Lucas test

Carry out the Lucas test with iso-propyl alcohol, n-butyl alcohol, scc.-butyl alcohol, cycZohexanol and iert.-butyl alcohol. Obtain an unknown alcohol from the instructor for test. 

The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another (Lucas test). Acohols are soluble in Lucas reagent (cone. HCl and ZnCla) while their halides are immiscible and produce turbidity in solution. In case of tertiary alcohols, turbidity is produced immediately as they form the halides easily. Primary alcohols do not produce turbidity at room temperature. 

A common analytic test used to classify alcohols is the Lucas test. This test is for alcohols with six or fewer carbon atoms (and therefore soluble in reagent mixture). They react with HCl/ZnCl2 to form an alkyl chloride, which is insoluble in the reaction mixture (causing the solution to turn cloudy). The time required for the reaction to occur indicates the type of alcohol. Table 3-1 summarizes the time required for each type of alcohol. 

Tbe Lucas test uses cone. HCI and ZnCl (to increase the acidity of the acid). 

Locating an organic compound, 1128 Lubrication of ground glass joints, 225 Lucas test, 261 reagent, 261 ... 

Alcohols Ceric ammonium nitrate Dichromate test Iodoform test Lucas test Positive for all alcohols Positive for 1° and 2° alcohols negative for 3° alcohols Positive for all alcohols of the general formula CH3CH(OH)R Immediate reaction for 3°, allylic or benzylic alcohols slower reaction (2-5 min) for 2° no reaction for 1° alcohols... 

Lucas test. Place 5 drops of each sample into separate clean, dry test tubes... 

The Lucas test is used to check for the presence of an alcohol functional group in an unknown compound. The test reaction is shown in the following equation ... 

The Lucas Test The Lucas reagent reacts with primary, secondary, and tertiary alcohols at predictable rates, and these rates can distinguish among the three types of alcohols. When the reagent is first added to the alcohol, the mixture forms a single homogeneous phase The concentrated HC1 solution is very polar, and the polar alcohol-zinc chloride complex dissolves. Once the alcohol has reacted to form the alkyl halide, the relatively nonpolar halide separates into a second phase. (R—OH dissolves, but R—Cl does not.)... 

The Lucas test involves adding the Lucas reagent to an unknown alcohol and watching for the second phase to separate (see Table 11-2). Tertiary alcohols react and show a second phase almost instantly because they form relatively stable tertiary carbocations. Secondary alcohols react in about 1 to 5 minutes because their secondary carbocations are less stable than tertiary ones. Primary alcohols react very slowly. Since the activated primary alcohol cannot form a carbocation, it simply remains in solution until it is attacked by the chloride ion. With a primary alcohol, the reaction may take from 10 minutes to several days. 

In the Lucas test, the Lucas reagent reacts with 1°, 2°, and 3° alcohols. The alcohols are distinguished by their reactivity with the Lucas reagent 33 alcohols react immediately 2C alcohols react within 5 minutes and 1 alcohols react poorly at room temperature. Because the compound reacts with the Lucas reagent in 4 minutes, then the structure of C4H10O, a 2° alcohol, is sec-butyl alcohol ... 

This ease of substitution provides a way in which primary, secondary and tertiary alcohols can be distinguished (Figure 7.1.14). In the Lucas test, an alcohol is shaken with zinc chloride in concentrated hydrochloric acid and allowed to stand. A tertiary alcohol reacts rapidly to give an opaque suspension of the insoluble chloroalkane. A secondary alcohol takes several minutes to become cloudy. A primary alcohol reacts very slowly, if at all. 

Whether an alcohol is primary, secondary, or tertiary is shown by the Lucas test, which is based upon the difference in reactivity of the three classes toward hydrogen halides (Sec. 16.4). Alcohols (of not more than six carbons) are soluble in the Lucas reagent, a mixture of concentrated hydrochloric acid and zinc chloride. (Why are they more soluble in this than in water ) The corresponding alkyl chlorides are insoluble. Formation of a chloride from an alcohol is indicated by the cloudiness that appears when the chloride separates from the solution hence, the time required for cloudiness to appear is a measure of the reactivity of the alcohol. 

JJ b.p. 128-30 negative halogen test Lucas test, primary... 

Although it is a secondary alcohol, -chloro-2-propanol behaves like a primary alcohol in the Lucas test. Can you suggest a reason for this behavior ... 

Lucas test When an alcohol is mixed with concentrated HC1 and ZnCl2 at room temperature,... 

Lucas test—distinguishes 1° from 2° from 3° alcohol by the rate of reaction... 

OH ( Remember that allylic cations are resonance-stabilized and are about as stable as 3° cations. Lucas cloudy in < 1 min. no reaction Thus, they will react as fast as 3" in the Lucas test,... 

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