Antiknock agent

An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs.

The mixture known as gasoline or petrol, when used in high compression internal combustion engines, has a tendency to knock (also called "pinging" or "pinking") and/or to ignite early before the correctly timed spark occurs (pre-ignition, refer to engine knocking).

Research

Early research into this effect was led by A.H. Gibson and Harry Ricardo in England and Thomas Midgley, Jr. and Thomas Boyd in the United States. The discovery that lead additives modified this behavior led to the widespread adoption of the practice in the 1920s and therefore more powerful higher compression engines. The most popular additive was tetraethyllead. However, with the discovery of the environmental and health damage caused by the lead, and the incompatibility of lead with catalytic converters found on virtually all US automobiles since 1975, this practice began to wane in the 1980s. Most countries are phasing out leaded fuel although different additives still contain lead compounds. Other additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol).

Typical agents

The typical antiknock agents in use are:

• Tetraethyllead (Still in use as a high octane additive)
• Alcohol
• Methylcyclopentadienyl manganese tricarbonyl (MMT)
• Ferrocene
• Iron pentacarbonyl
• Toluene
• Isooctane

Tetraethyllead

In the U.S., where tetraethyllead had been blended with gasoline (primarily to boost octane levels) since the early 1920s, standards to phase out leaded gasoline were first implemented in 1973. In 1995, leaded fuel accounted for only 0.6% of total gasoline sales and less than 2,000 tons of lead per year. From January 1, 1996, the Clean Air Act banned the sale of leaded fuel for use in on-road vehicles. Possession and use of leaded gasoline in a regular on-road vehicle now carries a maximum US$10,000 fine in the United States. However, fuel containing lead may continue to be sold for off-road uses, including aircraft, racing cars, farm equipment, and marine engines. The ban on leaded gasoline led to thousands fewer tons of lead being released into the air by automobiles.

Similar bans in other countries have resulted in sharply decreasing levels of lead in people's bloodstreams.

A side effect of the lead additives was protection of the valve seats from erosion. Many classic cars' engines have needed modification to use lead-free fuels since leaded fuels became unavailable. However, "Lead substitute" products are also produced and can sometimes be found at auto parts stores.

Gasoline, as delivered at the pump, also contains additives to reduce internal engine carbon buildups, improve combustion, and to allow easier starting in cold climates.

In some parts of South America, Asia, and the Middle East, leaded gasoline is still in use. Leaded gasoline was phased out in sub-Saharan Africa, starting 1 January 2006. A growing number of countries have drawn up plans to ban leaded gasoline in the near future.

Some experts speculate that Leaded petrol was behind a global crime wave in the late 80's and early 90's

To avoid deposits of lead inside the engine, lead scavengers are added to the gasoline together with tetraethyllead. The most common ones are:

• Tricresyl phosphate
• 1,2-Dibromoethane
• 1,2-Dichloroethane

MMT

Methylcyclopentadienyl manganese tricarbonyl (MMT) has been used for many years in Canada and recently in Australia to boost octane ratings. It also allows old cars, designed to use leaded fuel, to run on unleaded fuel without the need for additives to prevent valve erosion.

US Federal sources state that MMT is suspected to be a powerful neurotoxin and respiratory toxin, and a large Canadian study(Funded by automakers, who are against its use) concluded that MMT impairs the effectiveness of automobile emission controls and increases pollution from motor vehicles.

The compound is called a half-sandwich compound. The manganese atom in MMT is bonded to three carbonyl groups as well as to the methylcyclopentadienyl ring. These hydrophobic organic ligands make MMT highly lipophilic, which may increase bioaccumulation. A variety of related complexes are known, including ferrocene, which is also under consideration as an additive to gasoline.

Ferrocene

Ferrocene is the organometallic compound with the formula Fe(C₅H₅)₂. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds. The rapid growth of organometallic chemistry is often attributed to the excitement arising from the discovery of ferrocene and its many analogues.

Ferrocene and its numerous derivatives have no large-scale applications, but have many niche uses that exploit their unusual structure (ligand scaffolds, pharmaceutical candidates), robustness (anti-knock formulations, precursors to materials), and redox reactions (reagents and redox standards).

Ferrocene and its derivatives are antiknock agents added to the petrol used in motor vehicles, and are safer than the now-banned tetraethyllead. It is possible to buy, at Halfords in the UK, a petrol additive solution which contains ferrocene, intended to be added to unleaded petrol to enable its use in vintage cars designed to run on leaded petrol. The iron-containing deposits formed from ferrocene can form a conductive coating on the spark plug surfaces.

Iron pentacarbonyl

Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula Fe(CO)₅. Under standard conditions Fe(CO)₅ is a free-flowing, straw-colored liquid with a pungent odour.

This compound is a common precursor to diverse iron compounds, including many that are useful in organic synthesis. Fe(CO)₅ is prepared by the reaction of fine iron particles with carbon monoxide. Fe(CO)₅ is inexpensively purchased.

Iron pentacarbonyl is one of the homoleptic metal carbonyls; i.e. metal complexes bonded only to CO ligands. Other examples include octahedral Cr(CO)₆ and tetrahedral Ni(CO)₄.

Most metal carbonyls have 18 valence electrons, and Fe(CO)₅ fits this pattern with 8 valence electrons on Fe and five pairs of electrons provided by the CO ligands. Reflecting its symmetrical structure and charge neutrality, Fe(CO)₅ is volatile; it is one of the most frequently encountered liquid metal complexes.

Fe(CO)₅ adopts a trigonal bipyramidal structure with the Fe atom surrounded by five CO ligands: three in equatorial positions and two axially bound. The Fe-C-O linkages are each linear.

Fe(CO)₅ is the archetypal fluxional molecule due to the rapid interchange of the axial and equatorial CO groups via the Berry mechanism on the NMR timescale. Consequently, the¹³C NMR spectrum exhibits only one signal due to the rapid interchange between nonequivalent CO sites.

In Europe, iron pentacarbonyl was once used as an anti-knock agent in petrol in place of tetraethyllead. Two more modern alternative fuel additives are ferrocene and methylcyclopentadienyl manganese tricarbonyl. Fe(CO)₅ is used in the production of "carbonyl iron", a finely divided form of iron used in magnetic cores of high-frequency coils for electronics, and for manufacture of the active ingredients of some radar absorbent materials (e.g. iron ball paint). It is famous as a chemical precursor for the synthesis of various iron-based nanoparticles.

Iron pentacarbonyl has been found to be a strong flame speed inhibitor in oxygen based flames.

Toluene

Toluene is a clear, water-insoluble liquid with the typical smell of paint thinners, redolent of the sweet smell of the related compound benzene. It is an aromatic hydrocarbon that is widely used as an industrial feedstock and as a solvent. Like other solvents, toluene is also used as an inhalant drug for its intoxicating properties.

Toluene can be used as an octane booster in gasoline fuels used in internal combustion engines. Toluene at 86% by volume fueled all the turbo Formula 1 teams in the 1980s, first pioneered by the Honda team. The remaining 14% was a "filler" of n-heptane, to reduce the octane to meet Formula 1 fuel restrictions. Toluene at 100% can be used as a fuel for both two-stroke and four-stroke engines; however, due to the density of the fuel and other factors, the fuel does not vaporize easily unless preheated to 70 degrees celsius (Honda accomplished this in their Formula 1 cars by routing the fuel lines through the muffler system to heat the fuel). Toluene also poses similar problems as alcohol fuels, as it eats through standard rubber fuel lines and has no lubricating properties as standard gasoline does, which can break down fuel pumps and cause upper cylinder bore wear.

Toluene has also been used as a coolant for its good heat transfer capabilities in sodium cold traps used in nuclear reactor system loops.

Properties of xylenes and ethylbenzene are nearly identical to toluene, with the latter advertised by a refinery as "component of high performance fuels".

2,2,4-Trimethylpentane (isooctane)

2,2,4-Trimethylpentane, also known as isooctane, is an octane isomer which defines the 100 point on the octane rating scale (the zero point is n-Heptane). It is an important component of gasoline.

Isooctane is produced on a massive scale in the petroleum industry, usually as a mixture with related hydrocarbons. The alkylation process alkylates isobutane with isobutylene using a strong acid catalyst. In the NExOCTANE process, isobutylene is dimerized into isooctane and then hydrogenated to isooctane.

See also

• MTBE controversy