Org. Chem I Handout #1 DRAWING LEWIS STRUCTURES General rules to follow: 1.
2.
3.
Draw only the valence electrons Assign no more than 8 electrons to second‐row elements Each hydrogen only gets two electrons The number of bonds in most covalent molecules or polyatomic ions may be calculated using the equation: # of covalent bonds = # of electrons shared / 2 (electrons per bond) In neutral species: Element need ‐ available = # e‐ shared carbon 8 ‐ 4 = 4 hydrogen 2 ‐ 1 = 1 nitrogen 8 ‐ 5 = 3 oxygen 8 ‐ 6 = 2 How to draw Lewis structures: Step 1: Arrange the atoms next to each other with hydrogen and halogen atoms on the periphery (since they can only form one bond). Example: H
For CH4:
H
H
C
put H's on the periphery
H
H
For CH3OH:
H
O
C
H
H
Step 2: Count the electrons. Examples: CH4 CH3OH 1 C x 4 e‐ = 4 e‐ 4 H x 1 e‐ = 4 e‐ TOTAL = 8 e‐ 1 C x 4 e‐ 1 O x 6 e‐ 4 H x 1 e‐ TOTAL= 14 e‐ Copyright © Doug E. Frantz The University of Texas at San Antonio Org. Chem I Handout #1 Step 3: Add the bonds and lone pairs. H
For CH4:
H
8 e- total (2 per bond)
all e- accounted for
correct Lewis structure
H
C
H
H
H
add lone pairs
For CH3OH:
H
O
C
H
H
H
C
O
H
H
14 e- total
correct Lewis structure
does not satisfy
octect rule
only 10 eaccounted for so far
Formal Charges You can determine the formal charge of each atom using the following equation: Formal charge = # of valence e­ ­ # of e­ an atom “owns” For simplicity, an atom “owns” all of its unshared e‐ and half of its shared electrons. The formal charge of a molecule is the sum of the formal charges of all of its constituent atoms. Example: Calculate the formal charge on oxygen. H3C O CH3
CH3
1.
2.
# of e‐ owned by O = 2 (lone pair) + ½(6) = 5 formal charge on O = 6 – 5 = +1 Formal charges are always indicated with a + or – sign to indicate the charge as positive or negative. Resonance Structures Many times it is more realistic (based on physical data) to represent a molecule or ion as a composite of a number of classical structures. Example: The nitrate anion. O
O
O
O
O
O
N
N
N
O
O
O
!
O
O
N
The ! symbol is used to
indicate "partial" charge
(either positive or
negative)
O
!
classical, localize
contributing structures
!
non-classical, delocalized
hybrid resonance structure
dashed lines are using to
indicate "partial" bonding
character
Copyright © Doug E. Frantz The University of Texas at San Antonio