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Isopropyl alcohol

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Isopropyl alcohol (also isopropanol, iso, isopro, rubbing alcohol, or the abbreviation IPA) is a common name for propan-2-ol, a colorless, flammable chemical compound with a strong odor. It has the molecular 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.

Production

Isopropyl alcohol is produced by combining water and propene. There are two processes for achieving this: indirect hydration via the 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 predominantly 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 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. Pure (anhydrous) isopropyl alcohol is made by azeotropic distillation of the "wet" isopropyl alcohol using either diisopropyl ether or cyclohexane as azeotroping agents.

Availability

Although isopropanol is sometimes sold as "Isopropyl Rubbing Alcohol, 70%" (or 91%) and "Isopropyl Rubbing Alcohol, 99%", there is no isopropyl alcohol in the United States Pharmacopeia formula for rubbing alcohol, U.S.P.

Uses

Isopropyl alcohol is cheaply available. Like acetone, it dissolves a wide range of nonpolar compounds. It is also relatively nontoxic and dries quickly. Thus it is used widely as a solvent and as a cleaning fluid (by dissolving lipophilic contaminants such as oil). Examples of this use include 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 and other unwanted ink related marks very well (at the risk of damaging the non-stick surface of the white board). 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.

As a preservative (for biological specimens) isopropyl alcohol provides a cost-effective (when compared to pure ethanol) and comparatively non-toxic solution to formaldehyde and other synthetic preservatives. When used for the preservation of specimens in solution concentrations of 90-99% are optimal, though concentrations as low as 70% can be used in emergencies.

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.

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. Isopropanol is often sold in aerosol cans as a windscreen de-icer.

It can also be used to remove stains from any fabric, wood, cotton, etc.

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. The process is colloquially called salting out, and causes concentrated isopropanol to separate into a distinct layer.

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

2(CH₃)₂CH-OH → 2(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. Isopropyl alcohol has also been reported to form explosive peroxides.

Like many organic solvents, long term application to the skin can cause defatting.

Toxicology

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.

Isopropyl alcohol is about twice as toxic as ethanol, though isopropyl alcohol does not cause an anion gap acidosis as do ethanol and 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 metabolized. Isopropyl alcohol is more potent than ethanol as a CNS depressant, and its metabolite, acetone, is a CNS depressant in its own right. While around 15 g of isopropanol can be fatal if left untreated, it is not nearly as toxic as methanol or ethylene glycol.