Reactive halogen atoms

Finally — and this has to do with the nature of the bond strengths in such metal-metal bonded systems — the dimer also thermally dissociates. With a very reactive halogen atom donor,... 

A very reactive halogen atom, such as that of mi acyl or sulfonyl halide, is replaced by fluorine by the action of almost any inorganic fluoride. The most convenient method consists in heating gently a mixture of an acyl or sulfonyl chloride with zinc or antimony fluoride in an apparatus which permits the acyl fluoride to distil as it is formed. The acyl fluoride usually boils about 40° lower than the chloride, and its removal from the reaction mixture results in quantitative yields. Com- plete interchange also can be effected with hydrogen fluoride, but more elaborate equipment is required. Good results have been reported for the synthesis of formyl and acetyl fluorides from mixtures of fonnic or... 

The same tetraacetate (LXXI) was desulfurized, with Raney nickel to 2,3,4,6-tetra-0-acetyl-l,5-anhydro-D-glucitol (LXXIV). The requirement of a reactive halogen atom (in the carbohydrate component) rather limits the scope of this synthesis, but some further examples were supplied by Gehrke and Kohler in 1931. The reaction of 2,3,4-tri-O-acetyl-a-D-xylo-... 

Exciplex lasers (also called excimer lasers) use reactive halogen atoms to form excited pseudo-molecules with noble-gas atoms. Molecules such as XeF are stable on ly in excited electronic states and quickly dissociate after transition to the ground state. This makes possible a large population inversion and produces laser action in the ultraviolet region. A simple prototype for such behavior is the Hcj excimer, which is an entry in Table 11.1. 

The alkylation of 2-pyrazolin-5-ones at C-4 occurs readily (eq. 31) ii3, i32i wjih compounds having reactive halogen atoms. An example... 

Reduction of heterocyclic or aromatic ketones containing reactive halogen atoms are accompanied by exchange of these halogens nitro compounds, which are stabilized as aci-nitro compounds in alcoholic solution, are not reduced and in certain circumstances ether groups may be cleaved. 

The Acyl Chlorides.—These compounds are chiefly of value as reagents for the testing of amines, alcohols, and phenols. When an unknown containing a very reactive halogen atom is suspected of being an acyl halide, the usual experimental conditions arc reversed and a known amine is used as a reagent for the unknown. 

Only poly (organophosphazenes), obtained through the replacement of the highly reactive halogen atoms in poly-102 and poly-106 (X=F) by organic groups, can be applied as stable and useful materials. This is for some derivatives... 

The nitro-hydrocarbons are neutral substances but when a nitro-group is introduced into a phenol or amine the acidic properties are greatly increased or the basicity decreased. The presence of a nitro-group also tends to make halogen atoms in the same molecule much more reactive. 

The Alkyl Halides. Ethyl bromide and iodide (see below) are typical alkyl halides. Compounds of this class are of very great importance in synthetic work, owing to the reactivity of the halogen atom. This is illustrated by the following reactions ... 

The iodine atom in iodobenzene (unlike that in the corresponding aliphatic compounds) is very resistant to the action of alkalis, potassium cyanide, silver nitrite, etc. This firm attachment of the iodine atom to the benzene ring is typical of aromatic halides generally, although in suitably substituted nitio-compounds, such as chloro-2,4-dinitrobenzene, the halogen atom does possess an increased reactivity (p. 262). 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

DinitrophenyI derivatives. The halogen atom in 2 4-di-nitrochlorobenzene is reactive and coloured crystalline compounds (usually yellow or red) are formed with primary and with secondary amines ... 

Another teat, which indicates the reactivity of the halogen atom (chlorine and bromine), is based upon the fact that sodium chloride and sodium bromide are sparingly soluble in pure acetone ... 

Silicon halides are typically tetrahedral compounds. The siUcone—halogen bond is very polar thus the siUcon is susceptible to nucleophilic attack, which in part accounts for the broad range of reactivity with various chemicals. Furthermore, reactivity generally increases with the atomic weight of the halogen atom. 

The reactions of haloquinoxalines in which the halogen atom is bonded to the benzenoid ring have not been well studied, but by analogy with examples in the phenazine series it would seem probable that they are unlikely to be displaced with the same ease as those bonded directly to the heterocyclic ring. It is evident from the foregoing discussion that A-oxidation has a pronounced effect on their reactivity, and, by this means, considerable latitude in the specific functionalization of dihalo or polyhalo derivatives may be exercised. 

Dihaloquinoxalines are extremely reactive and both halogen atoms are replaceable, on occasions explosively (59RTC5), whereas in the case of dihalopyrazines, and tri- or tetra-halopyrazines, there is frequently a considerable difference in reactivity of the halogen atoms. When 2,3-dichloropyrazine is treated with ammonia at 130 °C, only one chlorine atom is displaced, giving 2-amino-3-chloropyrazine (66FES799). 

Halogen atoms at the 5-position of tetrazoles are reactive and easily replaced by nucleophiles. 5-Bromo-l-methyltetrazole is significantly more reactive than the 2-methyl isomer (77AHC(21)323). 

Enhanced reactivity of halogen atoms toward -nucleophiles... 

In some cases, diene polymers (for instance polychloroprene rubbers) can add to the growing polymer chain by 1,2 addition (also called vinyl addition). This creates labile hydrogen or reactive halogen on tertiary carbon atoms. A few percent of this type of structure in the rubber will assist cross-linking reactions. 

Reactivity is enhanced in conditions which promote the generation of halogen atoms, though this does not imply that all reactions proceed via the intermediacy of X atoms. The reversible thermal dissociation of gaseous I2 v 21 was... 

The halogen atom in benz-chloro substituted quinazolines is very stable (as in chlorobenzene), whereas the halogen atoms in positions 2 and 4 show the enhanced reactivity observed with halogen atoms on carbon atoms placed a and y to heterocyclic ring nitrogens. The chlorine atom in position 4 is more reactive than in position 2, and this property has been used to introduce two different substituents in the pyrimidine ring. ... 

The effect of substituents in the benzene ring on the reactivity of the halogen atom in 4-chloroquinazolines is worthy of investigation. 

Aminoquinazolines have been the subject of considerable investigation and a large number of derivatives have been prepared as potential antimalarials. The secondary and tertiary amino compounds can be prepared from the corresponding chloroquinazolines and the required primary or secondary amines. The reaction depends on the reactivity of the halogen atom, e.g, the 4-chloro atom reacts more readily than the 2-chloro atom in quinazolines and also on the basic strength of the amine used (see 6a). The reaction is... 

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