Stability halogens

For example, bromination of pentan-3-one gives mostly 2,2-dibromopentan-3-one. After one hydrogen is replaced by bromine, the enolate ion is stabilized by both the carbonyl group and the bromine atom. A second bromination takes place faster than the first. Notice that the second substitution takes place at the same carbon atom as the first, because that carbon atom bears the enolate-stabilizing halogen. 

To reduce dust and increase stability, halogenated biocides or other products, such as Oxone, are often pressed into tablets (Fig. 6.3-16) of different sizes and shapes... 

Aluminum should not be used as a material of construction for pumps, tanks, pipelines, valves, spray equipment and other handling equipment used for chlorinated solvents. Properly stabilized halogenated solvents are however commonly used in cleaning aluminum and other sensitive metals. 

Lee et al. (1977) report alopecia in rats exposed to a vapor of phenyl glycidyl ether, a material used in industry to stabilize halogenated compounds. Oral administration of this agent causes profound systemic symptoms and death from central nervous system depression. Controlled exposure to various levels of vapor caused no systemic effects in rats and dogs, but there was hair loss in the rats, predominantly in females. The hair loss was due to damage to anagen follicles, which caused abnormal keratinization of the hair shaft, and to increased conversion to telogen follicles. Lee et al. postulate that the effect was from percutaneous and follicular absorption, not from inhalation. The reliance of this data in man requires documentation. 

Pb(CH3)4 can be applied to extract aluminium and alkylaluminium alkoxide impurities from organoaluminium complexes used in the electrolytic preparation of tetraorganolead compounds [177]. Pb(CH3)4 was claimed to prevent corrosion of graphite by CO2 used as fluid coolant in nuclear reactors [178], and to stabilize halogenated aryl compounds, employed as dielectric, insulating, or cooling agents [179]. 

Uses Intermediate for pharmaceuticals stabilizer halogenated hydrocarbons surf, act. compd. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

Like hydroxyl groups and ammo groups however halogen substituents possess unshared electron pairs that can be donated to a positively charged carbon This electron donation into the TT system stabilizes the intermediates derived from ortho and from para attack... 

Comparable stabilization of the intermediate leading to meta substitution is not possible Thus resonance involving halogen lone pairs causes electrophilic attack to be favored... 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

Powdered antimony pentoxide is used primarily in plastics. Stabilizers used to prevent the particles from growing are caustic, and can react with the halogen in the formulation. This can result in color formation and a lower flame-retarding efficiency of the system. 

There are a relatively small number of producers of halogenated flame retardants, especially for brominated flame retardants, where three producers account for greater than 80% of world production. Table 10 gives estimates of the volumes of brominated and chlorinated flame retardants used worldwide. Volumes of flame retardants consumed in Japan have been summarized (61). Prices of halogenated flame retardants vary from less than 2.00/kg to as high as 13.00/kg. Cost to the user depends on the level of use of the specific flame retardant and other factors such as the use of stabilizers. 

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