Question 10.7: Using curved arrows to indicate the movement of electrons, d...

Using curved arrows to indicate the movement of electrons, draw the mechanism for each of the following reactions: (a) nucleophilic addition of CN^– to CH₃CHO and (b) electrophilic substitution of benzene with ⁺SO₃H. (Draw all resonance structures for the carbocation intermediate.)

Strategy For nucleophilic addition, draw Lewis structures with the nucleophile close to the electron-poor atom where attack will occur (in this case, the carbonyl carbon). For electrophilic substitution, draw Lewis structures with the electrophile close to the site of attack on the benzene ring. Remember that nucleophiles are attracted to and react with electron-poor atoms whereas electrophiles are attracted to and react with electron-rich areas of the molecule. Using this information, and the octet rule, determine which electrons are likely to be involved in the reaction and indicate their repositioning with curved arrows.

Setup (a) The nucleophile is CN^–. The site of attack is the carbonyl C in CH₃CHO, which is electron-poor because it is bonded to the more electronegative O atom.
(b) The electrophile is ⁺SO₃H. The site of attack is the electron-rich, delocalized pi bonds of the benzene ring.