Reduction reaction kinetics alkyl halides

The kinetics for the reduction of TCA in the presence of zinc or iron in addition to iron treated with catalytic amounts of nickel or copper are summarized in Table 7.10. Small amounts of nickel or copper (0.035 mol%) on the iron enhance the reaction rate. The reduction of other alkyl halides at ambient temperatures has been reported (Table 7.11). Iron (10 g surf, area 0.287 m g ) was added to 40 mL of an aqueous solution of the compound giving a surface area-to-volume ratio of... 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

The reactivity of 02 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution. Kinetic studies confirm that the reaction order is primary > secondary > tertiary and I > Br > Cl > F for alkyl hahdes, and that the attack by 02 - results in inversion of configuration (Sn2). Superoxide ion also reacts with CCI4, Br(CH2)2Br, CeCle, and esters in aprotic media. The reactions are via nucleophilic attack by 02 on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxyanions, water suppresses the nucleophilicity of 02 (hydration energy, lOOkcalmoL ) and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO and ROO ). 

A principal mechanistic question for alkyl halide reductions has been the degree of bonding of the metal to the halide at the transition state, that is, whether the reaction is inner-sphere in the Taube sense (see Scheme 1). The Ms symbol refers to solid metal, and less is known about reactions that occur at solid surfaces than those in solution because of problems with studying kinetics (unless the surface is an electrode). Rieke and coworkers emphasize quantities that can be measured, the ln(A Rx/A -Rx) for secondary (j-RX) and tertiary (/-RX) halides relative to primary ones (n-RX), which they summarize for 14 reactions [31]. The Co /RI, Cr /RBr, and Cr 7 Cl cases are completely established as Taube inner-sphere reactions [10]. They show the largest selectivity, ln(A ,-Rx/A -Rx) and ln(A 5-Rx)/A n-Rx) values of over three and one, respectively. At the other extreme, reaction of RBr with magnesium is extremely unselective [41], and is now usually accepted to be outer-sphere. 

Reduction of alkyl halides. LiAlH, is satisfactory for reduction of primary and secondary halides or tosylates to the hydrocarbons, but in the case of a tertiary halide the product is predominantly the olefin. Alkyl halides and tosylates, even tertiary, are reduced in good to high yield by sodium borohydride in 65% aqueous diglyme. When a relatively stable carbonium ion incapable of elimination is formed, yields are high, but yields are still satisfactory when elimination is possible. The reaction is very slow in the absence of water. A homogeneous solution required for kinetic studies is prepared from 80% (volume) aqueous diglyme, which can be made 1.80 M in the reagent. 

This reaction often occurs with Co(II) or Rh(II) systems, which are excellent one-electron reductants and poor two-electron reductants. For example, the 17-electron, d Co(II) complex KCo(CN)j reacts with alkyl halides by this path (Equation 7.9). These reactions display second-order kinetics, with the rate decreasing in the order RI > RBr >> RCl. Only activated halides such as X-CHjCOjMe react. Alkyl tosylates do not. ... 

A type of reaction of considerable mechanistic interest in the kinetics of electrochemical and solvolytic reactions is the reduction of alkyl halides RX where a nucleophilic substitution type of reaction occurs with the electrode acting as the nucleophile. The anion X and the hydrocarbon RH are the main products. In some cases, organometallic intermediates are produced, e.g., in the reduction of alkyl iodides or bromides at Pb. Here the process which leads to lead tetraalkyls is of considerable commercial significance. 

Reactions.— DMF is recommended as solvent for the sodium borohydride reduction of primary and secondary alkyl halides to the corresponding hydrocarbons this reaction, which proceeds at room temperature, shows the kinetic behaviour of an 5 fj2 process. The intermediacy of organoboranes in the analogous reduction of tertiary halides has been demonstrated "... 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

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