Identifying Alcohols

Alcohols

Alcohols are organic compounds that are amongst the most recognizable and familiar, as they have wide-ranging applications and uses in everyday life. Alcohols are organic molecules containing a hydroxyl functional group connected to an alkyl or aryl group (ROH). If the hydroxyl carbon only has a single R group, it is known as primary alcohol. If it has two R groups, it is a secondary alcohol, and if it has three R groups, it is a tertiary alcohol. Like many other organic compounds, alcohols can also be aromatic by containing a benzene ring. The simplest aromatic alcohol is phenol.

Identifying an alcohol in organic chemistry can be achieved by exploiting the different properties of the types of alcohols. While various instrumentation analysis methods, such as nuclear magnetic resonance (NMR) can be used, other qualitative tests can also be utilized. By coupling these tests together, the identity of an alcohol can be determined in a similar fashion to the way aldehydes and ketones can be identified.

Ferric Chloride Test

One method to differentiate between an aliphatic alcohol and an aromatic alcohol is by using iron(III) chloride. The iron chloride compound gives the solution a red-orange appearance. In the presence of an aromatic alcohol, like phenol, the chloride atoms are replaced by the aromatic alcohol, changing the coordination property of the center iron atom. This changes the color of the solution to a purple color. Aliphatic alcohols will not react with iron(III) chloride and, thus, the solution remains red-orange.

Jones Test

The Jones test utilizes chromium trioxide in the presence of sulfuric acid to act as a powerful oxidizing agent. In the presence of the Jones' reagent, a primary alcohol is first converted into an aldehyde and then into a carboxylic acid, while a secondary alcohol will be oxidized into a ketone. The oxidation state of chromium is the key to this test. Chromium is in the +6 oxidation state in the Jones' reagent. The Cr(VI) complexes in the reagent give it its bright reddish, orange color.

In the process of the reaction, chromium is reduced from Cr(VI) to the +3 oxidation state — Cr(III). First, the alcohol and chromic acid form a chromate ester. Then, a base (H₂O) cleaves the C-H bond of the alcohol, forming the carbonyl group while reducing Cr(VI) to Cr(IV). The carbon of the alcohol undergoes a 2-electron oxidation, and the Cr(IV) undergoes a 2-electron reduction, so this is a reduction-oxidation step.

Cr(IV) participates in further oxidation steps and is eventually reduced to Cr(III). Cr(III) is often present as hexaaquachromium(III) ions — [Cr(H₂O)₆]³⁺ — and Cr(III) complexes, where H₂O molecules are replaced by one or more sulphate ions — [Cr(H₂O)₅(SO₄)]⁺. These complexes give Cr(III) the characteristic green color.

Tertiary alcohols do not react with chromium, and thus no precipitate is made, keeping the solution orange. Therefore, the Jones test can help differentiate primary and secondary alcohols from tertiary alcohols.

Lucas Test

The Lucas test utilizes zinc(II) chloride in the presence of hydrochloric acid as a reagent. In the presence of an alcohol, the Lucas reagent will halogenate the alcohol, making an insoluble product in aqueous solutions. The reaction rate for this is highly dependent on the formation of a carbocation caused by the loss of the hydroxyl as water. A carbocation formed by a primary alcohol is unstable, and thus will not readily occur. This results in no observable reaction at room temperature. A tertiary alcohol will form a stable carbocation, resulting in a rapid reaction upon the addition of the Lucas reagent. This results in a second phase appearing in the reaction mixture because the final halogenated product is insoluble in water. Secondary alcohols will form a less stable carbocation than tertiary alcohols, but the reaction will occur at room temperature in a matter of minutes. Therefore, the Lucas Test can help differentiate between primary, secondary, and tertiary alcohols.

References

1. Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction to Organic Chemistry. Upper Saddle River, NJ: Prentice Hall.