Reações SN2 e Mecanismos

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CHAPTER 7 221
 
products are lower in energy than the reactants, because 
bromide is more stable than ethoxide. 
 
 
 
(e) In the transition state, the ethoxide ion is in the 
process of removing the proton, the double bond is in the 
process of forming, and the leaving group (bromide) is in 
the process of leaving. We use dotted lines to indicate 
the bonds that are in the process of being formed or 
broken, and we use  symbols to indicate the 
distribution of charge. Note that the negative charge is 
in the process of being transferred from the oxygen atom 
to the bromine atom, and the  symbols indicate that 
each location bears partial negative character in the 
transition state. Finally, brackets are drawn, together 
with the symbol that indicates that this is a transition 
state: 
 
 
7.80. 
(a) The nucleophile is iodide and the solvent is a polar 
aprotic solvent (DMF), indicating an SN2 reaction. The 
substrate (which is primary) has an electrophilic center 
shown here. 
 
Iodide attacks this electrophilic center in an SN2 process, 
ejecting the leaving group (highlighted above), as shown 
here: 
 
 
 
(b) This reaction occurs via an SN2 process. As such, the 
rate of the reaction is highly sensitive to the nature of the 
substrate. The reaction will be faster in this case, 
because the methyl ester is less sterically hindered than 
the ethyl ester. 
 
 
 
7.81. A strong base will remove the most acidic proton 
in the starting alcohol (the proton of the OH group), 
giving an anion that contains both a nucleophilic center 
and an electrophilic center, allowing for an 
intramolecular SN2-type process (bromide is ejected as a 
leaving group), as shown here. 
 
 
7.82. Iodide is a much stronger nucleophile than ethanol, 
so we expect the former to attack butyl bromide (a 
primary substrate) in an SN2 reaction to give butyl 
iodide. 
 
 
As a result of this initial rapid reaction, the concentration 
of iodide quickly decreases. Then, the slow rise in 
concentration of iodide indicates that another nucleophile 
is slowly ejecting the iodide ions. Indeed, there is a 
weak nucleophile present (ethanol). In the presence of 
ethanol, a slow SN2 process can occur in which butyl 
iodide functions as the substrate (iodide is an excellent 
leaving group) and ethanol functions as the nucleophile. 
The resulting oxonium ion is then deprotonated (by 
ethanol, this time functioning as a base), giving the 
product shown below, which is an ether. 
 
 
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