Electrophiles and Nucleophiles

Exercise Solutions
 
a. The curved arrow that starts at the sulfur lone pairs and ends at the carbon bearing the iodine atom indicates that the sulfur is donating a pair of electrons to form a new S-C bond.  This means CH3S- (methanethiolate) is a nucleophile in this reaction.  As there are only two reactants, the other reactant must by default be an electrophile.  We can verify this by noting that the iodoalkane is accepting a pair of electrons from the sulfur atom to form a new covalent bond.  (This is an example of the SN2 reaction.)

b. In the ionization of the bromoalkane to form a carbocation and bromide ion no new bonds are formed.  Therefore this ionization process does not involve a nucleophile or electrophile.  The reverse process, reaction of the carbocation with bromide ion to form a bromoalkane, does result in a new bond.  The curved arrow that starts at the bromide ion indicates a bromine lone pair is shared with the carbon bearing the formal positive charge resulting in a new C-Br bond.  Therefore bromide ion is a nucleophile.  The carbocation is accepting an electron pair to make a new C-Br bond, so it is an electrophile.

c. The hydroxide ion is sharing a lone pair with the hydrogen atom, resulting in a new O-H bond.  Therefore the hydroxide ion is a nucleophile.  We might also notice that hydroxide ion bears a formal negative charge, and that ions bearing a full negative charge are electron rich and therefore rarely act as electron acceptors (i.e., electrophiles).  Because there are only two reactants and one of these is clearly the nucleophile, the chloroalkane must therefore be the electrophile.  (This is an example of the E2 reaction.)

d. In the first step of the reaction, a p bond of benzene (C6H6) is shared with the nitronium ion (NO2+) to become a new C-N bond.  Therefore benzene is the nucleophile and the nitronium ion is the electrophile.  The product of this electrophilic attack on the benzene ring is called an arenium ion.  In the second step, a lone pair from a water molecule is used to make a new O-H bond with a hydrogen that is plucked from the arenium ion.  Because the water is donating the lone pair, it is the nucleophile.  This means the arenium ion is the electrophile in the second step.  (This is an example of electrophilic aromatic substitution.)

e. Tert-butyl carbocation: The central carbon of this ion bears a formal positive charge and has an open octet.  Both of these features suggest this cation will behave primarily as an electrophile.
f. Hydroxide ion: The oxygen atom has three lone pairs and a formal negative charge.  These features suggest this anion will behave principally as a nucleophile.
g. Aluminum chloride: The aluminum atom has an open octet, suggesting the molecule will behave as an electrophile.  Each chlorine has three lone pairs, suggesting a possibility for nucleophilic properties as well.  Neutral halogen atoms do not share electrons very often, however, because it would give them a formal positive charge.  Because of their high electronegativity, halogen atoms do not tolerate a positive formal charge very well.  The main exception to this rule is when a halogen can share a lone pair with a neighboring carbocation resulting in a structure in which all atoms have complete octets.  Recall from our studies of resonance that completion of octets is more important than location of formal charges.
h. Acetylene:  We predict this alkyne will behave primarily as a nucleophile because it has two p bonds to share.  A C-H bond is almost nonpolar because of the small difference in electronegativity of carbon and hydrogen.  Thus the molecule has no obviously electrophilic sites.
i. Formaldehyde: The lone pairs on oxygen and the C-O p bond suggest this molecule might behave an a nucleophile.  Because carbon is less electronegative than oxygen, the C=O bond is polarized with a d+ charge on carbon and a d- charge on oxygen.  This observation gives another reason why the molecule is a nucleophile, and also suggest it can be an electrophile.  Thus formaldehyde could be a nucleophile or electrophile, depending upon what else was in the reaction.  This dual reactivity is a common feature of many more complex molecules. Because this dual reactivity is due to the carbonyl group (C=O), any molecule containing this feature is expected to behave as both a nucleophile and electrophile.
j. Methyl chloride: The lone pairs on chlorine suggest this molecule could be a nucleophile (see answer g above for more discussion).  Carbon is less electronegative than chlorine, so the C-Cl bond is polar, with a d+ charge on the carbon and a d- charge on chlorine.  That the carbon bears a partial positive charge suggests that the carbon atom is electrophilic.  Thus this molecule could be a nucleophile (low probability) or electrophile.
k. Acetamide: This molecule contains a carbonyl group (C=O).  As mention in answer i above, this means it can be both nucleophilic and electrophilic.  The nitrogen lone pair is an addition nucleophilic site.
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