IUPAC nomenclature
Dr Margaret A.L. Blackie
mblackie@sun.ac.za
Naming of the Alkanes
Naming of the alkanes
1. Identify the main functional group
2. Identify the longest unbroken carbon
chain
CH3
CH3
H3C
CH
CH2
7 – carbon chain
CH2
CH2
CH3
CH
CH
CH2
CH3
8 – carbon chain
octane
base
This step is
not necessary
for the
alkanes –
there is no
functional
group.
NB: If the first substituent (side chain) has the same
number, use the second substituent to separate the
two etc …
3. Number the chain so that the first substituent
has the lowest possible number
CH3
CH3
CH2
CH3
H3C
CH
CH2
CH2
CH
CH
CH2
1
8
2
7
3
6
4
5
5
4
6
3
7
2
First substituent at carbon no. 2
Correct numbering:
CH3
8
1
First substituent at position no. 3
Substituents at 2, 5 and 6
4. Identify the substituents (side chains and
the position on the main chain or ring)
CH3
CH3
CH2
CH3
H3C
CH
CH2
CH2
CH
CH
CH2
1
2
3
4
5
6
7
2-methyl
5-ethyl
6-methyl
CH3
8
5. Use the prefixes (di-, tri-, tetra-) depending on
the number of identical substituents bonded to the
main chain
CH3
CH3
CH2
CH3
H3C
CH
CH2
CH2
CH
CH
CH2
1
2
3
4
5
6
7
2-methyl
5-ethyl
6-methyl
CH3
8
2,6-dimethyl
5-ethyl
6. Write down the side chains in alphabetical order,
preceded by the number: (don’t write down di-, tri-,
tetra-etc. in alphabetical order)
CH3
CH3
CH2
CH3
H3C
CH
CH2
CH2
CH
CH
CH2
1
2
3
4
5
6
7
5-ethyl-2,6-dimethyloctane
Dimethyl comes after ethyl, because e is
before m – the d of di is not considered.
CH3
8
The name should not be separated (there are some
exceptions which will be addressed later.)
Numbers separated by a comma
Dimethyloctane is
written as one word
5-ethyl-2,6-dimethyloctane
Numbers separated from words by a hyphen (N shows a
position and is considered as a number when naming the
amines and amides)
Note: The name is WRONG if any one of these three
rules are disobeyed!
Important: The name is WRONG if any one of these rules
are disobeyed!
5-ethyl-2,6-dimethyloctane
What is wrong with the names below?
5-ethyl-2-6-dimethyloctane
Name is wrong – the hyphen
between the numbers are incorrect
5,ethyl,2,6,dimethyloctane
The name is wrong – the comma
between a number and a word is
incorrect
5-ethyl-2,6-dimethyl octane
Name is wrong – There should be no
space between dimethyl and octane
Important points for purposes of
explanation
The longest unbroken carbon chain is not necessarily
straight
CH3
HC
H3C
H2C
CH2 CH
CH2
CH2
CH3
CH3
CH2
Longest straight chain
contains only 5 carbons
Longest unbroken chain
contains 8 carbons
Therefore the name is
octane and not pentane
If there are two options for an unbroken carbon chain:
Choose the chain with the most side chains as
the base
CH3
HC
H3C
CH2
Two substituents
H2C
CH2 CH
CH2
CH2
CH3
CH
CH3
CH3
Three substituents
Correct name: 2,6-dimethyl-4-propyloctane
Incorrect: 3-methyl-5-isobutyloctane
If there is no difference in the number of the position of the
first side chain, look at the following side chain, if there is no
difference at the second, look at the third, etc.
CH3
H3C
1
CH CH
2
3
CH3
CH3
CH2
4
CH CH3
5
6
CH3
H3C
6
CH CH
5
4
CH3
CH2
3
CH CH3
2
1
CH3
Correct numbering
First substituent is at position 2 in both numberings
Second substituent at positions 3 and 4 respectively.
Correct name : 2,3,5-trimethylhexane
When there is no difference in the numbers of the positions of
the substituents : choose the numbers according to alphabetical
order of the substituents, that is the first substituent
alphabetically gets the lowest number
CH3
CH3
H3C
1
CH2 CH
2
3
CH
4
CH2
CH2 CH3
5
6
CH3
3-methyl-4-ethylhexane
H3C
6
CH2 CH
5
4
CH
3
CH2
CH2 CH3
2
1
CH3
3-ethyl-4-methylhexane
Correct name
When a functional group occurs
At grade 12 level the following step is unnecessary,
but is included for educators.
Identifying the main functional group is the first step in
naming the compound.
Functional groups are prioritised according a list (see
next slide)
The main functional group is the functional group with
the highest priority.
The base is the longest unbroken carbon chain
containing the main functional group.
FUNCTIONAL GROUPS
Highest to lowest priority
General name
General
structure
Carboxylic acid
R
Name as
suffix
OH
-oic acid
OR'
-oate
O
Ester
R
O
Aldehyde
R
H
-al
R'
-one
O
Ketone
R
O
Alcohol
R OH
Alkene
Alkyne
-ol
-ene
R C C R'
-yne
The highest priority group must get
the lowest possible number
H3C
1
C
2
Subsituents:
3-bromo
4-methyl
H3C
O
CH
3
CH
4
CH3
5
Br
Base – pentan-2-one
Number from left to right
Main functional group – ketone – suffix ‘one’
Longest unbroken carbon
chain containing the main
functional group – 5 carbons ‘pentane’
Correct name:
3-bromo-4-methylpentan-2-one
H3C
O
H3C
1
C
2
CH
3
CH
4
CH3
5
Br
correct name:
3-bromo-4-methylpentan-2-one
3-bromo-4-methyl-2-pentanone is also acceptable
It is better to write the number directly in front
of the main functional group
H3C
O
H3C
1
C
2
CH
3
CH
4
CH3
5
Br
Correct name:
3-bromo-4-methylpentan-2-one
Main functional group – ketone – prefix ‘one’
Longest unbroken carbon chain containing
the main functional group – 5 carbons –
H3C
unsaturated chain - ‘pentene’-
Consider:
O
H3C
C
CH
C
CH2
Base – pent-4-ene-2-one
Br
correct name:
3-bromo-4-methylpent-4-ene-2-one
Number or no number?
O
H3C
C
CH2 CH2
CH3
Main functional group: ketone – prefix ‘one’
Number necessary because
the carbonyl group can be at
position 2 or 3
correct name: Pentan-2-one
H3C
CH2 CH2 CH2
CH
O
Main functional group : aldehyde – prefix ‘al’
Pentan-1-al is NOT
acceptable
No number on the aldehyde
because the carbonyl group
must be on the first carbon
correct name: Pentanal
Revise: the main functional group always gets the lowest possible number. The
carbonyl group is always at the end or beginning of the chain, therefore it must
be in position number 1.
Number or no number?
CH
H3C
H3C
CH
correct name: But-2-ene
CH2
H3C
CH
CH3
CH
correct name: Propene
CH2
CH2
But-1-ene is a possibility, so the
number is important.‘
There is no alternative,
propene is prop-1-ene
The locant is redundant in this
particular case and should be dropped.
The redundancy is associated with this particular
molecule not the nature of the functional group
so inclusion of the locant is acceptable
Questions
1.
2.
3.
4.
5.
On the previous slide But-2-ene and But-1-ene are called… ?
Give a brief explanation for the names given in question 1.
Write down the molecular formula for the above compounds.
Write down the structural formulae for 2 more possible structures.
Name the structures completed in question 4.
Answers
1. ISOMERS
2. Isomers are compounds with the same molecular formula but
different structural formulas .
4. Write down the structural formulae for 2 more possible structures.
H
H
CH2
H3C
C
H
C
C
C
C
H
CH
2-MethylPropene
H
H
H
H
Cyclobutane
Carboxylic acids and derivatives
CH2
H3C
O
C
OH
Carboxylic
acids
Propanoic acid
CH2
CH2
H3C
C
H3C
O
O
Cl
Propanoyl chloride
(not for exam)
O
C
-
Propanoate ion
Identify carboxylic acid
that produces the ester
This part is named as with the
corresponding ion - propanoate
O
H3C CH2 C
O CH3
Methylpropanoate
Identify the substituent on the
carboxylate oxygen
This part is named as the alkyl group – methyl
Esters