H3C C CH2 CH CI li CH3

2,2,4-trimethyipentane ("isooctane")

Gasoline that has been produced simply by distillation of crude oil has an octane number of about 50, much too low for use in modern vehicles. However, when added to gasoline in small amounts, the compounds tetramethyl-lead, Pb(CH3)4, and its ethyl equivalent prevent engine knocking and hence greatly boost the octane number of gasoline. For decades, they were added worldwide to gasoline consisting predominantly of unbranched alkanes and cycloalkanes. However, these additives have largely been phased out now in most developed countries owing to environmental concerns about lead, a topic discussed in Chapter 15.

In some European countries and in Canada, lead compounds were replaced by small quantities of an organic compound of manganese called MMT, which stands for methylcyclopentadienyl manganese tricarbonyl. The use of MMT has been controversial for health reasons, since manganese concentrations rise in air and soil as a result, and for technological reasons, since some car manufacturers claim it degrades emissions components on vehicles. Until 1995, MMT was banned in the United States; even though this ban was revoked, its use there is very small.

The alternative to using lead or manganese additives to boost octane ratings is to blend into gasoline significant quantities of highly branched alkanes,

TABLE 7-t

Octane Number

Compound

Octane Number

Benzene Toluene

118 116 116 112 116

fj-Xylene (1,4-dimethylbenzene)

Methanol

Ethanol

MTBE

BTX, or other organic substances such as MTBE (discussed later), which themselves have high octane numbers, A list of the common additives is shown in Table 7-1. Collectively, the benzene + toluene + xylene component in gasoline is called BTX.

Gasoline often also contains some trimethylated benzenes and ethylbenzene; the mixture is then called BTEX. Currently, most unleaded gasoline sold in the United States contains significant quantities of BTX (as high as 40% content in the past), ethanol (especially in the Midwest), or MTBE to boost the octane number. Unfortunately, the BTX hydrocarbons are more reactive than the alkanes that they replace in causing photochemical air pollution, so in a sense the lead pollution has been reduced at the price of producing more smog. In addition, the use of BTX in unleaded gasoline in countries, such as Great Britain, where few cars were equipped with catalytic converters resulted in the past in an increase in the BTX concentrations in outdoor air. Benzene in particular is a worrisome air pollutant since at higher levels it has been linked to increases in the incidence of leukemia (see Chapter 4).

The reformulated gasoline used in the second half of the 1990s in North America contained a maximum of 1% benzene and 25% (volume) aromatics in total, with a minimum of 2% oxygen (by mass). The second phase of reformulated gasoline, which entered the U.S. market in 2000, reduces the benzene and BTX components even further and lowers the sulfur content to 30 ppm. The EU planned to cut the maximum level of benzene in gasoline by 75% effective 2005.

The use of alcohols and compounds derived from them as additives or as "oxygenated" motor fuels in their own right is discussed in Chapter 8.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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