Review what we know about molecular formulas
Empirical and Molecular formulas tell us identity and
quantity of atoms. But how are these atoms connected?
Lewis structures are used to find
connectivity.
Works best for small molecules
As molecules get more complex, it becomes challenging to draw Lewis
structures using the method we gave you.
MJ Bojan
Chem 110
1
To aid in deducing the structure of molecules given
the molecular formula, we use the rules of valence.
Valence of atom is the number of covalent bond that it
typically forms
Rules of Valence
Element
H
C
N
O,S
F, Cl, Br
MJ Bojan
Chem 110
# Bonds
1
4
3
2
1
2
Using the rules of valence, structural formulas for
simple molecules can be deduced.
Molecular
formula
MJ Bojan
NH3
Condensed
molecular
formula
NH3
CO2
CO2
C2H6O
CH3CH2OH
C2H6O
or
structural
formula
CH3OCH3
Chem 110
3
Organic Chemistry is the chemistry of
compounds containing C bonded to H.
Millions of organic compounds are known.
Often contains O, N, S, and halogens
They are the main constituents of living matter:
DNA, RNA, proteins, enzymes, …
MJ Bojan
Chem 110
4
Organic Chemistry
Reasons for large numbers of organic compounds:
1. Carbon atoms can form strong, stable bonds to
other C atoms (thus forming rings, chains, etc.) and
to atoms such as H, O, N, S, halogens. (small size)
2. Carbon atoms form up to 4 bonds simultaneously:
(valence of 4) molecules can be branched.
3. Carbon atoms form multiple bonds with C or with O,
N, S: further structural variations are possible.
(Small size and valence of 4)
STRUCTURE AFFECTS FUNCTION
MJ Bojan
Chem 110
5
Classes of Organic Compounds
Classification is necessary to manage the large number
of compounds
HYDROCARBONS Simplest organic compounds
Contain only C and H
Classes of Hydrocarbons
H
H C
1) Alkanes
all single bonds
2) Alkenes
one or more double bonds
3) Alkynes
one or more triple bonds
4) aromatic
MJ Bojan
C C
H
H
alternating single, double
bonds in a ring
Chem 110
C
C
H
H
H
H
H
H
6
ALKANES
Name
Molecular
formula
Condensed molecular
formula
Lewis
Structures
H
methane
CH4
ethane
C 2H 6
H C
H
H
H H
CH3-CH3
H
C
C
H
H H
propane
C 3H 8
CH3CH2CH3
H
butane
C4H10
CH3CH2CH2CH3
pentane
C5H12
CH3CH2CH2CH2CH3
H
In general: CnH2n+2
MJ Bojan
Chem 110
H
H
H
C
C
C
H
H
H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
7
Structure of alkanes
All C atoms have tetrahedral geometry: the
bonds point at the vertices of a tetrahedron.
Bond angle = 109.5°
We can represent the chains as zig-zags
Both of these are
octane (C8H18):
or
an 8 carbon chain
MJ Bojan
Chem 110
8
What is the molecular formula?
n-octane
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
CH3
Iso-octane
CH 3
C
CH2 CH
CH3
MJ Bojan
Chem 110
CH3
CH 3
9
ISOMERS: are compounds that have the same
molecular formula, but different structures
Simplest example: butane (C4H10)
CH3−CH2−CH2−CH3
CH 3
2 isomers
CH
CH 3
CH 3
iso-butane
n-butane
MJ Bojan
Chem 110
10
ISOMERS: are compounds that have the same
molecular formula, but different structures
pentane has 3 isomers
CH3−CH2−CH2−CH2−CH3
CH 3
CH 2 CH
CH3
CH 3
CH3
CH 3
C CH3
CH3
iso-pentane
MJ Bojan
n-pentane
Neo-pentane
Chem 110
11
CYCLIC ALKANES:
Alkanes can form rings: CnH2n
Examples:
Cyclopentane
Cyclohexane
C5H10
C6H12
CH2
CH2
CH2
CH2
CH2
CH3
CH3
1,1-dimethylcyclobutane
MJ Bojan
Chem 110
12
New skills related to molecular formulas have
been added.
•  From now on know how to interpret:
–  Lewis structures
–  Condensed molecular formulas
–  Carbon backbone structures
•  To expand condensed molecular formulas
and carbon backbones into Lewis structures
use the rules of valence.
MJ Bojan
Chem 110
13
Can you interpret this structure?
This is the structure of
4-ethyl-2,3,3-trimethylnonane
What is the molecular formula of this compound?
How many −CH3 groups are there on this compound?
Mary J. Bojan
Chem 110
14