Isopropyl alcohol

Isopropyl alcohol (also isopropanol, iso, rubbing alcohol, or the abbreviation IPA) is a common name for 2-propanol, a colorless, flammable chemical compound with a strong odor. It has the chemical formula C₃H₈O and is the simplest example of a secondary alcohol, where the alcohol carbon is attached to two other carbons. It is an isomer of propanol.

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Manufacture

Isopropyl alcohol is produced by combining water and propylene.^[1] There are two processes for achieving this: indirect hydration (sulfuric acid process) and direct hydration. The former process, which can use low quality propylene, predominates in the USA while the latter process, which requires high-purity propylene, is more commonly used in Europe. These processes give predominantely isopropyl alcohol rather than propan-1-ol because the addition of water (or sulfuric acid) to propylene follows Markovnikov's rule.

The indirect process reacts propylene with sulfuric acid to form a mixture of sulfate esters. Subsequent hydrolysis of these esters produces isopropyl alcohol. Direct hydration reacts propylene and water, either in gas or liquid phases, at high pressures in the presence of solid or supported acidic catalysts. Both types of processes require that the isopropyl alcohol be separated from water and other by-products by distillation. Isopropyl alcohol and water form an azeotrope and simple distillation gives a material which is 87.9% by weight isopropyl alcohol and 12.1% by weight water.^[2] Pure (anhydrous) isopropyl alcohol is made by azeotropic distillation of the "wet" isopropyl alcohol using either diisopropyl ether or cyclohexane as azeotroping agents.

Uses

Sterilizing pads typically contain a 60-70% solution of isopropanol in water. Isopropyl alcohol is also commonly used as a cleaner and solvent in industry. It is also used as a gasoline additive for dissolving water or ice in fuel lines. Although Isopropanol is sometimes sold as "Isopropyl Rubbing Alcohol, 70% (or 91%)" and "Isopropyl Rubbing Alcohol, 99%" (harder to find but generally more useful for experimenters & cleaning uses), there is no isopropyl alcohol in the United States Pharmacopeia formula for rubbing alcohol, U.S.P. It is used as a disinfectant, and is a common solvent.

Isopropanol is a major ingredient in "dry-gas" fuel additive. In significant quantities, water is a problem in fuel tanks as it separates from the gasoline and can freeze in the supply lines at cold temperatures. The isopropanol does not remove the water from the gasoline. Rather, the isopropanol solubilizes the water in the gasoline. Once soluble, the water does not pose the same risk as insoluble water as it will no longer accumulate in the supply lines and freeze.

It is also a very good cleaning agent and often used for cleaning electronic devices such as contact pins (like those on ROM cartridges), magnetic tape deck and floppy disk drive heads, the lenses of lasers in optical disc drives (e.g. CD, DVD) and removing thermal paste from CPUs. It is also used to clean LCD and glass computer monitor screens (at some risk to the anti-reflection coating of the screen), and used by many music shops to give second-hand or worn records newer looking sheens. It cleans white boards very well and other unwanted ink related marks. Isopropyl alcohol also works well at removing smudges, dirt, and fingerprints from cell phones and PDAs. It is effective at removing residual glue from sticky labels.

Chemistry

Unlike ethanol or methanol, isopropanol can be separated from aqueous solutions by adding a salt such as sodium chloride, sodium sulfate, or any of several other inorganic salts.^[3] The process is colloquially called salting out, and causes concentrated isopropanol to separate into a distinct layer.

Being a secondary alcohol, isopropanol can be oxidised to the ketone acetone. This can be achieved using oxidising agents such as chromic acid, or by dehydrogenation of isopropanol over a heated copper catalyst:

(CH₃)₂CH-OH → (CH₃)₂C=O + H₂

Isopropanol may be converted to 2-bromopropane using phosphorus tribromide, or dehydrated to propylene by heating with sulfuric acid.

Isopropanol is often used as a hydride source in the Meerwein-Ponndorf-Verley reduction.

Like most alcohols, isopropyl alcohol reacts with active metals such as potassium to form alkoxides which can be called isopropoxides. The reaction with aluminium (initiated by a trace of mercury) is used to prepare the catalyst aluminium isopropoxide.

Isopropanol has a maximal absorbance at 204 nm in an ultraviolet-visible spectrum.

Safety

Isopropyl alcohol vapor is heavier than air and is highly flammable with a very wide combustible range. It should be kept away from heat and open flame. When mixed with air or other oxidizers it can explode through deflagration.^[4]

Isopropyl alcohol is oxidized by the liver into acetone. Symptoms of isopropyl alcohol poisoning include flushing, headache, dizziness, CNS depression, nausea, vomiting, anesthesia, and coma. Use in well-ventilated areas and use protective gloves while using. Poisoning can occur from ingestion, inhalation, or absorption.

Long term application to the skin can cause defatting.

Medicine/Toxicology

Isopropyl alcohol is about twice as toxic as ethanol. Isopropyl alcohol does not cause an anion gap acidosis (like ethanol or methanol). It produces an elevated osmolal gap, but generally no abnormal anion gap (though this may be seen as a result of hypotension and lactic acidosis). Overdoses may cause a fruity odor on the breath as a result of its metabolism to give acetone (which is not further metabolised)^[5]. While large quantities of isopropanol can be fatal if left untreated, it is not nearly as toxic as methanol or ethylene glycol.

References

1. ^ "Isopropyl Alcohol, by John E. Logsdon and Richard A. Loke". Kirk‑Othmer Encyclopedia of Chemical Technology. (Article Online Posting Date: December 4, 2000). John Wiley & Sons, Inc. Retrieved on 2007-08-30. 
2. ^ CRC Handbook of Chemistry and Physics, 44th ed. pp 2143-2184
3. ^ Merck Index of Chemicals and Drugs, 9th ed. monograph 5069
4. ^ Safety (MSDS) data for 2-propanol. Retrieved on 2006-09-28.
5. ^ Tiess, D: Z. ges. Hygiene 31, 530-531 (1985)