15 Important Rules for IUPAC Nomenclature

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IUPAC Nomenclature

Organic compounds are named in accordance with the International Union of Pure and Applied Chemistry (IUPAC). A variety of prefixes, suffixes, and infixes are used in chemistry to describe the type and position of functional groups in a compound.

15 Important Rules for IUPAC Nomenclature are as follows:

  1. The parent hydrocarbon chain is identified. In order of precedence, the following rules must be followed by this chain:
  2. It should have the maximum number of suffix functional group substituents. In contrast to halogen substituents, suffix means that the parent functional group should have a suffix. If there are multiple functional groups, the one with the highest group precedence should be used.
  3. It should have as many multiple bonds as possible.
  4. It should be as long as possible.
  5. It should have as many substituents or branches as possible listed as prefixes.
  6. It should have as many single bonds as possible.
  7. The parent functional group, if any, is identified in the highest order of precedence.
  8. The side-chains are identified. Side chains are carbon chains that do not belong to the parent chain but branch off from it.
  9. Identifying and naming the remaining functional groups, if any, using their ionic prefixes (such as hydroxy for -OH, oxy for =O, oxyalkane for O-R, and so on). Different side-chains and functional groups will be alphabetically grouped together. (The prefixes di-, tri-, and so on are not considered for alphabetical grouping.) For example, ethyl comes before dihydroxy or dimethyl because the “e” in “ethyl” alphabetically precedes the “h” in “dihydroxy” and the “m” in “dimethyl.” In either case, the “di” is ignored). When both side chains and secondary functional groups are present, they should be written as one group rather than separately.
  10. Double/triple bond identification
  11. The chain’s numbering. This is accomplished by first numbering the chain in both directions (left to right and right to left), and then selecting the numbering that adheres to these rules in order of precedence.
  12. The suffix functional group’s lowest-numbered locant (or locants). The locants are the carbon numbers to which the substituent is directly attached.
  13. Has the fewest locants for multiple bonds (The locant of a multiple bond is the number of the adjacent carbon with a lower number).
  14. Has the fewest number of prefix locants.
  15. Numbering the various substituents and bonds in relation to their locants. If there are multiples of the same type of substituent/double bond, a prefix indicating the number is added (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 listed before the name of the side chain in ascending order. The number will be written twice if there are two side-chains with the same alpha carbon. 2,2,3-trimethyl- is an example. When there are both double and triple bonds, “en” (double bond) comes before “yne” (triple bond). There is no need to number the main functional group when it is a terminal functional group (a group that can only exist at the end of a chain, such as formyl and carboxyl groups).

The stem names are derived from the names of hydrocarbons. Acyclic and cyclic saturated hydrocarbons (alkanes) in the range C1 –C12 are listed in table given below

IUPAC_Nomenclature

Aromatic systems are named similarly, but additional stem names are required. Important parent aromatic compounds include benzene, naphthalene, anthracene, and phenanthrene. The last three have fused rings and a fixed numbering system that includes only positions where substitution can occur.


In some cases, alternative names are displayed to emphasise that there is rarely a single ‘correct’ name. It can sometimes be advantageous to bend the rules slightly in order to provide a neat name rather than a fully systematic one. In most cases, this means using a lower priority functional group as the suffix name.

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Also Read: Redox Titrations- Definition, Principle, Types, Redox Indicators

Examples of IUPAC Nomeclature

image 4 15 Important Rules for IUPAC Nomenclature
• alkenes have higher priority than halides; suffix is –ene
• longest carbon chain is seven carbons: heptanes
• numbering is chosen to give lowest numbers for the double bonds; 2-ene denotes 2,3-double bond, 4-ene denotes 4,5-double bond
• the European system hepta-2,4-diene is less prone to errors than the US system 2,4-heptadiene
• an additional syllable -a- is used but is not obligatory; heptadiene is easier to say than heptdiene
image 5 15 Important Rules for IUPAC Nomenclature

• acids have higher priority than amines; remember ‘amino acids’
• suffix is -oic acid
• one of the methyls is part of the five carbon chain, the others are substituents
• note the use of 4,4-, which shows both methyls are attached to the same carbon; 4-dimethyl would not be as precise.

image 6 15 Important Rules for IUPAC Nomenclature
• highest priority group is ketone; suffix –one
• longest carbon system is the ring cyclohexane
• numbering is around the ring starting from ketone as position 1
• 2,5-diene conveys 2,3- and 5,6-double bonds
• note 2,5-dienone means two double bonds and one ketone; contrast endione which would be one double bond and two ketones

image 7 15 Important Rules for IUPAC Nomenclature
• simple ethers are best named as an alkyl alkyl ether
• the phenylmethyl group is commonly called benzyl
• an acceptable alternative is as an alkoxy alkane: the alternative ethoxytoluene would require an indication of the point of attachment
• the second alternative invokes a three-carbon chain with one carbon replaced by oxygen; this is indicated by using the extra syllable -oxa-, so the chain becomes 2-oxabutane

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