How the nucleophile affects elimination versus substitution

Here is an example of this idea at work a weak base (EtOH) leads to substitution while a strong base (ethoxide ion) leads to elimination. 

Temperature has an important role to play in deciding whether a reaction is an elimination or a substitution. In an elimination, two molecules become three (count them). In a substitution, two molecules form two new molecules. The two reactions therefore differ in the change in entropy during the reaction AS is greater for elimination than for substitution. In Chapter 12, we discussed the equation 

This equation says that a reaction in which A5 is positive becomes more favourable (AG becomes more negative) at higher temperature. Eliminations should therefore be favoured at high temperature, and this is indeed the case most eliminations you will see are conducted at room temperature or above. 

Now that you have seen a few examples of elimination reactions, it is time to return to our discussion of the two mechanisms for elimination. To summarize what we have said so far  

There are a number of factors that affect whether an elimination goes by an El or E2 mechanism. One is immediately obvious from the rate equations only the E2 is affected by the concentration of base, so at high base concentration E2 is favoured. The rate of an El reaction is not even affected by what base is present—so El is just as likely with weak as with strong bases, while E2 goes faster with strong bases than weak ones strong bases at whatever concentration will favour E2 over El. If you see that a strong base is required for an elimination, it is certainly an E2 reaction. Take the first elimination in this chapter as an example. 

We will shortly come back to these two mechanisms for elimination, plus a third, but first we need to answer the question when does a nucleophile start behaving as a base  

We have [eft detailed discussion of the formation of alkenes till this chapter, but we used the term elimination in Chapters 12 and 14 to describe the loss of a leaving group from a tetrahedral intermediate. For example, the final steps of the acid-catalysed ester hydrolysis shown below involve El elimination of ROH to leave a double bond C=0 rather than C=C. 

In Chapter 14, you even saw an El elimination giving an alkene. That alkene was an enamine—here is the reaction. 

Here is an example of this idea at work, weak base substitution 

We can also rationalize selectivity for elimination versus substitution, or attack of H versus attack on C in terms of hard and soft electrophiles (pp. 237-238). in an Sn2 substitution, the carbon centre is a soft electrophile—it is essentially uncharged, and with leaving groups such as halide the C-X a is a relatively low-energy LUMO. Substitution is therefore favoured by nucleophiles whose 

HOMOsare best able to interact with this LUIVIO—in other words soft nucleophiles. In contrast, the C-H o is higher in energy because the atoms are less electronegative. This, coupled with the hydrogen s small size, makes the C-H bond a hard electrophilic site, and as a result hard nucleophiles f avo u r el i m i nati on. 

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