The reverse of this is the formation of a bond by two radicals coming together and the individual electrons on the radicals jump into the same space to make a bond. They may not have changed their charge but they will both be radicals, and generally very reactive. If a bond is broken via this kind of process the resulting new species will have unpaired electrons on the two atoms that were bonded. In this case, the pair of electrons does not stay together through the process. Sometimes, bonds are broken and formed homolytically, meaning equally between the two atoms.
Sometimes, the curly arrows can represent a bond breaking and a new bond forming in a single step. One of the lone pairs on the oxygen jumps into the space between the H + and the OH – and a new bond is formed. Curly arrow used to show the creation of a bond. The bond is broken and two new species are formed.Ĭurly arrows can also be used to show bonds forming when electrons move into the space between two atoms.
The curly arrow in this figure represents the H-O bonding pair that ends up as the lone pair on the oxygen. If the electron pair originated as a bond, then it will end on an atom, which will then develop a negative charge relative to its original charge and the bond will be broken. The arrow must end in such a way that the receiving location can accommodate the electrons without violating any rules. It’s important to remember that every curly arrow must start at a pair of electrons, either in a bond, or a lone pair on an atom. We will be in essence extending this concept here but now, the electrons will be moving in such a way as to create or destroy bonds. The resonance structures involved electrons jumping around at thousands of times the rate that the atoms themselves could move, so it was just simpler to assume the atoms don’t move. We have explored this concept earlier when we looked at resonance structures of a single molecule or polyatomic ion species. Remember that the electrons are thousands of times lighter than the nuclei of the atoms themselves so although we often think and speak as if it’s the atoms that do the moving, we are better off to imagine the electrons as doing the moving while the atoms are relatively motionless. To keep track of these changes, we use curly arrows to represent the movement of electrons.
Many reactions take several steps to occur and each step may involve the creation or destruction of bonds. Generally, we can write an overall process as a simple balanced chemical reaction, but that reaction often does not happen in a single step as is implied by the single equation. When we draw out the reaction mechanisms, it’s important to keep track of where the electrons are, especially the ones involved in forming and breaking bonds. For example, a carbon-carbon double bond has two pairs of electrons in close proximity to each other, making for a concentration of negative charge whereas, if there is an electronegative atom bonded to a carbon, it will tend to inductively withdraw some of the electron density away from the carbon creating a positive charge concentration on that carbon. The charges in the substrate (the main organic molecule being reacted) will occur depending on the structure composition of the molecule. Chemical reactions in organic compounds tend to occur at reactive sites on the molecules, where there is buildup of either positive or negative charge.
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