Question

Match the following columns: Column-I Column - II (Conversion) (Reagent) (A) Hexan-1-ol to hexanal (P) ${\mathrm{C}}_5{\mathrm{H}}_5{\mathrm{NH}}^{\oplus }{\mathrm{CrO}}_3{\mathrm{Cl}}^{-}$ (B) But-2-ene to ethanoic acid (Q) ${\mathrm{KMnO}}_4$ in ${\mathrm{H}}^{\oplus }$ (C) Cyclohexanol to cyclohexanone (R) ${\mathrm{O}}_3/{\mathrm{H}}_2{\mathrm{O}}_2$ (D) Allyl alcohol to propenal $(\mathrm{S}){\mathrm{MnO}}_2$ (T) $\mathrm{Cu}/{300}^{\circ }\mathrm{C}$

Short answer

The correct matches for the given conversions and reagents are: A matches with P: (Hexan-1-ol to hexanal with PCC) B matches with Q: (But-2-ene to ethanoic acid with KMnO4 in H⁺) C matches with S: (Cyclohexanol to cyclohexanone with MnO2) D matches with R: (Allyl alcohol to propenal with O3/H2O2)

Step-by-step solution

Matching A (Hexan-1-ol to hexanal)

: Hexan-1-ol needs to be oxidized to hexanal. There are several reagents that can be used to achieve this. Among the given options, column (P) has the reagent that is used for this reaction. The reagent is C5H5NH⁺CrO3Cl⁻, also known as pyridinium chlorochromate (PCC). This reagent selectively oxidizes alcohols to aldehydes without proceeding to full oxidation like carboxylic acids. So, A matches with P.

Matching B (But-2-ene to ethanoic acid)

: But-2-ene needs to be converted to ethanoic acid, which is an oxidation of an alkene to a carboxylic acid. This can be accomplished with the reagent KMnO4 in H⁺, as provided in column (Q). Permanganate ion (MnO4-) is a strong oxidizing agent, and it can convert alkenes to diols and then further oxidize them to carboxylic acids. Thus, B matches with Q.

Matching C (Cyclohexanol to cyclohexanone)

: Cyclohexanol needs to be oxidized to cyclohexanone. The oxidation required needs to be mild since we don't want to oxidize it completely to carboxylic acid. The reagent in column (S) is MnO2, which can do selective oxidation without over-oxidizing. Manganese(IV) oxide (MnO2) is often used to oxidize secondary alcohols to ketones. So, C matches with S.

Matching D (Allyl alcohol to propenal)

: Allyl alcohol needs to be oxidized to propenal. Since this is an oxidation, we can exclude reagent (R) O3/H2O2 (Ozonolysis) and (T) Cu/300°C (Dehydrogenation). Therefore, the remaining option in column (D) is the reagent in column (R), O3/H2O2. Ozone can act as an oxidizing agent, and the subsequent use of hydrogen peroxide completes the oxidation process. Thus, D matches with R.

Key concepts

Oxidation Reactions

In organic chemistry, oxidation reactions involve the transfer of electrons, specifically the increase in the number of bonds to oxygen or the reduction of hydrogen content in a molecule. This is crucial for transforming one molecule into another more functional molecule.
For example, in the conversion of hexan-1-ol to hexanal, the primary alcohol is oxidized to an aldehyde. This reaction is significant because it requires the selective oxidation of the molecule without over-oxidation to a carboxylic acid. It is essential to choose the right oxidizing agent to achieve the desired level of oxidation. Oxidation reactions are widely used for:

• Transforming alcohols to aldehydes or ketones
• Converting alkenes to diols or carboxylic acids, as in the oxidation of but-2-ene to ethanoic acid
• Selective oxidation of alcohols without further oxidation

This way, chemists can prepare molecules that serve as intermediaries in the synthesis of more complex compounds.

Reagents in Organic Chemistry

Reagents are substances or compounds that facilitate a chemical reaction. In organic chemistry, choosing the right reagent can determine the success of a conversion process.
For instance, the reagent Pyridinium chlorochromate (PCC), represented as P(NH₄)Cl, is known for its ability to oxidize primary alcohols to aldehydes. This makes it a valuable reagent for converting hexan-1-ol to hexanal, without further oxidation to carboxylic acids.
Other examples of notable reagents in oxidation include:

• Potassium permanganate ( KMnO₄), a strong oxidizer used in acidic conditions to oxidize alkenes to carboxylic acids, as shown in the reaction converting but-2-ene to ethanoic acid
• Manganese dioxide ( MnO₂), useful for oxidizing secondary alcohols like cyclohexanol to ketones such as cyclohexanone
• Ozone ( O₃) in combination with hydrogen peroxide ( H₂O₂), for oxidizing allyl alcohol to propenal

Each of these reagents is chosen based on its strength, selectivity, and compatibility with the substrate in question.

Alcohols and Aldehydes Conversion

The conversion of alcohols to aldehydes is a fundamental process in organic chemistry, providing intermediates for synthesizing various compounds. Selective oxidation is key in avoiding full oxidation to carboxylic acids.
For example, the conversion of hexan-1-ol (a primary alcohol) to hexanal (an aldehyde) demonstrates the importance of using a selective reagent like PCC. This ensures that the primary alcohol is partially oxidized, maintaining the aldehyde functionality without further oxidation. Another common example is the oxidation of cyclohexanol (a secondary alcohol) to cyclohexanone (a ketone). Utilizing a reagent such as MnO₂ ensures that the alcohol is selectively oxidized to the carbonyl group, stopping at the ketone, which is crucial in many industrial and laboratory syntheses. Such transformations highlight the significance of alcohol oxidation in organic synthesis, providing pathways to more reactive or functionalized molecules that are useful in chemical, pharmaceutical, and materials sciences. Understanding these conversions allows chemists to tailor their reactions effectively.