Alkyne Nomenclature | Common vs IUPAC System – Organic Chemistry

Alkynes

Alkynes are unsaturated hydrocarbons which have at least one carbon-carbon triple bond and a general molecular formula CnH2n-2.  Their functional group is – C ≡ C –.

Alkyne Nomenclature:

  • Common System:

In the common system, alkynes are considered alkyl derivatives of acetylene (the smallest member of the family).

  • First names of alkyl groups present around the acetylenic group are written and then suffix acetylene is added.
  • Prefix n, iso, neo etc is used with names of propyl and higher alkyl groups as isomerism occurs in these groups.
  • Names of different alkyl groups are written in alphabetical order.
  • If alkyl groups have equal number of carbons then the one with no branch or less branch is written first. 
  • IUPAC System:

Rules of IUPAC System:

IUPAC System involves the following steps:

  Step-I    Chain selection:

  • Select the longest possible continuous carbon chain containing the carbon of the functional group and if the functional group has no carbon in it then the carbon to which the functional group is attached should be included in the selected chain.
  • If more than one longest chains of an equal number of carbons are there then the one which has more branches or substituents or side chains is selected.

   Step-II  Numbering:

  • Carbon atoms of the selected chain (parent chain) are numbered from one end to another end such that the locant or sum of locants of the functional group(s) should be smaller.
  • If the locant or sum of locants of the functional group(s) is the same for both ends then numbering is carried out from that end which gives the smallest locant to the first substituent.
  • If the locant of the first substituent is the same for both ends then numbering is carried out from that end which gives the smaller sum of locants of all the substituents.
  • If the sum of locants of all substituents is also the same for both ends then numbering is carried out from that end which gives smaller locant to more electronegative group(s) like halogen, NH2 etc.
  • If the locant of the more electronegative group is also the same for both ends then the last preference is given to the larger alkyl group.

Step-III Naming:

  • Names of substituents are written before the parent name and names of substituents are preceded by their locants. In the end, the parent name is written according to carbons in the selected chain and the parent name is preceded by locant(s) of functional group(s).
  • If the same substituent is occurring more than one time then prefixes di, tri, tetra, Penta etc are used with its name and the locant of each is written each time.
  • If substituents of more than one kind are there then their names are written in alphabetical order.
  • Two numbers are separated by a comma between them; a number and a name are separated by a hyphen between them while two names are separated by a space between them.           

Names of alkynes is derived from the name of corresponding alkanes by replacing the last three letters with the new suffix “yne”.

If triple bonds are more than one then the last two letters are dropped and the suffix diyne, triyne etc is added.


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