Aliphatic halides, electron attachment

Jorge Ayala determined the rate constants for thermal electron attachment to aliphatic halides and the halogen molecules to confirm values measured by other techniques. The electron affinities of the halogen molecules had been determined by endothermic charge transfer experiments [57-59]. In the case of the halogen molecules, the ECD results lead to the rate constant for thermal electron attachment rather than the electron affinity of the molecule. Two-dimensional Morse potentials for the anions were constructed based on these data. Freeman and Ayala searched for a nonradioactive source for the ECD. In 1975 the data on the electron affinities of atoms were summarized and correlations examined between these values and the position of the atoms in the Periodic Table [60]. A large number of the atomic electron affinities were measured by photoelectron spectroscopy [61]. A similar compilation of the electronegativities of elements was carried out. In this case some of the values were obtained from the work functions of salts [62], These results will be updated in Chapter 8. 

Phenols attached to insoluble supports can be etherified either by treatment with alkyl halides and a base (Williamson ether synthesis) or by treatment with primary or secondary aliphatic alcohols, a phosphine, and an oxidant (typically DEAD Mitsu-nobu reaction). The second methodology is generally preferred, because more alcohols than alkyl halides are commercially available, and because Mitsunobu etherifications proceed quickly at room temperature with high chemoselectivity, as illustrated by Entry 3 in Table 7.11. Thus, neither amines nor C,H-acidic compounds are usually alkylated under Mitsunobu conditions as efficiently as phenols. The reaction proceeds smoothly with both electron-rich and electron-poor phenols. Both primary and secondary aliphatic alcohols can be used to O-alkylate phenols, but variable results have been reported with 2-(Boc-amino)ethanols [146,147]. 

Although the mechanism is not understood, evidence strongly suggests this much the alkyl group R is transferred from copper, taking a pair of electrons with it, and attaches itself to the alkyl group R by pushing out halide ion (nucleophilic aliphatic substitution. Sec. 14.9). 

A halo group attached to a carbon which has a heteroatom attached to it has increased reactivity. This concept is amply explained by the electronic assistance of the heteroatom. For example, the halogen groups in a-haloethers and a-chloroamines are considerably more reactive than halo groups attached to aliphatic carbon atoms. The reactivity of halo groups is further enhanced if the halo group is directly attached to a C=0 double bond. For example, acid chlorides (III) are considerably more reactive than alkyl or aryl halides. Likewise, halo groups directly attached to C=N double bonds have an increased reactivity, quite comparable to that of acid chlorides. Thus imidoyl chlorides (IV) resemble acid chlorides in their chemical reactions. 

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