Nucleophilic addition lobes

Nucleophilic addition reactions occur by electron donation from the nucleophile to the IT antibonding orbital of the ketone. If the faces of the ketone are different, addition happens faster at the more accessible orbital lobe. Use SpartanView to display mesh electron-density surfaces of 2-norbomanone and camphor, and simultaneously display the tt antibonding orbital (LUMO) surface of each. Which face of each ketone is more reactive What is the stereochemistry of the alcohol produced by reaction of each with NaBH4 ... 

The structure of the relevant carbocation is shown at the right. Approach of a nucleophile toward the top lobe of its p orbital is partly blocked by the ethyl group two carbon atoms over and to a lesser extent by the ester function one carbon farther away (green). In addition, the hydroxy group on the bottom face of the ring directs nucleophilic addition of a water molecule from below by hydrogen bonding, as shown. 

The greater stability of the digonal vinyl cation 161 than of the bridged cation 162 would imply that, at least in the gas phase, race-mization is involved as stereochemical course of any reaction with nucleophiles since both lobes of the empty p-orbital of 161 are available for a nucleophilic attack. Efforts to rationalize (Bumelle, 1964) early reports of stereospecific trans addition to acetylene derivatives have recently been complemented by a theoretical calculation of the energy profile for addition of hydrogen fluoride to acetylene in the gas phase (Hopkinson et at., 1971). The results of these calculations suggest that the bridged cation is possibly a transition state and not an intermediate. 

FIGURE 7.56 Loss of optical activity occurs as soon as the free, planar carbocation is formed. Addition of any nucleophile will occur equally at the two available lobes of the 2p orbital to give a racemic mixture of products. 

Open cations permit addition of a nucleophile to each lobe of the empty orbital. Cyclic ions require opening in Sn2 fashion, leading to overall trans addition. 

---