Lucas' reagent

Lucas' reagent is a solution of zinc chloride in concentrated hydrochloric acid, used to classify alcohols of low molecular weight. The reaction is a substitution in which the chlorine replaces the hydroxy group. Even though this reaction is normally very unfavorable, the zinc ion complexes with the hydroxy group (by accepting a lone electron pair from O of -OH), making it a better leaving group. The remaining carbonium ion then combines with the chloride ion to form an alkyl halide.

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Lucas Test

Lucas test in alcohols is a test to differentiate between primary, secondary and tertiary alcohols. It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides.

When Lucas' reagent (ZnCl₂ in concentrated HCl solution) is added to the alcohol, H⁺ from HCl will protonate the -OH group of alcohol, so that the leaving group H₂O, being a much weaker nucleophile than OH^-, can be substituted by nucleophile Cl^-. Lucas' reagent offers a polar medium in which S_N1 mechanism is favored. In unimolecular nucleophilic substitution, the reaction rate is faster when the carbocation intermediate is more stabilized by greater number of electron donating alkyl group (R-) bonded to the positively charged carbon atom. Tertiary alcohols react immediately with Lucas reagent to produce turbidity while secondary alcohols do so in five minutes. Primary alcohols do not react appreciably with Lucas reagent at room temperature.

The reagent dissolves the alcohol, removing the OH group, forming a carbocation. The speed of this reaction is proportional to the energy required to form the carbocation, so tertiary, benzylic, and allylic carbocations react quickly, while smaller, less substituted, alcohols react more slowly. The cloudiness observed is caused by the carbocation immediately reacting with the chloride ion creating an insoluble chloroalkane.

Hence, the time taken for turbidity to appear is a measure of the reactivity of the class of alcohol with Lucas reagent, and this is used to differentiate between the three classes of alcohols:

• no visible reaction: primary alcohol
• solution turns cloudy in 3-5 minutes: secondary alcohol
• solution turns cloudy immediately, and/or phases separate: tertiary, allyl, or benzyl alcohol

The temperature at which the test is conducted is usually room temperature.