In the earlier days, the
conventional names for organic compounds were mainly derived from the source of
occurrence. However organic chemists realized the need for a systematic naming
for organic compounds since a large number of organic compounds are synthesized
in due course. This leads to setting up a system of nomenclature by "International
Union of Pure and Applied Chemistry, IUPAC" .
The IUPAC system of nomenclature
is a set of logical rules framed which are mainly aimed at giving an
unambiguous name to an organic compound. By using this system, it is possible
to give a systematic name to an organic compound just by looking at its
structure and it is also possible to write the structure of organic compound by
following the IUPAC name for that compound.
On this page, I have given a logical
introduction to IUPAC nomenclature. A concise and unified approach is followed
to help in giving IUPAC names to almost all types of compounds. This is not an
exhaustive reference to IUPAC nomenclature. However this is more than suffice
to all the students at various levels of their learning curve.
The systematic IUPAC name of
an organic compound consists of four parts.
1.
Root word
2.
Suffix(es) and
3.
Prefix(es)
4.
infix
The suffix is again divided into
primary and secondary.
The complete systematic IUPAC name
can be represented as:
* The root
word and 1osuffix together is known as base name.
* The Prefix(es), infix and 2o
suffix may or may not be required always.
1) Root
word: It indicates the number of carbon
atoms in the longest possible continuous carbon chain also known as parent
chain chosen by a set of rules. The root words used for different length of
carbon chain (upto 20) are shown below.
Number of carbon atoms in the parent chain
|
Root word
|
1
|
Meth
|
2
|
Eth
|
3
|
Prop
|
4
|
But
|
5
|
Pent
|
6
|
Hex
|
7
|
Hept
|
8
|
Oct
|
9
|
Non
|
10
|
Dec
|
11
|
Undec
|
12
|
Dodec
|
13
|
Tridec
|
14
|
Tetradec
|
15
|
Pentadec
|
16
|
Hexadec
|
17
|
Heptadec
|
18
|
Octadec
|
19
|
Nonadec
|
20
|
Icos
|
2) Suffix: It is again divided into two types.
i.
Primary suffix and
ii.
Secondary suffix
i) Primary suffix: It is used to indicate the degree of saturation or
unsaturation in the main chain. It is added immediately after the root
word.
Type of carbon chain
|
Primary suffix
|
Saturated
(all C-C bonds)
|
-ane
|
Unsaturated:
one C=C
|
-ene
|
Unsaturated:
two C=C
|
-diene
|
Unsaturated:
one C≡C
|
-yne
|
Unsaturated:
two C≡C
|
-diyne
|
Unsaturated:
one C=C & one C≡C
|
-enyne
|
ii) Secondary suffix: It is used to indicate the main
functional group in the organic compound and is added immediately after the 1o
suffix.
Note: If there are two or more
functional groups in a compound, the functional group with higher priority is
to be selected as main functional group, which must be indicated by a secondary
suffix. The remaining functional groups with lower priority are treated as substituents
and are indicated by prefixes.
The suffixes as well as prefixes
used for some important functional groups are shown in the following table in
the decreasing order of their priority.
Also note that different suffix is
used when carbon atom of the functional group is not part of the main chain.
Name of Functional group
|
Representation
|
Suffix
When carbon of the functional group is part of the parent chain
|
Suffix
When carbon of the functional group is NOT part of the parent chain
|
Prefix
|
carboxylic acid
|
-COOH
|
-oic acid
|
-carboxylic acid
|
carboxy-
|
Acid anhydride
|
|
-oic anyhydride
|
-carboxylic anhydride
|
-
|
Ester
|
-COOR
|
alkyl -oate
|
alkyl -carboxylate
|
alkoxycarbonyl-
|
Acid halide
|
-COX
|
-oyl halide
|
-carbonyl halide
|
halocarbonyl-
|
Acid amide
|
-CONH2
|
-amide
|
-carboxamide
|
carbamoyl-
|
Nitrile
|
-CN
|
-nitrile
|
-carbonitrile
|
cyano-
|
Aldehyde
|
-CHO
|
-al
|
-carbaldehyde
|
oxo-
|
Ketone
|
-CO-
|
-one
|
-
|
oxo-
|
Alcohol
|
-OH
|
-ol
|
-
|
hydroxy
|
Thiol
|
-SH
|
-thiol
|
-
|
mercapto
|
Amine
|
-NH2
|
-amine
|
-
|
amino-
|
Imine
|
=NH
|
-imine
|
-
|
imino-
|
Alkene
|
C=C
|
-ene
|
-
|
-
|
Alkyne
|
C≡C
|
-yne
|
-
|
-
|
Note: This is not the complete
reference.
3) Prefix: The prefix is used to indicate the side chains,
substituents and low priority functional groups (which are considered as
substituents). The prefix may be added immediately before the root word or
before the infix.
The prefixes used for some common
side chains and substituents are shown below. (the prefixes for functional
groups are already given)
Side chain or Substituent
|
Prefix
|
-CH3
|
methyl-
|
-CH2CH3 (or)
-C2H5
|
ethyl-
|
-CH2CH2CH3
|
propyl-
|
|
isopropyl-
|
-CH2CH2CH2CH3
|
butyl
|
|
sec-butyl
(or)
(1-methyl)propyl
|
|
isobutyl
(or)
(2-methyl)propyl
|
|
tert-butyl
(or)
(1,1-dimethyl)ethyl
|
-X
|
halo-
|
-OR
|
alkoxy-
|
-NO2
|
-nitro
|
Remember that the alkyl groups
along with halo, nitro and alkoxy have the same preference. They have lower
priority than double and triple bonds.
3) Infix: The infixes like cyclo, spiro, bicyclo are added between
the prefix(es) and root word to indicate the nature of parent chain.
* The "Cyclo" infix is
used to indicate the cyclic nature of the parent chain.
* The "Spiro" infix is
used to indicate the spiro compound.
* The "Bicyclo" infix is
used to indicate the bicyclic nature of the parent chain.
The infixes are some times called as
primary prefixes.
1) The first step in giving IUPAC
name to an organic compound is to select the parent chain and assign a root
word.
2) Next, the appropriate primary
prefix(es) must be added to the root word to indicate the saturation or
unsaturation.
3) If the molecule contains
functional group or groups, a secondary suffix must be added to indicate the
main functional group. This is optional and not necessary if the molecule
contains no functional group.
4) Prefix the root word with the
infix "cyclo" if the parent chain is cyclic; or with the infix
"spiro" if it is a spiro compound; or with the infix
"bicyclo" if the compound is bicyclic.
5) Finally add prefix(es) to
the name if there are side chains or substituents on the parent chain.
E.g. The IUPAC name of the following compound is
arrived in steps mentioned below.
Step-1
|
How many carbons are there in the
parent chain?
|
4
|
Root word = "but"
|
Step-2
|
Saturated or Unsaturated?
|
Saturated
|
1osuffix =
"ane"
|
Step-3
|
Is there any functional group?
|
Yes. There is an alcohol group on
2nd carbon.
|
2osuffix =
"2-ol"
|
Step-4
|
Are there any side chains or
substituents?
|
Yes. There is a methyl group on
3rd carbon.
|
2oprefix =
"3-methyl"
|
Now add them to makeup the IUPAC
name of the compound.
You will learn how to select a
parent chain?; how to number the carbon atoms and give the locants to the
functional groups, side chains ? etc., in the following section.
The following rules are helpful in
assigning the systematic IUPAC name of an organic compound.
1) The selection of parent chain: The first step in naming an organic compound is to select
the parent chain and give the root word depending on the number of carbons in
it.
The longest continuous carbon chain
containing as many functional groups, double bonds, triple bonds, side chains
and substituents as possible is to be selected as parent chain.
Illustrations:
i) In the following molecule, the longest
chain has 6 carbons. Hence the root word is "hex-". Note that the
parent chain may not be straight.
ii) The root word for the
following molecule is "hept-" since the longest chain contains 7
carbons.
Do not come under the impression that the ethyl groups (-C2H5)
are side chains and the longest chain contains 5 carbons.
The shaded part shows the longest
chain that contains 7 carbons. Also look at the alternate way of writing this
molecule in which the ethyl groups are expanded to -CH2CH3.
iii)
In the following molecule, there are three chains of equal length (7 carbons).
However
the chain with more number of substituents (that with 3 substituents as
shown in the following diagram) is to be taken as the parent chain
iv) The double bonds and triple
bonds have more priority than the alkyl side chains and some other substituents
like halo, nitro, alkoxy etc. Hence, whenever there are two or more chains with
equal number of carbons, the chain that contains double or triple bond is to be
selected as the parent chain irrespective of other chain containing more number
of substituents.
There are two chains with 6 carbons.
But the chain with the a double bond as shown in the diagram (II) is to be
selected as the parent chain.
Note: The double bond has more
priority than the triple bond.
v) However, the longest chain must
be selected as parent chain irrespective of whether it contains multiple bonds
or not.
E.g. In the following molecule, the longest
chain (shaded) contains no double bond. It is to be selected as parent chain
since it contains more carbons (7) than that containing double bond (only 6
carbons).
vi) The chain with main
functional group must be selected as parent chain even though it contains less
number of carbons than any other chain without the main functional group.
The functional group overrides all
of above rules since it has more priority than the double bonds, triple bonds,
side chains and other substituents.
E.g. The chain (shaded) with 6
carbons that includes the -OH functional group is to be selected as parent
chain irrespective of presence of another chain with 7 carbons that contains no
functional group.
There are other situations which
will decide the parent chain. These will be dealt at appropriate sections.
i) The positions of double bonds or
triple bonds or substituents or side chains or functional groups on the parent
chain are to be indicated by appropriate numbers (or locants). The locants are
assigned to them by numbering carbon atoms in the parent chain.
Even though two different series of
locants are possible by numbering the carbon chain from either sides, the
correct series is chosen by following the rule of first point of difference as
stated below.
The rule of first point of difference: When
series of locants containing the same number of terms are compared term by
term, that series which contains the lowest number on the occasion of the first
difference is preferred.
For example, in the following
molecule, the numbering can be done from either side of the chain to get two
sets of locants. However the 2,7,9 is chosen since it has lowest number i.e., 2
on the first occasion of difference when compared with the other set: 3,4,9.
Actually the so called “Least Sum
Rule” is the special case of above “Rule of First point of Difference”. Though
looking simple, the least sum rule is valid only to chains with two
substituents, a special case. However use of Least sum rule is not advisable
when there are more than two substituents since it may violate the actual rule
of first point of difference.
Therefore, while deciding the
positions, we should always use "the rule of first point of
difference" only.
ii) If two or more side
chains are at equivalent positions, the one to be assigned the lower number is
that cited first in the name.
In case of simple radicals, the group
to be cited first in the name is decided by the alphabetical order of the first
letter in case of simple radicals. While choosing the alphabetical order, the
prefixes like di, tri, tetra must not be taken into account.
In the following molecule, 4-ethyl-5-methyloctane,
both methyl and ethyl groups are at equivalent positions. However the ethyl
group comes first in the alphabetical order. Therefore it is to be written
first in the name and to be given the lowest number.
Note: The groups: sec-butyl and
tert-butyl are alphabetized under "b". However the Isobutyl and
Isopropyl groups are alphabetized under "i" and not under
"b" or "p".
iii) However, if two or more
groups are not at equivalent positions, the group that comes first
alphabetically may not get the least number.
E.g. In the following molecule,
5-ethyl-2-methylheptane, the methyl and ethyl groups are not at equivalent
positions. The methyl group is given the least number according to the rule of
first point of difference.
But note that the ethyl group is
written first in the name.
iv) The multiple bonds (double or
triple bonds) have higher priority over alkyl or halo or nitro or alkoxy
groups, and hence should be given lower numbers.
E.g. In the following hydrocarbon,
6-methylhept-3-ene, the double bond is given the lower number and is indicated
by the primary suffix 3-ene. The position of methyl group is indicated by
locant, 6.
v) The double bond is preferred over
the triple bond since it is to be cited first in the name.
Therefore the double bond is to be
given the lower number whenever both double bond and triple bond are at
equivalent positions on the parent chain.
E.g. In the following hydrocarbon,
hept-2-en-5-yne, both the double and triple bonds are at equivalent positions.
But the position of double bond is shown by 2-ene. The counting of carbons is
done from the left hand side of the molecule.
vi) However, if the double and
triple bonds are not at equivalent positions, then the positions are decided by
the rule of first point of difference.
E.g. In the following hydrocarbon,
hept-4-en-2-yne, the double and triple bonds are not at equivalent
positions. The triple bond gets the lower number
Again note that the 4-ene is written
first.
vii) Nevertheless, the main
functional group must be given the least number even though it violates the
rule of first point of difference. It has more priority over multiple bonds
also.
For example, in the following organic molecule,
6-methyloct-7-en-4-ol, the -OH group gets lower number (i.e., 4) by numbering
the carbons from right to left.
v) If the side chains themselves
contain terms like di, tri, tetra etc., the multiplying prefixes like bis,
tris, tetrakis etc., should be used.
E.g. The two 1,2-dimethylpropyl groups are
indicated by the prefix "bis" as shown below
vi) If two or more side chains of
different nature are present, they are cited in alphabetical order.
* In case of simple radicals, they
are alphabetized based on the first letter in the name of simple radical
without multiplying prefixes.
E.g. In the following molecule, the
ethyl group is written first since the letter 'e' precedes the letter 'm' of
methyl in the alphabetical order. We should not compare 'e' in the word 'ethyl'
and 'd' in the word 'dimethyl'
* However the name of a complex
radical is considered to begin with the first letter of its complete name.
E.g. In the following case, “dimethylbutyl” is
considered as a complete single substituent and is alphabetized under
"d".