Stable Structures
Stable carbon-containing compounds have four bonds to carbon.
H
CH4
CH3CH3
Methane
Ethane
H
C
C
H
H
C
C
H
Ethyne
(acetylene
H
Ethene
(Ethylene)
Three bonds to nitrogen...
H
N
N
H
H
H
H
Ammonia
C
H
H
H
H
Aminomethane
(methylamine)
H
C
C
N
H
Acetonitrile
Two bonds to oxygen
H
O
O
H
H
H
Water
C
O
H
C
H
H
H
Formaldehyde
Methanol
One bond to H, F, Cl, Br, I...
H
H
H
C
H
C
Br
H
C
Cl
H
Bromomethane
Chloroethene
(Vinyl Chloride)
Developing Reasonable Lewis Structures for Organic Molecules
1)
2)
3)
4)
5)
6)
7)
8)
9)
Draw the full structure of the molecules with the connectivity suggested by the
formula such that all valences are met (octets for C, N, O, F, Cl, Br, I)
Always show all valence shell electrons (including lone pairs)
a) Count the number of valence electrons in the entire molecule (each atom has the
same number of valence electrons as its group number in the periodic table).
b) From the number in a, subtract the number of bonding electrons (2 times the # of
lines in your structure). Any electrons that are unplaced are added to atoms that
lack a full octet (usually heteroatoms) as lone pairs.
c) Atome still lacking octets are completed using lone pairs from adjacent atoms to
form multiple bonds
Always show formal charges according to the following formula:
FC = # of valence electrons – (# of bonds to that atom) – (# of lone pair electrons)
Never draw structure with adjacent like charges
Do not place multiple charges on 1st or 2nd row atoms (+1 or -1 only), but try to avoid
charges (a neutral structure is always best).
Always place – charges on the more electronegative atom and + charges on the less
electronegative atom
Do not place charges on carbon if the overall molecule is neutral
Atoms below the 2nd row may have more than 8 electrons
Try to avoid O-O bonds
The periodic trend in Pauling's electronegativity...
For most (neutral) organic compounds, Lewis structures can be determined simply by
taking the connectivity implied (remember that there are 4 bonds to carbon, 3 to nitrogen, 2
to oxygen and 1 to hydrogen) and filling the octets.
CH3NH2
CH3OH
CH2O
H3CCN
H3CNO2
H2CNN
H2COH+
H3CSOCH3
HCONH2
H2NCONH2
(CH3O)2CH+
[CH2CHCH2]+
Functional Groups: "reactive centers that contain heteroatoms or multiple
carbon-carbon bonds.
H
H
H
C
H
C
N
H
C
H
O
H
C
Br
H
Heteroatoms: atoms in a molecule other than carbon or hydrogen.
Becoming familiar with common functional groups is very
helpful when trying to draw reasonable Lewis structures!!
Table 1.1 p. 4
The Functional Groups
Condensed structure
Name
O
COCl
C
Acid chloride
Nomenclature Suffix
(drop the e and add:)
-oyl chloride
Cl
OH
OH
Alcohol
-ol
CHO
Aldehyde
-al
O
C
H
C
C
Alkene
-ene
C
C
Alkyne
-yne
C o n d e n s e d s tru c tu re
N am e
N o m e n c la tu re S u ffix
( d r o p t h e e a n d a d d :)
O
C
H
N
CONH2
Amide
-amide
Amine
-amine
Carboxylic acid
-anoic acid
H
H
NH2
N
H
O
CO2H
C
OH
C o n d e n s e d s tru c tu re
N am e
O
C
CO2R
R
N o m e n c la tu re S u ffix
( d r o p t h e e a n d a d d :)
Ester
-anoate
Ether
ether
Ketone
-anone
Nitrile
-nitrile
Sulfide
sulfide
O
O
O
C
CO
C
S
N
CN
C o n d e n s e d s tru c tu re
S
S
O
N am e
N o m e n c la tu re S u ffix
( d r o p t h e e a n d a d d :)
S2
Disulfide
disulfide
SO3H
Sulfonic acid
-sulfonic acid
SO
Sulfoxide
O
S
OH
O
S
Thiol
SH
O
NO2
N
O
Nitro
-thiol
Rules for Drawing and Interpreting Resonance Structures
Resonance structures show how electrons are delocalized within a species. When
drawing and evaluating resonance structures, it is important to keep track of lone
pairs.
1) Only lone pair electrons and multiple-bond electrons (pi-electrons) move from one
resonance structure to another. Atoms remain in exactly the same place.
H2C
O
O
OH
O
C
C
C
C
H
H2C
H
H2C
H
H3C
H
2) From structure to structure, electron pairs move in the following ways:
lone pair to form an adjacent bond:
H
H
C
H
H
C
O
O
H
H
bond to form a lone pair:
O
O
S
S
H3C
H3C
CH3
CH3
bond to form a new bond:
H2C
H
H
C
C
CH2
H2C
CH2
3) Electrons move only to adjacent positions, but more than one pair of electrons can move
from one structure to another.
N
N
O
N
N
O
4) Always check to see what happens to the formal charges on specific atoms.
The net charge on each structure must be the same.
5) Structures that are identical in form are said to be degenerate. Such structures
contribute the same to the overall structure. Resonce "hybrids" are often drawn of
such structures.
H
C
H2C
H
H
=
C
CH2
H2C
CH2
C
H2C
CH2
6) When resonance structures are not degenerate, use the following rules to judge
which structures contribute more to the character of the species:
i) Resonance structures in which second period atoms (C-F) all have octets are favoured
over those with electon deficient atoms--resonance structures that have more than 8
electrons on such atoms are not valid.
H
H
C
H
H
C
O
O
H
H
ii) When
resonancestructures
structures
with
same
number
of bonds,
When comparing
comparing resonance
with
thethe
same
number
of bonds
(as in part
i above) structures
withformal
fewer formal
contribute
the character
a
structures
with fewer
chargescharges
contribute
moremore
to thetocharacter
of aofspecies
species.
H
O
C
H
H
O
N
C
H
H
N
H
iii) In cases where there are resonance structures with the same number of formal
charges, the favoured structure will be that in which the negative charge is on the more
electronegative atom or positive charge is on the more electropositive element.
N
N
O
N
N
O
Invalid Resonance Structures
i) have more than eight electrons on a second row element
N
N
O
N
N
X
O
N
N
O
ii) have less than eight electrons on O or N (or similarly electronegative element)
O
C
H
CH2
O
C
H
CH2
iii) have more than two atoms bearing formal charges (excepting polynitros and a few
others)
H
H
C
C
O
O
O
H
X
O
C
O
O
iv) have any atom with a formal charge greater than +/-1.
O
C
N
O
C
N
X
2O
C
N
HCO2
benzene
CH3NO2
H2CN2
HCONH2