Quick answer: Isopropyl alcohol (IPA, also called isopropanol or 2-propanol, CAS 67-63-0) is a colorless, highly flammable secondary alcohol with the molecular formula C₃H₈O and a molecular weight of 60.10 g/mol. It boils at 82.5°C, has a flash point of just 11.7°C - meaning it releases flammable vapor at virtually any indoor room temperature - and is fully miscible with water. These properties are what make it simultaneously one of the most useful industrial solvents and one of the more carefully regulated flammable liquids in common use.
Chemical Identity and Molecular Structure
| Parameter | Value |
|---|---|
| Chemical Name | Isopropyl Alcohol |
| IUPAC Name | Propan-2-ol |
| Common Synonyms | Isopropanol, IPA, 2-Propanol, sec-Propyl alcohol, Dimethyl carbinol, Rubbing alcohol |
| CAS Number | 67-63-0 |
| RTECS Number | NT8050000 |
| UN Number | 1219 |
| Molecular Formula | C₃H₈O / (CH₃)₂CHOH |
| Molecular Weight | 60.10 g/mol |
| Appearance | Colorless liquid |
| Odor | Sharp, alcoholic odor resembling a mix of ethanol and acetone |
Isopropyl alcohol is the simplest secondary alcohol: its hydroxyl group (-OH) attaches to a central carbon atom that is itself bonded to two methyl (-CH₃) groups. This is the structural detail that distinguishes it from its isomer, n-propanol (1-propanol), where the -OH group sits on a terminal (primary) carbon instead. That single structural difference - secondary vs. primary alcohol - is responsible for IPA's lower boiling point (82.5°C vs. n-propanol's 97°C) and its somewhat different reactivity, since secondary alcohols oxidize to ketones (IPA → acetone) rather than to aldehydes and carboxylic acids the way primary alcohols do.
Complete Physical Properties Data Table
| Property | Value |
|---|---|
| Density (20°C) | 0.786 g/cm³ (specific gravity ~0.79) |
| Boiling Point | 82.5°C (180.5°F) |
| Melting/Freezing Point | -88.5°C (-127°F) |
| Flash Point (closed cup) | 11.7°C (53°F) |
| Autoignition Temperature | ~460°C |
| Vapor Density (air = 1) | 2.1 |
| Vapor Pressure (20°C) | 33 mmHg |
| Vapor Pressure (25°C) | 44 mmHg |
| Refractive Index (20°C) | 1.3772 |
| Viscosity (20°C) | ~2.27 mPa·s |
| Evaporation Rate (n-BuAc = 1) | 1.7 |
| Explosive/Flammable Limits (vol. in air) | 2.0% – 12.7% |
| Water Solubility | Fully miscible |
Chemical Properties and Reactivity
Solubility: IPA is miscible with water, ethanol, ether, and chloroform in all proportions, and it dissolves a broad range of organic materials - oils, resins, alkaloids, ethyl cellulose, and polyvinyl butyral among them. This dual affinity for both polar (water) and moderately nonpolar (oils, resins) compounds is what makes it so versatile as a solvent.
Azeotrope formation: IPA and water form an azeotrope at approximately 80.37°C containing roughly 91% IPA by weight. At this specific ratio, both components evaporate together at the same rate, which means ordinary distillation cannot concentrate IPA beyond about 91% - reaching 99%+ purity requires additional separation techniques such as azeotropic distillation with an entrainer or molecular sieve dehydration. This is the direct explanation for why "91%" and "99%" are the two purity tiers most commonly seen on commercial IPA labels.
Oxidation: Because IPA is a secondary alcohol, oxidation (by agents such as chromic acid or other strong oxidizers) converts it to acetone, not to a carboxylic acid. This reaction is also the basis of one of IPA's major industrial roles, as a feedstock for acetone production.
Dehydration: Under acidic conditions and elevated temperature, IPA dehydrates to form propylene (C₃H₆).
Reactivity hazards: IPA reacts with active metals (potassium, aluminum, titanium) to form isopropoxides, and reacts violently with strong oxidizers such as hydrogen peroxide and nitric acid, generating enough heat to ignite its own vapors. It is also hygroscopic, gradually absorbing atmospheric moisture in imperfectly sealed containers, and can slowly form peroxides during prolonged storage.
pH: Neutral to slightly acidic in aqueous solution - IPA does not corrode metals through pH the way strong acids or bases do.
Why Concentration Changes Everything: 70% vs. 91% vs. 99% IPA
| Concentration | Water Content | Best For |
|---|---|---|
| 99% – 99.8% | ~0.2-1% | Electronics/semiconductor cleaning, anhydrous organic synthesis, moisture-sensitive precision work |
| 91% | ~9% (azeotropic) | General-purpose cleaning, industrial solvent use - the natural endpoint of simple distillation |
| 70% | ~30% | Skin antisepsis, hand sanitizers, surface disinfection - the concentration most medical protocols specify |
The counterintuitive part is that 70% IPA disinfects more effectively than 99% IPA. Pure (99%) isopropyl alcohol coagulates the surface proteins of bacteria almost instantly, forming a protective barrier that shields the interior of the cell before the alcohol can fully act. The water in a 70% solution slows that surface coagulation just enough to let the alcohol penetrate the cell membrane and denature proteins throughout the organism, producing a more complete kill. This is why CDC and hospital disinfection protocols specify 70% rather than higher concentrations for hand and surface antisepsis, while 99% is reserved for applications like electronics cleaning where residual water - not antimicrobial performance - is the concern.
IPA vs. Ethanol vs. Acetone: Property Comparison
IPA is frequently interchanged with ethanol and acetone in cleaning and solvent applications, but the three differ meaningfully:
| Property | Isopropyl Alcohol (IPA) | Ethanol | Acetone |
|---|---|---|---|
| CAS Number | 67-63-0 | 64-17-5 | 67-64-1 |
| Molecular Formula | C₃H₈O | C₂H₆O | C₃H₆O |
| Molecular Weight | 60.10 g/mol | 46.07 g/mol | 58.08 g/mol |
| Boiling Point | 82.5°C | ~78.4°C | 56°C |
| Flash Point | 11.7°C | ~13°C | -20°C |
| Water Miscibility | Full | Full | Full |
| Relative Solvency | Strong for oils/resins | Moderate | Strongest general solvency |
| Typical Distinguishing Use | Disinfection, electronics cleaning | Beverage/food-contact uses (where legally denatured or food-grade) | Fast-drying paint thinners, nail polish remover |
Ethanol is chemically the closest relative (a two-carbon primary alcohol vs. IPA's three-carbon secondary alcohol), but it is approved for food and beverage contact while IPA is toxic if ingested and never used in food products. Acetone, by contrast, isn't an alcohol at all - it's a ketone - and its much lower flash point and faster evaporation make it the more aggressive, faster-flashing solvent of the three.
Isopropyl Alcohol (IPA) Industrial Applications
Solvent & Cleaning
IPA is widely used as an industrial solvent and degreasing agent in electronics manufacturing, dissolving flux residues and oils on circuit boards while evaporating completely without leaving a conductive film. It also serves as a cleaning agent for precision instruments, optics, and machined parts across aerospace and semiconductor manufacturing.
Chemical Intermediate
IPA is a key feedstock in chemical synthesis, oxidized industrially to produce acetone, and used to manufacture isopropylamine, isopropyl acetate, isopropyl ether, and other isopropyl esters. In fine chemicals and pesticide production, it serves as an intermediate for compounds such as isopropyl xanthate and various fungicide and herbicide formulations.
Coatings, Inks & Adhesives
As a solvent for resins, alkyds, nitrocellulose, and shellac, IPA is used in paint and coating formulations, printing inks, and adhesive manufacturing, where its evaporation rate and solvency balance drying speed with working time.
Pharmaceutical & Personal Care
IPA is used in the production of hand sanitizers, antiseptics, and topical pharmaceutical formulations, typically at 70% concentration for antimicrobial applications. It also serves as a processing solvent in cosmetics and personal care product manufacturing.
Oil & Gas
In upstream operations, IPA functions as a defoaming agent in water-based fracturing fluids and as a dehydrating/antifreeze additive in fuel systems, preventing water accumulation and ice formation in pipelines and storage tanks.
Textile & Dyeing
IPA is used as a fixing agent in the dyeing industry and as a processing aid in textile finishing, helping stabilize dye uptake on fabrics.
Other Industrial Uses
Additional applications include use as an anti-fogging agent for glass and transparent plastics, a dispersant in pigment production, and a degreasing agent for animal-derived tissue membranes in biological processing.
FAQs
What is the molecular weight of isopropyl alcohol?
60.10 g/mol, based on the molecular formula C₃H₈O.
Is isopropyl alcohol the same as rubbing alcohol?
Generally yes - "rubbing alcohol" commonly refers to an IPA-water solution, typically at 70% concentration, though some rubbing alcohol products are ethanol-based instead. Checking the label or SDS is the only reliable way to confirm which alcohol a specific product contains.
Why does IPA evaporate faster than water?
IPA's boiling point (82.5°C) is far below water's (100°C), and its vapor pressure (33 mmHg at 20°C) is much higher than water's at the same temperature, so a larger fraction of IPA molecules have enough energy to escape into vapor at room temperature.
What happens when IPA oxidizes?
As a secondary alcohol, IPA oxidizes to acetone (a ketone) rather than to an aldehyde or carboxylic acid, which is the pathway primary alcohols like ethanol follow.
Can isopropyl alcohol be purified above 99%?
Yes, but not through simple distillation - the IPA-water azeotrope caps simple distillation at roughly 91% IPA. Higher purities (99%+) require azeotropic distillation, molecular sieves, or other specialized dehydration methods.
Is IPA more flammable than ethanol?
The two are broadly comparable - IPA's flash point (11.7°C) is close to ethanol's (~13°C), and both are classified as flammable liquids requiring similar fire-safety precautions. Neither should be treated as meaningfully "safer" than the other from a flammability standpoint.
