Amyl Alcohols

Abstract

Amyl alcohol describes any saturated aliphatic alcohol containing five carbon atoms. This class consists of three pentanols, four substituted butanols, and a disubstituted propanol, ie, eight structural isomers C 5 H 12 O: four primary, three secondary, and one tertiary alcohol. In addition, 2‐pentanol, 2‐methyl‐1‐butanol, and 3‐methyl‐2‐butanol have chiral centers and hence two enantiomeric forms. The odd‐carbon structure and the extent of branching provide amyl alcohols with unique physical and solubility properties and often offer ideal properties for solvent, surfactant, extraction, gasoline additive, and fragrance applications. Today the most important industrial process for amyl alcohol production is low pressure rhodium‐catalyzed hydroformylation (oxo process) of butenes. Mixtures of isomeric amyl alcohols (1‐pentanol and 2‐methyl‐1‐butanol) are often preferred because they are less expensive to produce commercially and have a more desirable combination of properties. With the exception of neopentyl alcohol (mp 53°C), the amyl alcohols are clear, colorless liquids under atmospheric conditions, with characteristic, slightly pungent and penetrating odors. Commercial primary amyl alcohol is a mixture of 1‐pentanol and 2‐methyl‐1‐butanol. Like the lower alcohols, amyl alcohols are completely miscible with numerous organic solvents and are excellent solvents for nitrocellulose, resin lacquers, higher esters, and various natural and synthetic gums and resins. However, in contrast to the lower alcohols, they are only slightly soluble in water. Important amyl alcohol reactions include dehydration, esterification, oxidation, amination, etherification, and condensation. Three significant commercial processes for the production of amyl alcohols include separation from fusel oils, chlorination of C‐5 alkanes with subsequent hydrolysis to produce a mixture of seven of the eight isomers (Pennsalt), and a low pressure oxo process. The oxo process is the principal one in practice today. The main effects of prolonged exposure to amyl alcohols are irritation to mucous membranes and upper respiratory tract, significant depression of the central nervous system, and narcotic effects from vapor inhalation or oral absorption. All the alcohols are harmful if inhaled or swallowed. 3‐Methyl‐1‐butanol has demonstrated carcinogenic activity in animal studies. The amyl alcohols are readily flammable substances; tert ‐amyl alcohol is the most flammable. Their vapors can form explosive mixtures with air. Amyl alcohols are best stored or shipped in either aluminum, lined steel, or stainless steel tanks.