Photochemical transformation reactions

Direct photolysis = transformation of a compound due to its absorption of UV light

Indirect photolysis = transformation of a compound due to its interaction with a reactant generated by the influence of UV light (photosensitizer or reactive oxygen species)

Direct photolysis and light absorption

Types of orbitals:

bonding: (single) or (double)

non-bonding: n (often lone pairs on hetero atoms such as N, O)

anti-bonding: * (single) or * (double)

Absorption of light causes electronic transitions:

important transitions are usually

n to * or to *

HOMO and LUMO

Ethylene

Light Energy

Energy E = hv = h(c/)where h = Plank’s constant

= wavelength

c = speed of light

Longer wavelengths = less energyBond E (kJ/mol) (nm)O-H 465 257C-H 415 288N-H 390 307C-O 360 332C-C 348 344C-Cl 339 353Br-Br 193 620O-O 146 820

Light hitting earth’s surface = 290-600nm

(why?)

Light absorption

AI

IC lo log

( )

( )[ ( ) ( ) ]

Beer-Lambert Law

where: A = absorbanceI = light intensity (emerging vs. incident) = wavelength = absorption coefficient of the medium = absorption coefficient of the compoundl = path length

Fate of excited species

Quantum yield: r() depends on chemical structure, solvent, pH, ionic strength, etc.

low activation energies: 10-30 kJ/mol in solution

“C”ompound

h excitation

“C*”

Physical processes:

•vibrational loss of energy (heat transfer)

•energy loss by light emission (luminescence)

•energy transfer promoting an electron in another species (photosensitization)

“C”

Chemical reactions:

•fragmentation

•intramolecular rearrangement

•isomerization

•H atom abstraction

•dimerization

•electron transfer from or to the chemical

Not “C”product(s)

Chemical processes

Reaction rates for direct photolysis in water:

k W Da0 02 3 .

Light absorption rate ( rate constant!)

Where:

W = incident light intensity

D = distribution function describing average pathlength of light vs. depth of concern (zmix)

= absorption coefficient for compound of interest

all at a specific wavelength ()

Example

Rate constant from light absorption rate

k k Sa a 0 ( ) ( ) S = light-screening factor

The specific light absorption rate must be adjusted to give a rate constant. First account for light scattering in the water:

dC

dtk C

pr a

,

( )

Then for quantum yield ():

First order process for dilute solutions.

Indirect photolysis

Important reactants (electrophiles)

Singlet oxygen (1O2)

[1O2 ]ss = 7 to 11 10-14 M summer day, mid-latitude

electrophile, important for

Diels-Alder rxns (electron-rich double bonds)

oxidation of reduced sulfur groups, anilines, and phenols.

Peroxy Radicals (ROO•)

formed by addition of 3O2 to excited chromophores or radicals

usually react via abstraction of H with alkyl phenols, aromatic amines, thiophenols, imines

reactivity depends on R

both are electrophiles, thus electron donating groups increase reactivity

Hydroxyl radicals

[ ] ( )( )[ ]

( . )[ ] ( . )[ ] ( . )[ ]OH noon

NO

HCO COss0

73

73

83

2 4

3 10

15 10 4 2 10 2 5 10

DOC

[OH]ss = 10-18 to 10-16 M summer day, mid-latitude

Although very reactive, concentration of OH in solution is often too low to be important

In Water, Primarily formed from photolysis of NO3- :

NO3- + h NO3

-*

NO2- + O

NO2 + O•-H2O

HO• + HO-

Reactions of OH in solution

H abstraction: aliphatic hydrogen, leaving stable radical.

RH + OH• R• + H2O

Methyl < primary < secondary < tertiary

benzylic or allylic H

Addition to multiple bonds:

electron donating groups will increase reaction rate.

Tropospheric photochemistry: Ozone

Ozone in the upper atmosphere or stratosphere acts as a protective layer screening harmful ultraviolet light (UV). Ozone found in the lower atmosphere or troposphere does not act as an essential screen but as a pollutant.

About 8 % of the total column ozone is in the troposphere.

Ozone is a green house gas and possibly contributes to the global warming.

Ozone is harmful for human being and crops in the troposphere.

Ozone oxidizes many chemical substances in the troposphere.

Ozone is continually monitored at many urban locations.

Tropospheric chemistry

NO2 + h NO + O

O + O2 O3

O3 + h O (1D) + O2 h < 310 nm

O (1D) + H2O 2HO•

OH concentrations highest during the day (max at noon)

Troposperic chemistry of pollutants• Reactivity is always a function of reaction rate and

concentration of reactant

• Reactive species include OH, NO3, O3, sometimes HNO3 and Cl

• OH almost always dominates, despite low concentrations (106 molecules/cm3), it is very reactive. “tropospheric vacuum cleaner”

Other Tropospheric Reactants

• Reactions with NO3 important for compounds containing (non-aromatic) double bonds, fused rings (PAHs), and S atoms. NO3 concentrations peak at night.

• O3 reacts with (non-aromatic) double bonds. O3 concentrations are higher during the day but can still be substantial at night.

• Cl atoms can be generated in marine environments at conc’s up to 104 molecules/cm3. May be important reactants in some situations.

Mechanisms of reaction with OH in the troposphere

H abstraction:

RH + •OH R• + H2O

Addition to double bonds or aromatic rings (favored):

+ OH

OH

+ OH

OH

Rate constants for reactions of OH

Can be estimated via AOP “atmospheric oxidation program”

Some values available through the free “Environmental Fate Database”

Examples: kOH in 10-12 cm3/molecule-shexane 5.51-hexene 37.5styrene 58toluene 6.4phenanthrene 31PCB 7 2.6

Factors affecting reactivity include electron density, stability of resulting radical, statistical factors (number of available sites)

Fate of species in

troposphere

Fate of radicals in

troposphere

Reactions of PCBs with OH during atmospheric transport

• Laboratory-measured rate constants between 5.0 - 0.4 1012 cm3s-1 (Anderson and Hites, 1996) Half-lives for gas-phase PCBs of 0.5 to 7 days at

relevant OH concentrations

• Single most important sink for PCBs on global scale (?)

• Reactivity decreases as number of chlorines increases.

log k = -0.22(#Cl) - 11.25

R2 = 0.95-13.0

-12.5

-12.0

-11.5

-11.0

0 1 2 3 4 5 6

Number of chlorine substituents(Anderson and Hites, 1996)

log

k

0

200

400

600

800

4 7 17 21 31 40 47 52 82 97 118

151

PCB Congener Number

Gas

-ph

ase

con

c (p

g/m

3 ) night

day

Approach• Examine data for daytime

depletion of PCBs.

• Derive environmental rate constants and compare with laboratory measurements.

• Examine relative reaction rates (relationship between rate constant and number of chlorine

substituents).

Diurnal variation in PCB concentrations

0

50

100

150

200

250

300

350

4 7 18 24 37 45 52 91 135

149

PCB Congener Number

Gas

-ph

ase

con

cen

trat

ion

(p

g/m

3)

night

day

Chicago, 7/26/1994

Congener Number

ke (7/24/97)

average ke

kOH (Anderson and Hites, 1996)

4 6.0 2.2 7 12.1 2.6 31 2.4 6.1 1.2 44 2.1 5.4 0.8 47 2.7 5.6 1.0 110 1.9 2.7 0.6

tClnCln o obskEnvironmental rate constants:

kobs = ke[OH]

Assume OH = 3 106 molecules/cm3

log k = -0.21(#Cl) - 10.44

R2 = 0.46-12.5

-11.5

-10.5

1 2 3 4 5 6 7

Number of Chlorines

log

k e

Slope of log ke vs. #Cl:

Chicago, 7/26/1994