NUCLEOPHILICITY

Dear students,

Today i am picking up a very important aspect of alkyl halide chapter i.e Nucleophilicity.I think it requires  very simple clarification (which is generally messed up  by every one)about two terms. Nucleophilicity and Basicity.

The nature of the nucleophile has a major effect on the SN2 reaction. Any species, either neutral or negatively charged , can act as a nucleophile as long as it has an unshared pair of electrons.Let us analyze the nucleophile order of the reactivity with Bromoethane and relative rates of their reactions.

• Water-1
• Acetate-500.
• Ammonia-700.
• Chloride-1000
• Hydroxide-10,000
• Methoxide-25000
• Iodide-1,00,000
• Cyanide-1,25,000
• Hydrosulfide-1,25,000

The reasons for the reactivity difference is only  nucleophilic's nature or "Nucleophilicity". The  termis usually taken to be a measure of the affinity of a nucleophile for a carbon atom in the SN2 reaction( Please note-The more strongly nucleophilic the reagent the more the reaction will be promoted) .The nucleophilicity of a reagent correlate with its basicity, as both involve the availability of electron pairs and ease with which they are donated.Basicity involve  electron pair donation to hydrogen, whereas nucleophilicity involves electron pair donation to another atom generally carbon.Basicity involves an equilibrium (thermodynamic) to what extent reaction is forward,whereas nucleophilicity usually involves a kinetic i.e what is rate of reaction.

Basicity is little affected by steric hindrance,whereas nucleophilicity is greatly affected by steric hindrance.

•  Nucleophilicity  roughly parallels basicity : when comparing nucleophiles that have the same reacting atom, then the two runs reasonably in parallel, and we find the stronger the base the more powerful the nucleophile:   EtO- > PhO- > MeCO_2..
• Nucleophilicity usually increasing going down a column of the periodic table.  HS- > OH-and in halide reactivity I->Br->Cl- . Going down the periodic table size increase.Size as well as Electronegativity governs polarisability. As the atom increases in size the hold the nucleus has on the peripheral electrons decreases, with the result that they become more readily polarisable, leading to the initiation of bonding at increasing nuclear separations.Also the larger the nucleophile ion less its solvation . This combination of factors makes larger, highly polarisable , weakly solvated iodide ion, a very better nucleophile than opposite character fluoride ion.On above conclusion we can also expect that the increase in reaction rate on transfer from a hydroxylic to a polar non-protic solvent  (polar a protic one) to be much less for iodide ion than Br -or Cl -. In this way polar aprotic solvent has following order F->Cl->Br->I- .

• Negatively charged nucleophiles are usually more reactive than neutral ones.As a result, SN2 REACTIONS are often carried out under basic conditions rather than neutral or acidic conditions.