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Chapter3 140330083709-phpapp01
- 1. Chapter 3:Chapter 3: RadicalRadical HalogenationHalogenation OO OO OO OO NN
- 2. The Ozone HoleThe Ozone Hole
- 3. Radical Halogenation andRadical Halogenation and Bond StrengthBond Strength Reactions require bondReactions require bond breakingbreaking and bondand bond makingmaking Bond strengthsBond strengths: Homolytic cleavage: Homolytic cleavage radicalsradicals ∆∆HH = D= DHºHº = Bond dissociation energy= Bond dissociation energy (kcal mol(kcal mol-1-1 )) This process contrasts withThis process contrasts with heterolyticheterolytic cleavagecleavage AA BB AA BB ++ -- ++ AA BB AA·· BB++ e.g.e.g. HH OOHH,, DDHºHº = +119= +119,, yet: Hyet: H22O + HO + H22O HO H33O + OH easyO + OH easy ++ -- ··
- 4. HH HH HH HH++ DDHºHº = +104 kcal mol= +104 kcal mol-1-1 ToTo functionalizefunctionalize alkanes, we need to breakalkanes, we need to break CC HH A 1-minute problem:A 1-minute problem: DDHºHº = ?= ? The simplest bond dissociation:The simplest bond dissociation: A. Same as H–HA. Same as H–H B. LargerB. Larger C. SmallerC. Smaller
- 5. We expectWe expect CC H to be less than for HH to be less than for H22.. But: Are all C–H bonds the same ?But: Are all C–H bonds the same ? DDHºHº s decrease along the series:s decrease along the series: CCHH44 > R> Rprimprim――HH > R> Rsecsec――HH > R> Rterttert――HH No!No! PrimaryPrimary SecondarySecondary TertiaryTertiary
- 6. CHCH33 HH DDHºHº = +105= +105 kcal molkcal mol-1-1
- 8. Calculate feasibility of a reaction:Calculate feasibility of a reaction: CHCH33––OH + HOH + H––I CHI CH33––I + HI + H––OHOH ΔΔH° =H° = ????
- 9. CHCH33––OH + HOH + H––I CHI CH33––I + HI + H––OHOH 93 71 57 11993 71 57 119 164 – 176 = –12 kcal mol164 – 176 = –12 kcal mol-1-1
- 11. Remember BH3! Why do we see such a trend?Why do we see such a trend? R isR is spsp22 -hybridized-hybridized.. Substitution stabilizes the radical. How?Substitution stabilizes the radical. How?
- 12. HyperconjugationHyperconjugation pp-Orbital-Orbital (with single(with single ee) overlaps with) overlaps with bonding molecular orbitalbonding molecular orbital ofof neighboring C-H (or any other) bond.neighboring C-H (or any other) bond. CC CC HH CC CC HyperconjugationHyperconjugation
- 13. HyperconjugationHyperconjugation Potential Energy DiagramPotential Energy Diagram C HC H HyperconjugationHyperconjugation EE C H bondC H bond pp-Orbital-Orbital MO picture ofMO picture of spsp33 1s(H)1s(H) Net stabilization: 3eNet stabilization: 3e Bonding MOBonding MO Antibonding MOAntibonding MO
- 15. Radical Halogenation:Radical Halogenation: Methane and Chlorine (Methane and Chlorine (kcalkcal molmol-1-1 )) CCHH33 HH ++ ClCl ClCl CCHH33 ClCl ++ HH ClCl Exothermic, but needs heat (∆) and/or light to start.Exothermic, but needs heat (∆) and/or light to start. 105105 5858 ∆∆HºHº = -25= -25 8585 103103 hhvv, ∆, ∆ CClCCl44 MechanismMechanism 1.1. InitiationInitiation: Cl: Cl22 2 Cl ∆2 Cl ∆HºHº = +58= +58 ““light the match”light the match” hhvv or ∆or ∆
- 16. How does the Cl–Cl bondHow does the Cl–Cl bond break?break? Thermally:Thermally: VibrationalVibrational energy getsenergy gets sufficiently large tosufficiently large to cause bond breaking.cause bond breaking. Photochemically:Photochemically: Absorption ofAbsorption of photon causesphoton causes excitation ofexcitation of bonding electronbonding electron to antibondingto antibonding
- 17. 2.2. PropagationPropagation ((“fire”“fire”): A): A radical chainradical chain mechanismmechanism aa.. CCHH44 ++ ClCl CCHH33 ++ HHClCl ∆∆HºHº == +2+2 bb.. CCHH33 ++ ClCl22 CCHH33ClCl ++ ClCl ∆∆HºHº == -27-27 up!up! down!down! [a. + b.][a. + b.]:: CCHH44 ++ ClCl22 CCHH33ClCl ++ HHClCl ∆∆HºHº = -25= -25 3.3. TerminationTermination: 2: 2ClCl ClCl22 CCHH33 ++ ClCl CCHH33ClCl CHCH33 + CH+ CH33 CHCH33 CHCH33 KillsKills propagationpropagation MechMech 105105 103103 5858 8585 Note:Note: Initiation stepInitiation step does not enterdoes not enter into equation. Only ainto equation. Only a few Clfew Cl∙∙ needed to convert all of the starting material.needed to convert all of the starting material.
- 18. Orbital Picture of HOrbital Picture of H·· AbstractionAbstraction Fast!Fast! PartialPartial radicalradical charactercharacter δδ∙∙ resemblesresembles productsproducts
- 19. Potential energy diagram of propagation stepsPotential energy diagram of propagation steps gives picture of the energetic “ups and downs”.gives picture of the energetic “ups and downs”.
- 21. Other Halogenations ofOther Halogenations of MethaneMethane Compare important DCompare important DHH º values:º values: ReactivityReactivity: F: F22 > Cl> Cl22 ~ Br~ Br22 > I> I22 won’t go!won’t go! FF22 ClCl22 BrBr22 II22 HF HCl HBr HI F Cl Br IHF HCl HBr HI F Cl Br I 3838 58 46 36 136 103 87 71 110 85 70 5758 46 36 136 103 87 71 110 85 70 57 explodesexplodes good !good ! CCHH33 XX Initiation OK for allInitiation OK for all Why?Why?
- 22. FF22 ClCl22 BrBr22 II22 HF HCl HBr HI F Cl Br IHF HCl HBr HI F Cl Br I 38 58 46 36 136 103 87 71 110 85 70 5738 58 46 36 136 103 87 71 110 85 70 57 Won’t go!Won’t go! Endothermic.Endothermic. Remember:Remember: CCHH33----HH 105 kcal mol105 kcal mol-1-1 CCHH33――XX
- 23. Reactivity determined in step a. by DReactivity determined in step a. by DHºHº == HH―X. Let’s―X. Let’s compare the position of the transition states alongcompare the position of the transition states along reaction coordinate.reaction coordinate. Hammond PostulateHammond Postulate.. not far, smallnot far, small δδ∙∙ on Con C far largerfar larger δδ∙∙ on Con C Early TSEarly TS fast , exothermic step ( F).fast , exothermic step ( F). Late TSLate TS slow , endothermic step ( Br, I).slow , endothermic step ( Br, I).
- 24. Selectivity for Differing C-HSelectivity for Differing C-H BondsBonds CHCH33CHCH22CHCH33 CHCH33CHCH22CHCH22ClCl + CH+ CH33CHCHCHCH33 Statistical (expected) 3 : 1Statistical (expected) 3 : 1 Reactivity (expected) Less (prim) More (sec)Reactivity (expected) Less (prim) More (sec) Found (25 ºC) : 43 : 57Found (25 ºC) : 43 : 57 Reactivity per H: 43/6 = 7.2 57/2 = 28.5Reactivity per H: 43/6 = 7.2 57/2 = 28.5 1 : 41 : 4 CHCH33 CHCH22 CHCH33 CHCH33 CHCH33 CHCH33CC HH prim, sec, tertprim, sec, tert ClCl -H-HClCl ClCl22, h, hvv
- 25. Transition states radical-like; reflect relative stabilities of productsTransition states radical-like; reflect relative stabilities of products For alkanes other than CH4, 1st pro- pagation step is exothermic
- 26. Tertiary?Tertiary? Statistical (expected) 9 : 1Statistical (expected) 9 : 1 Reactivity (expected) Less (prim) More (tert)Reactivity (expected) Less (prim) More (tert) Found (25 ºC) 64 : 36Found (25 ºC) 64 : 36 Normalized per H: 64/9 = 7 36/1 = 36Normalized per H: 64/9 = 7 36/1 = 36 1 : 51 : 5 Result:Result: Relative reactivity (selectivity) inRelative reactivity (selectivity) in chlorinations at 25ºC:chlorinations at 25ºC: Tert : Sec : Prim =Tert : Sec : Prim = ~ 5 : 4 : 1~ 5 : 4 : 1 CHCH33 CHCH33 CHCH33 ClClCHCH22 CHCH33 CHCH33 CHCH33 CHCH33 CHCH33CC CC ClClHHCC HH -H-HClCl ClCl22, h, hvv ++
- 27. Selectivity and OtherSelectivity and Other HalogensHalogens (CH(CH33))33CH +CH + FF22 FFCHCH22CH + (CHCH + (CH33))33CCFF 9:19:1 (CH(CH33))33CHCH ++ BrBr22 (CH(CH33))33CCBrBr only !only ! statistical !statistical !CHCH33 CHCH33 “Early TS” (CH(CH33))33CH +CH + FF22 FFCHCH22CH + (CHCH + (CH33))33CCFF 9:19:1 (CH(CH33))33CHCH ++ BrBr22 (CH(CH33))33CCBrBr only !only ! “Late TS”
- 28. Just to get a feel for the numbers……..Just to get a feel for the numbers…….. electivities vary extensively with the reagent employed, e.g., ICl, ROCl, Relectivities vary extensively with the reagent employed, e.g., ICl, ROCl, R22NNB
- 29. ProblemProblem Rank HRank H33CC∙∙, H, H22NN∙∙, and HO, and HO∙∙ in the order of increasingin the order of increasing reactivity (diminishing selectivity):reactivity (diminishing selectivity): a. Ha. H33CC∙∙, H, H22NN∙∙, HO, HO∙∙ b. Hb. H22NN∙∙, H, H33CC∙∙, HO, HO∙∙ c. Hc. H22NN∙∙, HO, HO∙∙, H, H33CC∙∙
Editor's Notes
- The first free flight of NASA’s X-43A hypersonic research aircraft. Most supersonic aircraft produce exhaust gases containing molecules such as nitric oxide (NO), whose radical reactions are destructive to the Earth’s stratospheric ozone layer. In the 1970s the United States abandoned plans to build a fleet of supersonic aircraft (SSTs, or supersonic transports) for just this reason. In contrast, the X-43A is hydrogen fueled, posing no risk to stratospheric ozone, and may represent the first step toward the development of environmentally acceptable high-speed flight
- Sept. 10, 2005: ESA: South Polar ozone hole makes big comeback/noticias.info/ This season's Antarctic ozone hole has swollen to an area of ten million square kilometres from mid-August - approximately the same size as Europe and still expanding. It is expected to reach maximum extent during September, and ESA satellites are vital for monitoring its development.This year's hole is large for this time of year, based on results from the last decade: only the ozone holes of 1996 and 2000 had a larger area at this point in their development. Envisat's Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) routinely monitors ozone levels on a global basis, continuing a dataset of measurements stretching back to mid-1995, previously made by the Global Ozone Monitoring Experiment (GOME) aboard the earlier ESA spacecraft ERS-2. ESA data form the basis of an operational near-real time ozone monitoring and forecasting service forming part of the PROMOTE (PROtocol MOniToring for the GMES Service Element) consortium, made up of more than 30 partners from 11 countries, including the Royal Dutch Meteorological Institute (KNMI). As part of the PROMOTE service, the satellite results are combined with meteorological data and wind field models so that robust ozone and ultraviolet forecasts can be made. In a first for ESA, these results are being used by the World Meteorological Organisation (WMO) to compile their regularly-updated Antarctic Ozone Bulletin. The precise time and range of Antarctic ozone hole occurrences are determined by regional meteorological variations. During the southern hemisphere winter, the atmospheric mass above the Antarctic continent is kept cut off from exchanges with mid-latitude air by prevailing winds known as the polar vortex. This leads to very low temperatures, and in the cold and continuous darkness of this season, polar stratospheric clouds are formed that contain chlorine. The stratospheric ozone layer that protects life on Earth from harmful ultraviolet (UV) radiation is vulnerable to the presence of certain chemicals in the atmosphere such as chlorine, originating from man-made pollutants like chlorofluorocarbons (CFCs). Now banned under the Montreal Protocol, CFCs were once widely used in aerosol cans and refrigerators. CFCs themselves are inert, but ultraviolet radiation high in the atmosphere breaks them down into their constituent parts, which can be highly reactive with ozone. As the polar spring arrives, the combination of returning sunlight and the presence of polar stratospheric clouds leads to splitting of chlorine into highly ozone-reactive radicals that break ozone down into individual oxygen molecules. A single molecule of chlorine has the potential to break down thousands of molecules of ozone. The PROMOTE atmospheric ozone forecast seen here has atmospheric ozone measured in Dobson Units (DUs), which stands for the total thickness of ozone in a given vertical column if it were concentrated into a single slab at standard temperature and atmospheric pressure – 400 DUs is equivalent to a thickness of four millimetres, for example. Developing out of the successful precursor Tropospheric Emission Monitoring Information Service (TEMIS), PROMOTE is a portfolio of information services covering the atmosphere part of the Earth System, operating as part of ESA's initial Services Element of Global Monitoring for Environment and Security (GMES). This is a joint initiative between ESA and the European Commission to combine all available ground- and space-based information sources and develop a global environmental monitoring capability for Europe.
- First ten entries in dist.female.first --------------------------------------- name freq cum.freq rank MARY 2.629 2.629 1 PATRICIA 1.073 3.702 2 LINDA 1.035 4.736 3 BARBARA 0.980 5.716 4 ELIZABETH 0.937 6.653 5 JENNIFER 0.932 7.586 6 MARIA 0.828 8.414 7 SUSAN 0.794 9.209 8 MARGARET 0.768 9.976 9 DOROTHY 0.727 10.703 10 JAMES 3.318 3.318 1 JOHN 3.271 6.589 2 ROBERT 3.143 9.732 3 MICHAEL 2.629 12.361 4 WILLIAM 2.451 14.812 5 DAVID 2.363 17.176 6 RICHARD 1.703 18.878 7 CHARLES 1.523 20.401 8 JOSEPH 1.404 21.805 9 THOMAS 1.380 23.185 10