Where do I put my CH3 groups in a condensed formula?

I’ve been getting a lot of office visits and emails over the same homework problem. Folks say, “I typed CH3CH2CH(CH3)CH(CH3)CH3” or “I typed CH3(CH3)CHCH…” or “I typed CH3CH(CH3)CH…” and it was marked incorrect. =(

Unfortunately, it is incorrect. The CH is attached to two identical CH3 groups so they need to be collected together in the condensed formula. (CH3)2CHCH… or …CHCH(CH3)2 are both acceptable.

From the conversations I’ve had I think the problem is conflating IUPAC (systematic) naming rules with condensed formula drawing rules. The former requires a distinction between parent chain atoms …CHCH3 and substituents -CH3, but the condensed formula does not.

Thanks for your questions.

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