ANSWERS CHEMISTRY-2X and CHEMISTRY 2Y ALL questions ...

Someday January 2004
11.00 a.m. – 12.00 p.m.
Chemistry – 2
Test 2
ANSWERS
CHEMISTRY-2X and CHEMISTRY 2Y
ALL questions should be answered, using a separate 12-page answer
book for each section.
Students taking just one module (e.g. Chem - 2Y only) should attempt
only questions from appropriate section.
Section A: Chemistry 2Y
Lecture Course Y2
Lecture Course Y3
GENERAL
Section B
Lecture Course X2
Lecture Course X3
GENERAL

O 3
[2]
Section A
For Students doing the Chemistry 2Y Course
Question Y2 - CG
The following mechanism has been proposed for the decomposition of ozone (O 3 ) in
the atmosphere:
k 1
O 3
O 2 + O
k 1 '
O + O 3 2 O 2
What is the intermediate?
[1]
Use steady state theory to show that
d ⎡O ⎤
3
k ⎡
1
O ⎤
⎣ ⎦ 3
= ⎣ ⎦
dt
'
k ⎡
1⎣O ⎤
2⎦+
k ⎡
2
O ⎤
⎣ 3⎦
[7]
If the reaction of O and O 3 is slow, show that the reaction is first order with respect to
k 2
Answer to Question Y2 - CG
O is the intermediate [1]
[ ]
dO =k O -k O O -k O O
dt
[ ]
[ ]
[ 3] 1[ 3]
=
'
dt k [ O ]
'
[ ] [ ][ ] [ ][ ]
1 3 1 2 2 3
k1[ O3]
'
1[ 2] 2[ 3]
2
[ 3]
2[ ][ 3 ]
'
[ ] [ ]
Þ O = k O +k O
dO3 kk
1 2
O
=k O O =
dt k O + k O
when k = k
'
2 1
dO k O
1 2
which is first order wrt O
1 2 2 3
3

Question Y3 - PK
(a) Give a brief account of “aromaticity”, compare the aromaticity of carbocyclic
(benzene) and heterocyclic compounds, such as furan (A) and pyridine (B), and
highlight the typical reactivity of aromatic compounds, such as benzene. Use
specific examples and molecular formulae.
[4]
O
N
A B
(b)
Predict the major monochlorination products of the following substances (C and
D) and give your reasoning. Use molecular formulae and curly arrows and
clearly show the mechanism.
[4]
O OC 2 H 5
C
D
OC 2 H 5
(c)
4-Bromopyridine (E) undergoes reaction with pyrrolidine to yield substitution
product (F). Propose a mechanism for this reaction. Use molecular formulae and
curly arrows.
[2]
N
Br
N
H
N
N
+
HBr
E
F
Answer to Question Y3 - PK
(a) As shown on my web-site; engage the correct electron pair in the conjugation
(furan, pyridine).
(b) C → m-; D → o- and p-.
(c) Nucleophilic aromatic substitution at 2-position:

N Br N
E
HN ..
H
N
Br
N
F
N
+
HBr
General Question 2Y - SKA
Draw all 7 isomers of C 4 H 10 O and name any 6 of them.
[10]
Answer to General Question 2Y - SKA
CH 3 CH 2 CH 2 OH, 1-butanol or n-butanol
(CH 3 ) 2 CHCH 2 OH, 2-methyl-1-propanol or isobutanol
(CH 3 ) 3 COH, 2-methyl-2-propanol or tert-butanol
CH 3 CH 2 CH(OH)CH 3 , R- & S- 2-butanol (sec-butanol)
(CH 3 CH 2 ) 2 O, diethyl ether
CH 3 CH 2 CH 2 OCH 3 , methyl n-propyl ether
(CH 3 ) 2 CHOCH 3 , methyl isopropyl ether
half mark per structure, 1 mark per name

Section B
For Students doing the Chemistry 2X Course
Question X2 - RAH
(a) Draw the mechanism and clearly show the stereochemistry of the product
formed when cyclohexene A is treated with bromine. [5]
(b)
(c)
A
Assign configurations, R or S, at each chiral centre in your product showing
your priority assignment clearly.
Draw the two possible conformations of your product and predict, with
reasoning, which will be more stable.
[2]
[3]
Answer to Question X2 – RAH (cont. over)
Testing ILO’s 1,3,8,11,12
(a)
Br
Br
Br
Br
Br
Mechanism [3], product [1], anti addition
(b) R,R or S,S (depending which drawn)
(c) chairs [2], diequatorial more stable
Br
[1]
[2]
[1]

Br
Br
Br
Br
Question X3 - JSJH
MgO has the rock salt (sodium chloride) structure.
(a) Assuming close packing of O 2- ions, describe the structure
[3]
(b) How many MgO formula units are there per unit cell
[1]
(c) Given that the lattice parameter, a, = 4.21 Å and again assuming close packing
of O 2- , calculate:
(i) the ionic radii of the O 2- and Mg 2+ ions
[3]
(ii) the crystallographic density
[3]
Atomic masses of O = 16, and Mg = 24, and Avogadro’s number = 6.022 x 10 23
Answer to Question X3 – JSJH (cont. over)
(a) A cubic close packed (or face centred cubic) array of oxygen ions with
magnesium cations occupying all the octahedral holes.
(b) Four.
(c) (i) a 2 + a2 = (4r) 2 - four ionic radii touch across the diagonal
= 2a 2 = 16r 2
= a 2 = 8r 2
= a= (8) 1/2 r
therefore if a = 4.21Å, then rO 2- = 1.49Å
each side of the unit cell comprises twice the radius of each ion, therefore:
a-(2rO 2- ) = 2rMg 2+
= rMg2+ = 0.62Å

I VII Cr VI Mo 0 [3]
Alternatively, the size of the octahedral hole = 0.414rO 2- =0.62Å
(ii) volume of unit cell = a 3 = 7.46x10 -23 cm 3
mass of 4 MgO units = 4(24) + 4(16) / 6.022x10 23
=2.656x10 -22 g
as density = mass per unit volume
then crystallographic density = 3.56g cm -3
General Question 2X - SKA
(a) Assign oxidation numbers to the atom underlined in each of the following
compounds.
Na 2 [Fe(CN) 5 (NO)] (C 6 H 6 )Cr(CO) 3 NaOCl
(b) Give the formulae of compounds which illustrate each of the following
oxidation states (only one example is required in each case).
[3]
(c) Which of the above compounds might act as oxidising agents?
[4]
Answer to General Question 2X - SKA
(a) Fe III Cr 0 Cl I (+1)
(b) NaIO 4 is the most likely, CrO 3 or K 2 Cr 2 O 7 but not CrCl 6 , Mo(CO) 6 or other
realistic answer
(c) Needs a high oxidation state so Fe III , Cl I , I VII and Cr VI but NOT Cr 0 or Mo 0 .
1 mark for each correct answer in each case.