Dicarboxylic acids naming paradox inside [1 question]

[DrTacoElf]

Ok lets say you have hexane dioic acid, and it is subsituted by one ethyl group and one propyl group. Each group is on the carbon alpha to a different carbonyl group. Which alkyl group would take precedence. In the kaplan book they chose ethyl but i'm am unsure to as why, because i thought it was based on size, what they did seems to have it based on alphabet.

Kaplan Answer: 2-ethyl-5-propylhexanedioic acid
My Answer (Is this Wrong, and if so why?) 5-ethyl-2-propylhexanedioic acid

Thanks!

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[txguy]

Nice question!

With the size thing, I think you are referring to placing substituents axial/equatorial on a six membered ring. (bigger groups go equatorial, if possible due to stability).

Just my opinion, but I would say this would not come up on the MCAT. Either way looks good to me--they would only catch you on the order in the naming, which you have correct by alphabet. If all else fails, go with what Kaplan has as their answer.

-tx

 

[liverotcod]

The alkyl substituents have equal priority (versus, say, a hydroxy or alkene), so I believe they get numbered in alphabetic order. But it's a pretty technical rule that often gets violated and I doubt would appear on an actual MCAT.

 

[SCB]

I agree that this would most likely not end up on the MCAT. In the practice tests I've taken (EK & AAMC) and two real MCATs I never saw anything that technical.

 

[CanIMakeIt]

The alkyl substituents have equal priority (versus, say, a hydroxy or alkene), so I believe they get numbered in alphabetic order. But it's a pretty technical rule that often gets violated and I doubt would appear on an actual MCAT.

Thats the correct rule ..... when both have equal priority, alphabetical order is used to determine the order.