Organic chemistry reactions involve electron-transfer reactions, and many of them use reaction mechanisms to show how electrons flow. This electron flow is typically represented using a double-sided curly arrow. In our first article, we discussed a method for identifying a nucleophile through visual inspection. Now, our second technique involves following the arrow trail and sniffing out the bond.
In the chapter on Fundamentals of Organic Reactions, I explained the different types of arrows that are commonly used in organic chemistry and their purposes.
A double-headed curly arrow is used to indicate the transfer of two electrons from point A to point B. The transfer is considered complete when these two electrons are used to form a two-electron covalent bond between A and B, written as A-B.
Here, point A is electron-rich and acts as a nucleophile, while point B is the region that lacks electrons and accepts the electrons from the nucleophile to form a two-electron covalent bond.
For example, consider the reaction between two substrates: an amine and a ketone. The flow of electrons from amine Nitrogen to form a bond with the carbonyl carbon of the ketone confirms the nucleophile to be the amine.
In this reaction, Nitrogen generously donates both electron pairs to the carbonyl carbon, thereby shifting its valency from trivalent to tetravalent and acquiring a positive charge. Simultaneously, the carbonyl carbon transfers the electrons to the electronegative Oxygen atom to accommodate the extra electrons and maintain its tetravalent state.
The act of electron-donation by Nitrogen results in a co-ordinate covalent bond, a classic example of a Lewis base electron-donation reaction. It's important to note that these Lewis base intermediates are not particularly strong and are reversible, implying that the bond can form and dissociate at or near room temperature.
The intermediate undergoes further proton-transfer reaction where the negatively charged Oxygen atom picks up a proton, acting like a base, from the positively charged Nitrogen atom. This nullifies the Nitrogen and Oxygen atoms' charges so that they return to the uncharged state. This step also fixates the intermediate co-ordinate covalent bond into a stable covalent bond.
The molecule formed is amine-alcohol, which can further undergo dehydration to form the final, more stable imine product.
The arrow paths reveal such intricacies in organic reactions, showing the bond formed between an electron-rich nucleophile and an electron-deficient electrophile. The arrow's base always indicates the pot of gold, like the mythical end of the rainbow, exposing the electron treasure, the nucleophile.
