Fluorine hydrogen-bond acceptor

The hydrogen bonds in aliphatic alcohol clusters can be modified in a systematic, yet subtle, way by replacing hydrogen atoms of the alkyl group by fluorine atoms [248, 249]. This leads to only modest changes in spatial extension, but it introduces polarity into the hydrophobic alkyl chains. Despite their polarity, the fluorine atoms are not considered to be attractive hydrogen bond acceptors [250]. Huorinated alkanes have quite remarkable properties that can be related to this combination of polarity and weak hydrogen bond propensity. Alcohols with... 

A second potential reaction path (C(2), TS(e ), Fig. 15) for twofold HFIP activation was calculated, which differs from TS(e ) with regard to the hydrogen bond from H O to HFIP. Here, a fluorine atom of the trifluormethyl group serves as hydrogen bond acceptor, and not a second hydroxy function. Both transition states are similar in energy but the corresponding precomplex C(2), consisting of H O and two HFIP molecules, lies 18.4 kJ moF above C(2). 

The substitution of hydroxyl groups by fluorine atoms has also been an option, considering that this alteration can be regarded a polar deoxygenation and that the fluorine atom may still serve as a hydrogen-bond acceptor or by exerting dipole and... 

Figure 7.11 Increase of the hydrogen bond acceptor ability of fluorine through the anomeric effect of geminal fluorinated atoms.

Table 6.2 SMARTS patterns for chemical features (HBA-F, hydrogen bond acceptor/electrostatic fluorine interaction HBD, hydrogen bond donor PI, positive ionizable Nl, negative ionizable)...

Avs(OH) = 216 cm-1 and with (OHO) = 29 kJ mol1 calculated from the temperature dependence of the lH chemical shift (Owen, 1989). Normally, however, F forms strong hydrogen bonds. This is in sharp contrast to fluorine s hydrogen-bond acceptor properties when it is covalently bonded then it is among the weakest acceptors. 

Summary of organic fluorine as hydrogen-bonding acceptor... 

Meanwhile, fluorinated alcohols cannot be good proton acceptors due to their electron-deficient lone pairs on the oxygen atoms. The (3 scale of both TFE and HFIP is 0.00, which is apparently smaller than those of ethanol (0.77), ether (0.47), water (0.18), or even toluene (0.11) [33]. Here, the (3 scale, i.e. the hydrogen-bond acceptor basicity of a solvent, describes the ability to accept a proton (donate an electron pair) in a solute-solvent hydrogen-bonding system [4]. These acidities (pFCa [31, 34]), hydrogen-bonding parameters (a and (3 scale... 

---