How do you determine if you have an electrophile or a nucleophile?

It's not always a black/white "it's definitely this or that", but...

PROPERTIES OF TYPICAL NUCLEOPHILE

• Should be relatively sterically unhindered for nucleophilic behavior.

Example:

• If negatively-charged, consider the possibility of either basic behavior or nucleophilic behavior.

Example:

• If it has a lone pair of electrons, consider the possibility of either basic behavior or nucleophilic behavior. If so, then it should definitely be in an aprotic solvent.

Example:

• #stackrel(..)("N")"H"_3#
• #"R"stackrel(..)("N")"H"_2#
• etc.

• Should be in an aprotic solvent, especially if the nucleophile is negatively-charged or has a lone pair (could undesirably steal a proton and deactivate itself):

Example:

AND has a conjugate acid #"pKa"# higher than the #"pKa"# of the solvent (remember the equilibrium lies on the side of the weaker acid, so the higher conjugate acid #"pKa"# means the equilibrium would favor protonating the nucleophile and deactivating it):

Example:

PROPERTIES OF TYPICAL ELECTROPHILE

• If positively-charged, generally it makes a nice electrophile.

Example:

• If a particular atom would possess a partial positive formal charge in a fairly polar bond:

Examples:

• 
• 
• #"HCl"#
• #"HBr"#
• #"HI"#
• etc.

or if the molecule is polarizable so that the electron distribution can be disturbed towards the more electronegative atom (away from the nucleophile):

Examples:

• #"HCl"#
• #"HBr"#
• #"HI"#
• #"Br"_2#
• #"I"_2#
• etc.

then it is probably an electrophile.