Thinkstock/Stockbyte/Getty Images
An alcohol contains a hydroxyl (-OH) functional group, whereas an alkene is defined by a carbon–carbon double bond. Each functional group participates in distinct chemical reactions, allowing chemists to identify unknown substances in the lab through selective tests.
Baeyer's test employs dilute potassium permanganate (KMnO₄) as the oxidizing reagent. Alkenes are oxidized to vicinal diols (glycols), which cause the purple KMnO₄ solution to fade to colorless. The disappearance of the purple color suggests the presence of an alkene. However, some alcohols, particularly secondary and tertiary ones, can also be oxidized by KMnO₄, so this test alone is not definitive.
Liquid bromine (Br₂), a brownish‑purple reagent, reacts rapidly with alkenes by adding across the double bond to form dibromides. When bromine is added to a solution of an unknown compound, a swift loss of color indicates the presence of a double bond. This test is far more selective for alkenes than Baeyer's test and is often used as a confirmatory method.
The Lucas reagent—concentrated HCl and ZnCl₂—provides a simple way to distinguish alcohols based on their steric environment. Tertiary alcohols react almost instantaneously, forming a cloudy precipitate, while secondary alcohols take several minutes to produce a visible suspension. Primary alcohols and alkenes do not react under these conditions, making the Lucas test a useful diagnostic tool for locating the alcohol functional group within a molecule.
Another classic test involves chromic acid, prepared from chromic anhydride in sulfuric acid. Primary and secondary alcohols are oxidized to aldehydes or ketones, turning the solution green, whereas tertiary alcohols remain unchanged. In addition, alcohols generally exhibit greater water solubility than alkenes, offering a simple physical property for initial screening.
Modern analytical techniques, such as IR spectroscopy, provide rapid confirmation. Alkenes display characteristic absorption bands in the ranges 1680–1640 cm⁻¹ (C=C stretch), 3100–3000 cm⁻¹ (C–H out‑of‑plane bending), and 1000–650 cm⁻¹ (C–C stretch). Alcohols, in contrast, show a broad, strong O–H stretch between 3550 and 3200 cm⁻¹, along with C–O stretching bands around 1100–1000 cm⁻¹.
