Docstoc

About Formal Charges

Document Sample
About Formal Charges Powered By Docstoc
					                           About Formal Charges
                                   CHEM 330 handout



The valence electrons of an atom in the bonded state (= one that is part of a molecule)
often cancel out its nuclear charge. Positive (nuclear) and negative (electronic) charges
are then electrostatically balanced. In some bonded states, however, an atom may find
itself surrounded by a greater or a fewer number of valence electrons than expected on
the basis of its nuclear charge. An electrostatic imbalance then occurs.

If the atom in question finds itself with more valence electrons than expected, then it will
be negatively charged. The atom will probably be inclined to behave as an electron
donor, and this propensity will govern much of its reactivity.

If the atom in question finds itself with fewer valence electrons than expected, then it will
be positively charged. The atom will probably be inclined to behave an electron
acceptor, and again this proclivity will control much of its reactivity.

Because organic reactions are movements of electrons, and because electrons always
move from electrostatically negative sites to positive ones, it is essential to know whether
the bonded state of an atom has created an electrostatic imbalance. This will allow us to
rationalize and predict the reactivity of such an atom, and – by extension – of the
molecule containing that atom.


Enter the formal charge


       The formal charge of an atom is a parameter that indicates whether
       the atom in question is electrostatically balanced or unbalanced.


The formal charge of an atom in a molecule is easily calculated from the complete Lewis
structure of the molecule (i.e., one that shows all bonding and nonbonding electron pairs).

One simply needs to count how many valence electrons (both bonding and nonbonding)
the atom in question has contributed to the molecule, i.e., how many valence electrons are
formally present around the atom, then decide whether the number of such valence
electrons corresponds to an electrostatic balance or an imbalance

The following examples illustrate how this is done.
CHEM 330                                           p. 2                         formal charges




Example 1: the formal charge on the C atom in methane, CH4

       Step 1: draw a complete Lewis structure of the molecule:

                                        H                     H

                                H       C   H             H   C    H

                                        H
                                                              H


       Step 2: count the number of valence electrons around the atom of interest.

       Each atom in a bonded pair of atoms has contributed one of its valence electrons
       to the electron pair that we call "bond." If we "shatter" the molecule so that each
       atom in a bonded pair retrieves one of the two electrons that form the bond (i.e., if
       we imagine the homolysis of each bond in the molecule), we will see how many
       valence electrons are present around each atom:

                           H
                                            "shatter"
                      H    C        H                         4H   +   C

                            H

       Conclusion: the C atom in methane is surrounded by 4 valence electrons

       Step 3: determine whether valence electrons balance the nuclear charge out.

       • Carbon is in group 4 of the periodic table, so it requires 4 valence electrons to
       balance the nuclear charge out.

       • The C atom in methane has 4 valence electrons

       • The C atom in methane is electrostatically balanced

                   Conclusion: the formal charge on C in methane is zero


       Notice that the formal charge on each H atom is also zero. Indeed, H
       atoms in any covalent molecule have always zero formal charge, as
       readily determined though the above logic.


                                    ===============
CHEM 330                                         p. 3                           formal charges




Example 2: the formal charge on the O atom in water, H2O

       Step 1: draw a complete Lewis structure of the molecule:

                               H                            H

                           H   O                    H       O           lone
                                                                        pairs


       Step 2: count the number of valence electrons around the atom of interest.

                               H
                                     "shatter"
                       H       O                        2H          +     O


       Step 3: determine whether valence electrons balance the nuclear charge out.

       • Oxygen is in group 6 of the periodic table, so it requires 6 valence electrons to
       balance out the nuclear charge.

       • The O atom in water has 6 valence electrons

       • The O atom in water is electrostatically balanced

                    Conclusion: the formal charge on O in water is zero


                  (the H atoms in water also have zero formal charge)

                                   ===============



Example 3: the formal charge on the N atom in NH4

       Step 1: draw a complete Lewis structure of the molecule:

                                                        H

                                   NH4 =    H           N       H

                                                        H
CHEM 330                                       p. 4                              formal charges




       Step 2: count the number of valence electrons around the atom of interest.

                        H
                                   "shatter"
                   H    N     H                       4H    +     N

                         H                                 4 valence electrons


       Step 3: determine whether valence electrons balance the nuclear charge out.

       • N is in group 5: it needs 5 valence electrons to balance out the nuclear charge.
       • The N atom in NH4 has 4 valence electrons: 1 fewer than it should.
       • The N atom in NH4 is electrostatically unbalanced

                       Conclusion: the formal charge on N in NH4 is + 1




       Important: formal charges are integral parts of a chemical structure and must be
       clearly indicated. This is done with encircled + or – signs. So, the correct way to
       draw NH4 is:

                                               H

                                         H     N      H

                                               H

Notice that the formal positive charge on the N atom is not balanced out by a negative
charge elsewhere in the molecule. Globally, therefore, the NH4 molecule is positively
charged, i.e., it is a cation. This particular cation is called the ammonium ion.

               The algebraic sum of formal charges of individual atoms
               in a molecule gives the total charge on the molecule

Why "formal" charge? Rigorously speaking, the + 1 charge present in NH4+ is
delocalized all over the molecule, i.e., each atom (N and 4 H's in this case) bears a share
thereof. For simplicity, however, it is convenient to think of it as if it were localized on
the N atom. That's why one calls it a formal charge: because for chemical reasoning it is
best to think of it as formally residing on the N atom.


                                   ===============
CHEM 330                                                 p. 5                                  formal charges




Example 4: the formal charge on the N atoms in hydrazoic acid, H–N=N=N:
H–N3 =     H        N        N   N

                                             "shatter"

   H       N             N       N                               proper way to draw hydrazoic acid:

    group 5          group 5         group 5                                  H–N=N=N
    5 val. e–        4 val. e–       6 val. e–
    zero             +1              –1                          or, since it is understood that each atom
                                                                 would have a complete Lewis octet:

                                                                                H–N=N=N

Notice that +1 and –1 formal charges in HN3 balance each other out. Overall, the
molecule is electrostatically neutral.

                                            ===============

Example 5: the formal charge on the C atom in the following four molecules:

                        CH3                 CH3                    CH3                CH2

               vacant
               orbital

                H                           H                       H

                C       H                   C      H                C     H             C      H

                H                           H                       H                   H



                 C                           C                      C                    C

                 +1                         zero                    –1                  zero



                    CH3                     CH3                     CH3                  CH2


Overall:       cationic                   neutral                 anionic             neutral

Called a "carbocation"                  a "radical"             a "carbanion"       a "carbene"

				
DOCUMENT INFO
Description: About Formal Charges