Organic Chemistry Workshop

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Transcription:

Organic Chemistry Workshop

Main Elements Carbon C Hydrogen H Nitrogen N Oxygen O Phosphorus P Sulfur - S

Valence Electrons

Drawing Lewis structures 1) Count the total valence electrons for the molecule: To do this, find the number of valence electrons for each atom in the molecule, and add them up. 2) Figure out how many octet electrons the molecule should have, using the octet rule: The octet rule tells us that all atoms want eight valence electrons (except for hydrogen, which wants only two), so they can be like the nearest noble gas. Use the octet rule to figure out how many electrons each atom in the molecule should have, and add them up. The only weird element is boron - it wants six electrons.

Drawing Lewis structures 3) Subtract the valence electrons from octet electrons: Or, in other words, subtract the number you found in #1 above from the number you found in #2 above. The answer you get will be equal to the number of bonding electrons in the molecule. 4) Divide the number of bonding electrons by two: Remember, because every bond has two electrons, the number of bonds in the molecule will be equal to the number of bonding electrons divided by two.

Drawing Lewis structures 5) Draw an arrangement of the atoms for the molecule that contains the number of bonds you found in #4 above: Some handy rules to remember are these: Hydrogen and the halogens bond once. The family oxygen is in bonds twice. The family nitrogen is in bonds three times. So does boron. The family carbon is in bonds four times. A good thing to do is to bond all the atoms together by single bonds, and then add the multiple bonds until the rules above are followed.

Drawing Lewis structures 6) Find the number of lone pair (nonbonding) electrons by subtracting the bonding electrons (#3 above) from the valence electrons (#1 above). Arrange these around the atoms until all of them satisfy the octet rule: Remember, ALL elements EXCEPT hydrogen want eight electrons around them, total. Hydrogen only wants two electrons.

Example

Formal Charge A concept know as formal charge can help us choose the most plausible Lewis structure where there are a number of structures which would all be satisfactory according to the rules used thus far. For example if we look at the cyanate ion, NCO-, we see that it is possible to write for the skeletal structure, NOC-, CNO-, or CON-. Using formal charge we can choose the most plausible of these three Lewis structures.

Formal Charge To determine the formal charge of an atom we first assign valence electrons to atoms in a Lewis structure as follows. All unshared (lone pair) electrons are assigned to the atom on which they are found. Half of the electrons in a bond between two atoms are assigned to one atom and half to the other.

After assigning all the valence electrons, we determine the formal charges as follows. The formal charge on an atom is the number of valence electrons on the free (nonbonded) atom minus the number of electrons assigned to that atom in the Lewis structure. We can state this mathematically as follows. FC = V - (L + 1/2 S)

Where V is the number of valence electrons on the free atom, L is the number of electrons present as lone pairs and S is the number of shared electrons. Note that the sum of the formal charges on all the atoms in a molecule will be zero while for an ion it will be equal to the charge on the ion. In using the concept of formal charge we should keep the following rules in mind.

Usually the most plausible Lewis structure is one with no formal charges (formal charges of zero on all atoms). Where formal charges are required, they should be as small as possible, and negative formal charges should appear on the most electronegative atoms. Adjacent atoms in a structure should not carry formal charges of the same sign. Formal charges on the atoms in a Lewis structure must total to zero for a molecule and to the net charge for a polyatomic ion.

Getting back to the example of the cyanate ion, the three Lewis structures with the formal charges indicated are shown below. Structure (a) would be the most plausible Lewis structure. a b c

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