Butyl fluorides

Halo-2-methylbutanes (X = F, Cl, Br) all produce the same carbocation upon dissociation, yet one of these halides is unreactive. Calculate the dissociation energies for 2-metbyl-2-butyl fluoride and 2-metbyl-2-butyl-cbloride. (Energies of fluoride and chloride are given at right.) Compare these to the dissociation energy of the corresponding bromo derivative. Which hahde is most likely to be unreactive ... 

Tertiary alkyl fluorides exhibit an additional downfield shift of about +25 ppm, which is also very sensitive to branching as seen in Scheme 3.6. The fluorine spectrum for /-butyl fluoride is shown in Fig. 3.8. The signal at -131 ppm is split into 10 peaks with a three-bond H—F coupling constant of 21 Hz. 

The proton and carbon spectra for /-butyl fluoride are provided in Figs. 3.9 and 3.10. 

A comment about the carbon NMR spectrum of t-butyl fluoride is appropriate. Because of the signal weakness of carbons such as the tertiary carbon of r-butyl fluoride, which bear fluorine but no hydrogens, many published tabulations of 13C spectra of compounds that contain such structural features fail to report these crucial signals. They can easily be missed, especially if you do not know what you are looking... 

Whether bound directly to the silicon or on a carbon bound to the silicon, a fluorine substituent within a silane is highly shielded compared to that in a hydrocarbon. For example, the fluorine of TMS fluoride absorbs more than 25-ppm upfield from that in t-butyl fluoride (Scheme 3.28). (For additional data on Si—F compounds, see Chapter 7, which deals with compounds that have heteroatom-fluorine bonds.)... 

Butyl ether, dl48 Butyl ethyl ketone, hi5 tert-Butyl fluoride, f22... 

Alkyl fluoride boron trifluoride systems were first investigated by Olah et al. (1957). 1 1 Addition complexes were observed at low temperatures, and their specific conductivity was measured. The specific conductivity of the propyl and butyl fluoride boron trifluoride systems was found three orders of magnitude larger than those of the methyl- and ethyl-fluoride systems. The latter systems on heating dissociated into their starting materials, whereas the former gave polymer mixtures (Olah, unpublished). 

The fact that the boron trifluoride t-butyl fluoride (and chloride) system and the boron trifluoride isopropyl fluoride systems on thermal decomposition yielded polymer mixtures may indicate the equilibrium in equation (1). On measuring absorption spectra of boron trifluoride—... 

Olah and co-workers in 1963 flrst observed the formation of stable alkylcarbonium ion complexes when t-butyl fluoride was dissolved in excess antimony pentafluoride (serving as both the Lewis acid and the solvent). 

In order to establish the identity of the trimethylcarbonium ion, the t-butyl fluoride-antimony pentafluoride system was investigated. It was found that when the vapour of t-butyl fluoride was passed over the surface of purifled liquid antimony pentafluoride (with exclusion of moisture and oxygen) a stable complex layer is formed on the top of the antimony pentafluoride. When this layer was separated and its proton magnetic resonance investigated (see subsequent discussion) the spectrum was found to be identical with that of the least-shielded species formed by decarbonylation of the t-butyloxocarbonium salt... 

Proton Magnetic Resonance Shifts of t-Butyl Fluoride and its Antimony Pentafluoride Complex (in SbFs)... 

For the study of the trimethylcarbonium ion, the system t-butyl fluoride. antimony pentafluoride was investigated (Olah et al., 1964b) (see Table 4). 

All the fluoroethanes are produced, but no butane is found, although fluorobutanes, ranging from butyl fluoride to perfluorobutane, have been identified. 

Butyl ethyl ketone, hl6 iert-Butyl fluoride, f20 Butyl glycol, b410 2,2 -(Butylimino)diethanol, b444 Butyl iodides, i30, i31, i44... 

The. S n reactions between HF and protonated methyl, ethyl, isopropyl, and /-butyl fluorides in the gas phase have been examined at the MP2/6-31+- -G(d,p) level of theory.112 113 The reaction of CH3FH+ clearly occurs via back-side attack as the transition state for this process is of lower energy than the transition state for frontside attack. The EtFH+ can react via a more stable back-side. S N2 reaction or an. S n 1 reaction via front-side attack since the. S N 1 pathway is 4.4 kJmol-1 lower in energy. No. S n2 path could be found for i-PrFH+ and the front- and back-side pathways had equal activation energies for /-BuFH+, which effectively reacts by an. S N1 mechanism. The conclusion is that the preference for back-side attack is reduced as the size of... 

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