Question

Total number of correct statements are: (i) Benzaldehyde cannot be obtained by Friedel craft acylation (ii) Nitrobenzene is used as a solvent during Friedel craft reaction (iii) Friedel craft acylation requires higher concentration of catalyst than Friedel craft alkylation. (iv) Benzaldehyde never reduces Fehling solution. (v) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}\) is used for selective reduction of only one \(-\mathrm{NO}_{2}\) group into \(-\mathrm{NH}_{2}\) (vi) In coupling reaction attacking electrophile is \(\mathrm{R}-\mathrm{C} \equiv \mathrm{O}^{\oplus}\) not \(\mathrm{Ph}-\mathrm{N} \equiv \mathrm{N}\) (vii) Polyalkylation is a disadvantage during monoalkylation of aromatic compound by Friedel craft reaction. (viii) Direct nitration of aniline gives good yield of para-nitro aniline.

Short answer

The total number of correct statements is 7 out of 8.

Step-by-step solution

Statement (i)

Benzaldehyde cannot be obtained by Friedel craft acylation. This statement is correct. Friedel craft acylation involves the formation of an acyl group on the aromatic ring, while benzaldehyde has a formyl group (-CHO) attached to it. Benzaldehyde can be synthesized using other methods, but not by Friedel craft acylation.

Statement (ii)

Nitrobenzene is used as a solvent during Friedel craft reaction. This statement is correct. Nitrobenzene is a polar solvent that can dissolve both the aromatic compound and the acylating agents, and it is commonly used in Friedel craft reactions.

Statement (iii)

Friedel craft acylation requires higher concentration of catalyst than Friedel craft alkylation. This statement is correct. Friedel craft acylation generally requires a higher concentration of Lewis acid catalysts (such as AlCl3) compared to Friedel craft alkylation, as acyl halides are less reactive towards electrophilic aromatic substitution reactions.

Statement (iv)

Benzaldehyde never reduces Fehling solution. This statement is correct. Fehling solution is used to differentiate between aliphatic aldehydes and aromatic aldehydes. Aliphatic aldehydes reduce Fehling solution and produce a red precipitate, whereas aromatic aldehydes, like benzaldehyde, do not reduce Fehling solution.

Statement (v)

\(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}\) is used for selective reduction of only one \(-\mathrm{NO}_{2}\) group into \(-\mathrm{NH}_{2}\). This statement is correct. Ammonium sulfide \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S}\) is used for selective reduction of only one nitrogroup (-NO2) into an amine group (-NH2) in the presence of other reducible groups.

Statement (vi)

In coupling reaction attacking electrophile is $\mathrm{R}-\mathrm{C} \equiv \mathrm{O}^{\oplus}\( not \)\mathrm{Ph}-\mathrm{N} \equiv \mathrm{N}$. This statement is correct. In a coupling reaction between an aromatic amine and a diazonium salt, the electrophile is the carbonyl group $\mathrm{R}-\mathrm{C} \equiv \mathrm{O}^{\oplus}\(, while \)\mathrm{Ph}-\mathrm{N} \equiv \mathrm{N}$ acts as a nucleophile.

Statement (vii)

Polyalkylation is a disadvantage during monoalkylation of aromatic compound by Friedel craft reaction. This statement is correct. Friedel craft alkylation reactions can lead to polyalkylation, which is the addition of more than one alkyl group to the aromatic ring. Polyalkylation is considered a disadvantage when trying to achieve monoalkylation because it leads to a complex mixture of products.

Statement (viii)

Direct nitration of aniline gives a good yield of para-nitro aniline. This statement is incorrect. Direct nitration of aniline leads to a mixture of ortho- and para-nitro aniline products, with a higher percentage of ortho-nitro aniline. To obtain a good yield of para-nitro aniline, one must protect the amino group before carrying out the nitration. In conclusion, 7 out of the 8 statements provided are correct.

Key concepts

Benzaldehyde Synthesis

Benzaldehyde, a very useful aromatic aldehyde, is characterized by the presence of a formyl group (-CHO). It is used as a precursor in the synthesis of various aromatic compounds and pharmaceuticals.
Friedel Craft acylation, a popular method for forming acyl groups on aromatic rings, cannot produce benzaldehyde. This is because acylation results in a carbonyl group rather than a formyl group. Benzaldehyde synthesis is typically carried out through methods such as the oxidation of toluene or from benzyl alcohol.

• Oxidation of toluene involves the use of oxidizing agents like potassium permanganate. In this method, toluene is slowly oxidized to produce benzaldehyde.
• Benzyl alcohol can also be oxidized to benzaldehyde using oxidizing agents such as chromium compounds.

These methods underline the specific reaction pathways needed for benzaldehyde versus those typically used in Friedel Craft acylation.

Fehling Solution Test

Fehling's solution is a chemical test used primarily to differentiate between aliphatic aldehydes and aromatic aldehydes. It contains copper(II) ions which react with aldehyde groups.
When an aliphatic aldehyde is present, the blue solution turns into a brick-red precipitate due to the formation of copper(I) oxide. However, this test does not show this response with aromatic aldehydes like benzaldehyde. Aromatic aldehydes lack sufficient reactivity to reduce the copper ions in Fehling's solution.

• Aliphatic aldehydes react because they can readily convert into their corresponding carboxylate ions.
• Aromatic aldehydes result in no change because their structure is more stable and less reactive.

This attribute is useful for distinguishing between different types of aldehydes during chemical analysis.

Aromatic Nitration

Aromatic nitration is a vital reaction in organic chemistry where a nitro group (-NO2) is introduced into an aromatic ring, such as benzene and its derivatives. This process usually employs nitric acid and sulfuric acid as reagents.
The reaction is an electrophilic aromatic substitution, where the existing hydrogen on the aromatic ring is replaced by a nitro group.

• Nitric acid serves as the nitronium ion source (\(NO_2^+\), the active electrophile in the reaction).
• Sulfuric acid acts as a catalyst and helps generate the nitronium ion.

For aniline derivatives, direct nitration typically results in a mix of ortho- and para-nitro products, because the amino group directs the nitration to these positions. However, to obtain primarily para-nitro product, the amino group is often protected beforehand. Understanding such nuances is crucial for effective synthesis in industrial and laboratory settings.