Question

Lucas reagent is (a) anhy. \(\mathrm{AlCl}_{3}\) and conc.HCl (b) anhy.ZnCl \(_{2}\) and conc.HCl (c) Zinc amalgam and conc.HCl (d) \(\mathrm{H}_{2} \mathrm{~N}-\mathrm{NH}_{2}\) and \(\mathrm{KOH}\)

Short answer

(a) anhy. AlCl₃ and conc.HCl (b) anhy.ZnCl₂ and conc.HCl (c) Zinc amalgam and conc.HCl (d) H₂N-NH₂ and KOH Answer: (b) anhy.ZnCl₂ and conc.HCl

Step-by-step solution

Recall the definition of Lucas reagent

Lucas reagent is a solution of anhydrous zinc chloride (ZnCl₂) in concentrated hydrochloric acid (HCl). It is used in organic chemistry as a reagent to test and differentiate between primary, secondary, and tertiary alcohols by observing the formation of alkyl halides.

Match the correct option for Lucas reagent

Now that we know the definition of Lucas reagent, let's match it with the given options: (a) anhy. \(\mathrm{AlCl}_{3}\) and conc.HCl: This option represents anhydrous aluminum chloride combined with hydrochloric acid, which is not the correct composition of Lucas reagent. (b) anhy.ZnCl \(_{2}\) and conc.HCl: This option represents anhydrous zinc chloride combined with hydrochloric acid, which matches the correct composition of Lucas reagent. (c) Zinc amalgam and conc.HCl: This option represents zinc amalgam combined with hydrochloric acid, which is not the correct composition of Lucas reagent. (d) \(\mathrm{H}_{2} \mathrm{~N}-\mathrm{NH}_{2}\) and \(\mathrm{KOH}\): This option represents hydrazine combined with potassium hydroxide, which is not the correct composition of Lucas reagent.

Choose the correct answer

From our analysis, the correct answer is: (b) anhy.ZnCl \(_{2}\) and conc.HCl, as this matches the definition and composition of Lucas reagent.

Key concepts

Organic Chemistry

Organic chemistry is a vast and fascinating branch of chemistry that focuses on the study of carbon-containing compounds. It encompasses a wide variety of substances, from simple molecules like methane to complex structures such as proteins and DNA. Carbon's unique ability to form four covalent bonds allows it to build extensive networks and diverse functional groups.

In organic chemistry, understanding the behavior of different functional groups is crucial. These groups, which include alcohols, ketones, and acids, dictate the chemical reactions of organic molecules. For example, alcohols are characterized by the presence of an -OH group, which makes them reactive and involved in various important reactions, like oxidation or the formation of esters.

Reagents like the Lucas reagent play a vital role in organic chemistry, allowing scientists to perform specific tests to identify and distinguish between similar functional groups. Such tools are fundamental in both research and industrial applications, aiding in the synthesis and characterization of new compounds.

Alcohol Test

Alcohol tests are essential in organic chemistry for identifying the structure and properties of alcohol compounds. Among these tests, the Lucas test is widely used to differentiate between primary, secondary, and tertiary alcohols.

The Lucas test employs the Lucas reagent, a mixture of anhydrous zinc chloride (ZnCl extsubscript{2}) and concentrated hydrochloric acid (HCl). This reagent specifically reacts with different alcohol types at varied rates. When alcohol is added to this reagent:

• Tertiary alcohols react immediately, producing a cloudy solution due to the formation of an insoluble alkyl halide.
• Secondary alcohols react more slowly, usually taking a few minutes to become cloudy.
• Primary alcohols might not react at all or may take a very long time to show any change.

This varying rate of reaction allows chemists to determine the type of alcohol present in a sample. By observing the timing and rate of cloudiness, one can distinguish among the three kinds of alcohols effectively.

Alkyl Halides Formation

Alkyl halides are a significant class of compounds in organic chemistry, formed when alcohols react with halogen acids in the presence of suitable catalysts. The Lucas reagent is one such catalyst used extensively for this purpose.

During the reaction with the Lucas reagent, alcohols undergo a substitution process, where the hydroxyl (-OH) group in the alcohol is replaced by a halogen atom, typically chlorine, leading to the formation of an alkyl halide. This reaction can be represented as follows:\[R-OH + HCl \rightarrow R-Cl + H_2O\]Here, \(R-OH\) represents the alcohol, and \(R-Cl\) signifies the alkyl halide formed.

Alkyl halides are critical intermediates in many chemical reactions, serving as building blocks in the synthesis of a wide range of compounds, including pharmaceuticals and agrochemicals. Additionally, they are used in the manufacturing of plastics, solvents, and more.