1.12 Naming a substituted alkene

[Jessie Chu 2G]

For part c of 1.12 in the O-chem book, can someone explain why the answer is 3,4,4-trimethyl-2-pentene instead of 2,2,3-trimethyl-3-pentene? Don't the functional groups get the highest priority when numbering?

Thanks!

[Niharika Reddy 1D]

Yes, functional groups get highest priority when numbering, but this alkene does not have any typical functional groups attached (atoms or atom groups that involve other elements in addition to carbon and hydrogen atoms). In the intro to ochem book, it says carbon-carbon double and triple bonds may be considered functional groups because they give molecules characteristic structures and properties. Therefore, the double bond (alkene) is given the highest priority and must have the lowest number. Methyl is an alkyl; it's just a substituent, not a functional group, and it has lower priority than an alkene (double bond). Since 2<3 for the numbering of the alkene (double bond), the correct name is 3,4,4-trimethyl-2-pentene.

[Chem_Mod]

Yes, functional groups get highest priority when numbering, but this alkene does not have any typical functional groups attached (atoms or atom groups that involve other elements in addition to carbon and hydrogen atoms). In the intro to ochem book, it says carbon-carbon double and triple bonds may be considered functional groups because they give molecules characteristic structures and properties. Therefore, the double bond (alkene) is given the highest priority and must have the lowest number. Methyl is an alkyl; it's just a substituent, not a functional group, and it has lower priority than an alkene (double bond). Since 2<3 for the numbering of the alkene (double bond), the correct name is 3,4,4-trimethyl-2-pentene.

Correct, the double bond gets the priority in numbering.