Indian National Chemistry Olympiad 2020 Theory (3 hours)

Roll No. Exam Centre:

Indian National Chemistry Olympiad 2020 Theory (3 hours)

Date: February 1, 2020 Do not write anything below this line

Question No

1

2

3

4

5

6

Total

Marks

16

22

24

13

29

15

119

Marks Obtained

Signature of Examiner

Instructions for students Write last four digits of your Roll No. at the top of all pages. This examination booklet consists of 29 pages of problems including answer boxes. Kindly check that the booklet has all the pages. If not, report to the invigilator immediately. All answers must be written in the appropriate boxes. Anything written elsewhere will not be considered

for assessment. Adequate space has been provided in the answersheet for you to write/calculate your answers. In case you

need extra space to write, you may request for additional blank sheets from the invigilator (Please draw a box and write the Q. No. in the box on this sheet for evaluation). Remember to write your roll number on the extra sheets and get them attached to your answersheet. Use only a pen to write the answers in the answer boxes. Answers written in pencil (except for graph) will be penalized. You must show the main steps in the calculations. For objective type question, mark X in the correct box. Some of the objective questions may have more than one correct answer. A copy of the Periodic Table of the Elements is provided at the end. Do not leave the examination room until you are directed to do so.

Fundamental Constants

Avogadro number Electronic charge

NA = 6.022 ? 1023 mol1 e = 1.602 ? 1019 C

Molar gas constant

R = 8.314 J K?1 mol1 = 0.08205 Latm K-1mol-1

1 atm = 101325 Pa

Mass of electron Speed of light 1 atomic mass unit

pH = log [H+]

me = 9.109 ? 1031 kg c = 2.998 ? 108 m s1 (1 amu) = 1.660 ? 1027 kg

pKa = log Ka

? Homi Bhabha Centre For Science Education, Tata Institute of Fundamental Research V.N. Purav Marg, Mankhurd, Mumbai 400 088

IUPAC Periodic Table of the Elements

1

1

H

hydrogen

1.008

[1.0078, 1.0082]

2

3

Li

lithium

6.94

[6.938, 6.997]

4

Be

beryllium

9.0122

11

Na

sodium

22.990

12

Mg

magnesium

24.305

[24.304, 24.307]

19

K

potassium

20

Ca

calcium

Key:

atomic number

Symbol

name

conventional atomic weight standard atomic weight

3

21

Sc

scandium

4

22

Ti

titanium

5

23

V

vanadium

39.098

37

Rb

rubidium

40.078(4)

38

Sr

strontium

44.956

39

Y

yttrium

47.867

40

Zr

zirconium

50.942

41

Nb

niobium

85.468

55

Cs

caesium

87.62

56

Ba

barium

88.906

57-71 lanthanoids

91.224(2)

72

Hf

hafnium

92.906

73

Ta

tantalum

132.91

87

Fr

francium

137.33

88

Ra

radium

89-103 actinoids

178.49(2)

104

Rf

rutherfordium

180.95

105

Db

dubnium

6

7

24

Cr

chromium

25

Mn

manganese

51.996

42

Mo

molybdenum

54.938

43

Tc

technetium

95.95

74

W

tungsten

75

Re

rhenium

183.84

106

Sg

seaborgium

186.21

107

Bh

bohrium

8

26

Fe

iron

55.845(2)

44

Ru

ruthenium

101.07(2)

76

Os

osmium

190.23(3)

108

Hs

hassium

18

2

He

helium

13

14

15

16

17

4.0026

9

27

Co

cobalt

58.933

45

Rh

rhodium

10

28

Ni

nickel

58.693

46

Pd

palladium

11

29

Cu

copper

63.546(3)

47

Ag

silver

12

30

Zn

zinc

65.38(2)

48

Cd

cadmium

5

B

boron

10.81

[10.806, 10.821]

6

C

carbon

12.011

[12.009, 12.012]

7

N

nitrogen

14.007

[14.006, 14.008]

8

O

oxygen

15.999

[15.999, 16.000]

9

F

fluorine

18.998

10

Ne

neon

20.180

13

Al

aluminium

26.982

14

Si

silicon

28.085

[28.084, 28.086]

15

P

phosphorus

30.974

16

S

sulfur

32.06

[32.059, 32.076]

17

Cl

chlorine

35.45

[35.446, 35.457]

18

Ar

argon

39.95

[39.792, 39.963]

31

Ga

gallium

69.723

32

Ge

germanium

72.630(8)

33

As

arsenic

74.922

34

Se

selenium

78.971(8)

35

Br

bromine

79.904

[79.901, 79.907]

36

Kr

krypton

83.798(2)

49

In

indium

50

Sn

tin

51

Sb

antimony

52

Te

tellurium

53

I

iodine

54

Xe

xenon

102.91

77

Ir

iridium

192.22

109

Mt

meitnerium

106.42

78

Pt

platinum

107.87

79

Au

gold

195.08

196.97

110

111

Ds Rg

darmstadtium roentgenium

112.41

114.82

80

Hg

mercury

200.59

81

Tl

thallium

204.38

[204.38, 204.39]

112

Cn

copernicium

113

Nh

nihonium

118.71

82

Pb

lead

207.2

114

Fl

flerovium

121.76

83

Bi

bismuth

208.98

115

Mc

moscovium

127.60(3)

84

Po

polonium

116

Lv

livermorium

126.90

85

At

astatine

117

Ts

tennessine

131.29

86

Rn

radon

118

Og

oganesson

57

La

lanthanum

138.91

89

Ac

actinium

58

Ce

cerium

140.12

90

Th

thorium

232.04

59

60

Pr Nd

praseodymium neodymium

140.91

91

Pa

protactinium

144.24

92

U

uranium

231.04

238.03

61

Pm

promethium

93

Np

neptunium

62

Sm

samarium

150.36(2)

94

Pu

plutonium

63

Eu

europium

151.96

95

Am

americium

64

Gd

gadolinium

157.25(3)

96

Cm

curium

For notes and updates to this table, see . This version is dated 1 December 2018. Copyright ? 2018 IUPAC, the International Union of Pure and Applied Chemistry.

65

Tb

terbium

158.93

97

Bk

berkelium

66

Dy

dysprosium

162.50

98

Cf

californium

67

Ho

holmium

164.93

99

Es

einsteinium

68

Er

erbium

167.26

100

Fm

fermium

69

Tm

thulium

168.93

101

Md

mendelevium

70

Yb

ytterbium

173.05

102

No

nobelium

71

Lu

lutetium

174.97

103

Lr

lawrencium

Indian National Chemistry Olympiad 2020

Problem 1

Chemistry of the artificial hair dyes

Dyeing of hair is an ancient art. Earlier, dyes were

obtained

from

plants

such

as indigo, henna, turmeric and amla. In dyeing with

natural henna, an aglycone molecule (structure shown

below), gets converted to orange colored Lawsone in

the presence of an oxidant (such as air).

Oxidant

Roll No

16 marks

Aglycone

Lawsone

Hydroquinone (1,4-dihydroxybenzene) and analogous 1,4-diimine derivatives also undergo similar transformations under the influence of an oxidant. A chemistry teacher at a hair salon once observed the barber preparing a low-cost hair colorant for a customer by mixing a spoonful of brownish-white crystalline solid with H2O2 and shaving cream. A little investigation indicated that the white solid was a chemical called PPD (p-phenylenediamine). On further reading, the teacher realized that PPD was one of the first synthetic chemical launched for hair dyeing market in the year 1907. Later he found that most of the hair colorants being sold in the market, including the so-called "natural/herbal" hair colours had PPD in them. Hair proteins have iso-ionic points (when the number of +ve and ?ve charged species in protein are equal) around pH = 4.86.8. Therefore, some of the hair dyes are also +vely charged species, which hair proteins absorb from neutral-alkaline solutions. In this problem, we will explore how colourless PPD leads to the intense black and numerous other hair colours. Under oxidizing condition, PPD gets converted to corresponding diimine A (C6H6N2).

1.1 Draw the structure of A.

?HBCSE, February 1, 2020

1

Indian National Chemistry Olympiad 2020

Roll No

In aqueous solutions in the pH range 7-10, a small fraction of A molecules exists in monoprotonated form B. 1.2 Draw the structure of B.

B

Usually colour in organic molecules originates due to extended conjugation. Black colour is generated from PPD by two pathways I and II. Pathway I: In many commercial hair colorants, the white powder contains a mixture of several compounds (called couplers) along with PPD. Under oxidizing conditions, PPD reacts with different couplers producing different hair colors, which in specific combination give black colour. For example, oxidized form of PPD (B) reacts with the coupler m-phenylenediamine in aqueous phase to form the thermodynamically stable product C, which on further oxidation produces a blue hair dye D (a cationic species having two rings). 1.3 Write the structure of C and dye D.

C C

D (Hair dye)

When resorcinol is used as a coupler, a yellow-green dye E is produced which reacts with diazomethane to form a compound having molecular formula C20H18N4O2. 1.4 Write the structure of E.

B

?HBCSE, February 1, 2020

E (Yellow-Green colour) 2

Indian National Chemistry Olympiad 2020

Roll No

Pathway II: When PPD is alone subjected to oxidation, the oxidized form A slowly gets polymerized to give an intermediate F, which on further oxidation gives an intense black hair colour.

1.5 Write the part-structure of the polymer (F).

F

The oxidative coupling of 4-dimethylamino aniline with phenol can take a different pathway. A 2 e oxidation of 4-dimethylamino aniline generates a cationic intermediate species G. Reaction of phenol with G, produces a colourless species leuco-indoaniline, which gets easily oxidized to give a dye, indoaniline. Quantum mechanical calculations indicate that conversion from G to leuco-indoaniline can go through a very low energy pathway involving formation of another intermediate species H. Due to - stacking interactions between two rings, H can easily rearrange to produce leuco-indoaniline.

1.6 Draw the most stable resonance form of G and possible structure of H.

[O]

G Rearrangement

leuco-indoaniline

?HBCSE, February 1, 2020

H 3

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