Halides alkyl iodides

Among alkyl halides alkyl iodides undergo nucleophilic substitution at the fastest rate alkyl fluorides the slowest... 

Compounds, eg, phenacyl halides, benzyl halides, alkyl iodides, or alkyl esters of sulfonic acids, react with DMSO at 100—120°C to give aldehydes (qv) and ketones (qv) in 50—85% yields (eq. 8) (41) ... 

Grignard reagents may be made from primary, secondary, and tertiary alkyl halides, as well as from vinyl and aryl halides. Alkyl iodides are the most reactive halides, followed by bromides and chlorides. Alkyl fluorides generally do not react. 

Direct Reaction of Zn with Alkyl Halides. The direct insertion see Insertion) reaction of Zn metal into alkyl halides - alkyl iodides being the ideal snbstrates - is a nseful reaction to prepare simple or polyfunctional organozinc halide compounds (equation 1). With primary alkyl iodides, the reaction requires an excess of Zn dnst (ca. 3 eqniv), previonsly treated with few mol % of 1,2-dibromoethane and TMSCl, and a temperature of 40 °C in THF. In these conditions, secondary alkyl iodides react at room temperatnre and benzylic and allylic bromides at 0 °C. The insertion see Insertion) into less activated C-X bonds may reqnire more reactive forms of zinc (Riecke zinc), higher temperatures, or the use of polar see Polar Compounds) solvent or cosolvent. 

Selective phosphonate ester dealkylation. Alkyl phosphonate esters are selectively and nearly quantitatively cleaved by bromotrimethylsilane in the presence of alkyl carboxylate esters, carbamates, acetylenes, ketones, and halides. Alkyl iodides do not exchange under the reaction conditions. The resulting bis(trimethylsilyl) phosphonates are hydrolyzed in acetone by a small excess of water. 

Particularly alkyl halides which have a perfluoroalkyl group at the /3-position undergo smooth carbonylation. Probably the coordination of fluorine to form a five-membered chelate ring accelerates the reaction. Double carbonylation to give the a-keto amide 915 is possible in Et NH with the fluorine-bearing alkyl iodide 914[769,770]. The ester 917 is obtained by the carbonylation of the /3-perfluoroalkyl iodide 916 in ethanol. 

The order of alkyl halide reactivity in nucleophilic substitutions is the same as their order m eliminations Iodine has the weakest bond to carbon and iodide is the best leaving group Alkyl iodides are several times more reactive than alkyl bromides and from 50 to 100 times more reactive than alkyl chlorides Fluorine has the strongest bond to car bon and fluonde is the poorest leaving group Alkyl fluorides are rarely used as sub states m nucleophilic substitution because they are several thousand times less reactive than alkyl chlorides... 

The alkyl halide can be primary secondary or tertiary Alkyl iodides are the most reac tive followed by bromides then chlorides Fluorides are relatively unreactive... 

Methyl and primary alkyl halides especially iodides work best Elimination becomes a problem with secondary and tertiary alkyl halides... 

Differences in solubility of the reactants may for example be utilized as follows. Sodium iodide is much more soluble in acetone than are sodium chloride or sodium bromide. Upon treatment of an alkyl chloride or bromide with sodium iodide in acetone, the newly formed sodium chloride or bromide precipitates from the solution and is thus removed from equilibrium. Alkyl iodides can be conveniently prepared in good yields by this route. Alkyl bromides are more reactive as the corresponding chlorides. Of high reactivity are a-halogen ketones, a-halogen carboxylic acids and their derivatives, as well as allyl and benzyl halides. 

With secondary and tertiary alkyl halides an Ea-elimination is often observed as a side-reaction. As the alkyl halide reactant an iodide is most often employed, since alkyl iodides are more reactive than the corresponding bromides or chlorides. With phenoxides as nucleophiles a C-alkylation can take place as a competing reaction. The ratio of 0-alkylation versus C-alkylation strongly depends on the solvent used. For example reaction of benzylbromide 4 with /3-naphth-oxide 5 in yV,A-dimethylformamide (DMF) as solvent yields almost exclusively the /3-naphthyl benzylether 6, while the reaction in water as solvent leads via intermediate 7 to formation of the C-benzylated product—l-benzyl-2-naphthol 8—as the major product ... 

The alkylation reaction is limited to the use of primary alkyl bromides and alkyl iodides because acetylide ions are sufficiently strong bases to cause dehydrohalogenation instead of substitution when they react with secondary and tertiary alkyl halides. For example, reaction of bromocyclohexane with propyne anion yields the elimination product cyclohexene rather than the substitution product 1-propynylcyclohexane. 

Phenylsulfonyl)nitromethane is preferentially C-alkylated by benzylic halides and primary alkyl iodides, affording secondary a-nitrosulfones338. 

Halide exchange, sometimes call the Finkelstein reaction, is an equilibrium process, but it is often possible to shift the equilibrium." The reaction is most often applied to the preparation of iodides and fluorides. Iodides can be prepared from chlorides or bromides by taking advantage of the fact that sodium iodide, but not the bromide or chloride, is soluble in acetone. When an alkyl chloride or bromide is treated with a solution of sodium iodide in acetone, the equilibrium is shifted by the precipitation of sodium chloride or bromide. Since the mechanism is Sn2, the reaction is much more successful for primary halides than for secondary or tertiary halides sodium iodide in acetone can be used as a test for primary bromides or chlorides. Tertiary chlorides can be converted to iodides by treatment with excess Nal in CS2, with ZnCl2 as catalyst. " Vinylic bromides give vinylic iodides with retention of configuration when treated with KI and a nickel bromide-zinc catalyst," or with KI and Cul in hot HMPA." ... 

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