Question

Give reasons for the following statements. (i) o- and p-nitrophenols can be separated by steam distillation. (ii) Sodium fusion extract has to be neutralized with dil.HNO \(_{3}\) before testing for the halogen with \(\mathrm{AgNO}_{3^{*}}\) (iii) Benzene does not decolourise bromine water though it contains three double bonds. (iv) Lassaigne's test for nitrogen is not given by diazonium compound. (v) Sodium fusion extract is boiled with \(\mathrm{HNO}_{3}\) for testing for halogen, if nitrogen and sulphur are present.

Short answer

Answer: The sodium fusion extract should be boiled with HNO\(_{3}\) for testing halogens if both nitrogen and sulfur are present because, in the presence of nitrogen and sulfur, the sodium fusion extract can form sodium thiocyanate (NaSCN), which can cause false positive results for halogens. Boiling the extract with HNO\(_{3}\) oxidizes NaSCN into sulfate ions, which prevents the formation of silver thiocyanate (AgSCN) and ensures a true result in the halogen test with AgNO\(_{3}\).

Step-by-step solution

(Reason for statement i)

(o- and p-nitrophenols can be separated by steam distillation because they have different boiling points and vapor pressures due to their different intermolecular forces. o-nitrophenol possesses an intramolecular hydrogen bond, which decreases the strength of its intermolecular hydrogen bonds with other molecules; as a result, it has a lower boiling point and higher vapor pressure than p-nitrophenol. Steam distillation is effective in separating compounds with different boiling points and vapor pressures.)

(Reason for statement ii)

(Sodium fusion extract has to be neutralized with dilute HNO\(_{3}\) before testing for halogen with AgNO\(_{3}\) because the sodium fusion extract is highly basic due to the formation of NaOH, which can cause a false positive result. Dilute HNO\(_{3}\) is used to neutralize the basic solution and avoid an incorrect reaction with AgNO\(_{3}\), which would lead to the precipitation of silver hydroxide instead of the halide salt.)

(Reason for statement iii)

(Benzene does not decolourise bromine water, even though it contains three double bonds, because benzene is highly stable and unreactive due to its delocalized \(\pi\) electron cloud spread across the entire ring. Instead of undergoing addition reactions typical of alkenes, benzene participates in electrophilic substitution reactions. Thus, benzene does not react easily with bromine water, which is a characteristic test for alkenes.)

(Reason for statement iv)

(Lassaigne's test for nitrogen is not given by diazonium compounds because this test detects the presence of nitrogen in organic compounds by the formation of sodium cyanide (NaCN) or sodium thiocyanide (NaSCN). However, diazonium compounds are highly reactive and unstable and tend to decompose into nitrogen gas and other products when heated. As a result, they do not form NaCN or NaSCN in the Lassaigne's test.)

(Reason for statement v)

(Sodium fusion extract is boiled with HNO\(_{3}\) for testing halogens if both nitrogen and sulfur are present because, in the presence of nitrogen and sulfur, the sodium fusion extract can form sodium thiocyanate (NaSCN), which can cause false positive results for halogens. Boiling the extract with HNO\(_{3}\) oxidizes NaSCN into sulfate ions, which prevents the formation of silver thiocyanate (AgSCN) and ensures a true result in the halogen test with AgNO\(_{3}\).)

Key concepts

Steam Distillation

Steam distillation is a technique used to separate components of a mixture based on their differing boiling points and vapor pressures. This method is particularly effective for substances that are temperature-sensitive or have high boiling points. In this process, steam is introduced into the mixture, lowering the boiling point of the more volatile substances. As a result, these components evaporate at lower temperatures than they would otherwise.

In the separation of o- and p-nitrophenols, steam distillation takes advantage of their different intermolecular forces. O-nitrophenol forms an intramolecular hydrogen bond, which decreases its intermolecular hydrogen bonding, leading to a lower boiling point and higher vapor pressure compared to p-nitrophenol. This allows o-nitrophenol to evaporate before p-nitrophenol during steam distillation, making it possible to separate the two compounds effectively.

• Effective for temperature-sensitive compounds
• Utilizes differences in vapor pressure
• Separates based on boiling point differences

Lassaigne's Test

Lassaigne's test is a qualitative test used to detect the presence of elements like nitrogen, sulfur, and halogens in organic compounds. The test involves fusing the organic compound with metallic sodium, which converts the elements into soluble salts.

For nitrogen, sodium reacts with it to form sodium cyanide (\( \text{NaCN} \)) or sodium thiocyanate (\( \text{NaSCN} \)). However, not all nitrogen-containing compounds give positive Lassaigne's tests. For example, diazonium compounds are unstable and decompose into nitrogen gas upon heating. Hence, they do not form \( \text{NaCN} \) or \( \text{NaSCN} \), giving negative results in this test.

• Detects elements like nitrogen and sulfur
• Forms soluble salts like sodium cyanide
• Diazonium compounds are an exception due to decomposition

Electrophilic Substitution

Electrophilic substitution is a common reaction mechanism in organic chemistry, particularly for aromatic compounds like benzene. Unlike alkenes, which undergo addition reactions, benzene undergoes substitution because of its stable delocalized \( \pi \)-electron cloud.

This stability prevents benzene from readily reacting with compounds like bromine water. Instead of adding bromine across the double bonds, benzene replaces one of its hydrogen atoms with a bromine atom. This type of reaction is an electrophilic substitution reaction, where the incoming substituent (like bromine) replaces a hydrogen atom on the benzene ring.

This is why benzene, despite having double bonds, does not decolourise bromine water. It highlights the unique stability and reactivity of the benzene ring:

• Common in aromatic compounds like benzene
• Involves substitution of hydrogen atoms
• Does not decolourise bromine water due to its reactivity style