One-step nucleophilic substitution

This one-step nucleophilic substitution is an example of the SN2 mechanism. The abbreviation SN2 stands for Substitution, Nucleophilic, bimolecular. The term bimolecular means that the transition state of the rate-limiting step (the only step in this reaction) involves the collision of two molecules. Bimolecular reactions usually have rate equations that are second order overall. 

Model 1 Examples of One-Step Nucleophilic Substitution Reactions... 

Which energy diagram in Figure 13.3 is most likely to be a one-step nucleophilic substitution ... 

Term Introduction One-step Nucleophilic Substitution Reaction = Sn2 Reaction ... 

The reactants in Model 7 carmot undergo a one-step nucleophilic substitution (Sn2) because the resulting transition states would be too crowded. How is this overcrowding problem attenuated (made smaller) in the first step of an SnI mechanism ... 

Explain why a one-step nucleophilic substitution reaction is called an Sn2 reaction. 

One-step nucleophilic substitution reactions at a chiral carbon are characterized by the inversion of the absolute configuration of this carbon. Your model set is not very useful for simulating this chiral inversion. Explain how the model set falls short and how an lunbrella in the wind can sort of simulate this inversion. 

The analogous reaction of 5-iodopyridazin-3(2H)-one led to the formation of pyridazino[4,5-6]indoles (3.82.) in a two step nucleophilic substitution, palladium catalysed intramolecular carbon-carbon bond formation sequence. The same reaction has also been carried out in one-... 

Fig. 5.53. Mechanistic aspects I of nucleophilic aromatic substitution reactions of aryldiazonium salts via radicals introduction of Nu=Cl, Br, CN or N02 according to Figure 5.52. Following step 2 there are two alternatives either the copper(II) salt is bound to the aryl radical (step 3) and the compound Ar-Cu(III)NuX decomposes to Cu(I)X and the substitution product Ar-Nu (step 4), or the aryl radical reacts with the cop-per(II) salt in a one-step radical substitution reaction yielding Cu(I)X and the substitution product Ar-Nu.

We can conclude that phosphate esters and other phosphate compounds react with water through bimolecular substitution in a concerted 5 2, or A D, mechanism with no metaphosphate intermediate. The appearance of the transition state is that it resembles metaphosphate monoanion, but the reality of the mechanism is that the reaction is a one-step bimolecular substitution. The metaphosphate ion can be formed in the gas phase (25) and there is evidence that metaphosphate can exist briefly in non-nucleophilic solvents 11,24. The reason that it is not an intermediate in water is presumably that there is no significant barrier for its reaction with water. 

A more convenient approach to the exchange of hydroxy groups by fluorine is one-step activation-substitution - the alcohol is treated with a sufficiently electron-deficient, fluorine-containing reagent which condenses with it, with liberation of a fluoride ion. This ion, in turn, effects nucleophilic replacement of the now present leaving group. Stereochemically, this process results in clean inversion at the carbon center. 

Both stages involve more than one step and these steps differ in detail among the various carboxylic acid derivatives and for different reaction conditions This chapter is organized to place the various nucleophilic acyl substitutions into a common mechanis tic framework and to point out the ways m which individual classes differ from the rest... 

The early chemistry leading to these derivatives was originally carried out via the 6a-(methylthio) derivative (17) which was prepared by way of a Schiff s base (39). The 6a-thiomethyl group could then be displaced by various nucleophiles giving rise to 6a-methoxy or other 6a-substituted penicillins. A stereo-specific one-step introduction of a methoxy group at C-6 in penicillins provided a simple entry to 6a-methoxy penicillins (40) in yields ranging from 50—62%. 

There are several reaction sequences which involve such intramolecular hydrogen abstraction steps. One example is the photolytically intitiated decomposition of N-haloamines in acidic solution, which is known as the Hofinann-Loffier reactionThe reaction leads initially to y-haloamines, but these are usually converted to pyrrolidines by intramolecular nucleophilic substitution ... 

To make this more specific. Table 5-3 gives examples of several reaction types that fit the RIP pattern. Consider nucleophilic substitution on saturated carbon. The concerted mechanism is the one-step bimolecular 5, 2 process ... 

One after the other, step through the sequence of structures corresponding to the three nucleophile substitution reactions shown above (reaction 1, reaction 2, reaction 3). Decide whether loss of Br occurs with or without the assistance of RO /ROH. The nucleophile-assisted and unassisted mechanisms are called Sn2 and SnI mechanisms respectively. Label each reaction as Sn2 or SnI as appropriate. 

The synchronous bimolecular mechanism for aromatic nucleophilic substitution involves unfavorable geometry (bonds made and broken are both in the plane of the ring and backside attack is not possible) and unfavorable energetics (one high-energy step is required... 

Today, we refer to the transformations taking place in Walden s cycle as nucleophilic substitution reactions because each step involves the substitution of one nucleophile (chloride ion, Cl-, or hydroxide ion, HO-) by another. Nucleophilic substitution reactions are one of the most common and versatile reaction types in organic chemistry. 

Two principle approaches from thiopyrylium salts to thiopyrans accompanied by C-substitution have been found, e.g., the one-step additions of C-nucleophiles or the two-step procedures involving primary conversions of the salts to nucleophilic intermediates followed by attacks with appropriate electrophiles. 

Bromo-4-chloro-lH-pyrazolo[3,4-d]pyrimidine could be easily fimc-tionalized at C-3 and C-4 in a one-pot two-step microwave-assisted process (Scheme 34) [55]. Ding and Schultz reported that nucleophilic substitution of the addition-elimination type at the C-4 position with amines and anilines smoothly occurred under acidic conditions in dioxane upon irradiation... 

Sn2 stands for substitution nucleophilic bimolecular. The lUPAC designation (p. 384) is AnDn- In this mechanism there is backside attack The nucleophile approaches the substrate from a position 180° away from the leaving group. The reaction is a one-step process with no intermediate (see, however, pp. 392-393 and 400). The C—Y bond is formed as the C—X bond is broken ... 

Now we get to the meaning of 2 in Sn2. Remember from the last chapter that nucleophilicity is a measure of kinetics (how fast something happens). Since this is a nucleophilic substitution reaction, then we care about how fast the reaction is happening. In other words, what is the rate of the reaction This mechanism has only one step, and in that step, two things need to find each other the nucleophile and the electrophile. So it makes sense that the rate of the reaction will be dependent on how much electrophile is around and how much nucleophile is around. In other words, the rate of the reaction is dependent on the concentrations of two entities. The reaction is said to be second order, and we signify this by placing a 2 in the name of the reaction. 

Two chemical approaches that involve nucleophilic substitution of the chlorine atoms attached to boron with linking reagents are reported in the literature. In both methods, the driving force is the formation of a stable by-product or one that can easily be stabilized. These substances contain chlorine. They can be distinguished by the number of steps, either one or two, required for preparing the polymer, starting from /i-chloroborazine. 

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