Charge cannot be created or destroyed. If the starting materials have no overall charge, then neither must the products. In the last example above, it is obvious why the bromine becomes negatively charged—it takes both electrons from the bond even though only one of them formally ‘belongs’ to it. It may be less obvious to you why the ammonium cation has to have a positive charge, but it must, in order to maintain overall neutrality. One way to think about it is to note that both of the electrons in the new N–H bond come from N, so N is one electron down on the deal. If the starting materials are charged, then the products must have, overall, the same charge. Here’s ammonia being protonated by H₃O⁺—both starting materials and products must have overall charge 1⁺.

When it is a π bond that is being broken rather than a σ bond, only the π bond is broken and the σ bond should be left in place. This is what commonly happens when an electrophilic carbonyl group is attacked by a nucleophile. Just as in the breaking of a σ bond, start the arrow in the middle of the π bond and end by putting the arrowhead on the more electronegative atom, in this case oxygen rather than carbon.

In this case the starting materials had an overall negative charge and this is preserved in the anionic product. The charge disappears from the hydroxide ion because it is now sharing a pair of electrons with what was the carbonyl carbon atom and a charge appears on what was the carbonyl oxygen atom because it now has one of the electrons in the old π bond.

مواضيع ذات صلة

Curly arrows are vital for learning organic chemistry

Mechanisms with several steps

Watch out for five-valent carbons

Drawing your own mechanisms with curly arrows

σ bonds as nucleophiles

π bonds as nucleophiles

Curly arrows show the movement of electrons

Curly arrows represent reaction mechanisms

Identifying an electrophile

Nucleophiles and electrophiles

Orbital overlap is essential for successful reaction

Reactions happen when electrons flow between molecules