Nucleophile and nucleophilicity

This presentation is about nucleophile and nucleophilicity and role of nucleophile in SN2 reaction.

1. NUCLEOPHILE NUCLEOPHILICITY AND ROLE OF NUCLEOPHILE IN SN2 REACTION

2. NUCLEOPHILE The term nucleophile is composed of two words nucleo and – phile.the - phile suffix used in lot of words mean lover of whatever comes in front. So the nucleophile is the lover of nucleus . Loving nuclei mean loving positive charges.So the lover of positive charges must be negatively charged. A nucleophile is an ion or molecule that donates a pair of electron to form a new bond. The term nucleophile is used for a compound that shares its lone pair with an atom other than proton. Nucleophile has an unshared pair of electron such as bromide and ammonia.

3. NUCLEOPHILE

4. NUCLEOPHILICITY Nucleophilicity is a measure how readily nucleophile is able to attack on electron deficient atom. In the case of SN2 reaction , ncleophilicity is a measure of how readily the nucleophile attack on carbon bonded to leavig group.nucleophilicity refers the rate of reaction of nucleophile with the carbon at the reaction site.

5. NUCLEOPHILICITY Nucleophilicity is a kinetic property.Many reactions of nucleophile are not reversible.A bond forms , a bond breaks and that the end of reaction.The problem with This from a measurement standpoint is that we often cannot determine an equilibrium constant for a reaction.And if we cannot do that we cannot develop a reactivity scale based on equilibria .

6. SN2 REACTIONS The abbreviation SN2 stands for nucleophilic substitution , bimolecular.This type of substitution reactions are bimolecular because both the nucleophile and haloalkane involve in rate determining step. This is the second order reaction .Doubling the concentration of alkyl halide doubles the rate of reaction.Doubling the concentration of nucleophile doubles the rate of reaction.Doubling the concentration of both nucleophile and alkyl halide quadruples the rate of reaction. Rate =k [A][B]

7. SN2 REACTION

8. Strength of the nucleophile A stronger nucleophile is an ion or molecule that reacts faster in SN2 reaction then a weaker nucleophile under the same condition.A strong nucleophile is much more effective than weak one in attacking at electrophilic carbon atom. Both methanol and methoxide ion have easily shared pairs of non-bonding electrons.But methoxide reacts with eclectrophile in the SN2 reaction about 1 million times faster than methanol.It is a generally true that a specie with a negative charge is stronger nucleophile than a smilar neutral specie.

9. STRENGTH OF NUCLEOPHILE

10. SOLVATION EFFECTS ON NUCLEOPHILICITIES The solvent in which nucleophilic substitution reactions are carried out has a marked effect on nucleophilicities . In polar aprotic solvents , Anoins are only weakly solvated and therefore free to participate in nucleophilic substitution reaction and basicity dictates nucleophilicity . In polar protic solvents , anoins are highly solvated by hydrogen bonding with solvent molecules and therefore are less free to participate in nucleophilic substitution reactions and polarizeability dictates nucleophilicity . So iodide is more polarizabile than other halides. so in polar protic solvents , it is a strong nucleophile . POLRAR APROTIC iodide>bromide>chloride> flouride POLAR PROTIC flouride <chloride<bromide<iodide

11. NUCLEOPHILICITY IS AFFECTED BY STERIC AFFECTS The nucleophile attacks from the back side of carbon-leaving group bond.As the leaving group departs with electron pair. The nucleophile supplies another electron pair to crbon . So nucleophilicity is affected by steric affects because a bulky nucleophile cannot approach the back side of carbon so easily as a less sterically hindered nucleophile can. Therefor tertiary butoxide with its three methyl groups is a poorer nucleophile than ethoxide ion even though tertiary butoxide ion is stronger base than ethoxide ion.

12. NUCLEOPHILICITY IS AFFECTED BY STERIC AFFECTS