Help! Nomenclature of this molecule:

[confusedgradu8]

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What would be the name of this molecule?


I thought it would be 2-methyl-trans-3-pentene.
But the books solution says its 4-methyl-trans-2-pentene.

Im not sure why...but I thought you have to pick the numbering with the lowest sum...

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[koopa_troopa]

The alkene is the main group of this molecule and should have the lowest number (2), which is why the book chose 2-pentene.

 

[DoctorRx1986]

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What would be the name of this molecule?


I thought it would be 2-methyl-trans-3-pentene.
But the books solution says its 4-methyl-trans-2-pentene.

Im not sure why...but I thought you have to pick the numbering with the lowest sum...

In the case of alkenes and alkynes, you start numbering at the carbon nearest the first carbon atom of the double bond. You assign the lowest numbers to double or triple bonds. Take this molecule, CH3CH=CHCH2CH2CH3. You start numbering it from the first carbon of the methyl group on the left. Upon doing so, you will realize the first carbon of the double bond will be a 2. The second carbon of that bond will be a 3. Once you realize this, you will see that the correct name of this molecule, stereochemistry assigned as well, is Trans-2-hexene.

Numbering the molecule beginning at the first carbon of the methyl group on the right would have resulted in assigning a 4 to the first carbon of the double bond and a 5 to the second carbon. In this case, you would incorrectly be naming the molecule as Trans-4-hexene. So, always remember to assign priority in numbering to a double bond and give it the lowest number.

 

[supertrooper66]

i don't think i've ever even seen nomenclature on a single practice test. definitely wasn't on my real mcat either!

but yea, like they said, you # the carbons based on what takes highest priority. methyl is like the LAST priority of everything haha.

 

[gogopogo]

i don't think i've ever even seen nomenclature on a single practice test. definitely wasn't on my real mcat either!

I agree. Much easier to be, like "certain classes of sulfonamides are blah blah blah..." and just expect it to be understood.

P.S. - Lactone: Cyclic ester (Looks like a guitar sorta)

 

[Quantum Chem]

Although this really doesn't apply to this molecule, I want to remind everyone that there are E and Z identifiers for some alkenes. Remember to rate each side of the alkene with the highest molecular weight substituent and if the two are facing the same side its Z and if they are opposite it would be E. So if done on this molecule it would be (E) 4-methyl-2-pentene.