F2 Taming the Dragon Called φθόριο Huaiju Wang Joint Group Meeting Mar. 30, 2018 Fragile organic molecules Mineral Sources of Fluorine Fluorite (fluorspar) Fluorapatite Cryolite CaF2 Ca5(PO4)3F Na3AlF6 Antozonite CaF2 Kraus, F.; et al. ACIE 2012, 51, 7847 History of Fluorine I think hydrofluoric acid My glass got corroded! has H and another 1771 element similar to Cl. Circa 1810 Carl Wilhelm Scheele Humphry Davy 1764 1810 Andreas Sigismund Marggraf André-Marie Ampère Mine too! It must be Let's call it "fluor-ine", an acid, say, "fluss- because, you know, spats-syran." chlor-ine. History of Fluorine Electrolysis of KHF2•HF 1886 in cooled platinum/iridium cell Henri Moissan Air F2 Cl2 History of Fluorine UF6 and fluorocarbons Freon and Teflon Circa 1940 Circa 1930 Manhattan Project GM and DuPont 4614 BIGELOW,PEARSON. COOK AND MILLER,JR. Val. 55 [CONTRIBUTION FROM THE CHEMICAL LABORATORYOB DUKE UNIVERSITY] The Action of Elementary Fluorine upon Certain Aromatic Organic Compounds under Various Conditions. I BY LUCIIJSA. BIGELOW,J. HERBERTPEARSON, LOUIS B. COOKAND WILLIAMT. MILLER,JR. Fluorine is the most reactive, as well as the most electronegative, of all the elements, and it is surprising, therefore, to find that the direct fluorination of organic compounds has received scant attention from Early Attempts with F2 earlier investigat0rs.l This is due in part to the enormous reactivity of the fluorine, which combines explosively with most organic compounds when the two are brought in contact; and in Dart also to the fact that the reactions of free fluorine, at least upon aromatic compounds, do F O O not seem to follow well-recog-F2 Me OH nized linesMe whichOH might be pre- O CCl , 0 ºC 4 F dicted by analogy and studied OH readily. The work to be de- scribed in this paper representsF2 grey solid the initial stage of a careful yellow solid white solid study of the controlled reactionsCCl4, 0 ºC of elementary fluorine upon a variety of aromaticCl organic com- F Cl Cl F Cl Cl pounds, under a number ofF dif-2 F Cl fering types of experimental Cl F Cl Cl CCl , 0 ºC conditions. 4 F Cl Cl Cl F The Apparatus F Cl The fluorine was generatedF2 by C Cl Cl the electrolysis2 6of molten potassium Cl F bifluoride, at 2503OOo, invapor an electro- over Cu Cl lytic cell made of heavy copper, and F illustrated in detail in Fig. 1. FThis2 F F generator wasC2 Hdesigned6 several years F ago by Paul M. Gross andvapor J. S. Buckover Cu F F in this Laboratory, and was similar in principle to several which have been described previously in thv. Iircmture. It dilfcred significantly from there. however. in that thecopper pipe which held the graphite anode also 5crved a5 the wit for the fluorine, and wzs supponcd by mmnc of H fluorite ring, which was hoth insulating. and enrlre~y Bigelow, L. A.; et al. JACS 1933 – 1938 inert. even under severc upernring conditions. The gac, :dtcr heing %elfree at rheanode, was passed through a copper tuhe approximatrly 63 X 3.5 cm. filled with anhydrous (I)'The most imporia~tlht~r~~ur~ irlercncc~ OIC as lollv~~Umsm Lum.9, rrnd , la. 1513 (1886): 10s. 102 256 BOIJ (Ip.80). llumiitoa. J. I'hjr Chcn , 23. 572 1'*1.1., Fichtrr md llllmpr* lido Chim. .t<m 9, wr1 (tWG,, Bancroft and Jsm. l'r~niLlrriwhrm S,r 63. 181 (1929 Din- crolt and Whcnrty. I'roc. Sit. :l'd See., 11. IS3 (1531); Wbc3rly. J Ph,.i Cl,r!n. 36, :XI21 (1931 Rulf and Krim. 1. axorg Chm.. POI. 245 (1931). A Thermodynamic Impasse gas phase H3C H Cl Cl H3C Cl H Cl ΔrHº = –24 kcal/mol gas phase H3C H F F H3C F H F ΔrHº = –103 kcal/mol gas phase H3C CH3 2 CH3• ΔrHº = 90 kcal/mol gas phase 1 H3C H /2 O O H3C OH ΔrHº = –27 kcal/mol gas phase H3C H 2 O O O C O 2 H OH ΔcHº = –192 kcal/mol “The action of fluorine on a carbon compound can be likened to a combustion process where the products are carbon tetrafluorides and hydrogen fluoride” – Alan M. Lovelace Enthalpy of reaction calculated from JANAF table A Thermodynamic Trick gas phase H3C H F F H3C F H F ΔrGº = –103 kcal/mol Initiation gas phase F F 2 F• ΔrGº = 30 kcal/mol gas phase H3C H F F CH3• F• H F ΔrGº = –6 kcal/mol Propagation gas phase H3C H F• CH3• H F ΔrGº = –36 kcal/mol gas phase CH3• F F H3C F F• ΔrGº = –68 kcal/mol Termination gas phase CH3• F• H3C F ΔrGº = –98 kcal/mol gas phase 2 CH • 3 H3C CH3 ΔrGº = –70 kcal/mol Lagow, R. J.; et al. Prog. Inorg. Chem., 1979, 26, 161 Chambers: Fluorine in Organic Chemistry Final Proof 4.8.2004 6:47pm page 92 92 Chapter 4 Chambers: Fluorine in Organic Chemistry Final Proof 4.8.2004 6:47pm page 92 3 X ½ 92 Chapter 4 Kinetics to the Rescue Figure 4.1 168 RICHARD J. LAGOW and JOHN L. MARGRAVE I are more demanding than isopropyl groups. Indeed, van der Waals3 volumes, ˚ 3 repulsion˚ 3X ½ CH3 16:8 A compared to CF3 42:6 A , further illustrateF this point [5] (Figure 4.2). ¼ ¼electron density 5 ½ Figure 4.1 H F H H F F are more demanding than isopropyl groups. Indeed, van der Waals volumes, ˚ 3 ˚ 3 CH3 16:8 A compared to CF3 42:6 A , further illustrate this point [5] (Figure 4.2). ¼ I ¼ I I I 5 16.8A3 I 42.6AI 3 ½ Fig. 1. The stericH protection of the carbon backbone by fluorineF of a polytetrafluoro- ethylene chain.H The helicalH configuration of fluorine with a repeatF distance of 16.8 K Figure 4.2 (- - - -) results from the steric crowding of adjacent fluorine.F The nonbonding electron cloud of the attached fluorine atoms would tend to repel some of the3 incident fluorine molecules as they 3approach the carbon Table 4.1 skeleton.Steric parametersThis16.8A reduces the [4]number of effective collisions,42.6A making it possible to increase the total number of collisions and still not accelerate the reaction rate as Figure 4.2 the reaction proceeds toward completion. This sheath of fluorine atoms is one of Substituent Taft Es Charton n the reasons for the inertness of Teflon and other fluorocarbons and also explains the greater success commonly reported in the literature when the hydrocarbon Table 4.1 Steric parameters [4] H to be fluorinated is partially fluorinated in advance1.24 by some other process or is 0 prechlorinated. þ F Substituent Taft0.78E Charton n — þ s OH C. Kinetic Control of the Reactions0.69 of Elemental Fluorine — H þ1.24 0 CH3 F þ 0.780 — 0.52 The most crucial element in the controlþ of direct fluorination, a kinetic CH2CH3 OHconsideration, is the dilution technique. In 0.69most0.07 previous work on reactions — of 0.56 CH elemental fluorine, dilution with nitrogenþÀ or0 helium has been employed. 0.52 How- CH CH3 2 3 0.47 0.76 ð ÞCH2ever,CH3 the concentration of fluorine in theÀ 0.07reactor has been kept at a 0.56constant C CH3 value (usually 10% or greater) by introducingÀ 1.54 a specified mixture of fluorine and 1.24 3CH CH3 0.47 0.76 ð Þ ðnitrogen,Þ2 for example, a 10: 1 nitrogen-to-fluorineÀÀ ratio, relatively rapidly into a CH2F C CH3 1.540.24 1.24 0.62 ð reactor.Þ3 Such a dilution scheme correspondsÀÀ to the horizontal straight line in CHF CH2F 0.240.67 0.62 0.68 2 Fig. 2. The rate of reaction between aÀ hydrocarbon compound and a 10% CHF2 À0.67 0.68 CF3 fluorine mixture is relatively high and Àthe 1.16very exothermic process leads to 0.91 CF3fragmentation (Table 111) and in some cases,À1.16 to combustion. The initial 0.91stages of reaction are most critical and nearly all theÀ fragmentation occurs at this time. A III ELECTRONICIII ELECTRONIC EFFECTS EFFECTS OF OF POLYFLUOROALKYL POLYFLUOROALKYLChambers, R. D. Fluorine in Organic Chemistry, Blackwell Publishing, 2004 GROUPS [6] GROUPS [6] In this section we will deal with the effects of a polyfluoroalkyl group as a whole attached In this sectionto a saturated, we will and deal therefore with not the formally effects charged, of a polyfluoroalkyl carbon atom. The effect group of fluorine as a whole and attached to a saturated,fluorinated and therefore groups directly not bonded formally to reaction charged, centres carbon such as atom. intermediate The effect carbocation of fluorine and and carbanion sites will be treated in separate sections. fluorinated groups directly bonded to reaction centres such as intermediate carbocation and carbanionA Saturated sites will besystems treated in separate sections. 1 Strengths of acids A SaturatedAs fluorine systems is the most electronegative element, it could be expected that the introduction of a fluorine atom or polyfluoroalkyl group into the carbon chain of an organic acid, such 1 Strengths of acids As fluorine is the most electronegative element, it could be expected that the introduction of a fluorine atom or polyfluoroalkyl group into the carbon chain of an organic acid, such Elemental Fluorine as an F Source With Enough Dilution F 10% (v/v) F2/N2 CFCl3, –78 ºC Me OAc 10% (v/v) F2/N2 Me OAc 0.1 eq. PhNO2 Me H excess NaF Me H 50% H H F H CFCl3/CHCl3, –25 ºC AcO AcO H H Me 10% (v/v) F /N Me Me 2 2 0.1 eq.