Three students, Manish, Ramesh and Rajni, were determining the extra elements present in an organic compound given by their teacher. They prepared the Lassaigne's extract (LE) independently by the fusion of the compound with sodium metal. Then they added solid FeS04 and dilute sulphuric acid to a part of Lassaigne's extract. Manish and Rajni obtained Prussian blue colour bit Ramesh got red colour. Ramesh repeated the test with the same Lassaigne's extract, but again got red colour only. They were surprised and went to their teacher and told him about their observation. Teacher asked them to think over the reason for this. Can you help them by giving the reason for this observation? Also, write the chemical equations to explain the formation of compounds of different colours.
In the Lassaigne's test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of
ferri ferrocyanide.
6NaCN + FeS04→ Na4[Fe(CN)6] + Na2S04
3Na4[Fe(CN)6] + 2Fe2(S04)3 → Fe4[Fe(CN)6]3 + 6Na2S04
In compounds containing nitrogen and sulphur together, the sodium metal should be in slight excess otherwise in Lassaigne's test, sodium thiocyanate (NaCNS) is formed which gives red colour with Fe3+ ions and decomposes as follows:
On the basis of above results, it is clear that Ramesh used less sodium and hence NaSCN was formed in the Lassaigne's extract which gave red colouration due to Fe(SCN)3 formation while Manish and Rajni used excess sodium and hence, NaCN was formed in the Lassaigne's extract which gave Prussian blue colour of Fe4[Fe4(CN)6].
Classify the following reactions in one of the reaction type studied in this unit.
(a) CH3CH2Br + HS– ———–> CH3CH2SH + Br–
(b) (CH3)2C=CH2 + HCl ———–> (CH3)2CCl—CH3
(c) CH3CH2Br + HO– ———–> CH2=CH2 + H2O + Br–
(d) (CH3)3C—CH2OH + HBr ———–> (CH3)2 C Br CH2CH2CH3 + H2O
Electronegativity of carbon atoms depends upon their state of hybridization. In which of the following compounds, the carbon marked with asterisk is most electronegative?
(a) 2-Ethyl-3-methylpentane
(b) 3,4-Dimethylhexane
(c) 2-sec-Butylbutane
(d) 2,3-Dimethylbutane
What are electrophiles? Explain electrophile substitution reaction with the help of example.
Describe the method, which can be used to separate two compounds with different solubilities in a solvent S.
Two or more compounds having same the molecular formula but different functional groups are called functional isomers. Which of die following pairs are not functional group isomers?
(a) II and III (b) II and IV
(c) I and IV (d) I and II
By mistake, an alcohol (boiling point 97 °C) was mixed with a hydrocarbon (boiling point 68 °C). Suggest a suitable method to separate the two compounds. Explain the reason for your choice.
In the Lassaigne’s test for ntrogen in an organic compound, the Prussian blue colour is obtaine d due to the formation of:
(a) Na4[Fe(CN)6] (b)Fe4[Fe(CN)6]3
(c) Fe2[Fe(CN)6] (d)Fe3[Fe(CN)6]4 .
(a) What do you understand by Homolytic fission?
(b) What are carbanions? Give an example.
(a) Which is more suitable method for the purification of a compound in liquid state which decomposes at or below its boiling point?
(b) How will you separate a mixture of ammonium chloride and common salt?
The IUPAC name for
(a) 1-Hydroxypentane-l, 4-dione
(b) 1,4-Dioxopentanol
(c) l-Carboxybutan-3-one
(d) 4-Oxopentanoic acid
Name a suitable technique of separation of the components from a mixture of calcium sulphate and camphor.
Will CCl4 give white precipitate of AgCl on heating it with silver nitrate? Give reason for your answer.
0.15 g of an organic compound gave 0.12 g of Ag Br by the Carius method. Find percentage of Br in the compound.
(a) 1 -Chloro-2-nitro-4-methylbenzene
(b) l-Chloro-4-methyl-2-nitrobenzene
(c) 2-Chloro-1 -nitro-5-methylbenzene
(d) m-Nitro-p-chlorotoluene
In which of the following representations given below spatial arrangement of group/atom is different from that given in structure ‘A'?
What are hybridisation states of each carbon atom in the following compounds? CH2=C=O, CH3CH=CH2, (CH3)2CO, CH2=CHCN, C6H6.
Explain the terms inductive and electromeric effects. Which electron displacement effect explain the following correct orders of acidity of the carboxylic acids?
(a) Cl3CCOOH > Cl2CHCOOH > ClCH2 COOH
(b) CH3CH2COOH > (CH3)2 CHCOOH > (CH3)3CCOOH
Write the name of element which is confirmed on adding Na2[Fe(CN)5NO] in sodium extract solution due to appearance of violet colouration.
Covalent bonds can undergo fission in two different ways. The correct representation involving the heterolytic fission of CH3 – Br is
The addition of HC1 to an alkene proceeds in two steps. The first step is the attack of H+ ion to >C = C< portion which can be shown as
(a) What is the basic principle involved in the estimation of nitrogen by Dumas method.
(b) In a Dumas nitrogen estimation method, 0.30 g of an organic compound gave 50 cm3 of N2 collected at 300 K and 715 mm Hg pressure. Calculate the percentage composition of nitrogen in the compound. (Vapour pressure of water at 300 K is 15 mm Hg)
Why is an organic compound fused with Sodium for testing nitrogen, halogens and sulphur?
Electronegativity of carbon atoms depends upon their state of hybridization. In which of the following compounds, the carbon marked with asterisk is most electronegative?
Nucleophile is a species that should have
(a) a pair of electrons to donate
(b) positive charge
(c) negative charge
(d) electron deficient species
In DNA and RNA, nitrogen atom is present in the ring system. Can Kjeldahl method be used for the estimation of nitrogen present in these? Give reasons.
If a liquid compound decomposes at its boiling point, which method(s) will you choose for its purification? It is known that the compound is stable at low pressure, steam volatile and insoluble in water.
Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles
(a) CH3COOH + HO– ———–> CH3COO– + H2O
(b) CH3COCH3 + CN ———–> (CH3)2 C(CN)(OH)
(c) C6H5 + CH3CO ———–> C6H5COCH3
Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens ?
Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur and halogens.
