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This is important throughout organic chemistry, but will be especially important when trying to determine the products of elimination and substitution (E1, E2, SN1, SN2)reactions.  There are generally three trends to remember when discussing how nucleophilic a reactant is:

1)      Size - Generally, the more linear and/or smaller the nucleophile, the more nucleophilic it will be.  This is because it can react at more sites and will not be sterically hindered if it is smaller or linear.

2)       Electronegativity- The more electronegative an atom is, the less nucleophilic it will be.   This is because more electronegative atoms will hold electron density closer, and therefore will be less likely to let that electron density participate in a reaction.  We see this in calculations and experiments that show nucleophilicity decreases as you get closer to fluorine on the periodic table (C > N > O > F)

3)      Polarizability- The more polarizable an atom is, the more nucleophilic it will be.   Polarizability is defined as the ability to distort the electron cloud of an atom, which allows it interact with a reaction site more easily.  Generally, polarizability increases as you travel down a column of the periodic table (I > Br > Cl > F)

 Below is a table of relative nucleophilic strength.  This is relative because nucleophilic strength is also dependant on other factors in the reaction, such as solvent. 

VERY Good nucleophiles

HS-, I-, RS-

Good nucleophiles

Br-, HO-, RO-, CN-, N3-

Fair nucleophiles

NH3, Cl-, F-, RCO2-

Weak nucleophiles

H2O, ROH

VERY weak nucleophiles

RCO2H

 

As shown above, as a general rule, the anion of a reactant will be a better nucleophile than the neutral form.  (i.e. RCO2- is a better nucleophile than RCO2H)

Step 2 is learning about the electrophiles.  Please visit our recent post on this topic –> electrophiles

For more information on this and other topics of organic chemistry interest, please visit organic chemistry