Nitrations with Acetyl Nitrate. I. The Nature of the Nitrating Agent and the Mechanism of Reaction with Simple Alkenes

Abstract

Acetyl nitrate in acetic acid-acetic anhydride solution reacts within five minutes with cis- and trans-2-butenes, 2-methylpropene, 2-methyl-2-butene, cyclopentene, cyclohexene and 1-methylcyclohexene to give β-nitro acetates in 20–64% yields and β-nitroalkenes in 3–50% yields. Small amounts of β-nitro nitrates are present in the β-nitro acetate fraction. Under these conditions 1-butene, 3-methyl-l-butene, 2,3,3-trimethyl-l-butene, 2,4,4-trimethyl-2-pentene, 3-chloro-2-methyl-l- propene, 2-bromopropene and 2-acetoxypropene are unreactive; 2,3-dimethyl-2-butene is reactive but little β-nitro acetate is formed. Evidence is presented which indicates that protonated acetyl nitrate (AcOHNO2)+ is the principal nitrating agent for alkenes and for anisole in nitric acid-acetic anhydride nitrating mixture. A small amount of sulfuric acid has a strong accelerating effect on the nitrations and is sometimes useful for realizing reactions with the less reactive alkenes. The β-nitro acetates could be converted to β-nitro alcohols by the action of methanol and sulfuric acid, and to mixtures of a- and β-nitroalkenes by the action of triethylamine in dioxane or ether. 3-Nitro-1-butene was isomerized to 2-nitro-2-butene by triethylamine in chloroform. The β-nitro acetates obtained from cis- and trans-2-butene were stereoisomeric, and were shown to result primarily from stereospecific cis addition to the double bond. It is proposed that the formation of both the β-nitro acetates and β-nitroalkenes occur by essentially concerted reactions involving a protonated form of molecular acetyl nitrate. © 1960, American Chemical Society. All rights reserved.