Question

Choose the incorrect option for following sequence: $$ \stackrel{\mathrm{CH}_{3}}{\mathrm{O}}^{\mathrm{CrO}_{2} \mathrm{Cl}_{2}}{\longrightarrow}(\mathrm{P}) \frac{\mathrm{Zn}-\mathrm{Hg}}{\mathrm{HCl}}{\longrightarrow}(\mathrm{Q}) $$ (A) Formation of (P) is Etard reaction (B) Formation of (Q) from (P) is Clemmensen reduction (C) (Q) has common name Toluene (D) (P) has common name Benzoic acid

Short answer

The incorrect option for the given sequence is: (D) (P) has the common name Benzoic acid.

Step-by-step solution

Identify the components and reactions involved in given sequence

We are given a reaction sequence with two consecutive reactions: 1. The first reaction has a molecule with a CH3 group (Toluene) and Chromium oxide chloride (\( \mathrm{CrO_2Cl_2} \)) as reactants. The product of this reaction is unknown and labeled as (P). 2. The second reaction has (P) as the reactant and uses a mixture of Zinc amalgam (Zn-Hg) and Hydrochloric acid (HCl) as a reagent. The product of this reaction is unknown and labeled as (Q). We are given four answer options and need to identify the incorrect one.

Identify the reaction and product of the first step

In the first reaction, the given molecule is toluene, and the reagent is \( \mathrm{CrO_2Cl_2} \), which suggests that the Etard reaction is taking place. The Etard reaction is the oxidation of a methyl group on an aromatic ring to an aldehyde group. In this case: Toluene \(\xrightarrow[\mathrm{CrO_{2}Cl_{2}}]{\mathrm{Etard \:reaction}}\) Benzaldehyde (P) Now we must analyze the answer options given: (A) Formation of (P) is Etard reaction - This statement is correct, as described above.

Identify the reaction and product of the second step

In the second reaction, the reactant is benzaldehyde (P), and the reagents are Zinc amalgam (Zn-Hg) and hydrochloric acid (HCl), indicating Clemmensen reduction is taking place. The Clemmensen reduction reduces an aldehyde or ketone to a hydrocarbon by removing the carbonyl group. In this case: Benzaldehyde (P) \(\xrightarrow[\frac{\mathrm{Zn-Hg}}{\mathrm{HCl}}]{\mathrm{Clemmensen \:reduction}}\) Toluene (Q) Now we can analyze the remaining answer options: (B) Formation of (Q) from (P) is Clemmensen reduction - This statement is correct, as described above. (C) (Q) has the common name Toluene - This statement is correct, as shown in the second reaction. (D) (P) has the common name Benzoic acid - This statement is incorrect, as the common name of (P) is Benzaldehyde, not Benzoic acid. Thus, the incorrect option for the given sequence is: (D) (P) has the common name Benzoic acid.

Key concepts

Etard Reaction

The Etard reaction is a chemical transformation essential to organic chemistry, particularly when dealing with the oxidation of aromatic methyl groups.

Simply put, the Etard reaction involves the conversion of a methyl group, which is attached to an aromatic ring, into an aldehyde group. This reaction uses chromium oxide chloride (\r\( \mathrm{CrO_2Cl_2} \)) as a reagent, and it's typically run in the presence of a suitable solvent, like carbon disulfide or chloroform. It's a highly useful reaction because it provides a direct method to obtain aldehydes from methyl groups on aromatic rings.

Understanding the fundamental steps of the Etard reaction can help us identify that statement (A) in our exercise is correct, affirming the formation of Benzaldehyde (P) from Toluene through this well-known oxidation process.

Clemmensen Reduction

Like the Etard reaction, the Clemmensen reduction is another pivotal transformation in the realm of organic chemistry. In essence, it's the process of converting aldehydes or ketones into hydrocarbons through the removal of their oxygen atom.

This reduction is accomplished using a blend of zinc amalgam and hydrochloric acid. The Clemmensen reduction is particularly notable for its ability to provide a straightforward route to synthesize hydrocarbons from more oxidized carbonyl compounds, even when the substrates are sensitive to strong acids or other harsh conditions.

Knowing the details of the Clemmensen reduction helps us see why option (B) is correct in our exercise. The resultant compound, Toluene (Q), is indeed produced from Benzaldehyde (P) through this reduction technique.

Aromatic Compound Oxidation

Oxidation reactions of aromatic compounds are central to forming various functional groups pivotal in organic synthesis. When the term 'aromatic compound oxidation' is mentioned, it broadly refers to reactions that increase the number of bonds between carbon and oxygen (or other electronegative elements) in an aromatic ring.

The Etard reaction is one choicest representative of this kind of transformation, which is particularly adept at converting methyl groups to aldehyde groups. However, oxidation can also lead to the formation of carboxylic acids, alcohols, and even ketones depending on the reactants and conditions.

Recognizing the breadth and importance of aromatic compound oxidation underscores the misinterpretation presented in option (D), where Benzaldehyde (P) is incorrectly called Benzoic acid, which would instead be a product of further oxidation.

Benzaldehyde Synthesis

Benzaldehyde is an aromatic aldehyde with substantial utility in the flavor, fragrance, and pharmaceutical industries. Its synthesis can be accomplished via several methods, one of which is prominently featured in our exercise - the Etard reaction.

The synthesis of benzaldehyde typically involves the oxidation of a methyl group on a benzene ring, which, in our example, starts with Toluene as the precursor. The Etard reaction offers a fairly straightforward approach to achieve this transformation, showcasing the practicality of selective oxidations within complex molecules.

Through the sequence presented, we observe the correct synthetic pathway leading from Toluene to Benzaldehyde (P), facilitating clear understanding for our exercise which is critical for effective learning in organic chemistry.