SNI reaction

Nucleophihc substitution reactions fall into two categories, called S l reactions and 8 2 reactions. (The numbers 1 and 2 refer to a specific aspect of the kinetics of the reactions [ Chap- 

Commonly encountered leaving groups include Cr, Br , and F. A carbocation is an unstable species and, being positively charged, is prone to nucleophilic attack. Because the carbocation is planar about the carbon that bears the positive charge, it is equally likely that a nucleophile will attack from either side. 

In the reaction of (CH3)3CBr with H2O, the product is the same regardless of whether the nucleophile (water) attacks from the front or from the back. When a molecrtle loses a leaving group from a chiral carbon, however, as would be the case with the thalidomide molecule, attack from one side of the carbocation will yield one enantiomer whereas attack from the other side will yield the other enantiomer. The result is a racemic mixtrue. The conversion of a single enantiomer to a racemic mixture of both enantiomers is called racemization. 

Nucleophilic attack on opposite sides of the carbocation leads to two diJBferent products, which are mirror images of each other. 

Sample Problems 10.6 and 10.7 show how to draw mechanisms for addition and substitution reactions. 

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Mannosides are difficult to obtain since here a 2-O-acyl group blocks the -position. 2-O-Benzyl-a-mannosyl bromides give, however, high yields of pure -glycosides with a heterogeneous silver silicate catalyst preventing anomerization and SnI reaction of the bromide H. Paulsen, 1981 B, Q. 

FIGURE 8 8 Inversion of configuration predomi nates in SnI reactions because one face of the car bocation is shielded by the leaving group (red)... 

FIGURE 8 9 A polar solvent stabilizes the transition state of an SnI reaction and in creases its rate... 

Section 11 14 Benzylic carbocations are intermediates in SnI reactions of benzylic halides and are stabilized by electron delocalization... 

The SnI mechanism is an ionization mechanism. The nucleophile does not participate until after the rate-deter-rnining step has taken place. Thus, the effects of nucleophile and alkyl halide structure are expected to be different from those observed for reactions proceeding by the Sn2 pathway. Flow the structure of the alkyl halide affects the rate of SnI reactions is the topic of the next section. 

The relative reactivity of alkyl halides in SnI reactions is exactly the opposite of Sn2 ... 

CleaiTy, the steric crowding that influences reaction rates in Sn2 processes plays no role in SnI reactions. The order of alkyl halide reactivity in SnI reactions is the same as the order of caibocation stability the more stable the caibocation, the more reactive the alkyl halide. 

Identify the compound in each of the following pairs that reacts at the faster rate in an SnI reaction ... 

Partial but not complete loss of optical activity in SnI reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile. Ionization of the alkyl halide gives a carbocation-halide ion pair-, as depicted in Figure 8.8. The halide ion shields one side of the carbocation, and the nucleophile captures the carbocation faster from the opposite side. More product of inverted configuration is formed than product of retained configuration. In spite of the observation that the products of SnI reactions are only partially racemic, the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular- mechanism. 

Functional group transfonrrations that rely on substitution by the SnI mechanism are not as generally applicable as those of the Sn2 type. Hindered substrates are prone to elimination, and reanangement is possible when carbocation intennediates are involved. Only in cases in which elimination is impossible are SnI reactions used for functional group transfonnations. 

With secondary and tertiary alcohols, this stage is an SnI reaction in which the alkyloxonium ion dissociates to a carbocation and water. 

The caibocations fonned as intennediates when allylic halides undergo SnI reactions have their positive chaige shaied by the two end caibons of the allylic system and may be attacked by nucleophiles at either site. Pioducts may be fonned with the same pattern of bonds as the starting allylic halide or with allylic rearrangement. 

The 5/) -hybridized carbon of an acyl chloride is less sterically hindered than the sp -hybridized carbon of an alkyl chloride, making an acyl chloride more open toward nucleophilic attack. Also, unlike the Sn2 transition state or a carbocation intennediate in an SnI reaction, the tetrahedral intennediate in nucleophilic acyl substitution has a stable anangement of bonds and can be fonned via a lower energy transition state. 

A stepwise mechanism is also possible this is the two-step unimolecular SnI reaction ... 

For each reaction, plot energy (vertical axis) vs. the number of the structure in the overall sequence (horizontal axis). Do reactions that share the same mechanistic label also share similar reaction energy diagrams How many barriers separate the reactants and products in an Sn2 reaction In an SnI reaction Based on your observations, draw a step-by-step mechanism for each reaction using curved arrows () to show electron movements. The drawing for each step should show the reactants and products for that step and curved arrows needed for that step only. Do not draw transition states, and do not combine arrows for different steps. 

Benzene rings have a dramatic effect on SnI reaction rates. This depends on the position of the ring relative to the leaving group. Consider the following reactions. 

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