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Question 1 and 2 of the worksheet?

Shang 22 In this figure, P2 is lower energy than P1. By the Hammond postulate, TS2 is also lower energy as well as structurally more like the reactants. Thus the activation energy. EA[2) is also lower Trends in Nucleophile Strength For this discussion, we have to assume the rate factors above apply in reverse that a faster reaction rate is associated with a more exothermic reaction. For nucleophiles, the neutral nucleophiles (amines, alcohols, ethers) tend to be less nucleophilic than negatively charged ones. Within the set of anion nucleophiles, general trend correlates faster rates with relative stability of the negative charge. More basic anions are typically also more nucleophilic, as both of those trends are associated with a relatively high energy. The size of the nucleophilic tom is also important-larger atoms have (a) more volume for the excess electron density to spread out, and (b) greater polarizability, so that the electron density can reach out to bond with an electrophilic Resonance stabilization, because it removes electron density from the nucleophilic atom, also decreases nucleophile strength. Der er noen Questions Examine the lists in section 9.3a of Karty (2ed, p. 448 and 451). 1. Draw Lewis structures for thiocyanate (NCS) and aride (N, ) ions. Then suggest why the azide ion is more reactive than thiocyanate. 2. Phosphines like PICsh are sometimes used as nucleophiles. Where would you rank a neutral phosphine compared to an amine, and why?

Answer 1

IN ONON: a ti NCS ( Nors: I 2 o ti

1) Given above are the lewis structures of the azide and thiocynate ion respectively, along with their resonanace structures and formal charges on each atom.

Both of these structures have the same number of resonance structures, but differ in their stability. In Azide ion the -ve charge is shared equally b/w the two nitrogen ends in the resonance structures, hence all the resonance structures have same energy. In contrast, in the thiocynate ion, the -ve charge is shared b/w the N and S atoms, but the -ve charge is stable on N only as it is the most electronegative in the given species, hence all the resonace structures do not have the same energy and structure 2 is the most stable form of thiocynate ion. This makes thiocynate ion an ambident nucleophile and less reactive than the azide ion.