Cyanide is a Carbon Nucleophile

Why does cyanide attack from carbon and not nitrogen? Isn't nitrogen more electronegative than carbon and, therefore, better able to carry the negative charge? Look at the highest-occupied molecular orbital (the HOMO) of cyanide. This is where the "most available" pair of electrons resides, that is, the "nucleophilic site".

Build cyanide. Click on (New) to bring up Spartan's entry model kit. Select sp carbon () and click anywhere on screen. Select sp nitrogen () and click on the triple free valence on carbon. Click on (Delete) and click on the free valence on carbon. Click on (Minimize) and then click on (View).

 

Select Calculations from the Setup menu. Select Equilibrium Geometry from the top menu to the right of "Calculate", and Hartree-Fock and 3-21G(*) from the two bottom menus. Change "Total Charge" from Neutral to Anion (click on the down arrow to the right). Click on OK. Select Surfaces from the Setup menu. Click on Add at the bottom of the dialog, select HOMO from the "Surface" menu and click on OK. Select Submit from the Setup menu, supply a name "cyanide", and click on Save.

 

 

When completed, double click on the line "HOMO..." in the Surfaces dialog. You can alter the display style by first clicking on the graphic and then selecting an entry from the "Style" menu at the bottom right of the screen.

The HOMO is more heavily concentrated on carbon, and acetonitrile will be the dominant product. Nitrogen is more electronegative than carbon, and it will hold its electrons more tightly.