Chemical Change
Chapter 2
Dr. Suzan A. Khayyat
1
types of chemical reaction
Chemical
reactions
Thermal
chemical
Reaction
Photochemical
Reaction
Photooxidation Reaction
Photoaddition Reaction
Photohydrogenation
Pericyclic Reaction
Photodissociation
Dr. Suzan A. Khayyat
2
•
The Jablonski Diagram
•
The energy gained by a molecule when it absorbs a photon causes an electron to
be promoted to a higher electronic energy level. Figure 3 illustrates the principal
photophysical radiative and non-radiative processes displayed by organic
molecules in solution. The symbols So, S1, T2, etc., refer to the ground electronic
state (So), first excited singlet state (S1), second excited triplet state (T2), and so
on. The horizontal lines represent the vibrational levels of each electronic state.
Straight arrows indicate radiative transitions, and curly arrows indicate nonradiative transitions. The boxes detail the electronic spins in each orbital, with
electrons shown as up and down arrows, to distinguish their spin.
•
Note that all transitions from one electronic state to another originate from the
lowest vibrational level of the initial electronic state. For example, fluorescence
occurs only from S1, because the higher singlet states (S2, etc.) decay so rapidly by
internal conversion that fluorescence from these states cannot compete.
Dr. Suzan A. Khayyat
3
Jablonski energy diagram
Singlet State
(S 1,S2, ......)
1(n,
Triplet State
(T1, T2, ...)
Photochem.
 
A
b
s
o
r
p
t
i
o
n
F
l
u
o
r
e
s
c
e
n
c
e
ISC
Biological
Response
n 
 
photochem. &
singlet oxygen
Phosphorescence
Ground State
So
Jablonski energy diagram
Dr. Suzan A. Khayyat
4
Jablonski diagram
•
Figure 3. The basic concepts of this Jablonski diagram are presented in the Basic
Photophysics module. This version emphasizes the spins of electrons in each of
the singlet states (paired, i.e., opposite orientation, spins) compared to the
triplet states (unpaired, i.e., same orientation, spins).
Dr. Suzan A. Khayyat
5
Photooxygenation Reaction
1
hv
Sens (S0)
1
Sens* (S1)
3
*
Sens (T1)
+ 3O
2
1
*
Sens (S1)
3
*
Sens (T1)
1
1
Sens (S0) + O2
Dr. Suzan A. Khayyat
6
)1O2(
1
+
g
1
g
37.5 Kcal/mol
3
g
22.4 Kcal/mol
1
Highest occupied molecular orbital of O2
Dr. Suzan A. Khayyat
7
C6H5
C6H5
H3C
N
H
N
H
H3C
N
OH
CH CH3
N
CH3
N
HOOCH2C-H2C
H
N
N
CH CH3
OH
H
N
Cl
C6H5
C6H5
Tetraphenylporphyrine (TPP)
Cl
Cl
HOOC-H2C-H2C
CH3
Hematoporphyrine( HP)
Cl
COONa
I
I
ONa
O
O
I
I
Ros Bengal(RB)
Dr. Suzan A. Khayyat
8
Types of singlet oxygen reactions
H
1
+
1)
A
O2
OOH
O
X
1
2)
+
3)
+
O2
1
O2
O
X
B
C
Dr. Suzan A. Khayyat
O
O
9
1- Ene Reaction
O*2
H
C
C
O
OH
C
C
C
C
Cis cyclic mechanism for the reaction of 1O2 with
mono-olefins.
Dr. Suzan A. Khayyat
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Dr. Suzan A. Khayyat
11
H
OOH
C
C
C
+
1
O2
Dr. Suzan A. Khayyat
C
C
C
12
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13
Dr. Suzan A. Khayyat
14
Dr. Suzan A. Khayyat
15
2-Cycloaddition Reaction (Diels Alder)
Dr. Suzan A. Khayyat
16
Direct addition reaction to produce(1,2-dioxetane)
Dr. Suzan A. Khayyat
17
Dr. Suzan A. Khayyat
18
Dr. Suzan A. Khayyat
19
Dr. Suzan A. Khayyat
20
Photosensitized oxidation
H3C
+
CH3
O
O2
hv , sens
H3C
O
O
H3C
CH3
C
H3C
C
CH3
O
H3C
+
O2
hv , sens
CH3
CH2
C
H3C
C
OOH
O
C2H5O-CH=CH-OC2H5 +
O2
hv , sens
Dr. Suzan A. Khayyat
CH3
O
C2H5O-CH-CH-OC2 H5
21
Photodissociation: processes and examples
• Hydrocarbons:
/
RCH2R +
CH2=CH2+
hv
RCR/
hv
H2 + H2C=C: (
+ H2
HC
CH)
2H + H2C=C:
H2 + HC
CH
2H + HC
CH
Dr. Suzan A. Khayyat
22
Carbonyl Compounds
1- Keetones:
• Norrish Type I:
The Norrish type I reaction is the photochemical cleavage or homolysis
of aldehydes and ketones into two free radical intermediates. The
carbonyl group accepts a photon and is excited to a photochemical
singlet state. Through intersystem crossing the triplet state can be
obtained. On cleavage of the α-carbon carbon bond from either state,
two radical fragments are obtained.
Dr. Suzan A. Khayyat
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Dr. Suzan A. Khayyat
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Dr. Suzan A. Khayyat
25
Norrish type II
• A Norrish type II reaction is the photochemical intramolecular abstraction
of a γ-hydrogen (which is a hydrogen atom three carbon positions
removed from the carbonyl group) by the excited carbonyl compound to
produce a 1,4-biradical as a primary photoproduct
Dr. Suzan A. Khayyat
26
Dr. Suzan A. Khayyat
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Dr. Suzan A. Khayyat
28
RCHO +
hv
RH +
2C2H4 +
C=O
+
CO
+
hv
CO
CO
CH2=CHCH2CH2 CHO
Dr. Suzan A. Khayyat
29
Complete the next equations
O
H2C
hv
H2C
hv
O
Dr. Suzan A. Khayyat
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O
CH3
H3C
hv
C
H2
O
CH3
hv
H3C
CH3
CH3
Dr. Suzan A. Khayyat
31
2- Esters:
hv
RCH2CH2CH2
COOR\
RCH=CH2 +
hv
\
RCOOCH2CH2R
RCOOH
Dr. Suzan A. Khayyat
+
CH3COOR\
CH2=CHR\
32
Photocycloaddition
2+2 Intermolecular cycloaddition
O
O
R
+
hv
H3CO
H3CO
R
R\
OCH3
O
OCH3
R\
O
O
Dr. Suzan A. Khayyat
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Dr. Suzan A. Khayyat
34
O
2
O
O
O
hv
+
O
Dr. Suzan A. Khayyat
35
2+2 Intramolecular cycloaddition
hv
Dr. Suzan A. Khayyat
36
2+4 Cycloaddition
+
Dr. Suzan A. Khayyat
37
hv
+
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38
Dr. Suzan A. Khayyat
39
Photoaddition and photocyclization
reactions
NH2
H
N
hv
H
N
+
+
+
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40
Direct and photosensitized reactions
direct
trans
sensitized
cis
Dr. Suzan A. Khayyat
41
Isomerization and rearrangements
Dr. Suzan A. Khayyat
42
direct
Triplet
sensitized
Dr. Suzan A. Khayyat
45
hv
H
hv
H
+
+
Benzvalene
Dr. Suzan A. Khayyat
bicyclohexadiene
fulvene
46
hv
CN
C6H5
C6H5
C6H5
Dr. Suzan A. Khayyat
C6H5
CN
47
Synthetic applications of electrocyclisation reactions:
The conversion of ergosterol to vitamin D2 proceeds through a ring-opening (reverse)
electrocyclisation to give provitamin D2, which then undergoes a second rearrangement (a [1,7]sigmatropic shift). Stereochemical control in the sigmatropic shift process will be described in a
later section of this course.
H
sunlight
H
HO
ergosterol
H
photochemicallypromoted electrocyclisation
(antarafacial, conrotation)
H
provitamin D2
HO
[1,7]-sigmatropic shift.
H
HO
Dr. Suzan A. Khayyat
vitamin D2
48
Photochemistry in solution
O
H2
(CH3) C
C
H2
C
liq
(CH3)
CO +
C3H8 + H3C
CHCHO
gas
H2
(CH3)2 C
Dr. Suzan A. Khayyat
O
C
O
C
H2
C
(CH3)2
49
Factors determining reactivity
• 1•
•
•
•
The excess energy possessed by the species (which
may help overcome activation barriers).
2- The intrinsic reactivity of the specific electronic
arrangement.
3- The relative efficiencies of the different competing
pathways for loss of the particular electronic state.
4- The type of orbital (s, p, σ, or, π, etc.) and its
symmetry.
5- Explicit in the correlation rules for orbital symmetry
and spin that are introduced first at the end of this section.