1. Environmental Chemistry Chapter 3: The Detailed Chemistry of the Atmosphere Copyright © 2007 DBS

2. Review: How to Draw Lewis Structures 1.Determine the sum of valence electrons 2.Use a pair of electrons to form a bond between each pair of bonded atoms 3.Arrange the remaining electrons to satisfy octet rule (duet rule for H) 4.Assign formal charges (valence – directly surrounding e - )

3. CH 4 and H 2 O but unlike methane, two e - pairs are bonding and two are non-bonding The non-bonding e - pairs take up more space than bonding pairs, so the H- to-O-to-H bond angle is compressed Methane, CH 4 Water, H 2 O

4. VSEPR Valence Shell Electron Pair Repulsion Theory assumes that the most stable molecular shape has the electron pairs surrounding a central atom as far away from one another as possible No. e - pairs around central atom Shape of moleculeBond angle 4 pairs, all bonding: CH 4, CF 4, CF 3 Cl, CF 2 Cl 2 Tetrahedral109.5° 4 pairs, three bonding, one non-bonding: NH 3, PCl 3 Triangular pyramid~107° 4 e - pairs, two bonding, two non-bonding: H 2 O, H 2 S Bent~105°

5. Lewis Structures of Free Radicals Free radicals possess an unpaired e - The unpaired e - is not in actual use as a bonding e - Carbon centered radical in which the carbon atom has one unpaired e - forms 3 bonds rather than four Oxygen forms one rather than 2 bonds: O – H A halogen forms no bonds: Cl H―C ― H | H

6. Lewis Structures of Free Radicals Choice of assigning the unpaired e - Hydroperoxy radical, HO 2 : Complicated in molecules containing multiple bonds For hydroxy formyl, HOCO a reasonable structure is: Does not go to O since C must have 4 valence H-O-O H-O-C=O

7. Lewis Structures of Free Radicals A simple formula, ClO does not indicate which atom carries the e -. Draw Lewis structures for: OH CF 2 Cl ClO NO O – H F – C – F | Cl Cl – O N = O

8. Hydroxyl Radical: The Atmosphere’s Detergent OH is the prominent oxidizing species in the atmosphere Despite very low atmospheric concentrations, currently estimated at 10 6 molecules cm -3, corresponding to a mean tropospheric volume mixing ratio of 4 x 10 -8 ppmv The lifetimes of most atmospheric gases are, therefore, largely determined by [OH] and the corresponding reaction coefficients Radical reactions that are spontaneous produce stable products with strong bonds

9. Hydroxyl Radical: The Atmosphere’s Detergent The major route for the formation of the hydroxyl radical in the troposphere is: NO 2 + h ( < 400 nm) → NO + O O + O 2 + M → O 3 O 3 + h ( < 320 nm) → O 2 + O* O* + H 2 O → 2 OH NO 2 + H 2 O → NO + 2 OH Others: O* + CH 4 → OH + CH 3 OH HNO 2 → OH + NO H 2 O 2 + h → 2OH

10. Interactions with Hydroxyl Radical Usually it reacts by adding itself to a molecule at the multiple bond It can also abstract hydrogen atom to produce carbon centered radicals OH addition does not occur to O=O bonds since the bonding that would result will be weak For example, in the case of SO 2, the OH radical adds to the sulfur atom forming a strong bond but not to an oxygen atom Hydroxyl radicals do not add to CO 2 since C=O bonds are very strong However, it adds to CO, the addition favors conversion of triple bond to stable double bond

11. Radicals React with O 2 to produce Peroxy and Hydroperoxy Radicals Predominant fate is ‘add-on’ reaction with O 2, e.g. CH 3 + O 2 → CH 3 OO HOO / HO 2 (hydroperoxy) and CH 3 OO are called peroxy radicals - Less reactive than other radicals - Do not readily abstract H - Do not react with O due to low conc. Main reactions: HOO + NO → OH + NO 2 R-OO + NO → RO + NO 2 (where R = carbon chain) H 3 C – O – O Successive reactions will completely oxidize the organic compound

12. H Atom Abstraction by O 2 from Nonperoxy Radicals Gases that undergo decomposition by absorbing UV-A or visible light can generate free radicals. e.g., formaldehyde H 2 CO + UV-A (<338 nm) → H + HCO If there is no suitable hydrogen atom for O 2 to abstract then it adds-on H-abstraction occurs provided a new bond is formed peroxy radical CH 3 -O + O 2 → H 2 C=O + HOO H-C=O + O 2 → C=O + HOO

13. Fate Decision tree illustrating the fate of gases emitted into the air HNO 3, HCl, NH 3, etc H 2 CO CH 4 + OH  H 2 O + CH 3

14. Fate of Free Radicals Decision tree illustrating the fate of airborne free radicals ROO· + NO  NO 2 + RO· CH 3 · + O 2  CH 3 OO·

15. Oxidation of CH 4 Produced in inefficient (anaerobic) burning of fuels Predominant HC in atmosphere No multiple bonds Not soluble in water, does not absorb sunlight Slow oxidation initiated by hydroxyl radical (hydrogen abstraction reaction) CH 4 + OH → CH 3 + H 2 Oabstraction CH 3 + O 2 → CH 3 OOO 2 adds forming peroxy CH 3 OO + NO → CH 3 O + NO 2 transfer of O CH 3 O + O 2 → H 2 CO + HOOO 2 absracts H …conversion of methane to formaldehyde

16. H 2 CO + UV-A (338 nm) → H + HCO unstable H + O 2 → HOOO 2 abstracts HCO + O 2 → CO +HOOO 2 abstracts Note: CO is a stable intermediate and can further undergo transformations C O + OH → HO-C=O H-O-C=O + O 2 → O=C=O + HOO ….. Production of CO 2 as the final product CH 4 + 5O 2 + NO + 2OH + UV-A → CO 2 + H 2 O + NO 2 + 4HOO

17. Notice the radicals consumed and produced. What happens to the HO 2 produced? What happens to the NO 2 produced? (see fate of free radicals)

18. Reaction intermediates during hydride oxidation

19. Problems 3-4 3-5 3-6

20. Part 2

21. Photochemical Smog Saturated hydrocarbons such as CH 4 react with hydroxyl radical by hydrogen abstraction Hydrocarbons with double bond (e.g., ethene) react with OH by addition because of lower activation energy …formation of carbon centered radical Oxidation of Reactive Hydrocarbons Energetics favor addition over abstraction

22. Photochemical Smog Carbon centered radical reacts with O 2 to produce a peroxy radical which in turn oxidizes NO to NO 2 Photochemical decomposition of NO 2 to NO and O and formation of ozone results in photochemical smog NO 2 → NO + O (1) O + O 2 → O 3 (2) NO + O 3 → 2NO 2 + O 2 (3) NO 2 is the only significant source of O

23. Formation of Aldehydes Decomposition of carbon centered radical Aldehydes further decompose in sunlight RHCO + sunlight → R + HCO ….further increase in the number of radicals Original C=C is split into 2 aldehydes

24. Overall RHC=CHR + OH + 2O 2 + NO → 2RHC=O + HOO + NO 2

25. Mechanism of the RHC=CHR oxidation process in the smog NO is oxidized by the C-O-O· Addition of O 2 to radical center Energetics favor addition over abstraction Cleavage allows formation of aldehyde double bond Reaction with O 2 allows 2 nd aldehyde to form Photolysis follows. Peroxy radicals are formed. NO is oxidized to NO 2 Radicals formed: HO 2 (2); OH (1), RO (1), NO 2 (3) Also CO

26. Problems 3-8

27. The Fate of Free Radicals Rate of reaction between two radicals increase as the radical concentration increases R + R’ → R-R’ stable molecule e.g., OH +NO 2 → HNO 3 OH + NO → HNO 2 OH + NO (HONO accumulates only in the night) When the concentration of NOx is low, 2OH → H 2 O 2 2HOO → H 2 O 2 + O 2 sunlight Starts AM cycle

28. Fate of Other Radicals O 2 + R-C=O (peroxyacetylnitrate) Peroxyacetylnitrate is eye irritant and toxic to plants Thus in the afternoon hours a build up of oxidizing agents such as nitric acid, hydrogen peroxide and PAN is encountered

29. Hourly Variation of Concentration of Gases Early morning traffic increases the emissions of both nitrogen oxides and VOCs as people drive to work Later in the morning, traffic dies down and the nitrogen oxides and volatile organic compounds begin to react forming nitrogen dioxide, increasing its concentration As the sunlight becomes more intense later in the day, nitrogen dioxide is broken down and its byproducts form increasing concentrations of ozone As the sun goes down, the production of ozone is halted. The ozone that remains in the atmosphere is then consumed by several different reactions Source: http://jan.ucc.nau.edu/~doetqp-p NO → NO 2 HC → Aldehydes

30. Role of NO 3 Nitrate radical produced from NO 2 and O 3 NO 2 + O 3 → NO 3 + O 2 Photolysis yields NO 2 and O Abstracts H from RH during evening NO 3 + RH → HNO 3 + R Similar to OH

31. Part 3

32. Oxidation of SO 2 (g) Addition of OH followed by the formation of SO 3 SO 3 + H 2 O (g) → H 2 SO 4 (g ) H 2 SO 4(g) + nH 2 O → H 2 SO 4 (aq)

33. Oxidation of SO 2 (aq) Determination of total sulfur content in water SO 2 is soluble in water. It exists in the dissolved form if there is significant cloud or mist in the atmosphere. The oxidation to sulfuric acid occurs in the aqueous phase after rain drops accumulate on earth. SO 2 (g) + H 2 O (aq) ⇌ H 2 SO 3 (aq) Typically SO 2 conc. is 0.1 ppm or (0.1/10 6 ) =1 x 10 -7 atm From Henry’s law, K H = 1 M atm -1 = [H 2 SO 3 ]/P  [H 2 SO 3 ] = 1 M atm -1 x 1x10 -7 atm = 1 x 10 -7 M (or moles/L) But H 2 SO 3 dissociates readily with a dissociation constant of K = 1.7 x 10 -2 M -1 H 2 SO 3 ⇌ H + + HSO 3 - As HSO 3 dissociates, more of SO 2 dissolves until it reaches an equilibrium with H + and HSO 3 1.7 x 10 -2 M -1 (or K) = [H + ][HSO 3 - ]/[H 2 SO 3 ] 1.7 x 10 -2 M -1 (or K) = [HSO 3 - ] 2 /[H 2 SO 3 ] = [HSO 3 - ] 2 / 1 x 10 -7 M …[H + ] = [HSO 3 - ] [HSO 3 - ] 2 = 17 x 10 -10 M 2 = 4 x 10 -5 M  Total dissolved S is 4 x 10 -5 M

34. Oxidation of SO 2 (aq) Dissolved SO 2 is oxidized by trace amounts of H 2 O 2 and O 3 Sunlight is a dominant factor in forming O 3 and H 2 O 2 If strong acids are present in the droplet they control the pH. Any freshly dissolved SO 2 has no effect [HSO 3 - ] = K x [H 2 SO 3 ]/[H + ] =1.7 x 10 -2 x 10 -7 /[H + ] =1.7x10 -9 /[H + ] …inversely proportional to H + Since strong acids dissociate readily, [H + ] concentration controls the overall concentration of HSO 3 - Acidity of the droplet has effect on the rate of SO 2 oxidation At pH below 5 H 2 O 2 dominates oxidation and above pH 5 ozone or other catalytic reactions dominate the oxidation

35. Hydrogen abstraction reactions dominate chemistry in both stratosphere and troposphere …….but the radicals that dictate the chemistry are different Stratosphere: OH, O, Cl, and Br abstract H atom from stable molecule such as CH 4 Troposphere: hydroxyl and NOx radicals are the primary reactants

36. Processes Involving Loosely Bound Oxygen Atoms A Y-O structure from which O atom can be detached readily

37. Examples of “Loose O Atom” Reactions Reaction with atomic oxygen Y―O → Y + O 2 Photochemical decomposition Y―O + sunlight → Y + O Reaction with NO Y―O + NO → Y + NO 2 Abstraction of oxygen from Ozone Y―O + X → Y + XO O 2 ―O + Cl → O 2 + ClO