A nucleophile is a species (an ion or a molecule) which is strongly attached to a region of positive charge on an another ion or molecule.
Nucleophiles are either fully negative ions, or they make contain a strong Δ charge somewhere on a molecule. The most common nucleophiles that occur are oxygen and nitrogen.
For this example the reacting molecule will be represented as Nu-H. The Nu part of the reactant molecule contains the nucleophilic oxygen or nitrogen atom.
In this particular reaction the overall effect is that the I ¯ by Nu ¯ and the product of the reaction if hydrogen fluoride.
Since the overall reaction of this reaction appears to be a substitution by the nucleophile the reaction could be described as a nucleopihilic substitution. However, the reaction occurs in two distinct stages. The first stage of the reaction is an addition reaction, which is then followed by an elimination reaction which produces the overall product of the reaction: HI: therefore the reaction is appropriately named nucleophilic addition-elimination
In the mechanism the positively charged carbon is attacked by the nucleophile and a bond is formed. During the bonding of the carbon with the nucleophile there is also a change in the bonding between the carbon and the oxygen, two electrons in one of the bonds contained within in the double bond is taken and placed on the oxygen alone making it negatively charged and removing the double bond between the two atoms.
This stage of the reaction occurs in two steps. The in the first step the carbon- oxygen double bond is reformed. In order for this to occur the electrons from the oxygen must be transferred onto the Iodine atom forming an Iodide ion.
The last step of the reaction is that the Iodide ion removes the hydrogen off the original nucleophile. The hydrogen is removed as a positive ion thus leaving the pair of electrons behind on either the oxygen or the nitrogen atom in the nucleophile. These electrons cancel the positive charge that was left on the nucleophile in the earlier step.