Reagent Lucas

To 1 ml. of the alcohol in a small test-tube, add quickly 6 ml. of Lucas reagent at 26-27°, close the tube with a cork, shake, and allow to stand. Observe the mixture during 5 minutes. The following results may be obtained —... 

The reason for this is that reaction (i) is usually much slower than (ii) and (iii) so that the main reaction appears to be (Iv) (compare the preparation of tertiary butyl chloride from tertiary butyl alcohol and concentrated hydrochloric acid, Section 111,33). If the reaction is carried out in the presence of P3rridine, the latter combines with the hydrogen chloride as it is formed, thus preventing reactions (ii) and (iii), and a good yield of the ester is generally obtained. The differentiation between primary, secondary and tertiary alcohols with the aid of the Lucas reagent is described in Section III,27,(vii). 

Lucas reagent is prepared by dissolving 68 g. (0-5 mole) of anhydrous zinc chloride (fused sticks, powder, etc.) in 62-6 g. (0-6 mole) of concentrated hydrochloric acid with cooling to avoid loss of hydrogen chloride. 

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. 

Problem 13.53 How does the Lewis theory of acids and bases explain the functions of (o) ZnCl, in the Lucas reagent (b) ether as a solvent in the Grignard reagent M... 

Secondary and tertiary alcohols react via the SnI mechanism with the Lucas reagent. The reaction occurs via a carbocation intermediate. Thus, it is possible to form both SnI and El products. The temperature must be kept low to avoid the formation of El product. 

The reagent composed of HC1 and ZnCl2 is called the Lucas reagent. Secondary and tertiary alcohols react with the Lucas reagent by the SnI mechanism. 

When a primary alcohol reacts with the Lucas reagent, ionization is not possible— the primary carbocation is too unstable. Primary substrates react by an Sx2 mechanism, which is slower than the SN1 reaction of secondary and tertiary substrates. For example, when butan-l-ol reacts with the Lucas reagent, the chloride ion attacks the complex from the back, displacing the leaving group. 

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. 

When cw-2-methylcyclohexanol reacts with the Lucas reagent, the major product is 1-chloro-1-methylcyclohexane. Propose a mechanism to explain the formation of this product. 

A test used to determine whether an alcohol is primary, secondary, or tertiary. The test measures the rate of reaction with the Lucas reagent, ZnCl2 in concentrated HC1. Tertiary alcohols react fast (seconds), secondary alcohols react more slowly (minutes), and primary alcohols react very slowly (hours), (p. 481)... 

Predict the major products (including stereochemistry) when cix-3-methylcyclohexanol reacts with the following reagents, (a) PBr3 (b) SOCl2 (c) Lucas reagent... 

Bromine Na Metal Chromic Acid Lucas Reagent... 

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 ... 

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. 

A tertiary alcohol reacts immediately with the Lucas reagent, and a secondary alcohol reacts within five minutes a primary alcohol does not react appreciably... 

Refer to the isomeric pentyl alcohols of Problem 1(a), p. 515. (a) Indicate which (if any) will give a positive iodoform test, (b) Describe how each will respond to the Lucas reagent, (c) Describe how each will respond to chromic anhydride, (d) Outline all steps in a possible synthesis of each, starting from alcohols of four carbons or less, and using any necessary inorganic reagents. 

Concentrated HCI solution reacts with tertiary alcohols directly but only reacts with primary and secondary alcohols in the presence of a ZnCl2 catalyst. A solution of ZnCl2 salt in HCI is known as Lucas reagent . 

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