Intro to Organic Chemistry Question 1.21

[Jade Bowman 1A]

The answer to part a in the book says that the name of the cycloalkene is 5-methylcyclopenta-1,3-diene. Can you also write the name like 5-methyl-1,3-cyclopentadiene?

[Chem_Mod]

yes absolutely

[jgreynoso 2J]

How come it is not 1,4 for the pentadiene and 3 for the methyl? Do you want lower numbers for the double bonds first?

Thank you.

[Chem_Mod]

Multiple bonds have completely higher priority than prefixed substituents. You will choose the lowest possible numbers for the double bonds (which can potentially force the number of the substituent to become a high number, but this is ok)

[jgreynoso 2J]

Thank you! So how come on page 43 of the organic chem book, question 1.35, the numbering starts with the tert and then the 3 is on the substituent? Should not the numbering follow the double bond initially on the tert-butyl, causing the substituent to have a number of 5?

[Chem_Mod]

1.35 is tricky. Actually, due to the resonance in the benzene ring, if you shift all the double bonds by 1 position, it represents the exact same molecule.

Another way to think about it is that you can draw benzene as a hexagon with a circle inside, rather than a hexagon with 3 double bonds. Once you do this, the lowest way to number would be 1,3 rather than 1,5.