Question

Match the columns: Column-I Column-II (Reaction) (Colour change) (A) Ceric Ammonium nitrate \(+\mathrm{ROH}\) (P) Yellow to Red (B) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} / \mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{ROH}\) (Q) Colourless to pink (C) (R) Pink to colourless (D) \(\mathrm{KMnO}_{4} / \mathrm{H}^{+}+\mathrm{R}-\mathrm{CH}_{2}-\mathrm{OH}\) (S) Orange to green (T) Blue to red

Short answer

The matched pairs for the given reactions and their color changes are: (A) ↔ (P) (B) ↔ (S) (D) ↔ (R)

Step-by-step solution

List given reactions and color changes

First, let us list the given chemical reactions (Column-I) and color changes (Column-II) in a clear form. Column-I: (A) Ceric Ammonium Nitrate + ROH (B) \( K_2Cr_2O_7 / H_2SO_4 + ROH \) (C) (D) \( KMnO_4 / H^+ + R-CH_2-OH \) Column-II: (P) Yellow to Red (Q) Colorless to Pink (R) Pink to Colorless (S) Orange to Green (T) Blue to Red

Analyze each reaction and find their corresponding color change

We will now go through each reaction in Column-I and determine the color change observed in the reaction. (A) Ceric Ammonium Nitrate + ROH: Ceric Ammonium Nitrate (CAN) is an orange-yellow colored compound, and when it reacts with alcohol (ROH), it gets reduced to cerous ion, which is colorless. Hence, the color change observed is from Yellow to Red. So, (A) matches with (P). (B) \( K_2Cr_2O_7 / H_2SO_4 + ROH \): Potassium dichromate in acidic solution (Orange color) gets reduced to chromium(III) ion (Green color) upon reaction with alcohol (ROH). Hence, the color change observed is from Orange to Green. So, (B) matches with (S). (D) \( KMnO_4 / H^+ + R-CH_2-OH \): Potassium permanganate in acidic solution (Purple color) gets reduced to colorless manganese(II) ion upon reaction with primary alcohol (R-CH2-OH). Hence, the color change observed is from Pink to Colorless. So, (D) matches with (R).

Write the matched pairs

Now that we have matched the reactions in Column-I with their corresponding color changes in Column-II, let's write down the matched pairs: (A) ↔ (P) (B) ↔ (S) (D) ↔ (R) No matching pairs were found for options (Q) and (T).

Key concepts

Ceric Ammonium Nitrate Reaction

The Ceric Ammonium Nitrate (CAN) reaction is a classic test in organic chemistry known for its distinctive color change. When CAN, which is usually an orange-yellow compound due to the presence of ceric (Ce⁴⁺) ions, is added to an alcohol (ROH), a reduction reaction occurs. Here, the ceric ions are reduced to cerous (Ce³⁺) ions. This change manifests itself as a visually striking transition from a yellow to a red color.

Students may often confuse this color change thinking red indicates the initial color due to the common association of red with danger or errors. However, it is crucial to remember that in this particular reaction, the initial color is yellow and the final color after the reaction is red. This is a result of the reduced cerous ion having a different absorbance spectrum, hence altering the color we observe. The CAN test is a qualitative one often used for detecting the presence of alcohols in a given sample.

Potassium Dichromate Reaction

Potassium dichromate, with its characteristic orange color, is another reagent that is commonly used in organic chemistry reactions involving oxidations. In the presence of an acid like sulfuric acid, Potassium dichromate (K₂Cr₂O₇) can act as a strong oxidizing agent. When it reacts with alcohols (ROH), which are often oxidizable substrates, the dichromate ion reduces to chromium(III) ion (Cr³⁺), resulting in a color change from orange to green.

The presence of Chromic Acid (H₂CrO₄) or Chromium Trioxide (CrO₃) is what gives potassium dichromate its vibrant orange color. Upon reduction, the formation of chromium(III) gives the solution a green hue. This change is particularly useful for not only identifying the presence of alcohols but also can provide insights into the type of alcohol present, be it primary, secondary, or tertiary, based on how readily the oxidation occurs. Understanding this reaction is fundamental for students, as it is widely applicable in both analytical and synthetic chemistry.

Potassium Permanganate Reduction

Potassium permanganate (KMnO₄) is another well-known oxidizing agent in organic chemistry. It is highly recognizable due to its deep purple color in solution, which can serve as a self-indicator for redox reactions. In an acidic environment, potassium permanganate can oxidize primary alcohols (R-CH₂-OH) to carboxylic acids. During this process, the permanganate ion (MnO₄^-) is reduced to manganese(II) ion (Mn²⁺), causing the solution to undergo a dramatic color shift from pink to colorless.

It’s key for students to note that the purple color of the permanganate solution turns pink when diluted, and it is from this pink color that the permanganate solution transitions to colorless upon completion of the reaction. This reaction is not only indicative of the presence of primary alcohols but also the double bonds, as potassium permanganate can also oxidize alkenes. The visual cue of the reaction reaching completion is incredibly helpful in a laboratory setting, eradicating the need for additional indicators.