Strange MCAT Ochem practice question

[ponybreeder4]

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Which of the following molecules has 3 chemically equivalent hydrogen atoms?
(a) Cyclohexane
(b) 2-methylbutane
(c) 2,3-dimethylbutane
(d) Pentane



dont both c and d work? am i missing something?
the answer says just d

 

[EP123]

For 2,3-dimethylbutane, the attached methyl groups are equivalent to the adjacent main chain methyl groups (C1 and C4). If you look at it as an ethane with 2 methyl groups attached on both sides, then there are really only 2 equivalent hydrogen atoms. The one on the methyls and the CH.

 

[monomania]

I thought the same thing at first, but c) actually has two equivalent hydrogens. To see this, first draw the butane chain "cis" style, then add in the two methyl groups. There are two planes of symmetry. You'll see that the first set of equivalent hydrogens are those attached to the tertiary carbons, whereas the second set are those attached to the primary carbons. Hope that makes sense.

 

[Doodl3s]

You're right, I'm curious about this too since i always considered myself good at orgo >_<

the only thing i can come up with is the fact that C actually has like 12 equivalent hydrogens.... if you draw it you'll see 4 methyl groups in the same environment...

B has 6 equivalent (2 methyls attached to the same carbon)
A has 2 sets of 6 equivilent hydrogens. (or six sets of 2)

D then would TECHNICALLY have 6 identical, since the two ends are in the same environment, but .... i dunno really lol... the other ones all have ALOT more than 3 identical though, i can tell you that much!

EDIT: according the other answers, this question is really "which of these have 3 Different types of hydrogens"?

SECOND EDIT: on looking up equivalent hydrogens, i get it now. So to solidify if i understand this, A has 1 equivalent hydrogen, B has 4, C has 2, an D has 3

 

[ponybreeder4]

ah yes. thank u for the help guys. now that i drew it cis it all makes sense...bit of a brain fart there

 

[UTsksk]

yeah, doodl3s got it right 😀

 

[0Complications]

Yeah, Dood3s right. I was going to send them an email last night when I got that question and dispute it's validity then I realized they were speaking about discrete environments.

Still not the greatest question.