a) Hex-lene-5yne
b) He X-lyne-5ene
which to prefer while
numbering - Double Bond
or triple bond?
Pls refer to attachment
Answers
The question you asked is a very good and important one.
When a double bond and triple bond are present in the same hydrocarbon molecule, we usually follow these rules given by IUPAC:
=> In all such hydrocarbons, the suffix should ALWAYS be -yne and NOT
-ene.
=> If one bond is present closer to the ends of the molecule than the other, whether it is a double or triple bond, we always begin numbering from that bond.
=> Otherwise, in all such cases, we give preference to the double bond, i.e the double bond gets the lower numbering and the molecule ends in -yne.
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=> In this molecule, both the double bond and the triple bond are present at the ends of the molecule.
=> So, we begin numbering from the double bond, or from the right of the molecule.
=> The double bond gets the lowest numbering of 1 and the triple bond gets the number 5.
=> The suffix has to be "-yne" and not "-ene".
Accordingly, the IUPAC name of the molecule will be:
(a) Hex-1-en-5-yne.