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Alkenes longer than ethene have an added complication. It is essential to state the relative position of a double bond. Butene for example can have the double bond in two different positions (Hydrogen atoms ommitted): C=C-C-C or C-C=C-C (C-C-C=C is identical to C=C-C-C). The first is still called Butene, the second is But-2-ene. The 2 comes from the double bond being attached to the second carbon in the chain.
Alkenes longer than ethene have an added complication. It is essential to state the relative position of a double bond. Butene for example can have the double bond in two different positions (Hydrogen atoms ommitted): C=C-C-C or C-C=C-C (C-C-C=C is identical to C=C-C-C). The first is still called Butene, the second is But-2-ene. The 2 comes from the double bond being attached to the second carbon in the chain.
Using butene as an example again it can be seen that there is enough space for more than one double bond: C=C=C-C, C=C-C=C or even C=C=C=C. These are respectively called But-1,2-diene, But-1,3-diene and but-triene. Again the numbers 1,2 and 3 signify the position the carbon bond. but-triene does not need any positioning numbers because there is no other way of arranging the double bonds. The 'di' and 'tri' signify that there is more than one double bond in the chain.
Using butene as an example again it can be seen that there is enough space for more than one double bond: C=C=C-C, C=C-C=C or even C=C=C=C. These are respectively called But-1,2-diene, But-1,3-diene and but-triene. Again the numbers 1,2 and 3 signify the position the carbon bond. but-triene does not need any positioning numbers because there is no other way of arranging the double bonds. The 'di' and 'tri' infixes signify that there is more than one double bond in the chain. The sum of the positioning numbers should always be as low a possible ie. although But-3,4-diene is understandable as C-C=C=C, the sum of the numbers (3+4=7) is more than the sum of But-1,2-diene (1+2=3) which is an identical compound.
===Alkynes===
===Alkynes===