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CHEMISTRY 2500
Topic #2: Isomers and Naming Organic Molecules
Fall 2012
Dr. Susan Findlay
Isomers

What happens when an organic molecule is simply described by
its molecular formula?

In a few cases, the molecular formula gives enough information.
Draw:
CH4
C2H6
C3H8
CH4O
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Isomers
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Usually, molecular formula is not enough. Draw:
C4H10
C5H12
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For practice, draw each of the molecules’ structural formula then convert it to line-bond notation.
Isomers
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Molecules which have the same molecular formula but different
connectivity are referred to as constitutional isomers.
Constitutional isomers often behave differently due to their
different shapes. In some cases, they even have different
functional groups!
Draw all of the constitutional isomers for C3H8O, and identify the
functional group in each molecule.
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For practice, draw each of the molecules’ structural formula then convert it to line-bond notation.
Isomers
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Now we will add just one carbon atom… Draw all of the
constitutional isomers for C4H8O.
If you Google this question, you’ll find websites telling you there are 1, 5 or 15 structures.
They’re wrong. Be very careful asking the Internet for homework help…
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Please don’t actually do this before class; you’ll spoil the learning experience.
Isomers
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Nomenclature (Naming Organic Molecules)
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When you have to draw a lot of isomers, one way to be sure
that no two are the same is to name them all. If the names are
different, the isomers are different. If the names are the same,
you drew the same thing twice.
Nomenclature also makes it easier for us to talk about organic
chemistry in situations where we can’t just draw the molecules.
The rules for organic nomenclature are the same as those you
learned in CHEM 2000 (see next page); however, we will now
deal with a few more functional groups, a few more different
kinds of substituents and molecules which contain more than
one kind of functional group.
For those who have not taken (or do not recall) CHEM 2000,
nomenclature is described in detail in Sorrell pages 8-18. You
are responsible for this information.
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Organic Nomenclature (CHEM 2000 Rules)
To name an organic molecule, work back-to-front:


O
Br

CH3O
Br
CH3O
5

Br
O
4
2
3
Br
Find the principal functional group.
Find the longest chain (or ring) including the principal functional
group and all double/triple bonds.
Number the chain starting at whichever end gives the principal
functional group the lower number. (For a ring, start numbering
at the principal functional group.)
1
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
Name the principal functional group, numbering if necessary.
-2-one
Name the main chain (or ring), numbering the C=C or CC bonds
if necessary. If this gives a name in which the next letter after the
‘e’ of ‘ane’, ‘ene’ or ‘yne’ is a vowel, drop the ‘e’.
-3-penten-2-one

Name and number the substituents on the main chain. If a
substituent appears more than once, use a prefix to indicate how
many there are and include a number for each appearance. List
the substituents in alphabetical order (not counting prefixes)
followed by the main chain (or ring) name.
3,4-dibromo-5-methoxy-3-penten-2-one

Finally, add cis- or trans- (or E- or Z- or R- or S-) to the front of
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the name if necessary.
trans-3,4-dibromo-5-methoxy-3-penten-2-one
Organic Nomenclature
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Essentially, the name of an organic compound consists of up to
four fields, each of which gives specific information about the
compound: (diagram from Sorrell, p.9; 3rd field re-named)
Saturation index
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Organic Nomenclature
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The simplest names only involve the second and third fields
which indicate the number of carbon atoms in the main chain
(second field) and the number and type of multiple bonds (third
field). For example:
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Hexane has six carbon atoms and no multiple bonds (third field =
“ane”).
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Hex-1-ene has a single double bond starting with carbon #1.
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Hex-1-yne has a single triple bond starting with carbon #1.
(also called 1-hexene)
(also called 1-hexyne)
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Organic Nomenclature
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The following prefixes are used to indicate the length of the
main chain/ring:
#C
Compound
root
#C
Compound
root
1
meth
6
hex
2
eth
7
hept
3
prop
8
oct
4
but
9
non
5
pent
10
dec
For rings, add cyclo before the compound root:
e.g. cyclopentane
cyclopentene
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Organic Nomenclature
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In cases of multiple multiple bonds, use the prefixes di, tri or
tetra to indicate how many double (or triple) bonds are in the
molecule and give a number for the location of each multiple
bond.

hexa-1,3-diene is not the same as hexa-1,5-diene
(1,3-hexadiene and 1,5-hexadiene are also acceptable names)

hex-1-en-3-yne is not the same as hex-1-en-5-yne
(1-hexen-3-yne and 1-hexen-5-yne are also acceptable names)
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Organic Nomenclature
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Attaching groups to the chain introduces the first and fourth
fields.
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Halogens are not considered “principal functional groups” so
halogens are treated in the same way as carbon chains branching
off the main chain. They are described in the “substituents” section
at the beginning of the name. Substituents take last priority when
numbering the main chain, so substituting Cl for one of the H in
hex-1-ene will not change the numbering:
e.g. 5-chlorohex-1-ene (or 5-chloro-1-hexene)
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Organic Nomenclature
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Alcohols, amines and carbonyl-containing functional groups are
considered candidates for “principal functional group”. Since there
can only be one kind, use the table on p.16 to determine which.
The principal functional group takes priority when numbering the
main chain. As such, substituting OH for one of the H in 5chlorohex-1-ene may result in a re-numbering:
5-chlorohex-1-en-3-ol is correct:
5-chlorohex-1-en-6-ol is NOT. It must be 2-chlorohex-5-en-1-ol:
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Organic Nomenclature
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If there are multiple functional groups, all those which are not the
principal functional group are treated as substitutents.
Aldehydes are higher priority than alcohols, so turning the terminal
methyl group in 5-chlorohex-1-en-3-ol into an aldehyde relegates
the alcohol (hydroxy group) to substituent status, giving us
2-chloro-4-hydroxyhex-5-enal:
No number is necessary for principal functional groups which must
be terminal since they, by definition, include carbon #1. These
principal functional groups include:
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Organic Nomenclature
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Principal functional groups (from highest to lowest priority):
Functional Group
Suffix
Carboxylic acid
-oic acid
Sulfonic acid
-sulfonic acid
Ester*
-oate
Acid Chloride
-oyl chloride
Amide*
-amide
Nitrile
-nitrile
Aldehyde
-al
Ketone
-one
Alcohol (including phenol)
-ol
Thiol
-thiol
Amine
-amine
*Esters and amides have two ‘halves’ to be named. For details on
naming esters and/or amides, consult your CHEM 2000 notes
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and/or Sorrell p.702.
Organic Nomenclature
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Substituents (including functional groups which were lower priority
than the principal functional group):
Name
Name
Name
-CH3
methyl
-OCH3
methoxy
-F
fluoro
-CH2CH3
ethyl
-OCH2CH3
ethoxy
-Cl
chloro
-CH2CH2CH3
propyl
-OCH2CH2CH3
propoxy
-Br
bromo
-CH2CH2CH2CH3
butyl
-OCH2CH2CH2CH3
butoxy
-I
iodo
-NH2
amino
etc.
etc.
-CH(CH3)2
isopropyl
-OCH(CH3)2
isopropoxy
-NO2
nitro
-CH2CH(CH3)2
isobutyl
-OCH2CH(CH3)2
isobutoxy
-CN
cyano
-CH(CH3)CH2CH3
s-butyl
-OCH(CH3)CH2CH3
s-butoxy
-C(CH3)3
t-butyl
-OC(CH3)3
t-butoxy
-OH
hydroxy
-C6H5
phenyl
-OC6H5
phenoxy
=O
oxo
-CH2C6H5
benzyl
-OCH2C6H5
benzoxy
-SH
mercapto
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Organic Nomenclature
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Other names you should know:
benzene
toluene*
aniline*
phenol
benzaldehyde
benzoic acid
* = not official IUPAC name but a very common name that you’re likely to encounter
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Organic Nomenclature
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And now for some practice… Name the following:
If you’re not yet comfortable with line-bond structures, redraw each of these molecules
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as the structural formula first.
Organic Nomenclature
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Organic Nomenclature
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Classification of Carbon Atoms

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Carbon atoms are often classified according to how many other
carbon atoms they are bonded to. In the last example on the
previous page, there are examples of primary, secondary,
tertiary and quaternary carbon atoms:
These classifications only apply to saturated carbons (no
multiple bonds). The carbon atoms that form the benzene ring
are described as aryl to indicate that they are part of an
aromatic ring. Other carbon atoms from double bonds are
classified as alkenyl while those from triple bonds are classified
as alkynyl. We will use this terminology throughout the course.
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Classification of Carbon Atoms
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Occasionally, we also need to refer to carbon atoms (or other
groups) in terms of distance from each other. For this purpose,
Greek letters are used.
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 refers to the atom attached to a group
 is the next atom
 is the one after that
 is the one after that
 is the one after that
etc.
In one example, the “group” of interest is a
carboxylic acid, and the Greek letters refer to
the carbon atoms:



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In another example, the “group” of interest is
a carbocation, and the Greek letters refer to the
hydrogen atoms:


H

H3C
C
+

 CH
3
C
H2
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