Heptane Synthesis

Heptane or ''n''-heptane is the straight-chain alkane with the chemical formula H₃C(CH₂)₅CH₃ or C₇H₁₆. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale (the 100 point is 100% iso-octane). Octane number equates to the anti-knock qualities of a comparison mixture of heptane and iso-octane which is expressed as the percentage of iso-octane in heptane, and is listed on pumps for gasoline (petrol) dispensed globally.

History

Normal heptane was discovered in 1862 by Carl Schorlemmer, who, while analyzing pyrolysis products of the cannel coal mined in Wigan, identified, separated by fractional distillation and studied a series of liquid hydrocarbons inert to nitric and sulfuric acids. One of them, which he called hydride of heptyl (oenanthyl), had an empirical formula of C₇H₁₆, density of 0.709 at 18 °C and boiled between 98 and 99 °C. In the next year he identified the same compound in the Pennsylvanian oil. By 1872 he switched his nomenclature to the modern one.

During the American Civil War and shortly thereafter Californians discovered that some pines gave turpentine with unusual properties. It took until 1879 to identify heptane as the cause of that (see below for details), and only by the end of the century was this fact accepted by European chemists.

Uses

Heptane and its many isomers are widely used in laboratories as a non-polar solvent. It is often preferred to hexane due to heptane being comparatively less toxic and less volatile. As a liquid, it is ideal for transport and storage. In the grease spot test, heptane is used to dissolve an oil spot to show the previous presence of organic compounds on a stained paper. This is done by shaking the stained paper in a heptane solution for about half a minute.

Aqueous bromine may be distinguished from aqueous iodine by its appearance after extraction into heptane. In water, both bromine and iodine appear brown. However, iodine turns purple when dissolved in heptane, whereas the bromine solution remains brown.

Heptane is commercially available as both pure and mixed isomers for use in paints and coatings, as the rubber cement solvent "Bestine", the outdoor stove fuel "Powerfuel" by Primus, as pure ''n''-heptane for research and development and pharmaceutical manufacturing and as a minor component of gasoline (petrol). On average, gasoline is about 1% heptane.

Heptane is also used as an adhesive remover by stamp collectors. Since 1974, the United States Postal Service has issued self-adhesive stamps that some collectors find difficult to separate from envelopes via the traditional method of soaking in water. Heptane-based products like Bestine, as well as limonene-based products, have become popular solvents for removing stamps more easily.

Octane rating scale

''n''-Heptane is defined as the zero point of the octane rating scale. It is a lighter component in gasoline and burns more explosively, causing engine pre-ignition ( knocking) in its pure form, as opposed to octane isomers, which burn more slowly and give less knocking. It was originally chosen as the zero point of the scale because of the availability of very high purity ''n''-heptane, unmixed with other isomers of heptane or other alkanes, distilled from the resin of Jeffrey pine and from the fruit of ''Pittosporum resiniferum''. Other sources of heptane and octane, produced from crude oil, contain a mixture of different isomers with greatly differing ratings, and do not give as precise a zero point.

Isomers and enantiomers

Heptane has nine isomers, or eleven if enantiomers are counted:
* Heptane (''n''-heptane), H₃C–CH₂–CH₂–CH₂–CH₂–CH₂–CH₃,
* 2-Methylhexane (isoheptane), H₃C–CH(CH₃)–CH₂–CH₂–CH₂–CH₃,
* 3-Methylhexane, H₃C–CH₂–C^*H(CH₃)–CH₂–CH₂–CH₃ (chiral),
* 2,2-Dimethylpentane (neoheptane), H₃C–C(CH₃)₂–CH₂–CH₂–CH₃,
* 2,3-Dimethylpentane, H₃C–CH(CH₃)–C^*H(CH₃)–CH₂–CH₃ (chiral),
* 2,4-Dimethylpentane, H₃C–CH(CH₃)–CH₂–CH(CH₃)–CH₃,
* 3,3-Dimethylpentane, H₃C–CH₂–C(CH₃)₂–CH₂–CH₃,
* 3-Ethylpentane, H₃C–CH₂–CH(CH₂CH₃)–CH₂–CH₃,
* 2,2,3-Trimethylbutane, H₃C–C(CH₃)₂–CH(CH₃)–CH₃, also known as pentamethylethane and triptane.

Preparation

The linear ''n''-heptane can be obtained from Jeffrey pine oil. The six branched isomers without a quaternary carbon can be prepared by creating a suitable secondary or tertiary alcohol by the Grignard reaction, converting it to an alkene by dehydrogenation, and hydrogenating the latter. The 2,2-dimethylpentane isomer can be prepared by reacting ''tert''-butyl chloride with ''n''-propyl magnesium bromide. The 3,3-dimethylpentane isomer can be prepared from ''tert''-amyl chloride and ethyl magnesium bromide.

Health risks

Acute exposure to heptane vapors can cause dizziness, stupor, incoordination, loss of appetite, nausea, dermatitis, chemical pneumonitis, unconsciousness, or possible peripheral neuropathy.

In a CDC study, it was found that prolonged exposure to heptane may also cause a state of intoxication and uncontrolled hilarity in some participants and a stupor lasting for 30 minutes after exposure for others. Prolonged exposure can also lead to skin dryness or cracking, since the substance defats skin.

According to information from the New Jersey Department of Health and Senior Services, ''n''-heptane can penetrate the skin, and further health effects may occur immediately or shortly after exposure to it. Exposure to ''n''-heptane may lead to short-term health effects like irritation of the eyes, nose, or throat, headache, dizziness, or loss of consciousness; and chronic health effects, like reduced memory and concentration, sleep disturbance, and reduced coordination due to its effects on the nervous system.

Upon chronic exposure, it can pose a reproductive hazard or cancer risks.

References

External links

International Chemical Safety Card 0657
(''n''-heptane)


(2-methylhexane)





Phytochemical database entry

{{Authority control

Alkanes
Hydrocarbon solvents