Bromides reactivity order

As usual, the reactivity order with respect to halogen in C=C-X is I > Br > Q. The organic halide in the couplings with alkynylzinc halides is usnaily an iodide, though also activated vinylic bromides (e.g. BrCH=CHCOOC2H5) have been found to react smoothly. We... 

Problem 7.50 List the following alkyl bromides in order of decreasing reactivity in the indicated reactions. 

Cyclopentadienyl iodide (5-iodo-l,3-cyclopentadiene) reacts approximately 10 times as rapidly as cyclopentenyl iodide (3-iodocyclopentene) with tetrabutylammonium bromide under the same conditions (Breslow, R. Canary, J. W., J. Am. Chem. Soc., 1991, 113, 3950). Under solvolysis conditions (SnI), the reactivity order is reversed. Provide a rationalization based on the frontier orbitals of the system. What would you predict for the analogous 3- and 7-membered ring systems ... 

The reactivity order of the halides is I > Br > Cl F. Whereas magnesium and lithium react well with chlorides, bromides, and iodides, zinc is satisfactory... 

A notable feature in all these coupling protocols is that the coupling rates of iron-phosphorus systems, of the (salen)iron complex 5, the Fe(acac)3 catalyst, and catalyst 10 with respect to the alkyl halide are rather uncommonly bromide> iodide>chloride (entries 3, 4, 9, 13), whereas the reactivity order for iron-amine catalyst systems is iodide>bromide>chloride (entries 1, 5, 6). 

Generally, the reaction rates of aryl halides follow the order iodides > bromides > chlorides > fluorides. This fact can be used for the selective substimtion in polyhalogenated systems. For instance, 2-bromo -chlorotoluene gives 76% of 5-chloro-2-methylphenol by treatment with sodium hydroxide at 200 °C. Nevertheless, polyhalogenated systems which contain fluorides have a variable behaviour depending on the reaction temperature. At lower temperatures preferential hydrolysis of the fluoride takes place and at >200 °C the usual reactivity order iodides > bromides > chlorides > fluorides is observed. For instance, l,2-dibromo-3,4,5,6-tetrafluorobenzene affords 2,3-dibromo-4,5,6-trifluorophenol in 87% yield by treatment with potassium hydroxide at 85 °C. Under the same conditions, 1,4-dibromo-2,3,5,6-tetrafluorobenzene produces a 78% yield of 2,5-dibromo-3,4,6-trifluorophenol. However, 4-fluorobromobenzene with NaOH at 200 °C gives 4-fluorophenol in 70-79% yield. ... 

The SET reactions can be initiated by organic anions such as carbanions and heteroatom anions, reducing organic reagents such as tetrakis(N,N-dimethylamino)ethene, low valent metal, and electrochemical reduction. Irradiation by UV light is sometimes useful for the acceleration of SET reactions, but it is not necessarily required when good electron donors, for instance thiolates instead of thiols, are used. The chain reactions of, in particular, less reactive perfluoroalkyl bromides are inhibited by species such as nitrobenzene, para-dinitrobenzene, and hydroquinone [3,6]. The reactivity order of perfluoroalkyl hahdes... 

Arrange the following alkyl bromides in order of decreasing reactivity in an 8 2 reaction l-bromo-2-methylbutane, l-bromo-3-methylbutane, 2-bromo-2-methylbutane, and 1-bromopentane. 

Reduction of halides. The TiCl3 3THF Mg system reduces bromides and iodides almost quantitatively at 20°. The reactivity order is I>Br>Cl>F. Alkenes are by-products in the reaction of aliphatic alicyclic halides. The source of the hydrogen that replaces halogen is not knovirn, but may be THF. 

The replacement of an a hydrogen of an alkyl halide by halogen decreases Sn2 reactivity. Chloroform, however, is about one thousandfold more reactive in basic hydrolysis than methylene chloride . Every bromine-containing halo-form studied (Table 7) is at least 600 times as reactive toward hydroxide ions in 66.7% aqueous dioxan as methylene bromide ". Toward weakly basic nucleophiles, such as thiophenoxide ion, the predicted reactivity order is obeyed haloforms have been found to be less reactive than the corresponding methylene halides . The reaction of haloforms with sodium thiophenoxide is strongly accelerated, however, by the presence of hydroxide ions - . These observations are quite unexplainable in terms of scheme (22). 

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