Soluções do Capítulo 5

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Chapter 5 Suggested solutions for Chapter 5 25 Problem 2 Each of these molecules is nucleophilic. Identify the nucleophilic atom and draw a mechanism for reaction with a generalized electrophile giving the product in each case. H H R: MeO OMe R OMe NH 5 MeO OMe Me Me Purpose of the problem The recognition of nucleophilic sites is the other half of the battle in starting to understand mechanisms. Here is some practice: later you will do this too automatically. Reactions occur when the two meet. Suggested solution This time there are three anions but only two of them (the alkyne and the sulfur anions) have lone pair electrons. We can start our arrows from these and they are the points where the electrophile will attach itself. R R E E $ E The last anion is like the anion we discussed on p. 125 of the main text. The negative charge does not show a pair of electrons on Al but just an imbalance of protons and electrons. All the valency electrons are in the bonds and we must use these σ electrons in the reaction. The arrow should start halfway along the σ bond and emerge through the H atom. H H H AI MeO OMe OMe One nucleophile has a π bond from C to N. The nitrogen atom also has a lone pair of electrons This point is explored in Chapter 7. and you could start your arrow either there or halfway down the π bond - it doesn't matter which. E H E H NH N NH N Me Me Me Me Me Me Me Me The remaining two nucleophiles have lone pairs. One is symmetrical (NH₂-NH₂, hydrazine) and will attack through one nitrogen atom. You may have drawn the product as a cation or you may have removed a proton from it. Either answer is correct. E E © N N H H H H H Finally, the phosphorus compound has four atoms with lone pairs! There are three OMe groups and the phosphorus atom itself. However, the lone pairs on the oxygen atoms are probably in orbitals (and are certainly in some kind of orbital with the principal quantum number 2) while that on the phosphorus atom is in a 3sp³ orbital and is of higher energy. It reacts. OMe MeO OMe MeO OMe MeO E