UNIVERSITY COLLEGE LONDON
EXAMINATION FOR INTERNAL STUDENTS
MODULE CODE : CHEM0035
ASSESSMENT : CHEM0035A6UC/ CHEM0035A6UD
PATTERN
MODULE NAME : CHEM0035 - Organic Photochemistry Radicals and
Heterocycles
LEVEL: : Undergraduate
DATE : 19-May-2022
TIME : 10:00
Controlled Condition Exam: 3 Hours exam
You cannot submit your work after the date and time shown on
AssessmentUCL – you must ensure to allow sufficient time to upload and
hand in your work
This paper is suitable for candidates who attended classes for this
module in the following academic year(s):
Year
2020/21 and 2021/22
Additional material
Periodic table.pdf and Physical constants.pdf
Special instructions
N/A
Exam paper word
count
N/A
TURN OVER
CHEM0035 page 2 of 5
Candidates should attempt ALL questions. The paper is marked out of 90, and the
numbers in square brackets in the right-hand margin indicate the provisional allocation
of marks to the subsections of a question.
1. Answer ALL parts.
(a) (i) Explain what is meant by the term photostationary state in
relation to the E-Z isomerization of a double bond. [2]
(ii) 1,3-Pentadiene is a widely used triplet quencher. Explain how such
a quencher can reveal information about the excited states involved
in a photochemical reaction and why 1,3-pentadiene is effective in
this role. [3]
(iii) Irradiation of ketone A, in D2O and CD3CN as the co-solvents,
affords products B, C and deuterated species D. Provide
mechanisms to account for each of these products. [7]
(iv) Irradiation of valerophenone E by comparison leads to similar
photochemical reaction pathways but at a significantly reduced rate
compared to ketone A. Suggest a reason for this reduced rate. [2]
(b) (i) Explain why the bond angle in triplet carbenes is greater than in
singlet carbenes, with reference to the orbitals involved. [2]
(ii) Treatment of diazo compound F with rhodium(II) acetate leads to
two products G and H in a 5:1 ratio. Provide mechanisms to
account for each of these products. [6]
(iii) Propose an explanation for why G is the major product in this
reaction, compared to H as the minor. [2]
[Question 1 continued overleaf
CHEM0035 page 3 of 5
Question 1 continued]
(iv) Suggest a structure for the product formed in the following reaction.
Draw mechanisms to account for the formation of the product, and
comment on any chemo- and regioselectivity. [6]
2. Account for each of the following observations as fully as possible, drawing
mechanisms and explaining any issues of regio- or stereoselectivity.
(a) When PhCH3 reacts with Br·, abstraction of hydrogen occurs almost
exclusively at the methyl group. [4]
(b) Treatment of aryl bromide A with tributyltin hydride and AIBN gives
products B and C in a ratio which is dependent on the concentration of
tributyltin hydride. [10]
(c) Reaction of aryl bromide D with tris(trimethylsilyl)silane and AIBN gives
the tricyclic product E. [9]
[Question 2 continued overleaf
CHEM0035 page 4 of 5
Question 2 continued]
(d) A mixture of compounds F and G, when treated with catalytic Ru(bpy)3Cl2
under irradiation by blue light, is converted to a ketone product H. [7]
3. Answer ALL parts.
(a) Show how the following transformation could be carried out and explain
the product selectivity. No mechanism is required. [6]
(b) Provide the missing reagents and detailed mechanisms for the following
reactions. Suggest chemical structures for A and B. [6]
[Question 3 continued overleaf
O
O
F
O
+
AcO-NH4
+
heat
reagents?
N
N
F
C4H9SNa
A
C11H11FN2
B
C16H22N2S
CHEM0035 page 5 of 5
Question 3 continued]
(c) Consider the reactions below and propose a chemical structure for C and
D, as well as a detailed mechanism for their formation. Compound D
reacts with triflic acid, rapidly eliminating trifluoroacetamide and yielding E.
In a final step, E is nitrated to form the final compound F. Propose
structures for E and F (no mechanisms required), and explain any
regioselectivity. [8]
(d) (i) Propose reagents and intermediate structures for the following
transformation (No mechanisms are necessary). More than one
step is required. [5]
(ii) Suggest a structure for G and account for its formation by providing
a detailed mechanism. [5]
END OF PAPER