Question

The compound which reacts fastest with Lucas reagent at room temperature, is
A. $Butan - 1 - ol$
B. $Butan - 2 - ol$
C. $2methylpropan - 1 - ol$
D. $2methylpropan - 2 - ol$

Answer 1

The Lucas reagent is the catalyst that is used for alcohols. This helps to convert alcohols into alkyl halides. The primary, secondary and tertiary carbon atoms react differently with this reagent, that is, they can react to give a product in either five minutes or sometimes only after heating.

Complete step by step answer:
The Lucas reagent is a common reagent in organic chemistry that consists of anhydrous zinc chloride $(ZnC{l_2})$ and concentrated hydrochloric acid $(HCl)$ . This reagent is used to convert hydrocarbons containing hydroxy groups into alkyl halides. The zinc chloride reacts with the oxygen of the hydroxy functional group in the alcohols and converts the hydroxy group into a good leaving group. The hydroxy group on leaving forms a nucleophile. The alkyl part will form a carbocation. The speed of the reaction depends on the stability of the carbocation. The order of stability of carbocation is :
${1^o}C < {2^o}C < {3^o}C$
Therefore, to answer this question we must find whether the carbon is primary, secondary or tertiary.
In option a, that is, $Butan - 1 - ol$ we can see that it is a primary alcohol. Therefore, on reacting with Lucas reagent we get a precipitate after heating only. This formation of a precipitate resembles the end of the reaction.
In option b, the compound is a secondary alcohol that is the hydroxy group attached to a secondary carbon atom. Therefore, the lucas reagent reaction will end after $5\min s$ .
The third option contains the compound $2methylpropan - 1 - ol$ .
This shows that the carbon is secondary and therefore, the alcohol is secondary. Thus, the reaction will end in turbidity after some time.
The fourth option finally mentions a tertiary carbon as it is bonded to two other alkyl groups and that will form a tertiary alcohol. This will lead to the alcohol reacting very quickly with the regent.

So, the correct answer is Option D.

Note: The Lucas reagent helps to replace the hydroxy group of alcohol with the halide group. The order of rate of the reaction depends on the stability of carbocation. Therefore, the tertiary carbon reacts the fastest since it is the most stable.