Study the following examples of straight-chain alkynes:

Alkyne Structure	IUPAC Name	Model
	ethyne
	propyne
	but-1-yne
	but-2-yne
	pent-1-yne
	pent-2-yne
	hex-1-yne
	hex-2-yne
	hex-3-yne

Note from the above names that the locant is only the first number of the two which identify the position of the triple bond. Thus although the triple bond in hex-3-yne is between carbons numbered 3 and 4 in the chain, only the first is required to position the triple bond, the second being understood to be the next higher number.
This observation requires that the triple bond be included in the main chain that is used for the root name and requires a change in the rules for picking the carbon chain used for the root name:

Changes and additions to the rules for naming compounds are thus:

• the root name is chosen for the longest continuous chain of carbon atoms which contains either the carbon(s) of the functional group or the carbon bearing the functional group.

• for double or triple C,C bonds, the whole bond must be in the main chain and the locant identifies only the lowest numbered carbon of the two.

• the functional group is given the lowest possible number.

Note also the position of the locant in the name. IUPAC now puts the locant immediately in front of the group that it is positioning: hex-3-yne rather than 3-hexyne.

Examine the following names for other alkynes:

Alkyne Structure	IUPAC Name	Model
	3-methylbut-1-yne
	2,2-dimethylhex-3-yne
	3-propylhept-1-yne

Note the last example, which is named as a heptyne: the longest continuous chain of carbons is eight, but this does not contain the functional group.

If more than one C,C triple bond is present, the name can be produced by using diyne, triyne, etc:

Alkyne Structure	IUPAC Name	Model
	oct-2,4-diyne
	2,2,9,9-tetramethyldec-3,5,7-triyne

Next page : Functional Groups with Suffix Only III Alkenes.

Index