Chapter 7: Problem 8

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

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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\(_{3}\).)

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Short Answer

Step by step solution

(Reason for statement i)

(Reason for statement ii)

(Reason for statement iii)

(Reason for statement iv)

(Reason for statement v)

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