Polarized multiple bonds without leaving groups

4 POLARIZED MULTIPLE BONDS WITHOUT LEAVING GROUPS 

Acid catalysis, often by proton transfer or hydrogen bonding to Y, enhances the ability of this electron sink to accept electrons. Since the lone pair on an sp nitrogen is easier to protonate than a lone pair on an sp oxygen (see chart. Appendix), acidic catalysis occurs much more readily with nitrogen. The carbocation thus formed is a much better electron sink. Additional examples of this sink are  

Which of the following is the most reactive toward nucleophilic attack Which is the next most reactive Which is the least reactive  

Answer The most reactive is the protonated aldehyde. The protonated imine is next, for it is slightly more stable. The least reactive is the neutral imine because the anion formed upon nucleophilic attack is more basic than the anion formed upon attack of the neutral aldehyde. 

The most common representatives of this electron sink are the nitriles with Y as N. Nitriles are much less reactive than ketones. The electronegativity of Y polarizes the triple bond so that carbon bears a partial plus charge. The two perpendicular double bonds can be treated separately thus attack by a nucleophile is identical to the doubly bonded electron sink, C=Y. Some examples are Ph-C=N, and acylium, CH3-C=0 .  

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Section 6.4 Polarized Multiple Bonds Without Leaving Groups... 

Polarized multiple bonds without leaving groups (6.4) ... 

C = Y / Polarized multiple bond without L Protonated carbonyls R2C=0H Iminium ions R2C=NRj Aldehydes RHC=0 Ketones R2C=0 Imines R2C=NR The larger partial plus carbon is more reactive Addition. Following Substitution or Elimination (8.5.1) Polarized triple bonds R-C=Y without leaving groups... 

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