Chloroformate
Chloroformates are a class of reactive organic compounds defined by the general formula ROC(O)Cl, where R is an alkyl or aryl group, consisting of a carbonyl group bonded to a chlorine atom and an alkoxy or aryloxy substituent.[1][2] They represent esters of the unstable chloroformic acid (HOC(O)Cl) and are valued for their versatility as acylating agents in organic synthesis due to their acid chloride-like reactivity.[3] These compounds are typically synthesized by reacting phosgene (COCl₂) with alcohols (ROH) or phenols (ArOH) under controlled low-temperature conditions to prevent decomposition, though phosgene-free alternatives using carbon monoxide and nitrite esters with platinum-group catalysts have been developed for safer production.[3][4] Physically, chloroformates are clear, colorless liquids with low freezing points, boiling points often exceeding 100°C, and good solubility in organic solvents, but they hydrolyze rapidly in moist air or water to produce the corresponding alcohol, carbon dioxide (CO₂), and hydrochloric acid (HCl), with hydrolysis half-lives ranging from 1.4 to 53.2 minutes depending on the substituent.[3] In industrial and laboratory applications, chloroformates function as key intermediates for manufacturing pharmaceuticals, pesticides, herbicides, perfumes, polymers, dyes, and food additives, including the production of peroxydicarbonates used as polymerization initiators.[3] They also play roles in analytical chemistry as derivatizing agents for enhancing volatility in gas chromatography and in peptide synthesis for temporary protection of amino groups.[4] Key reactions involve nucleophilic acyl substitution with amines, alcohols, or thiols to form carbamates, carbonates, or thiocarbonates, respectively, enabling efficient construction of complex molecules.[5] Despite their utility, chloroformates are highly corrosive and pose significant health risks as direct-acting irritants and respiratory toxicants, with animal studies indicating acute lethality at low concentrations (e.g., LC50 values around 45–410 ppm in rats), underscoring the need for stringent safety protocols during handling.[3]Properties
Physical properties
Chloroformates, a class of organic compounds with the general formula ROC(O)Cl where R is an alkyl or aryl group, are typically clear, colorless liquids at room temperature. They exhibit high volatility, evaporating readily and often producing a strong, pungent odor due to their high vapor pressures and tendency to form vapors at ambient conditions.[6] Boiling points of chloroformates vary depending on the R group but generally range from around 70°C for simple alkyl derivatives to over 150°C for aromatic ones, reflecting increasing molecular weight and intermolecular forces. For instance, methyl chloroformate has a boiling point of 71–72°C, ethyl chloroformate boils at 93–95°C, and benzyl chloroformate at 103°C under reduced pressure or higher under standard conditions. Freezing points are notably low, often below -50°C for alkyl chloroformates, such as -61°C for methyl chloroformate and -80°C for ethyl chloroformate, allowing them to remain liquid over a wide temperature range. Densities for common alkyl chloroformates fall between 1.2 and 1.4 g/cm³ at 20°C, with methyl chloroformate at 1.22 g/mL and ethyl chloroformate at 1.135 g/mL, indicating they are denser than water. Regarding solubility, chloroformates are poorly soluble in water—methyl chloroformate, for example, shows limited miscibility—but they are fully miscible with common organic solvents such as dichloromethane, diethyl ether, and ethanol. The following table summarizes key physical properties for representative chloroformates:| Compound | Boiling Point (°C) | Freezing Point (°C) | Density (g/mL at 20°C) | Appearance |
|---|---|---|---|---|
| Methyl chloroformate | 71–72 | -61 | 1.22 | Clear, colorless liquid |
| Ethyl chloroformate | 93–95 | -80 | 1.135 | Clear, colorless liquid |
| Benzyl chloroformate | 103 (at 20 mmHg) | -20 | 1.195 | Clear, colorless to pale yellow liquid |