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IUPAC Nomenclature Chemistry Class - XI

Sudhansu Bhushan Roy
30/03/2017 0 0

IUPAC nomenclature of organic chemistry :

The  IUPAC nomenclature of organic chemistry  is a systematic method of naming organic chemical compounds as recommended  by the International Union of Pure and Applied Chemistry (IUPAC). Ideally, every possible organic compound should have a name from which an unambiguous structural formula can be created.

For ordinary communication, to spare a tedious description, the official IUPAC naming recommendations are not always followed in practice, except when it is necessary to give an unambiguous and absolute definition to a compound, or when the IUPAC name is simpler (e.g. ethanol instead of ethyl alcohol). Otherwise the common or trivial name may be used, often derived from the source of the compound (see below). In addition, very long names may be less concise than structural formulae.

Basic Principles :

In chemistry, a number of prefixes, suffixes and infixes are used to describe the type and position of functional groups in the compound.

The steps to naming an organic compound are:

  1. Identification of the parent hydrocarbon chain. This chain must obey the following rules, in order of precedence:
  • It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest precedenceshould be used.
  • It should have the maximum number of multiple bonds
  • It should have the maximum number of single bonds.
  • It should have the maximum length.
  1. Identification of the parent functional group, if any, with the highest order of precedence.
  2. Identification of the side-chains. Side chains are the carbon chains that are not in the parent chain, but are branched off from it.
  3. Identification of the remaining functional groups, if any, and naming them by the their ion names (such as hydroxy for -OH, oxy for =O, oxyalkane for O-R, etc.). Different side-chains and functional groups will be grouped together in alphabetical (The prefixes di-, tri-, etc. are not taken into consideration for grouping alphabetically. For example, ethyl comes before dihydroxy or dimethyl, as the "e" in "ethyl" precedes the "h" in "dihydroxy" and the "m" in "dimethyl" alphabetically. The "di" is not considered in either case). When both side chains and secondary functional groups are present, they should be written mixed together in one group rather than in two separate groups.
  4. Identification of double/triple bonds.
  5. Numbering of the chain. This is done by first numbering the chain in both directions (left to right and right to left), and then choosing the numbering which follows these rules, in order of precedence:
  • Has the lowest-numbered locant(or locants) for the suffix functional group. Locants are the numbers on the carbons to which the substituent is directly attached.
  • Has the lowest-numbered locants for multiple bonds (The locant of a multiple bond is the number of the adjacent carbon with a lower number).
  • Has the lowest-numbered locants for double bonds.
  • Has the lowest-numbered locants for prefixes.
  1. Numbering of the various substituents and bonds with their locants. If there is more than one of the same type of substituent/double bond, a prefix is added showing how many there are.

Table (di - 2 tri - 3 tetra - 4 then as for the number of carbons below with 'a' added)

The numbers for that type of side chain will be grouped in ascending order and written before the name of the side-chain. If there are two side-chains with the same alpha carbon, the number will be written twice. Example: 2,2,3-trimethyl- . If there are both double bonds and triple bonds, "en" (double bond) is written before "yne" (triple bond). When the main functional group is a terminal functional group (A group which can only exist at the end of a chain, like formyl and carboxyl groups), there is no need to number it.

  1. Arrangement in this form: Group of side chains and secondary functional groups with numbers made in step 3 + prefix of parent hydrocarbon chain (eth, meth) + double/triple bonds with numbers (or "ane") + primary functional group suffix with numbers. Wherever it says "with numbers", it is understood that between the word and the numbers, the prefix (di-, tri-) is used.
  2. Adding of punctuation:
    1. Commas are put between numbers (2 5 5 becomes 2,5,5)
    2. Hyphens are put between a number and a letter (2 5 5 trimethylheptane becomes 2,5,5-trimethylheptane)
    3. Successive words are merged into one word (trimethyl heptane becomes trimethylheptane)
      Note: IUPAC uses one-word names throughout. This is why all parts are connected.

The finalized name should look like this:
Note: # is used for a number. The group secondary functional groups and side chains may not look the same as shown here, as the side chains and secondary functional groups are arranged alphabetically. The di- and tri- have been used just to show their usage. (di- after #,#, tri- after #,#,#, etc.).

Here is a sample molecule with the parent carbons numbered:

For simplicity, here is an image of the same molecule, where the hydrogens in the parent chain are removed and the carbons are shown by their numbers:

Now, following the above steps:

  1. The parent hydrocarbon chain has 23 carbons. It is called tricosa-.
  2. The functional groups with the highest precedence are the two ketone groups.
    1. The groups are on carbon atoms 3 and 9. As there are two, we write 3,9-dione.
    2. The numbering of the molecule is based on the ketone groups. When numbering from left to right, the ketone groups are numbered 3 and 9. When numbering from right to left, the ketone groups are numbered 15 and 21. 3 is less than 15, therefore the ketones are numbered 3 and 9. The smaller numberis always used, not the sum of the constituents numbers.
  3. The side chains are: an ethyl- at carbon 4, an ethyl- at carbon 8, and a butyl- at carbon 12.
    Note:The -O-CH3at carbon atom 15 is not a side chain, but it is a methoxy functional group
    1. There are two ethyl- groups. They are combined to create, 4,8-diethyl.
    2. The side chains are grouped like this: 12-butyl-4,8-diethyl. (But this is not the final grouping, as functional groups may be added in between.)
  • The secondary functional groups are: a hydroxy- at carbon 5, a chloro- at carbon 11, a methoxy- at carbon 15, and a bromo- at carbon 18. Grouped with the side chains, this gives 18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15-methoxy
  • There are two double bonds: one between carbons 6 and 7, and one between carbons 13 and 14. They would be called "6,13-diene", but the presence of alkynes switches it to 6,13-dien. There is one triple bond between carbon atoms 19 and 20. It will be called 19-yne
  • The arrangement (with punctuation) is: 18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15-methoxytricosa-6,13-dien-19-yne-3,9-dione
  • Finally, due to Cis-trans isomerism, we have to specify the relative orientation of functional groups around each double bond. For this example, we have (6E,13E)

The final name is (6E,13E)-18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15

 *   *  *

How to name organic compounds using the IUPAC rules?

In order to name organic compounds you must first memorize a few basic names. These names are listed within the discussion of naming alkanes. In general, the base part of the name reflects the number of carbons in what you have assigned to be the parent chain. The suffix of the name reflects the type(s) of functional group(s) present on (or within) the parent chain. Other groups which are attached to the parent chain are called substituents.

  • Alkanes - Saturated hydrocarbons ;

The names of the straight chain saturated hydrocarbons for up to a 12 carbon chain are shown below. The names of the substituents formed by the removal of one hydrogen from the end of the chain is obtained by changing the suffix -ane to -yl.

Number of Carbons


























  • There are a few common branched substituents which you should memorize. These are shown below. 

Here is a simple list of rules to follow. Some examples are given at the end of the list. 

  1. Identify the longest carbon chain. This chain is called the parent chain.
  2. Identify all of the substituents (groups appending from the parent chain).
  3. Number the carbons of the parent chain from the end that gives the substituents the lowest numbers. When compairing a series of numbers, the series that is the "lowest" is the one which contains the lowest number at the occasion of the first difference. If two or more side chains are in equivalent positions, assign the lowest number to the one which will come first in the name.
  4. If the same substituent occurs more than once, the location of each point on which the substituent occurs is given. In addition, the number of times the substituent group occurs is indicated by a prefix (di, tri, tetra, etc.).
  5. If there are two or more different substituents they are listed in alphabetical order using the base name (ignore the prefixes). The only prefix which isused when putting the substituents in alphabetical order isiso as in isopropyl or isobutyl. The prefixes sec- and tert- are not used in determining alphabetical order except when compared with each other.
  6. If chains of equal length are competing for selection as the parent chain, then the choice goes in series to:
    a) the chain which has the greatest number of side chains.
    b) the chain whose substituents have the lowest- numbers.
    c) the chain having the greatest number of carbon atoms in the smaller side chain.
    d)the chain having the least branched side chains.
  7. A cyclic (ring) hydrocarbon is designated by the prefix cyclo-which appears directly in front of the base name.

In summary, the name of the compound is written out with the substituents in alphabetical order followed by the base name (derived from the number of carbons in the parent chain). Commas are used between numbers and dashes are used between letters and numbers. There are nospaces in the name.

Here are some examples :

 Alkyl halides
The halogen is treated as a substituent on an alkane chain. The halo- substituent is considered of equal rank with an alkyl substituent in the numbering of the parent chain. The halogens are represented as follows:









  • Here are some examples:

Alkenes and Alkynes - Unsaturated Hydrocarbons :

Double bonds in hydrocarbons are indicated by replacing the suffix -ane with -ene. If there is more than one double bond, the suffix is expanded to include a prefix that indicates the number of double bonds present (-adiene-atriene, etc.). Triple bonds are named in a similar way using the suffix -yne. The position of the multiple bond(s) within the parent chain is(are) indicated by placing the number(s) of the first carbon of the multiple bond(s) directly in front of the base name.

Here is an important list of rules to follow:

  1. The parent chain is numbered so that the multiple bonds have the lowest numbers (double and triple bonds have priority over alkyl and halo substituents).
  2. When both double and triple bonds are present, numbers as low as possible are given to double and triple bonds even though this may at times give "-yne" a lower number than "-ene". When there is a choice in numbering, the double bonds are given the lowest numbers.
  3. When both double and triple bonds are present, the -en suffix follows the parent chain directly and the -yne suffix follows the -en suffix (notice that the e is left off, -en instead of -ene). The location of the double bond(s) is(are) indicated before the parent name as before, and the location of the triple bond(s) is(are) indicated between the -en and -yne suffixes. See below for examples.
  4. For a branched unsaturated acyclic hydrocarbon, the parent chain is
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