Structure and Nomenclature
Stable carbon-containing compounds have four bonds to carbon.
H
H
C
H
H
H
C
C
C
H
H
H
H
H
C
H
H
ethene
(ethylene)
ethane
C
ethyne
(acetylene)
Three bonds to nitrogen...
N
H
H
H
H
H
ammonia
N
C
H
H
H
H
aminomethane
(methylamine)
H
C
C
N
H
acetonitrile
1
Two bonds to oxygen...
H
H
O
H
O
O
C
H
C
H
H
water
H
H
methanol
f ormaldehyde
One bond to H, F, Cl, Br, I...
H
H
H
Br
C
C
H
bromomethane
H
H
C
Cl
chloroethene
(vinyl chloride)
2
Developing Reasonable Lewis Structures for Organic Molecules
1)
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).
2) 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) Atoms still lacking octets are completed using lone pairs from adjacent atoms to
form multiple bonds.
3) Always show formal charges according to the following formula:
FC = # of valence electrons – (# of bonds to that atom) – (# of lone pair electrons)
4) Never draw structures with adjacent like charges.
5) Formal charges of 0 is always better than charge separation.
6) Do not place multiple charges on 1st or 2nd row atoms (+1 or -1 only), and try to avoid
charges altogether (a neutral structure is always best).
7) Always place – charges on the more electronegative atom and + charges on the less
electronegative atom.
8) Do not place charges on carbon if the overall molecule is neutral.
9) In general, H only needs 2 electrons, B and Al need only 6 electrons, and atoms
below the second row may have more than 8 electrons.
10) Try to avoid O-O bonds.
The periodic trend in Pauling's electronegativity...
Drawing Organic molecules - Condensed and Line Structures.
Hydrocarbon fragments:
Name
Expanded form
Condensed form
H
methyl
H
C
CH3
H
H H
methylene
methine
CH2
C
CH
C
H
5
Writing condensed structures…
H
H
H H
O
C
C
C
C
H
H H
H
O
Full Structure (expanded form)
The condensed form of most functional groups is written backwards if the group
is found on the left hand side of the structure...
H
O CH2 CH3
HOCH2CH3 not OHCH2CH3
6
Writing line structures…
H
H
H H
O
C
C
C
C
C
H
H H
H
H
H
Expanded Structure
When drawing line structures:
•use lines to represent carbon-carbon bonds.
•ignore carbon-hydrogen bonds in hydrocarbon units.
•always draw heteroatoms and hydrogen atoms attached to heteroatoms.
•draw all the atoms for functional groups (for the most part).
O
O
C
H
C
C
N
S
H
O
H
C
H
CN
N
SH
SH
7
Line structures
are very
usf for
ul for
cyclic molecules
molecules.
Line structures
are very
useful
cyclic
H H
H
H C
C
C
H
H
H
H
H
C
C
C
O
C
C
H
OH
C
H
H
O H
H
C
C
N
O
NO2
H
These examples demonstrate how ‘clean’ line structures look
compared to expanded or even condensed structures.
Practice!
H
O
C
H
C
C
H H
C
C
C
H
H
H
H
H H
CN
9
Practice!
Cl H
H
H
C
N
C
C
H
H H
H
H
O
C
CH3
NH
10
Ultimately, organic molecules are usually drawn as a mixture of expanded,
condensed, and line structures. Be careful however, molecules can be drawn
many different ways and look completely different...
O
O
H3C CH2
CH2
O
C
CH2
C
H2
C
H
H3C
O
CH3CH2CH2COCH2CHO
H
H
O
O
C
C
C
C
C
H
H H
H H
C
H2
C
H2
O
H
O
CH3(CH2)2CCH2CH
H H
C
O
O
H
CHO
O
O
H
H
Warnings:
•The end of a line represents a methyl group not a hydrogen atom.
•If you draw ‘C’, you must draw all atoms attached to it.
O
11
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.
HCONH2
H2NCONH2
(CH3O)2CH+
[CH2CHCH2]+
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 any and all lone pairs
(unshared electron pairs).
1. Only lone pair electrons and multiple-bond electrons (pi-electrons) move from one
resonance structure to another. Atoms never move.
H2C
O
O
C
C
H
H2C
H
H2C
OH
O
C
C
H
H3C
H
13
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
H2C
C
H
H
CH2
H2C
C
=
CH2
H2C
C
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
OO
N
CC
H
HH
N
N
H
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 (Fatal Flaws which result in a 0!)
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 (except polynitros and a few
others)
O
H
H
C
C
O
O
H
X
O
O
C
O
iv) have any 2nd row atom with a formal charge greater than +/-1 (except S).
O
C
N
O
C
N
X
2O
C
N
HCO2
benzene
CH3NO2
H2CN2
HCONH2