the entire range of refinery processes, alkylation is a very important process that enhances the yield of high-octane gasoline. However not all refineries have an alkylation plant. The oil and gas journal annual survey of worldwide refining capacities for January 2007 lists many countries with no alkylation plants at their refineries.
A primary factor in deciding to install alkylation is usually economics. Refinery alkylation units are complex and there's substantial economy of scale. In addition to a suitable quantity of feedstock, the price spread between the value of alkylate product and alternate feedstock disposition value must be large enough to justify the plant. Alternative outlets for refinery alklylation feedstocks include sales as
LPG, blending of C4 streams directly into gasoline and feedstocks for chemical plants. Local market conditions vary widely between plants. Variation in the RVP specification for gasoline between countries and between seasons dramatically impacts the amount of butane streams that can be blended directly into gasoline. The transportation of specific types of LPG streams can be expensive so local disparities in economic conditions are often not fully mitigated by cross market movements of alkylation feedstocks.
Another factor in the decision to build an alkylation plant concerns the availability of a suitable catalyst. If
sulfuric acid is used, significant volumes are needed. This requires access to a suitable plant for the supply of fresh acid and the disposition of spent acid. If a sulfuric acid plant must be constructed specifically to support an alkylation unit, this will have a significant impact on both the initial capital requirements and ongoing operating costs. The second main catalyst option is
hydrofluoric acid. Consumption rates for HF acid in alkylation plants are much lower than for sulfuric acid. HF acid plants can process a wider range of feedstock mix with proplyenes and butylenes. HF plants also produce alklyate with better octane than sulfuric plants. However due to the hazardous nature of the material, HF acid is produced at very few locations and transportation must be managed rigorously.
Further Information
Get more info on 'Alkylation'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://alkylation.totallyexplained.com">Alkylation Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
