Halide affinities

Alternative negative ion-based methods for measuring carbene and diradical enthalpies of formation have been developed, which can give BDEs indirectly. A common approach for this involves the use of halide affinity measurements. The relationship between enthalpy of formation and halide affinity is illustrated by Eq. 5.14. 

A variation of the halide affinity approach was used by Riveros et al. in the investigation of the enthalpy of formation of o-benzyne. Reaction of bromo- or iodobenzene with base in an ICR leads predominantly to the formation the expected M-1 anion, but also leads to the formation of solvated halide ions (Eq. 5.15). By using substrates with known halide affinities, it was possible to assign limits to the enthalpy of formation of the benzyne product. Ultimately, the experiment is comparable to that outlined in Eq. 5.14, although the acidity and halide affinity measurements are made in a single step. 

The next term in Equation (5) is the halide affinity of the metal halide, Ax (MtXJ. This is a very important term for the final value of AH4e and a term which, were it generally available, would facilitate worthwhile conclusions as to which metal halide would be the best anionogen. However, only two halide affinities for the metals of interest are known to us, viz. the fluoride affinities of BF3 and WF5 [26, 27]. In Scheme 3 the role of the halide affinity in the Born-Haber cycle for the BIE [Equation (iv)] is emphasised and an alternative route for the same energy contribution to AH4B is given. The individual BDE D(XBMt-X) is, like the halide affinity, known only for a very few examples of interest,... 

Figure 2. Activity as a function of halide affinity the same experiment as in Figure 1.

Alkyl chlorides and bromides (RX) react with Mg+ 54, Al+, Ga+ and In+ 54,55 by halide transfer whenever this process is exothermic, i.e. when the halide affinity of the metal cation... 

Many synthetic reactions, that proceed via carbocations, produce these intermediates from mixtures of alkyl halides and Lewis acidic metal halides. The concentration of carbocations produced under these conditions depends on the tendency of the alkyl halides to ionize ( carbocation stability ) and the strengths of the Lewis acids in a certain solvent. Because the ionizing abilities of alkyl halides can be derived from the schemes and correlations given in Sections B through F, we shall now concentrate on the relative halide affinities of the Lewis acidic metal halides. For this purpose, we consider Eq. (13) which describes the exchange of a chloride ion between the Lewis acids R+ and MCI,. 

Of course, the gas-phase thermochemistry of ions is not solely restricted to the measurement of the quantities described above a wide range of other ion affinities have been measured, including methyl cation affinities, hydride affinities and halide affinities . Further, such measurements can often be related to unknown neutral thermochemistry via the appropriate thermochemistry cycle. For example, the phosphorus-carbon double bond strength (the sum of the a and n bond contributions) in HP=CH2 was recently estimated via mass spectrometric measurements to be 101 7 kcaF (ref. 43). 

As such, while the concentration of active —CH2—CF2—I decreases and that of unreactive —CF2—CH2—I increases with conversion, (Figure 2.3) [51], the total (—CH2—CF2—I -I- —CF2—CH2—I) iodine functionality remains at least 95%, even at larger levels of Mn2(CO)io [51]. This is adequate for block copolymer synthesis, if both halide chain ends can be activated, and this is where the high Mn(CO)5 halide affinity [59] comes into play. Indeed, as seen above, Mn(CO)5" was able to activate not only the CF2—I based initiators, but even the CH3—I, CH3—( 2)5—I, as well as H—CF2—CF2—CH2—I models of the reverse PVDF—CF2—CH2—12,1-chain end... 

---