Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

For SN1 reactions

Table 1 The maximum secondary a-deuterium KIEs expected for SN1 reactions with various leaving groups at 25°C.a... Table 1 The maximum secondary a-deuterium KIEs expected for SN1 reactions with various leaving groups at 25°C.a...
Hydroxyl ions increase the rate of SN2 reactions but have little or no effect on SN1 reactions. The k0H /kHi0 ratio for benzoyl chloride in 50% water/acetone is about 600, as compared with values of 104-105 for SN2 reactions and values of 102— 103 for SN1 reactions. In 25% aqueous acetone 0h / h2o is about 104 for both 4-nitro and 4-methoxybenzoyl chloride133, but increasing the water content makes ( oH-/ H2o)4-MeO ( oh-/ h2oM N02, in agreement with the... [Pg.243]

In accord with this assignment is the observation of Hudson and Moss144 that, with 95% aqueous acetone solvent, lithium perchlorate increased the rate of hydrolysis of 2,4,6-trimethylbenzoyl chloride, whereas chloride ions have no effect this is in contrast to the behaviour of 4-nitrobenzoyl chloride whose rate of hydrolysis is decreased by lithium perchlorate and increased by chloride ions. The influence of solvent sorting145 is probably small for SN1 reactions in 90% aqueous acetone and neglecting ion pairing an observed salt effect reflects the effect of the ion atmosphere on the transition state. The same observation will not hold for either dioxan/water (because of the low dielectric constant of dioxan) and acetone/water of high water content (because of the extensive solvent sorting). [Pg.247]

The rates of SN reactions are sensitive to the nature and composition of the solvent. This is easy to understand for SN1 reactions because the ionizing... [Pg.237]

Shiner (1971c) has shown that for SN1 reactions the cr-deuterium isotope effect for any particular leaving group reaches a limiting value. In our notation... [Pg.136]

Further examination of Table 8.2 shows that allyl chloride and benzyl chloride have much faster rates for SN1 reactions than would be expected for primary systems. Examination of the carbocations reveals that the reason for this enhanced reactivity is the significant resonance stabilization provided by the adjacent double bond or benzene ring. Resonance stabilization increases with the substitution of additional phenyl groups, as illustrated by the reaction rates of diphenylmethyl and triphenylmethyl chloride (Table 8.2). [Pg.273]

In Chapter 4, Sn2 reactions were defined and presented in the context of the various conditions necessary for such reactions to take place. However, as mentioned in the introductory comments of Chapter 4, there are additional fundamental mechanistic types relevant to organic chemistry that are essential to understand in order to advance in this subject. In this chapter, discussions of organic chemistry reaction mechanisms are advanced to the study of SN1 reactions. While conditions required for SN1 reactions to proceed are quite different from those essential for SN2 reactions, the products of SN1 reactions, in many cases, resemble those derived from SN2 mechanisms. Additionally, unlike SN2 reactions, SN1 reaction mechanisms allow routes for unwanted or, in some planned cases, preferred side reactions. [Pg.83]

Polar protic solvents are especially good for Sn1 reactions. [Pg.266]

Large a-deuterium KIEs were found for carbenium ion reactions and small KIEs for Sn2 reactions, and since the Crl 11(D) stretching vibration becomes stronger as the sp3 hybridized substrate is converted into the sp2 hybridized carbenium ion in an SN1 reaction, an inverse KIE, not the large normal KIEs observed for SN1 reactions, should be observed. Therefore, it was suggested that the magnitude of the KIE was... [Pg.230]

POTENTIAL ENERGY DIAGRAMS FOR MULTISTEP REACTIONS THE Sn1 mechanism... [Pg.159]

The conclusion that SN1 reactions on enantiomerically pure substrates should give racemic products is nearly, but not exactly, what is found. In fact, few S jl displacements occur with complete racemization. Most give a minor (0%-20%) excess of inversion. The reaction of (J )-6-cbloro 2,6 dimethyloctane with I420, for example, leads to an alcohol product that is approximately 80% racemized and 20% inverted (80% R,S + 20% S is equivalent to 40% R + 60% S). [Pg.375]

Note that in the S l reaction, which is often carried out under acidic conditions, neutral water can act as a leaving group. This occurs, for example, when an alkyl halide is prepared from a tertiary alcohol by reaction with HBr or HC1 (Section 10.6). The alcohol is first protonated and then spontaneously loses H2O to generate a carbocation, which reacts with halide ion to give the alkyl halide (Figure 11.13). Knowing that an SN1 reaction is involved in the conversion of alcohols to alkyl halides explains why the reaction works well only for tertiary alcohols. Tertiary alcohols react fastest because they give the most stable carbocation intermediates. [Pg.378]

The nature of the nucleophile plays a major role in the SN2 reaction but does not affect an S l reaction. Because the SN1 reaction occurs through a rale-limiting step in which the added nucleophile has no part, the nucleophile can t affect the reaction rate. The reaction of 2-methyl-2-propanoI with HX, for instance, occurs at the same rate regardless of whether X is Cl, Br, or 1. Furthermore, neutral nucleophiles are just as effective as negatively charged ones, so S 1 reactions frequently occur under neutral or acidic conditions. [Pg.378]

The properties of a solvent that contribute to its ability to stabilize ions by solvation are related to the solvent s polarity. SN1 reactions take place much more rapidly in strongly polar solvents, such as water and methanol, than in less polar solvents, such as ether and chloroform. In the reaction of 2-chloro-2-methylpropane, for example, a rate increase of 100,000 is observed on going from ethanol (less polar) to water (more polar). The rate... [Pg.379]

Just as the L2 reaction is analogous to the SK-2 reaction, the SN1 reaction has a close analog called the El reaction (for elimination, unimolecular). The El reaction can be formulated as shown in Figure 11.21 for the elimination of HC1 from 2-chloro-2-methylpropane. [Pg.391]

El eliminations begin with the same uni molecular dissociation we saw in the Sfsjl reaction, but the dissociation is followed by loss of H+ from the adjacent carbon rather than by substitution. In fact, the El and SN1 reactions normally occur together whenever an alkyl halide is treated in a protic solvent with a non-basic nucleophile. Thus, the best El substrates are also the best SN1 substrates, and mixtures of substitution and elimination products are usually obtained. For example, when 2-chloro-2-methylpropane is warmed to 65 °C in 80% aqueous ethanol, a 64 36 mixture of 2-methyl-2-propanol (Sjql) and 2-methylpropene (El) results. [Pg.392]

The reaction of an alkyl halide or los3 late with a nucleophiJe/base results eithe in substitution or in diminution. Nucleophilic substitutions are of two types S 2 reactions and SN1 reactions, in the SN2 reaction, the entering nucleophih approaches the halide from a direction 180° away from the leaving group, result ing in an umbrella-like inversion of configuration at the carbon atom. The reaction is kinetically second-order and is strongly inhibited by increasing stork bulk of the reactants. Thus, S 2 reactions are favored for primary and secondary substrates. [Pg.397]

The S il reaction occurs when the substrate spontaneously dissociates to a carbocation in a slow rate-limiting step, followed by a rapid reaction with the nucleophile. As a result, SN1 reactions are kinetically first-order and take place with racemization of configuration at the carbon atom. They are most favored for tertiary substrates. Both S l and S 2 reactions occur in biological pathways, although the leaving group is typically a diphosphate ion rather than a halide. [Pg.397]

For over 35 years, the quinone methide species has been invoked as a reactive intermediate in bioreductive alkylation and in other biological processes.8 29 Generally, there is only circumstantial evidence that the quinone methide species forms in solution. Conceivably, the O-protonated quinone methide (i.e., the hydroquinone carbocation) could be the electrophilic species. If so, bioreductive alkylation may simply be an SN1 reaction. Also, there are questions concerning the mechanism of quinone methide... [Pg.218]

The etherification between alcohol 10 and imidate 67 was one of the key transformations in the successful preparation of compound 1. The use of HBF4 as the catalyst for the etherification was crucial for obtaining high levels of diastereose-lectivity and relatively high conversion to the desired product 18. The fact that sec-sec ethers have rarely, if ever, been obtained with high levels of diastereocontrol in Sn2 fashion under typical SN1 reaction conditions prompted us to investigate the complex mechanistic details of this exceptional reaction. [Pg.214]

Acyclic phosphoranes, ArnP(OR)5 n with n = 0 - 3 have been shown to hydrolyse by an SN1(P) mechanism for n. = 1, 2 and 3 but by an S 2(P) or addition-elimination mechanism for n. = O23. This duality of mechanism is analogous to the classical S l vs S 2 mechanisms observed for solvolysis reactions at tetrahedral carbon. [Pg.58]

In steric terms there is a relief of crowding on going from the initial halide, with a tetrahedral disposition of four substituents about the sp3 hybridised carbon atom, to the carbocation, with a planar disposition of only three substituents (cf. five for the SN2 T.S.) about the now sp2 hybridised carbon atom. The three substituents are as far apart from each other as they can get in the planar carbocation, and the relative relief of crowding (halide - carbocation) will increase as the substituents increase in size (H- Me- Me3C). The SN1 reaction rate would thus be expected to increase markedly (on both electronic and steric grounds) as the series of halides is traversed. It has not, however, proved possible to confirm this experimentally by setting up conditions such that the four halides of Fig. 4.1 (p. 82) all react via the SN1 pathway. [Pg.84]

These YA values are found not to run in parallel with the dielectric constant values for the solvents concerned. Obviously the dielectric constant value for the solvent must be involved in some way in YA, as separation of opposite charges is a crucial feature of the rate-limiting step in an SN1 reaction formation of the T.S. leading to the ion-pair intermediate (47). But specific solvation of the separating charges must also be involved and YA will reflect those, and quite possibly other properties of the solvent as well. It is common to describe YA as representing a measure of the ionising power of the solvent A. [Pg.390]

Several interesting observations have been made on this reaction. First, the rate of isomerization was found to be the same as the rate of dehydration. All attempts to dehydrate the starting complex by conventional techniques were found to lead to isomerization. On the basis of this and other evidence, the mechanism proposed involves the aquation in the complex followed by anation. In this process, water first displaces Cl- in the coordination sphere and then is displaced by the Cl-, possibly by an SN1 mechanism. A trigonal bipyramid transition state could account for the Cl- reentering the coordination sphere to give an cis product. The rate law for this reaction is of the form... [Pg.732]

The role of steric effects is unclear but the anomeric effect could also contribute to an increase in electron density at nitrogen. X-ray data for the two TV-acyloxy-TV-alkoxyamides, a urea and a carbamate outlined above show clear evidence, both from bond lengths and conformations, of an anomeric interaction RO-N bonds are short when compared to alkoxyamines. This interaction is responsible for SN1, SN2, homolytic and rearrangement reactions of /V-acyloxy-TV-alkoxyamides (vide infra) and has also been supported computationally. Acyloxylation of the hydroxamic esters results in both pyramidalisation as well as anomeric donation from the... [Pg.58]


See other pages where For SN1 reactions is mentioned: [Pg.349]    [Pg.238]    [Pg.118]    [Pg.129]    [Pg.154]    [Pg.100]    [Pg.136]    [Pg.147]    [Pg.136]    [Pg.349]    [Pg.238]    [Pg.118]    [Pg.129]    [Pg.154]    [Pg.100]    [Pg.136]    [Pg.147]    [Pg.136]    [Pg.305]    [Pg.373]    [Pg.378]    [Pg.380]    [Pg.1315]    [Pg.391]    [Pg.242]    [Pg.251]    [Pg.241]    [Pg.704]    [Pg.707]    [Pg.89]    [Pg.147]    [Pg.246]   


SEARCH



Potential Energy Diagrams for Multistep Reactions The SN1 Mechanism

SN1 reactions

© 2024 chempedia.info