Identification of Primary, Secondary, and Tertiary Alcohols: An Experiment in Spectrophotometry, Organic Chemistry, and Analytical Chemistry

Abstract

A simple method is presented that enables students to distinguish in a few minutes between primary, secondary and tertiary alkyl alcohols. This method is based on peculiarities of absorption spectra in the near-UV region of alkyl nitrites, the products of alcohol nitrosation. This procedure consists of adding 1–2 drops of alcohol to the acidified solution of NaNO2, extracting the nitrate obtained with hydrocarbon solvent and taking the spectrum of a highly diluted nitrite solution in the same solvent. All tertiary nitrites have a maximum at ca. 400 nm whereas all primary and secondary nitrites do not have such a maximum. The discrimination between two latter nitrites is based on the ratio of the heights of two adjacent maxima at ca. 345 and 360 nm, this ratio being equal to 1.3–1.4 for 11 primary nitrites examined (with the exception of methylnitrite) and to 3.0–3.7 for 5 secondary nitrites. The semiquantitative method is also proposed for the analysis of primary and secondary alcohols in their mixture either with the calibration curve or with linear expression (the latter works well when the primary alcohol content in the mixture is more than 20%). The assignments are proposed for students, including nitrosation procedure, taking spectra, plotting graphs, deducing linear dependence and investigating of further possibilities of alcohol mixture analysis.