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Fluoro-Alkane, -Alkenes, -Alkynes and –Aromatics

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Fluoro-Alkane, -Alkenes, -Alkynes and –Aromatics Fluoro-alkane, -alkenes, -alkynes and –aromatics Fluoroalkanes Properties of fluoroalkanes Consider these typical bond strengths: C-C bond 88kcal/mol C-F bond 111kcal/mol C-H bond 106kcal/mol This implies that the carbon – carbon bonds are the weakest link in fluoroalkanes. 1 Features to bear in mind: (i)Bond Strengths tend to increase as the total number of fluorines increases I.e. Bond strengths F F F F F < < F C-F bond shortens (strengthens) as more F's are added 2 The C-F bond shortens (gets stronger) as more fluorines are added due to an increase in back bonding (halo effect). BDE Fluorine makes the atoms close to it bond stronger also !! ↑ shorter ↑ stronger no significant changes Some C-C bond strengthening also occurs on substitution of F for H. For example the CF3-CH2CH3 bond is ~5kcal stronger than the CF3CH2-CH3 bond. (i)-(CF2)n- is more stable than -(CH2)n- due to better chemical stability. PTFE shows resistance to oxidation and microbial attack. (Generally related to BDE’s of C-H and C-F). 3 (iii) Electron Pair Repulsions (Intramolecular) are important to PTFE. 1,3 repulsions force the F atoms out of linearity, which is in contrast to the hydrogen containing polymer analogue. 4 Boiling Points Fluorocarbons are obviously heavier than their hydrocarbon counterparts, and thus we might initially expect them to have correspondingly higher boiling points. However, the boiling points of hydrocarbons and perfluorocarbons are very similar. (B.Pts in oC) # of Carbons 1 2 3 4 5 6 FC -128 -78 -38 -1 29 57 HC -161 -88 -42 -0.5 36 69 The increase in molecular weight that increases the boiling point, therefore must be offset by the decrease in attractive intermolecular bonding forces in the fluorocarbon systems. (I.e. the electron pairs on fluorines in one molecule have a tendency to repel the other fluorinated molecules). 5 Surface properties Fluorinated solids / surfaces display unusual properties. PTFE has a coefficient of friction described as “wet ice sliding on wet ice”. This low friction and oil repellency is almost certainly due to the electron pair repulsions. Chemical Inertness The electron clouds of the fluorines “screen” the carbon atoms of the chain, and thus discourage the approach and attack of incoming reactants. (The carbons are the weakest link). Thus fluorocarbons find uses as inert fluids (see later). Carbon tetrafluoride is fantastically stable and inert. But the hydrolysis of CF4 is predicted to be exothermic by 73kcal/mol. (i.e. very thermodynamically favorable). 6 So it is a large kinetic barrier to hydrolysis (i.e. a high activation energy) that causes the stability. In fact under the right conditions perfluorocarbons like PTFE can generate a lot of heat (e.g. for use in pyrotechnics or car airbag inflation). Usually this is achieved using metals → metal fluorides; an exothermic reaction). Solubility Perfluorocarbons have a profound ability to dissolve partially fluorinated compounds. Also certain fluorocarbons have excellent oxygen and nitrogen solvating ability, and prompted their application as artificial blood.(“Oxygen transport fluids”) 7 Reactions of Perfluorocarbons Generally perfluorocarbons are very inert and unreactive. The only reaction they are prone to undergo, usually under forcing conditions, is Defluorination (Note that in hydrocarbon systems, reduction = increasing saturation In fluorinated systems, reduction = increasing unsaturation) Birch Reduction Heating with strong reducing agents (electron donors) can afford defluorination. CF3 CF3 Fe F F 500oC These include zero valent metals and also organic electron transfer agents, like Thiolate. 8 The product with Thiolate anion was a little unexpected. SPh SPh PhS- PhS SPh F F PhS SPh SPh SPh 9 Electron donation to Perfluorodecalin gives a radical anion, which expels fluoride anion to leave a tertiary radical. Further reduction produces an anion which expels fluoride ion to generate a new C=C double bond. This process continues until pefluoronaphthalene is generated. As you will see later, such compounds are prone to nucleophilic attack (more precisely, Nucleophilic Aromatic Substitution), and Thiolate, being a good nucleophile, ends up substituting at every fluorinated position. 10 Fluoroalkenes Unsaturation tends to increase reactivity of fluorocarbon systems. E.g. CF3 F3C CF2 FF F3C PerFluoroIsoButene Perfluoromethyldecalin PFIB is ten times more toxic than phosgene, whereas perfluoromethyldecalin is used as artificial blood. (A slight difference !!!) 11 Preparation of Fluoroalkenes Synthesis of CF2=CF2 HF / SbF5 Industrially: CHCl3 CHF2Cl Pt, 700oC 2 CHF2Cl F2C=CF2 via carbene :CF2 dimerisation H-Cl Laboratory: By controlled heating of PTFE under vacuum, you can trap TFE. Heat, vacuum PTFE F2C=CF2 This is a unique “unzipping” of a polymer back to a monomer. 12 At greater temperatures (700°C) and pressures, higher mass compounds are formed. PTFE → CF2=CF2 + CF2=CFCF3 + CF2=C(CF3)2 F2C=CF2 HFP F2C=CF2 :CF2 F2C=CFCF3 :CF2 PFIB F2C=C(CF3)2 13 HexafluoroPropene HFP is a very important fluoroalkene for synthetic reactions since it does not homopolymerise (one of the few). Commercially is produced by the controlled pyrolysis of CF2=CF2 (see previous). In the lab it can be generated via the following synthetic scheme: O E.C.F. O 1) H2O O F2 F2 C C 2) NaOH Cl HF F3C C F F3C C O- +Na F2 F2 heat -CO2 - F - F F F CCF2 C F C F3C CF2 3 _ 14 Reactions of Fluorinated Alkenes Fluorinated alkenes are typically electrophilic, and therefore they react readily with nucleophiles. E.g. The reaction can either be nucleophilic addition or substitution. Notice that in both cases, this reaction is Regiospecific. (CH3O- attached to the CF2 end). It is important to be able to explain what factors influence the reactivity of the fluoroalkenes, and thus why these reactions are regioselective. 15 General Rules for Ionic Reactions of Fluoroalkenes F is more Cl δ+ reactive δδ+ than _ CCF strongly stabilizing inductive withdrawal CF offset by lp repulsions This highlights the different substituent effects of “fluorine” and “fluoroalkyl”. 16 Orientation of Attack The nucleophilic reactions are very specific for attack at the terminal =CF2 group. If we consider the two possible intermediate anions which would be produced by attack at either end of HFP, and consider their relative stabilities, we can explain the regioselectivity of these reactions. The RHS reaction has two favorable fluoroalkyl stabilizations and one fluorine destabilization The LHS reaction has one favorable fluoroalkyl stabilization and two fluorine destabilizations. Clearly the RHS reaction pathway is preferred. 17 This also explains the following regiochemistry with CTFE: - Cl Nuc- F Nuc - F Cl 2C CF C NucFC C F CFCl F _ F Relative Reactivity Among Fluoroalkenes CF2=CF2 < CF2=CFCF3 < CF2=C(CF3)2 TFE HFP PFIB Their corresponding intermediates (and approximately their transition states) are: → Increasing in Anion stability → Which show an increasing stability. Hence PFIB is more reactive than HFP than TFE. (PFIB reacts with neutral methanol). 18 Nucleophilic Epoxidation Sodium Hypochlorite (NaOCl) is a good nucleophilic epoxidising agent (Recall mcpba is an electrophilic epoxidising agent) 19 “Mirror Image” Chemistry Reactions with Fluoride Ion create a Mirror Image Organic Chemistry. (Nothing to do with chirality!) - (CF ) C- Chemistry Compare: F F2CC(CF3)2 3 3 + Chemistry H H2CC(CH3)2 (CH3)3C+ Oligomerisation Alkenes can be polymerized by acid, whereas fluoroalkenes can be oligomerised using fluoride ion. 20 And even for the generation of strained bicyclic systems. Such systems can give rise to long lived anions. E.g. F CsF _ F F Cs+ F 21 The anions can be observed at low temperature by 19F NMR. Negative sigma complexes can be formed and observed by NMR. E.g. FF N N N N F CsF _ N FFN Cs+ 22 Negative Friedal Crafts Processes Fluorinated systems can be used for a process analogous to FC alkylations, but using anions / nucleophiles instead of cations / electrophiles. E.g. CF3 F3C CF F F- N F2CCFCF3 (CF3)2FC- F N C3F7 C3F7 C3F7 C3F7 C3F7- F F F N C3F7 N C3F7 C3F7 N C3F7 thermodynamic kinetic (See fluoroaromatic section later for more detail.) 23 Electrophilic Attack On Fluorinated Alkenes Consider the following reaction: F3C H HSO3F CF CH CH (DIMER) 3 2 H CF3 H3C H When the reaction was performed with Deuterio-fluorosulfonic acid, there was no D incorporation. F3C H DSO3F CF CH CH 3 2 H CF3 H3C H This implies the mechanism does not involve proton (or D+) transfer from the acid. 24 Here there was no change in the location of the deuterium, which again implies that the reaction is induced by the acid, but there is no H+ transferred. F3C H HSO3F CF CD CH 3 2 D CF3 H3C D .........So how does this reaction proceed? 25 26 Notice that the +ve charge stays well away from the powerfully electron withdrawing CF3 group. There is a Hydrogen shift (hydride, :H-) to produce a more stable allylic cation. Certain fluorinated cations are observable by NMR. E.g. F CF CF SbF5 F 3 F pC H OCH SO , -60oC 6 4 3 2 + F F OCH3 The NMR shifts imply (confirm) that the +ve charge lies almost totally at C1 and C3. 27 Notice that for a positive charge: α Fluorine is strongly stabilizing β Fluorine is strongly destabilizing Therefore we would (correctly) predict that the cation derived from HFP is less stable than that derived from pentafluoropropene. F F F less stable CF3CF CF2 + F F +ve charge at C1 and C3, so an F at C2 is a β F = destabilizing H F F more stable CF3CH CF2 + F F 28 Free Radical Additions For reactivity towards free radicals we observe: CHCl=CHCl << CH2=CH2 << CCl2=CH2 This implies that substituents tend to enhance the reactivity towards free radical addition if the substituents are at the same end of the double bond.
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