Animated Solution for Chemistry - Organic Chemistry: Match the following : $\begin{array}{ll}\text{Test / Method}& \text{Reagent}\\ \text{I. Lucas test}& \text{(A) }{C}_6{H}_{5}S{O}_{2}Cl\text{ / KOH}\\ \text{II. Dumas method}& \text{(B) }HN{O}_3\text{ / }AgN{O}_3\\ \text{III. Kjeldahl’s method}& \text{(C) }CuO\text{ / }C{O}_2\\ \text{IV. Hinsberg test}& \text{(D) Conc. HCl and }ZnC{l}_2\\ & \text{(E) }{H}_{2}S{O}_4\end{array}$

Organic chemistry is not just about drawing structures; it's about identifying and quantifying what you have in a flask. This problem is a beautiful tour through four of the most fundamental techniques in an organic chemist's toolkit. Let's break down each method, understand the underlying chemistry, and see exactly why these specific reagents are chosen.

Differentiating Alcohols

Imagine you have three unlabelled test tubes containing a primary, a secondary, and a tertiary alcohol. How do you tell them apart? Enter the Lucas test. The Lucas reagent is a powerful mixture of concentrated hydrochloric acid ($\text{HCl}$) and anhydrous zinc chloride (${\text{ZnCl}}_2$).

When an alcohol reacts with this reagent, it undergoes a nucleophilic substitution reaction ($S_{N}1$) to form an alkyl chloride. Because alkyl chlorides are insoluble in the aqueous reagent, they create a cloudy suspension or turbidity. The magic lies in the kinetics. Tertiary alcohols form highly stable carbocations and react almost instantaneously, turning the solution cloudy immediately. Secondary alcohols take about 5 to 10 minutes. Primary alcohols, lacking carbocation stability, do not react at room temperature. Thus, the reagent ${\text{Conc. HCl and ZnCl}}_2$ is the perfect match for the Lucas test.

Estimating Nitrogen

Once you know what functional groups are present, you often need to know exactly how much of an element is in your compound. The Dumas method is a classic technique for the quantitative estimation of nitrogen.

In this method, a known mass of the organic compound is strongly heated with cupric oxide ($\text{CuO}$) in an atmosphere of carbon dioxide (${\text{CO}}_2$). The carbon and hydrogen in the compound are oxidized to carbon dioxide and water, respectively, while the nitrogen is liberated as free nitrogen gas (${\text{N}}_2$). Any traces of nitrogen oxides formed are reduced back to elemental nitrogen by passing the gases over a heated copper spiral. The volume of the collected nitrogen gas is then measured to calculate its percentage. Therefore, the combination of $\text{CuO}$ and ${\text{CO}}_2$ is the hallmark of the Dumas method.

The Acid Digestion

While the Dumas method is elegant, Kjeldahl's method is the workhorse of nitrogen estimation, especially in agriculture and food science.

The process begins with digestion. The organic compound is heated with concentrated sulfuric acid (${\text{H}}_2{\text{SO}}_4$). This intense acidic environment decomposes the organic matter, oxidizing the carbon and hydrogen, while the nitrogen is quantitatively converted into ammonium sulfate, $({\text{NH}}_4{)}_2{\text{SO}}_4$.

Once the digestion is complete, the solution is treated with an excess of sodium hydroxide to liberate ammonia gas, which is then absorbed in a known volume of standard acid. By titrating the unreacted acid, we can determine exactly how much nitrogen was in the original sample. The critical first step, however, relies entirely on concentrated ${\text{H}}_2{\text{SO}}_4$.

Sorting Amines

Finally, we come to the Hinsberg test, a brilliant chemical method for distinguishing between primary, secondary, and tertiary amines. The Hinsberg reagent is benzene sulfonyl chloride ($C_6{H}_{5}S{O}_{2}Cl$).

When a primary amine reacts with this reagent, it forms an N-alkylbenzenesulfonamide. Because this molecule still has an acidic hydrogen atom attached to the nitrogen, it dissolves readily in an aqueous base like potassium hydroxide ($\text{KOH}$) to form a clear solution.

A secondary amine reacts to form an N,N-dialkylbenzenesulfonamide. Lacking an acidic hydrogen, this product is completely insoluble in aqueous $\text{KOH}$ and forms a distinct solid precipitate. Tertiary amines, having no replaceable hydrogen atoms, simply do not react with the Hinsberg reagent under these conditions. Thus, the combination of $C_6{H}_{5}S{O}_{2}Cl$ and $\text{KOH}$ is the definitive reagent for this test.

By carefully analyzing the chemical principles behind each technique, the matching becomes straightforward: - Lucas test (I) pairs with Conc. HCl and ${\text{ZnCl}}_2$ (D). - Dumas method (II) pairs with $\text{CuO}$ / ${\text{CO}}_2$ (C). - Kjeldahl's method (III) pairs with ${\text{H}}_2{\text{SO}}_4$ (E). - Hinsberg test (IV) pairs with $C_6{H}_{5}S{O}_{2}Cl$ / $\text{KOH}$ (A).

This perfectly aligns with option (c). Mastering these reagents not only helps you solve matching questions but also deepens your intuition for practical organic chemistry.