[amani]

hello
i want to ask you about the nucleoiphile that react with alkyl halide in sn2 reaction ,how do we know which one is more reactive than another ?and what the relationship between its reactive and electronegativity and its size and why
thank you for your helping

[venkatjohnny]

hi,
nucleophilicity and basicity are almost same for some compounds, but based upon size we can tell that with increase in size basicity increases but not the nucleophilicity.(steric factor)
By considering electronegativity factor, the more the electronegativity lesser the nucleophilicity(tries to pull the electron density!!!)
For more details refer "Art of writing Organic reaction mechanism" by Robert b. Grossmon

[g-bones]

A quick way to check which nucleophile is stronger is to check the pKa of the conjugate acid.  the lower the pKa the weaker the nucleophile, usually. for example HCl has a really low pKa, so Cl- is not very nucleophilic