Lucas' reagent

Lucas sample ( Lucas ) test is a test reaction in the organic chemistry for the determination of primary, secondary and tertiary alcohols. Alcohols react at different rates or not at all depending on the position of the hydroxyl group in the molecule of the alcohol. Since must completely dissolve in the added Lucas reagent of alcohol, the test is limited to alcohols with less than six carbon atoms.

The reagent

When Lucas reagent - named after Howard J. Lucas - a solution of zinc chloride in concentrated hydrochloric acid is referred to; The acid -reactive component and zinc chloride is used as catalyst. This mixture was used prior to the publication of his tests by Norris and Taylor for other purposes, such as Lucas himself wrote.

Implementation

To carry out the Lucas test a few drops of alcohol to be tested are added to the Lukas reagent and shaken. The test is positive if it forms a milky liquid. This is done, depending on the position of the hydroxy group at different rates:

• Primary alcohols: reaction only by heating the sample

• Secondary alcohols: reaction within a short time ( about five minutes)

• Tertiary alcohols: instantaneous reaction

The reaction time is longer, the smaller the ratio of reagent to the alcohol and the lower the temperature is.

Explanation

Low molecular weight alcohols dissolve very well in water and also in the Lucas reagent. Respond depending on the position of the hydroxy group, the alcohols with the reagent by the OH group is substituted by a chlorine atom. The inert primary alcohols do not react noticeably, and the solution remains clear. Secondary alcohols react in a few minutes, and the resulting chlorinated hydrocarbon tarnishes the first solution and then precipitates as insoluble phase. The much more reactive tertiary alcohols show instantly turbidity and phase separation:

Exceptions

An exception is allyl alcohol, wherein the reaction - the tertiary alcohols same - occurs immediately. In addition, the resulting allyl chloride in the Lucas reagent is soluble, so that the necessary turbidity absent.