Lucas' reagent 1767428 208052837 2008-04-25T05:52:27Z 84.71.43.138 {{Unreferenced|date=February 2008}} '''Lucas' reagent''' is a solution of [[zinc chloride]] in concentrated [[hydrochloric acid]], used to classify [[alcohol]]s of low molecular weight. The reaction is a [[Nucleophilic substitution reaction|substitution]] in which the chlorine replaces the [[hydroxyl]] group. Even though this reaction is normally very unfavorable, the zinc ion complexes with the hydroxyl group (by accepting a lone electron pair from O of -OH), making it a better [[leaving group]]. The remaining carbocation then combines with the chloride ion to form a [[chloroalkane]]. ==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 [[halide]]s. [[Image:Lucas test.png|center|400px]] When Lucas' reagent (ZnCl<sub>2</sub> in concentrated [[Hydrochloric acid|HCl]] solution) is added to the [[alcohol]], H<sup>+</sup> from HCl will protonate the -[[Hydroxyl|OH]] group of alcohol, so that the leaving group H<sub>2</sub>O, being a much weaker [[nucleophile]] than OH<sup>-</sup>, can be substituted by nucleophile Cl<sup>-</sup>. Lucas' reagent offers a polar medium in which S<sub>N</sub>1 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 alcohol|tertiary]], [[allyl alcohol|allyl]], or [[benzyl alcohol]] The test is usually conducted at room temperature. [[Category:Chemical tests]] [[de:Lucas-Probe]] [[fr:Test de Lucas]]