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Question

Define back bonding and explain its effects on acid strength.


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Solution

Part 1: Back bonding:

  • The exchange of electrons between an atomic orbital on one atom and an antibonding orbital on another atom is known as back bonding.
  • Back bonding is a type of bonding that occurs between atoms in a compound when one atom has a single electron and the other has a vacant orbital next to it.
  • It occurs as electrons pass from one atom’s atomic orbital to another atom’s or ligand’s anti-bonding orbital.
  • As a result, the bonding takes on a partial double-bond character. It reduces the bond length while increasing the bond order.
  • It has an effect on molecule properties such as hybridization and dipole moment.

Part 2: Effect of back bonding on acid strength:

  • When the back bonding is effective, the lack of electrons is balanced and the lewis acid character decreases.
  • Due to this, acid strength decreases down the halogen group.
  • Back bonding is effective only when the size of the valence shell matches.
  • For example, in BF3 both boron and fluorine have their valence electrons in 2p but in BBr3, the valence electrons of bromine are in 4p while valence electrons of boron are in 2p, so the size does not match.
  • Therefore, back bonding decreases down the halogen group.

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